Securities registered pursuant to section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes o    No x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act. Yes o    No x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes x    No o

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 229.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files). Yes x    No o

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§ 229.405 of this chapter) is not contained herein, and will not be contained, to the best of registrant's knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. o

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. o

Indicate by check mark whether the registrant is a shell company (as defined by Rule 12b-2 of the Act). Yes o    No x

The aggregate market value of the voting stock held by non-affiliates of the registrant as of June 30, 2017, the last business day of the registrant's most recently completed second fiscal quarter, was approximately $343,111,833 (computed by reference to the price at which the registrant's common stock was last sold on such date, as reported by The Nasdaq Global Select Market). Shares of common stock held by the registrant's officers and directors and beneficial owners of 10% or more of the outstanding shares of the registrant's common stock have been excluded from the calculation of this amount because such persons may be deemed to be affiliates of the registrant; however, the treatment of these persons as affiliates of the registrant for purposes of the calculation of this amount is not, and shall not be considered, a determination as to whether any such person is an affiliate of the registrant for any other purpose.

As of March 6, 2018, the number of outstanding shares of the registrant's common stock was 152,394,550.

Portions of the registrant's definitive proxy statement for its 2018 annual meeting of stockholders are incorporated in Part III of this report by reference, to the extent stated therein.

For the Fiscal Year Ended December 31, 2017

Unless the context indicates otherwise, all references to “Clean Energy,” the “Company,” “we,” “us,” or “our” in this report refer to Clean Energy Fuels Corp., together with its majority and wholly owned subsidiaries.

We serve fleet vehicle operators in a variety of markets, including heavy-duty trucking, airports, refuse, public transit, industrial and institutional energy users, and government fleets. We believe these fleet markets will continue to present a growth opportunity for natural gas vehicle fuel for the foreseeable future. As of December 31, 2017, we serve nearly 1,000 fleet customers operating over 46,000 natural gas vehicles, and we own, operate or supply over 530 natural gas fueling stations in 42 states in the United States and four provinces in Canada. We estimate this number of stations is approximately four times the number of CNG fueling stations operated by our largest competitor in today’s market, and we believe our natural gas fueling operations cover more states and provinces than any of our competitors. We believe we are the only company in the United States or Canada that provides both CNG and LNG vehicle fuel on a significant scale.

Natural Gas Vehicles for America ("NGV America") estimates that, as of December 31, 2017, there were approximately 1,750 natural gas fueling stations in the United States and 153,000 natural gas vehicles on American roads, including approximately 39,500 heavy -duty vehicles (such as heavy -duty trucks, refuse trucks and buses), 25,800 medium-duty vehicles (such as delivery vans and shuttles) and 87,000 light-duty vehicles (such as passenger cars, small utility vehicles, trucks and vans).

Domestic and Plentiful Supply.    Technological advances in natural gas drilling and production, including the widespread deployment of horizontal drilling techniques and the use of hydraulic fracturing, have unlocked vast natural gas reserves. The United States has proven, abundant and growing reserves of natural gas, and produces the highest volume of natural gas in the world.

Less Expensive.    Due to the abundance of natural gas, the cost of natural gas in the United States is less than the cost of crude oil on an energy equivalent basis. Based on projections from the U.S. Energy Information Administration, we believe natural gas will remain cheaper than gasoline and diesel for the foreseeable future. In addition, because the price of the natural gas commodity makes up a smaller portion of the cost of a GGE of CNG or LNG relative to the commodity portion of the cost of a GGE of diesel or gasoline, the price of a GGE of CNG or LNG is less sensitive to increases in the underlying commodity cost.

Cleaner.    Natural gas contains less carbon than any other fossil fuel and, as a result, produces fewer carbon dioxide emissions when burned.  The California Air Resources Board ("CARB") has concluded that a vehicle fueled by natural gas has fewer greenhouse gas emissions than a comparable vehicle fueled by gasoline or diesel, on a well-to-wheel basis. Additionally, a study from Argonne National Laboratory, a research laboratory operated by the University of Chicago for the U.S. Department of Energy, indicates that natural gas vehicles produce between 13% and 21% fewer greenhouse gas emissions than comparable gasoline- and diesel -fueled vehicles.

Safer.    As reported by NGV America, CNG and LNG are relatively safer than gasoline and diesel because they dissipate into the air when spilled or in the event of a vehicle accident. When released, CNG and LNG are also less combustible than gasoline or diesel because they ignite only at relatively high temperatures. The fuel tanks and systems used in natural gas vehicles are subjected to a number of federally required safety tests, such as fire, environmental hazard, burst pressure and crash testing, according to the U.S. Department of Transportation National Highway Traffic Safety Administration. CNG and LNG are stored in above-ground tanks and therefore will not contaminate soil or groundwater in the event of a spill or leak.

CNG Sales.    CNG is natural gas that is compressed and dispensed in gaseous form. CNG is typically delivered by obtaining natural gas from local utilities or third-party marketers and then compressing and storing it at a fueling station and dispensing it directly into a vehicle. Some of the natural gas we obtain from third parties for CNG sales is purchased under take-or-pay contracts that require us to purchase minimum volumes of natural gas.

Through our subsidiary NG Advantage, LLC (“NG Advantage”), we also transport and sell CNG for non-vehicle purposes via virtual natural gas pipelines and interconnects. NG Advantage transports CNG to industrial and institutional energy users that do not have direct access to natural gas pipelines. NG Advantage also transports CNG between pipelines for customers that desire to take advantage of commodity price differences. NG Advantage uses a fleet of 94 high-capacity tube trailers to transport CNG and we anticipate that NG Advantage may need to purchase or lease additional trailers in the future to transport CNG in support of its operations.

LNG Production and Sales.    LNG is natural gas that is cooled at a liquefaction facility to approximately -260 degrees Fahrenheit until it condenses into a liquid. We obtain LNG from our own liquefaction plants and from third -party suppliers. We own and operate LNG liquefaction plants near Houston, Texas and Boron, California, which we call the "Pickens Plant" and the "Boron Plant," respectively. The Pickens Plant has the capacity to produce 35.0 million gallons of LNG per year and includes a tanker trailer loading system and a 1.0 million gallon storage tank that can hold up to 840,000 usable gallons. The Boron Plant is

capable of producing 60.0 million gallons of LNG per year and has a dual tanker trailer loading system and a 1.8 million gallon storage tank that can hold up to 1.5 million usable gallons. In 2017, we purchased 39% of our LNG from third-party suppliers and we produced the remainder of our LNG at the Pickens Plant and the Boron Plant. Some of the natural gas we obtain from third parties for LNG sales is purchased under take-or-pay contracts that require us to purchase minimum volumes of natural gas.

We sell LNG for use as a vehicle fuel on a bulk basis to fleet customers, who often own and operate their fueling stations, and through our network of public access fueling stations. We deliver LNG via our fleet of 84 tanker trailers to fueling stations, where it is stored and then dispensed in liquid form into vehicles. We contract with third parties to provide tractors and drivers. The need to liquefy and transport LNG generally causes LNG to cost more than CNG. We sell LNG through supply contracts that are priced on an index-plus basis, such that LNG sales under these contracts increase or decrease as a result of an increase or decrease in the cost of natural gas. We also sell LNG vehicle fuel on a per fill-up basis at prices we set at public access stations based on prevailing market conditions. Additionally, we sell LNG for non-vehicle purposes, including to customers who use LNG in oil fields, and for industrial, utility, marine and rail applications.

O&M Services.   We perform O&M services for CNG and LNG fueling stations that we do not own. For these services, we generally charge a fixed or a per-gallon fee based on the volume of fuel dispensed at the station. We have an operations team that performs preventive maintenance and is available to respond to service requests.

AFTC. Under separate pieces of U.S. federal legislation, we have been eligible to receive the AFTC alternative fuels tax credit for our natural gas vehicle fuel sales made between October 1, 2006 and December 31, 2017. The AFTC credit is equal to $0.50 per gasoline gallon equivalent of CNG that we sold as vehicle fuel, $0.50 per liquid gallon of LNG that we sold as vehicle fuel through 2015, and $0.50 per diesel gallon equivalent of LNG that we sold as vehicle fuel in 2016 and 2017. Based on the service relationship with our customers, either we or our customers claim the credit. AFTC revenue for the 2017 calendar year will be recognized and collected in 2018. AFTC is not available, and may not be reinstated, for vehicle fuel sales after December 31, 2017.

Station Construction and Engineering.    We design and construct fueling stations and facility modifications and sell or lease some of these stations to our customers. We charge construction or other fees or lease rates based on the size and complexity of the project. Since 2008, we have served as the general contractor or supervised qualified third-party contractors to build 436 natural gas fueling stations. We acquired the additional stations that we own but did not build through acquisitions of assets or businesses. We use a combination of custom designed and off-the-shelf equipment to build fueling stations. Equipment for a CNG station typically consists of dryers, compressors, dispensers and storage tanks. Equipment for a LNG station typically consists of storage tanks and dispensing equipment. Many of our fueling stations have separate public access areas for retail customers, which generally have the look, feel and dispensing rates of gasoline and diesel fueling stations. We also offer assessment, design and modification solutions to provide operators with code-compliant service and maintenance facilities for natural gas vehicle fleets. For example, our NGV Easy Bay product is a natural gas vapor leak barrier developed specifically for natural gas vehicle facilities.

RNG Sales.    RNG is produced from waste streams such as landfills, animal waste digesters and wastewater treatment plants. RNG production plants are connected to natural gas pipelines, which allow RNG to be transported to vehicle fueling stations where it can be compressed and dispensed as CNG, and to LNG liquefaction facilities where it is converted to LNG. We purchase RNG from third -party producers, and we sell that RNG for vehicle fuel use through our fueling infrastructure under the brand name Redeem.    

Sales of RINs and LCFS Credits.   We generate RIN Credits when we sell RNG for use as a vehicle fuel in the United States, and we generate LCFS Credits when we sell RNG and conventional natural gas for use as a vehicle fuel in California and Oregon. We can sell these credits to third parties who need the RINs and LCFS Credits to comply with federal and state emission compliance requirements. Generally, the amount of RINs and LCFS Credits we generate increases as we sell higher volumes of natural gas as a vehicle fuel, but the amount of credits we sell and our revenue from these sales can vary depending on a number of factors, including the market for these credits, which has historically been volatile and subject to significant price fluctuations, any changes to the federal and state programs under which the credits are generated and sold, and our ability to strictly comply with these programs.

Vehicle Acquisition and Finance.    We offer vehicle finance services, including loans and leases, to help our customers acquire natural gas vehicles. As appropriate, we apply for and receive federal, state and local incentives associated with natural gas vehicle purchases and pass these benefits through to our customers. We may also secure vehicles to place with customers and/or pay deposits with respect to these vehicles before receiving a firm order from our customers, which we may be required to purchase if our customers fail to purchase the vehicle as anticipated.

In July 2017, we agreed to purchase 146 used natural gas heavy -duty trucks at a favorable price from a single seller, with the intention of selling the trucks to our customers.

Grant Programs.    We apply for and help our fleet customers apply for federal, state and local grant programs in areas in which we operate. These programs can provide funding for natural gas vehicle conversions and purchases, natural gas fueling station construction and natural gas vehicle fuel sales.

Former Activities.    Before March 31, 2017, we produced at our own production plants a portion of the RNG that we sold. On March 31, 2017, we completed the sale to BP Products North America, Inc. (“BP”) of certain assets related to this RNG production business, including two RNG production facilities, a 50% ownership interest in joint ventures formed to develop two new RNG production facilities, and third-party RNG supply contracts (we refer to this asset sale transaction as the “BP Transaction”). After the BP Transaction, we obtain all of the RNG we sell from third-party producers, including BP.

Trucking. We believe heavy-duty trucking represents one of the greatest opportunities for natural gas to be used as a vehicle fuel in the United States, and as of December 31, 2017, fuel over 3,000 heavy-duty trucks. Because these high-mileage vehicles consume substantial amounts of fuel, they can derive significant benefits from the lower cost of natural gas. Many well-known shippers, manufacturers, retailers and other truck fleet operators have started to adopt natural gas fueled trucks to move their freight. Such companies include Honda, Frito-Lay, FedEx, Anheuser-Busch, Verizon, Bimbo, The Home Depot, AT&T, Colgate-Palmolive, Costco Wholesale, Lowes, Pepsi, UPS, MillerCoors, HP, Unilever, Starbucks, Kraft, Kroger, P&G, Hertz and Owens Corning.

Airports. We estimate that vehicles serving airports in the United States, including airport delivery fleets, rental car and parking passenger shuttles and taxis, consume an aggregate of approximately two billion gallons of fuel per year. Additionally, many U.S. airports face emissions challenges and are under regulatory directives and political pressure to reduce pollution, particularly as part of any expansion plans. As a result, many of these airports have adopted various strategies to address tailpipe emissions, including rental car and hotel shuttle consolidation and requiring or encouraging service vehicle operators to switch their fleets to natural gas. To assist in this effort, airports are contracting with service providers to design, build and operate natural gas fueling stations in strategic locations on their properties.

As of December 31, 2017, we serve customers at 39 airports, including Atlanta Hartsfield Jackson International, Baltimore Washington International, Dallas-Ft. Worth International, Denver International, Dulles International (Washington D.C.), George Bush International (Houston), Las Vegas, Logan International (Boston), LaGuardia (New York City), John F. Kennedy International (New York City), Los Angeles International, Newark International, Oakland International, Orlando, Phoenix Sky Harbor International, San Francisco International, San Diego International, SeaTac International (Seattle) and Tampa International.

Refuse. According to INFORM, there are nearly 200,000 refuse trucks in the United States that collect and haul refuse and recyclables, which collectively consume approximately two billion gallons of fuel per year. We estimate that approximately 55% of new refuse trucks in 2017 operate on natural gas, up from approximately 3% of new refuse trucks in 2008. Refuse haulers are increasingly adopting trucks that run on CNG to realize operating savings and to address their customers' demands for reduced emissions.

As of December 31, 2017, we fuel over 11,000 refuse vehicles for customers including Waste Management and Republic Services, as well as other waste haulers such as Atlas Disposal, Burrtec, Recology, South San Francisco Scavenger, Waste Connections and Waste Pro, among others. We also provide vehicle fueling services to municipal refuse fleets, including fleets in Dallas, Los Angeles, San Antonio and New York City, among other locations.

Public Transit. According to the American Public Transportation Association, there are over 71,000 municipal transit buses operating in the United States. In many areas increasingly stringent emissions standards have limited the fueling options available to public transit operators. Also, transit agencies typically fuel at a central location and use high volumes of fuel. We estimate that transit agencies in the United States consume approximately 1.5 billion gallons of fuel per year. Many transit agencies have been early adopters of natural gas vehicles, and over 25% of existing transit buses and over 35% of new transit buses operate on natural gas.

As of December 31, 2017, we fuel close to 9,000 transit vehicles for customers including Los Angeles Metropolitan Transit Authority, Foothill Transit (Los Angeles County, California), Orange County Transit Authority, Santa Monica Big Blue Bus, Dallas Area Rapid Transit Phoenix Transit, New Jersey Transit, Jacksonville Transportation Authority, NICE Bus (Nassau County, New York) and Washington Metro Area Transportation Authority, as well as public transit customers in British Columbia.

Industrial and Institutional Customers. NG Advantage uses its virtual natural gas pipelines and interconnects to serve a number of customers that do not have direct access to natural gas pipelines or desire to take advantage of commodity price differences. We also transport LNG to customers via virtual natural gas pipelines.

Government Fleets. In 2015, 2016 and 2017, approximately 18%, 16% and 19% of our revenue, respectively, was derived from contracts with government entities, such as municipal transit fleets. As government regulations on pollution continue to become more stringent, government agencies are evaluating ways to make their fleets cleaner and run more economically.

Construction and Operation of CNG and LNG Stations. To construct a CNG or LNG fueling station, we must satisfy permitting and other requirements and either we or a third -party contractor must be licensed as a general engineering contractor. Each CNG and LNG fueling station must be constructed in accordance with federal, state and local regulations pertaining to station design, environmental health, accidental release prevention, above-ground storage tanks and hazardous waste and other materials. For fueling stations we operate, we are also required to register with certain state agencies as a retailer/wholesaler of CNG and LNG. We also may benefit from any grant programs or similar government incentives that may be available for the construction of natural gas fueling stations.

Transfer of LNG. Federal safety standards require each transfer of LNG to be conducted in accordance with specific written safety procedures. These procedures must require that qualified personnel be in attendance during all LNG transfer operations, and these procedures must be implemented, and copies of the procedures must be available/displayed, at each LNG transfer location.

Construction and Operation of LNG Liquefaction Plants. To build and operate LNG liquefaction plants, we must apply for facility permits or licenses that address many factors, including storm water and wastewater discharges, waste handling and air emissions related to production activities and equipment operation. The construction of LNG plants must also be approved by local planning boards and fire departments.

Vehicle Finance. State agencies generally require the registration of finance lenders. For example, in California, pursuant to the California Finance Lenders Law, one of our subsidiaries is required to be registered as a finance lender with the California Department of Corporations.

Generation and Sale of RIN Credits and LCFS Credits. In February 2010, the U.S. Environmental Protection Agency ("EPA") finalized the RFS (which was established by the Energy Policy Act of 1992/2005), which creates RINs that can be generated by the production and use of RNG in the transportation sector and sold to fuel providers that are not compliant under the RFS. In addition, CARB has adopted the Low Carbon Fuel Standard, which encourages low carbon "compliant" transportation fuels (including CNG, LNG and RNG) in the California marketplace by allowing producers of these fuels to generate LCFS Credits that can be sold to noncompliant regulated parties.

Sale of Natural Gas Vehicle Fuel: AFTC. We have been eligible to receive the AFTC alternative fuels tax credit for our natural gas vehicle fuel sales made between October 1, 2006 and December 31, 2017. On February 9, 2018, AFTC was retroactively extended from January 1, 2017 to December 31, 2017, and AFTC revenue for the 2017 calendar year will be recognized and collected in 2018. AFTC is not available, and may not be reinstated, for vehicle fuel sales after December 31, 2017. See the additional discussion regarding AFTC under “Our Products, Services and Other Business Activities” above.

Sale of Natural Gas Vehicle Fuel, Operation of Fueling Stations and Production of LNG: Greenhouse Gas Emissions Regulation. California has enacted laws and regulations that require specified greenhouse gas emissions reductions, and the federal government and many other state governments are considering similar measures. These regulations, if and when adopted and

As of December 31, 2017, we employed 431 people. We have not experienced any work stoppages and none of our employees is subject to collective bargaining agreements. We believe our employee relations are good.

We operate our business in one reportable segment. For information about our revenue, operating loss and long-lived assets in different geographic areas, see Note 17 to our consolidated financial statements included in this report.

In 2015, 2016 and 2017, we incurred pre-tax losses of $133.8 million, $12.4 million and $83.3 million, respectively. We may continue to incur losses, the amount of our losses may increase, and we may never achieve or sustain profitability, any of which would adversely affect our business, prospects and financial condition and may cause the price of our common stock to fall. In addition, to try to achieve or sustain profitability, we may take actions that result in material costs or material asset or goodwill impairments. For instance, in 2017, we recorded significant charges in connection with CEC’s natural gas fueling compressor business, our closure of certain fueling stations, our determination of an impairment of assets as a result of the foregoing, and certain other actions. Any similar actions in the future could have material adverse consequences, including material negative effects on our financial condition, our results of operations and the trading price of our common stock.

As of December 31, 2017, our total indebtedness was approximately $260.9 million in principal amount, and approximately $140.2 million, $54.5 million, $54.6 million, $4.2 million, $3.8 million and $3.6 million of this principal amount matures in 2018, 2019, 2020, 2021, 2022 and thereafter, respectively ($25.0 million matures in July 2018 and $110.5 million matures in October 2018). We expect our total interest payment obligations relating to our indebtedness to be approximately $15.6 million in 2018.

We may be required to pursue one or more alternatives to meet our debt obligations. For instance, we are permitted to issue up to 14.0 million shares of common stock to repay a portion of the principal amount of our outstanding 7.5% convertible notes. Any repayment of our debt with equity would increase the number of our outstanding shares and may significantly dilute the ownership interests of our existing stockholders. Additionally, any shortfall of cash from operations to service our debt may lead us to seek capital from other sources, such as selling assets, restructuring or refinancing our debt or obtaining additional equity capital or debt financing. Our ability to engage in any of these activities, should we decide to do so, would depend on the capital markets and the state of our industry, business and financial condition at the time, and we may not be successful in obtaining additional capital on desirable terms, at a desirable time or at all. Any failure to make payments on our debt when due, either in cash or stock, could result in a default on our debt obligations.

As of December 31, 2017 we had total liquidity of $177.5 million consisting of cash and cash equivalents of $36.1 million and short-term investments of $141.4 million. Our business plan calls for approximately $15.0 million in capital expenditures for 2018 and as much as $45.0 million by NG Advantage, the majority of which will be financed by third parties, as well as additional capital expenditures thereafter. We will also require capital to make principal and interest payments on our indebtedness, pay our other operating expenses, and for any mergers, acquisitions or strategic investments, transactions or relationships we may pursue. If we cannot fund any of these activities with cash provided by our operations, we may seek to obtain additional capital from other sources, such as by selling assets or pursuing debt or equity financing.

•The adoption of natural gas engines that are well-suited for heavy-duty trucks is essential to our success in this market. We have no influence over the development, production, cost, availability or sales and marketing of these engines. Cummins Westport is the only natural gas engine manufacturer for the heavy -duty truck market in the United States, and we have no control over whether and the extent to which Cummins Westport will remain in the natural gas engine business or whether other manufacturers will enter this business.

•Our success in the heavy -duty truck market depends on the expansion of the U.S. natural gas heavy-duty truck market. Operators may not adopt heavy-duty natural gas trucks due to cost, availability, actual or perceived performance issues or other factors, many of which are beyond our control. To date, adoption and deployment of natural gas trucks have been slower and more limited than we anticipated.

•Truck and other vehicle operators may not fuel at our stations due to lack of access or convenience, fuel prices or other factors.

•Our ANGH stations may experience mechanical or operational difficulties, which could require significant costs to repair and could reduce customer confidence in our stations.

•We may fail to accurately predict demand in any of the locations in which we build and open ANGH stations. As a result, we have built stations that we may not open for fueling operations and we may open stations that fail to generate the volume or profitability levels we anticipate, either or both of which could occur due to a lack of sufficient customer demand at the stations or for other reasons. For any stations that are completed but unopened, we would continue to have substantial investments in assets that do not produce revenue. For any stations that are open and underperforming, we may decide to close the stations, as we did in 2017 with 42 fueling stations. These station closures, and any future station closures we implement, could result in substantial costs, such as lease termination, severance or other similar fees and non-cash asset impairments or other charges, and could also harm our reputation and reduce our customer base and prospects for future growth.

•We agreed to purchase 146 used natural gas heavy -duty trucks, and we may not resell all such trucks to our customers.

We have, and expect to continue to seek, long-term CNG, LNG and RNG station construction, maintenance and fuel sales contracts with various government bodies. These government contracts accounted for approximately 18%, 16% and 19% of our annual revenue in 2015, 2016 and 2017, respectively. In addition to normal business risks, including the other risks discussed in these risk factors, our contracts with government entities are often subject to unique risks, some of which are beyond our control. For example, long-term government contracts and related orders are subject to cancellation if adequate appropriations for subsequent performance periods are not made. Further, the termination of funding for a government program supporting any of our government contracts could result in the loss of anticipated future revenue attributable to the contract, which could have a negative impact on our operations.

Our results in certain periods have also been impacted by transactions or events that have resulted in significant cash or non-cash gains or losses, which may not recur regularly, in the same amounts or at all in other periods. For example, in 2015 and 2016, our results were positively affected by $31.0 million and $26.6 million of revenue in the respective periods from the AFTC tax credit, and we expect to recognize all of the AFTC revenue associated with our vehicle fuel sales made in 2017 during 2018, due to the reinstatement of AFTC for 2017 on February 9, 2018. AFTC, however, is not available and may not be reinstated for vehicle fuel sales made after December 31, 2017, in which case we will not recognize revenue associated with AFTC after 2018. In addition in 2016, our results were positively affected by an aggregate of $34.3 million of gains related to repurchases or retirements of our outstanding convertible debt, and in 2017, our results were also positively affected by a $70.7 million gain related to the BP Transaction. Our results for 2017, however, were negatively affected by $81.1 million of charges in connection with our closure of certain fueling stations, our determination of an impairment of assets as a result of the foregoing, and certain other actions, and by a $6.5 million loss in connection with the CEC Combination.

We are a party to two long-term natural gas purchase agreements with a take-or-pay commitment, and we may enter into additional similar contracts in the future. These take-or-pay commitments require us to pay for the natural gas we have agreed to purchase, irrespective of whether we sell the gas. If the market for natural gas as a vehicle fuel declines or fails to develop as we anticipate, if we lose natural gas vehicle fueling customers, or if demand under any existing or future sales contract does not maintain its volume levels or grow, these take-or-pay commitments may exceed our natural gas demand. In addition, we are involved in various firm commitment natural gas supply arrangements, and we may establish additional similar arrangements in the future. These arrangements require us to supply specified volumes of natural gas over specified periods of time, and subject us to deficiency payments or other penalties if we are unable to deliver the committed volumes as and when required. If we fail to generate sufficient demand for our take-or-pay purchase commitments or satisfy our firm supply commitments, our supply costs or operating expenses could increase without a corresponding increase in revenue, which could cause our margins, performance and liquidity to be negatively impacted.

We lend to certain qualifying customers a portion, and occasionally all, of the purchase price of natural gas vehicles they agree to purchase. These financing activities involve a number of risks, including, among others, that the equipment financed consists mostly of vehicles, which are mobile and easily damaged, lost or stolen, and that the borrower may default on payments, enter bankruptcy proceedings and/or liquidate. As of December 31, 2017, we had $8.3 million outstanding in loans provided to customers to finance natural gas vehicle purchases.

As of December 31, 2017, there were 151,650,969 shares of our common stock outstanding, 8,613,854 shares of our common stock underlying outstanding options, 1,832,575 shares of our common stock underlying restricted stock units and 14,991,521 shares of our common stock underlying outstanding convertible notes (assuming conversion at the stated conversion price). All outstanding shares of our common stock are eligible for sale in the public market, subject in certain cases to the requirements of Rule 144 under the Securities Act. Also, shares issued upon the exercise, vesting and settlement or conversion of outstanding stock options, restricted stock units and convertible notes may be eligible for sale in the public market, to the extent permitted by Rule 144 and the provisions of the applicable stock option, restricted stock unit and convertible note agreements or if such shares have been registered under the Securities Act. If these shares are sold, or if it is perceived that they may be sold, in the public market, the trading price of our common stock could decline.

As of December 31, 2017, 13,030,978 shares of our common stock held by our co-founder and board member T. Boone Pickens were pledged as security for loans made to Mr. Pickens. We are not a party to these loans. If the price of our common stock declines, Mr. Pickens may be forced to provide additional collateral for the loans or to sell shares of our common stock in order to remain within the margin limitations imposed under the terms of the loans. Any sales of our common stock following

As of December 31, 2017, our co-founder and board member T. Boone Pickens beneficially owned approximately 9.03% of our outstanding common stock (including 13,030,978 outstanding shares of common stock and 725,000 shares underlying outstanding options). As a result, Mr. Pickens is able to influence or control matters requiring approval by our stockholders, including the election of directors and mergers, acquisitions or other extraordinary transactions. Mr. Pickens may have interests that differ from yours and may vote in ways with which you disagree or that may be adverse to your interests. This concentration of ownership may also have the effect of delaying, preventing or deterring a change of control of our Company, which could deprive our stockholders of an opportunity to receive a premium for their shares of our common stock as part of a sale of our Company and could affect the market price of our common stock. Conversely, this concentration of ownership may facilitate a change of control at a time when you and other investors may prefer not to sell.

Our corporate headquarters are located at 4675 MacArthur Court, Suite 800, Newport Beach, California 92660, where we occupy approximately 48,000 square feet of office space. Our lease for this facility expires in June 2021.

There were approximately 53 stockholders of record as of March 6, 2018. We believe there are approximately 60,095 additional stockholders whose shares of our common stock are held on their behalf by brokerage firms or other agents.

The following graph compares the five-year total return to holders of our common stock relative to the cumulative total returns of the Nasdaq Global Market Index, and the Russell 2000 Index. The graph assumes that $100 was invested in our common stock and on each of these indices on December 31, 2012 (the last trading day before the beginning of our fifth preceding fiscal year). We chose to include the Russell 2000 Index because it includes issuers with similar market capitalizations as us and due to the lack of a comparable industry or line-of-business index or peer group, as we are the only actively traded public company whose only line of business is to sell natural gas for use as a vehicle fuel and the associated equipment and services necessary to use natural gas as a vehicle fuel.

The consolidated statements of operations data for the years ended December 31, 2015, 2016 and 2017 and the consolidated balance sheet data as of December 31, 2016 and 2017 are derived from our audited consolidated financial statements included in this report. The consolidated statements of operations data for the years ended December 31, 2013 and 2014, and the consolidated balance sheet data as of December 31, 2013, 2014 and 2015 are derived from our audited consolidated financial statements that are not included in this report. Historical results are not indicative of the results to be expected in the current period or any future period.

We serve fleet vehicle operators in a variety of markets, including heavy-duty trucking, airports, refuse, public transit, industrial and institutional energy users, and government fleets. We believe these fleet markets will continue to present a growth opportunity for natural gas vehicle fuel for the foreseeable future. As of December 31, 2017, we serve nearly 1,000 fleet customers operating over 46,000 natural gas vehicles, and we own, operate or supply over 530 natural gas fueling stations in 42 states in the United States and four provinces in Canada.

In evaluating our operating performance, our management focuses primarily on: (1) the amount of CNG, LNG and RNG gasoline gallon equivalents delivered (which we define as (i) the volume of gasoline gallon equivalents we sell to our customers, plus (ii) the volume of gasoline gallon equivalents dispensed at facilities we do not own but where we provide O&M services on a per-gallon or fixed fee basis, plus (iii) our proportionate share of the gasoline gallon equivalents sold as CNG by our joint venture with Mansfield Ventures, LLC called Mansfield Clean Energy Partners, LLC (“MCEP”), plus (iv) our proportionate share (as applicable) of the gasoline gallon equivalents of RNG produced and sold as pipeline quality natural gas by the RNG production facilities we owned or operated before completion of the BP Transaction, discussed under “2017 Developments” below), (2) our station construction cost of sales, (3) our gross margin (which we define as revenue minus cost of sales), and (4) net loss attributable to us. The following tables present our key operating data for the years ended December 31, 2015, 2016, and 2017:

AFTC. We have been eligible to receive the AFTC alternative fuels tax credit for our natural gas vehicle fuel sales made during all periods covered in the performance overview. The AFTC, which had previously expired on December 31, 2016, was reinstated on February 9, 2018 to apply to vehicle fuel sales made from January 1, 2017 through December 31, 2017. As a result, all AFTC revenue for vehicle fuel we sold in the 2017 calendar year, expected to total approximately $25.0 million, will be recognized and collected subsequent to December 31, 2017. The AFTC credit is equal to $0.50 per gasoline gallon equivalent of CNG that we sold as vehicle fuel, $0.50 per liquid gallon of LNG that we sold as vehicle fuel through 2015, and $0.50 per diesel gallon of LNG that we sold as vehicle fuel in 2016 and 2017.

SAFE&CEC S.r.l. On November 26, 2017, we, through our former subsidiary IMW Industries Ltd. (formerly known as Clean Energy Compression Corp.) (“CEC”), entered into an investment agreement with Landi Renzo S.p.A. (“LR”), an alternative fuels company based in Italy, pursuant to which we and LR agreed to combine our respective natural gas compressor subsidiaries, CEC and SAFE S.p.A, in a new company known as “SAFE&CEC S.r.l.” (we refer to this combination transaction as the “CEC Combination”). SAFE&CEC S.r.l. is focused on manufacturing, selling and servicing natural gas fueling compressors and related equipment for the global natural gas fueling market. Upon the closing of the CEC Combination, which occurred on December 29, 2017, we own 49% of SAFE&CEC S.r.l. and LR owns 51% of SAFE&CEC S.r.l.

The operating results of CEC have been fully consolidated into our consolidated results through December 29, 2017 (effectively the full year of 2017). As of December 31, 2017 we have an investment balance of $27.9 million representing our 49% interest in SAFE&CEC S.r.l., and beginning in 2018 we will account for our interest in SAFE&CEC S.r.l. using the equity method of accounting as we have the ability to exercise significant influence over SAFE&CEC S.r.l.’s operations. We recognized a loss of $6.5 million in connection with the CEC Combination, and incurred working capital adjustments, funding for certain post-closing commitments, and transaction fees of which $4.0 million was unpaid as of December 31, 2017. See Note 3 to our consolidated financial statements for more information.

Asset Impairments, Other Charges, and Inventory Valuation Provision. During the year ended December 31, 2017, we undertook an evaluation of our operations with the intent of minimizing and eliminating assets we believe are underperforming. As a result of this evaluation, we identified 42 fueling stations where the volume and profitability levels are not expected to be sufficient to support our investment in the fueling station assets, and we determined to close these stations by the end of 2017.

As a result of these decisions and the steps taken to implement them, we determined that impairment indicators were present for CEC’s assets and the closed station assets, and we recorded asset impairment charges of $32.3 million related to CEC and $20.4 million related to the station closures during 2017. We also recorded during this period an additional $5.1 million of other charges relating to the station closures, consisting of lease termination fees, write-offs of deferred losses, and an increase in asset retirement obligations. In addition, we incurred $3.1 million in cash charges during this period related to our workforce reduction and $13.2 million in non-cash charges during this period related to an inventory valuation provision.

In addition, during the fourth quarter of 2017, the Company incurred a cash charge of $7.0 million related to payments for LCFS Credits as further discussed below in Volume-Related Revenue. As a result of these steps, we incurred, on a pre-tax basis, aggregate cash and non-cash charges of $81.1 million during 2017. See Note 2 for more information.

Purchase of Natural Gas Heavy -Duty Trucks. In July 2017, we entered into an arrangement to purchase 146 used natural gas heavy -duty trucks from a single seller by January 31, 2018, with the intention of selling the trucks to our customers. As of December 31, 2017, this arrangement represented an outstanding commitment to purchase 124 natural gas heavy -duty trucks for $7.9 million. Subsequent to December 31, 2017, 66 of the trucks were sold to third parties.

NG Advantage Customer Arrangement. In June 2017, our subsidiary NG Advantage, LLC (“NG Advantage”) entered into an arrangement with one of its customers for the purchase, transportation and sale of CNG over a five-year period. The arrangement is customary and ordinary course, and provided for the payment by the customer of a nonrefundable amount of $13.4 million to reserve a specified volume of CNG transportation capacity. This amount was paid to NG Advantage in two installments, with $8.4 million paid on July 3, 2017 and $5.0 million paid on October 2, 2017.

Contribution of Milton CNG Fueling Station. On July 14, 2017, we contributed to NG Advantage all of our right, title and interest in and to a CNG fueling station located in Milton, Vermont. We had purchased this CNG station from NG Advantage in October 2014 in connection with our initial investment in NG Advantage, and at that time, we entered into a lease agreement with NG Advantage to lease the station back to NG Advantage. This lease agreement was terminated contemporaneously with the contribution of the station to NG Advantage in July 2017. As consideration for the contribution, NG Advantage issued to us Series A Preferred Units with an aggregate value of $7.5 million. The Series A Preferred Units provide for an accrued return in the event of a liquidation event with respect to NG Advantage and will convert into common units of NG Advantage if and when it completes a future equity financing that satisfies certain specified conditions; however, the Series A Preferred Units do not, in themselves, increase our controlling interest in NG Advantage. As a result, immediately following the contribution, our controlling interest in NG Advantage remained at 53.3%.

Termination of ATM Program. On May 31, 2017, we terminated our equity distribution agreement (the “Sales Agreement”) with Citigroup Global Markets Inc. (“Citigroup”), as sales agent and/or principal. The Sales Agreement was terminable at will upon written notification by us with no penalty. Pursuant to the Sales Agreement, we were entitled to issue and sell, from time to time, through or to Citigroup shares of our common stock having an aggregate offering price of up to $200.0 million in an “at-the-market” offering program (the “ATM Program”). The ATM Program commenced on November 11, 2015 when we and Citigroup entered into the original equity distribution agreement, which was amended and restated on September 9, 2016 and again on December 21, 2016 prior to its termination. See “Liquidity and Capital Resources-Sources of Cash” below for more information about the ATM Program.

BP Transaction. On February 27, 2017, our subsidiary Clean Energy Renewable Fuels ("Renewables") entered into an asset purchase agreement (the “APA”) with BP Products North America, Inc. (“BP”), pursuant to which Renewables agreed to sell to BP certain assets related to its RNG production business (the “BP Transaction”), consisting of Renewables’ two existing RNG production facilities, Renewables’ 50% ownership interest in joint ventures formed to develop two new RNG production facilities, and Renewables’ third-party RNG supply contracts (the “Assets”). The BP Transaction was completed on March 31, 2017 for a sale price of $155.5 million, plus BP’s assumption of the obligations under the Canton Bonds (as defined in Note 11 to our consolidated financial statements) which totaled $8.8 million as of March 31, 2017.

On March 31, 2017, BP paid Renewables $30.0 million in cash and delivered to Renewables a promissory note with a principal amount of $123.5 million, which was paid in full on April 3, 2017. In addition, on June 22, 2017, BP paid Renewables an additional $2.0 million related to the determination of certain post-closing adjustments. Renewables recognized a gain of $70.7 million as of December 31, 2017 from the BP Transaction. Pursuant to the APA, the valuation date of the BP Transaction was January 1, 2017, and the APA included certain provisions to adjust the purchase price based on the amount of cash accumulated

by Renewables from the valuation date to the closing date, net of permitted cash outflows. Control of the Assets was not transferred until the BP Transaction was completed on March 31, 2017. Accordingly, the full operating results of Renewables are included in the consolidated statement of operations through March 31, 2017. The net proceeds from the BP Transaction, net of $1.0 million cash transferred to BP, were $142.2 million.

In addition, under the APA, BP is required, following the closing of the BP Transaction, to pay Renewables up to an additional $25.0 million in cash over a five-year period based on the market price of RINS and LCFS credits and volumes of RNG sold ("BP Earn-out"). We recognized $0.8 million of BP Earn-out for the period ended December 31, 2017, and this amount is included in the gain on BP Transaction of $70.7 million.

Renewables Options. In September 2013, Renewables adopted a unit option plan and granted unit option awards thereunder (the “Renewables Option Awards”) to certain individuals. In connection with the closing of the BP Transaction, all holders of outstanding Renewables Option Awards entered into a surrender agreement with us and Renewables, pursuant to which (i) all Renewables Option Awards held by holders who were not members of Renewables’ Board of Managers were surrendered and canceled in full in exchange for, upon the closing of the BP Transaction and Renewables’ receipt of any BP Earn-out, a cash payment in an amount determined based on such holder’s percentage ownership of Renewables following a cashless “net exercise” of such holder’s Renewables Option Awards, and (ii) all Renewables Option Awards held by members of Renewables’ Board of Managers were surrendered and canceled in full in exchange for, upon the closing of the BP Transaction and Renewables’ receipt of any BP Earn-out, awards of shares of our common stock (the “Company Stock Awards”). The number of shares of our common stock subject to each Company Stock Award was calculated by dividing the cash payment to which the applicable holder would have been entitled as described in (i) above by the closing price of our common stock on March 31, 2017, the closing date of the BP Transaction. All Company Stock Awards were granted under our 2016 Performance Incentive Plan and are fully vested upon grant, and the shares subject to such awards are freely tradable upon issuance, subject to applicable securities laws relating to shares held by our affiliates.

Debt Repurchase. In February 2017, we purchased from one of our directors and significant stockholders, T. Boone Pickens, the 7.5% Convertible Note due July 2018 having an outstanding principal amount of $25.0 million held by Mr. Pickens for a cash purchase price of $21.75 million. See Note 11 to our consolidated financial statements more information about our outstanding debt.

As of December 31, 2017, we had total indebtedness of $260.9 million in principal amount, of which approximately $140.2 million is expected to become due in 2018. Certain of the agreements governing our outstanding debt, which are discussed in Note 11 to our consolidated financial statements, have certain non-financial covenants with which we must comply. As of December 31, 2017, we were in compliance with all of these covenants.

Market for Natural Gas as a Vehicle Fuel. CNG and LNG are generally less expensive than gasoline and diesel on an energy equivalent basis. Additionally, according to studies conducted by the California Air Resources Board ("CARB") and the Argonne National Laboratory, a research laboratory operated by the University of Chicago for the U.S. Department of Energy, CNG and LNG are cleaner than gasoline and diesel fuel, based on greenhouse gas emissions produced by vehicles operated by these fuels. According to the U.S. Energy Information Administration, demand for natural gas fuels in the United States has increased in

Our Performance. In light of the factors described above, in 2017, we took steps to minimize and eliminate assets we believe are underperforming in an effort to better align our activities with current and anticipated natural gas fueling demand, and to try to lower our operating expenses by streamlining our operations. These steps included a workforce reduction and other measures to reduce overhead costs, which resulted in cash severance costs and certain non-cash stock-based compensation charges; our decision to close certain of our natural gas fueling stations by the end of 2017, which resulted in an impairment of these station assets and certain other cash and non-cash charges; our determination that the assets of CEC were impaired, which resulted in a non-cash charge; and our decision to position CEC’s compressor business for industry consolidation with a potential strategic partner rather than continue to manage it ourselves, which resulted in the CEC Combination in the fourth quarter of 2017. See "2017 Developments" above and Notes 2 and 3 to our consolidated financial statements for more information.

See "Results of Operations" below for more discussion of our performance in 2015, 2016 and 2017.

Volume-Related Revenue. The amount of CNG, LNG and RNG GGEs we delivered increased by 13.9% from 2015 to 2017.

In particular, the amount of RNG we sell for vehicle fuel, which is delivered in the form of CNG or LNG and is distributed under the brand name Redeem, has experienced rapid growth in recent years, increasing by 56.7% from 50.1 million GGEs in 2015 to 78.5 million GGEs in 2017. We believe this demand for Redeem is largely attributable to the lower greenhouse gas emissions that it produces relative to gasoline and diesel fuel.

Although we continue to record revenue from the sale of RINs and LCFS Credits generated from our continued sales of our Redeem RNG vehicle fuel and CNG and LNG, the amount of revenue we receive from sales of these credits decreased in 2017 as a result of our sale of our former RNG production facilities and other related assets in the BP Transaction. This decrease has adversely affected our results of operations, in particular our volume-related revenue, and reduced our effective price per gallon (discussed under “Results of Operations” below). In addition, we recognized no revenue from sales of LCFS Credits during the third and fourth quarters of 2017 because CARB had restricted our ability to sell and transfer LCFS Credits pending completion of an administrative review. We were, however, required to settle preexisting contractual obligations to transfer LCFS Credits to third parties by making cash payments totaling $7.0 million, the equivalent value of the LCFS Credits we would have otherwise transferred to satisfy our obligations. These payments are reported in Asset impairments and other charges in our consolidated statements of operations. In November 2017, CARB invalidated certain LCFS Credits we had generated in prior periods and released the restriction on our ability to sell and transfer LCFS Credits. We are disputing CARB’s invalidation of LCFS Credits.

We believe the critical accounting policies discussed below affect our more significant estimates made in preparing our consolidated financial statements. See in Note 1 to our consolidated financial statements for more information about these and our other significant accounting policies.

If the recent volatility of our market capitalization is sustained, we may perform impairment tests more frequently and it is possible that our goodwill could become impaired, which could result in a material charge and adversely affect our results of operations.

See Note 1 to our consolidated financial statements for information about recently adopted accounting changes and recently issued accounting standards, including our expected adoption in the first quarter of 2018 of ASU 2014-09 related to revenue from contracts with customers, which will provide a single, comprehensive revenue recognition model for all contracts with customers.

The discussions below compare our results of operations in 2017, 2016 and 2015. Historical results are not indicative of the results to be expected in the current period or any future period.

2017 Compared to 2016

Revenue.    Revenue decreased by $61.1 million to $341.6 million for 2017, from $402.7 million for 2016. This decrease was primarily due to the absence of AFTC revenue recorded in 2017 as well as lower volume -related revenue, compressor revenue and station construction sales.    

Volume -related revenue decreased by $19.0 million between periods primarily due to reduced revenue received from sales of RINs and LCFS Credits, which was due in large part to the effects of the BP Transaction (see Note 3 for more information) as well as the impact of restrictions imposed on our LCFS Credit account by CARB (see "Key Trends" for more information). The decrease in volume -related revenue between periods was partially offset by an increase of 22.4 million gallons delivered.

This increase in gallons delivered was due to a 24.2 million increase in CNG gallons delivered, which was primarily attributable to 33 new refuse customers, seven new transit customers, and one new trucking customer. This increase was partially offset by a 0.7 million decrease in LNG gallons delivered as non-vehicle fuel and a 1.1 million decrease in RNG gallons delivered as non-vehicle fuel. Our effective price per gallon charged was $0.76 for 2017, a $0.10 per gallon decrease from $0.86 per gallon

Station construction sales decreased by $13.0 million between periods, principally due to a decrease in large, full -station projects and station upgrade projects.

Compressor revenue decreased by $3.8 million between periods due to lower compressor sales, which we believe was primarily due to continued low global demand.

AFTC revenue decreased by $26.6 million between periods due to the absence of AFTC revenue recorded in 2017.

Cost of sales.    Cost of sales increased by $0.2 million to $255.8 million for 2017, from $255.6 million for 2016. This increase was primarily due to a $13.2 million inventory valuation provision charge recorded in 2017, comprised of $7.8 million related to station construction inventory and $5.4 million related to compressor inventory (see "2017 Developments" and Note 2 for more information regarding developments related to this compressor inventory). This increase was partially offset by a $10.0 million decrease in station construction costs between periods due to lower station construction sales, and a $1.4 million decrease in compressor costs between periods due to lower compressor sales.

Our effective cost per gallon decreased by $0.03 per gallon between periods, to $0.49 per gallon for 2017 from $0.52 for 2016, excluding the $7.8 million inventory valuation provision charge discussed above. Our effective cost per gallon is defined as the total costs associated with delivering natural gas, including gas commodity costs, transportation fees, liquefaction charges, and other site operating costs, plus the total cost of providing O&M services at stations that we do not own and for which we receive a per-gallon or fixed fee, including direct technician labor, indirect supervisor and management labor, repair parts and other direct maintenance costs, all divided by the total GGEs delivered less GGEs delivered by non-consolidated entities, such as entities that are accounted for under the equity method. The decrease in our effective cost per gallon was primarily due to the sale of Renewables’ two RNG production facilities in the BP Transaction, resulting in no cost of sales to operate these facilities in the last nine months of 2017.

Selling, general and administrative.    Selling, general and administrative expenses decreased by $9.8 million to $95.7 million for 2017, from $105.5 million for 2016. This decrease was primarily driven by continued cost reduction efforts and reduced administrative costs due to the BP Transaction in 2017.

Depreciation and amortization.    Depreciation and amortization decreased by $2.7 million to $56.6 million for 2017, from $59.3 million for 2016, primarily due to the sale of Renewables’ two RNG production facilities in the BP Transaction.

Asset impairments and other charges. During 2017, we recorded employee terminations, asset impairments and other charges of $67.9 million related to our station closures, CEC's natural gas fueling compressor business, and payments for LCFS Credits. See "2017 Developments" and Note 2 for more information.

Interest expense.    Interest expense decreased by $11.8 million to $17.8 million for 2017, from $29.6 million for 2016. This decrease was primarily due to a reduction of outstanding indebtedness between periods.

Other income (expense), net.    Other income (expense), net, increased by $0.4 million, to $0.1 million for 2017, from $(0.3) million for 2016. This increase was primarily due to a $0.7 million decrease in losses from asset disposals, partially offset by a $0.4 million increase in the loss from foreign currency transactions not in our subsidiaries' functional currency.

Income tax benefit (expense).    Income tax benefit (expense) increased by $3.2 million to $1.9 million for 2017, from $(1.3) million for 2016. The increase in income tax benefit was primarily due to the deferred tax benefit attributable to the reduction of goodwill amortization following the BP Transaction.

Loss from noncontrolling interest.    In 2017, we recorded a $2.2 million loss for the noncontrolling interest in the net loss of NG Advantage, compared to a $1.6 million loss for 2016. The noncontrolling interest in NG Advantage represents a 46.7% minority interest, which was held by third parties during the applicable periods.

Gain from extinguishment of debt, net. Gain from extinguishment of debt, net decreased by $31.1 million to $3.2 million for 2017, from $34.3 million for 2016. This decrease was primarily due to our repurchase of a lower principal amount of debt at higher prices in 2017 compared to 2016.

Gain from sale of certain assets of subsidiary. In 2017, we recorded a gain of $70.7 million related to the BP Transaction. We recorded no comparable gain in 2016.

Loss from formation of equity method investment. In 2017, we recorded a loss of $6.5 million related to the CEC Combination. There was no comparable transaction in 2016.

2016 Compared to 2015

Revenue.     Revenue increased by $18.4 million to $402.7 million for 2016, from $384.3 million for 2015. This increase was primarily due to increased volume -related revenue, including higher RINs and LCFS Credits sales, and increased station construction sales, partially offset by lower AFTC revenue and compressor sales.

Cost of sales.     Cost of sales decreased by $ 2.9 million to $ 255.6 million for 2016, from $258.5 million for 2015. This decrease was primarily due to a $24.1 million decrease in compressor costs between periods as a result of decreased compressor sales, and a $3.5 million decrease in gas commodity costs as a result of a decrease in natural gas prices (discussed below). These decreases were partially offset by a $24.7 million increase in costs related to increased station construction sales between periods.

Selling, general and administrative.     Selling, general and administrative expenses decreased by $8.2 million to $105.5 million for 2016, from $113.7 million for 2015. This decrease was primarily driven by a $4.9 million decrease in employee-related expenses between periods and a $3.8 million decrease in travel and promotional expenses between periods. These decreases were partially offset by a $0.5 million increase in research and development costs between periods.

Depreciation and amortization.     Depreciation and amortization increased by $4.1 million to $59.3 million for 2016, from $55.2 million for 2015, due to purchases of property and equipment, primarily for our stations.

Gain from extinguishment of debt, net. Gain from extinguishment of debt, net, was $34.3 million for 2016 as a result of repurchasing and retiring $284.6 million in principal amount of, and $3.6 million in accrued and unpaid interest on, certain of our outstanding indebtedness for an aggregate purchase price of $185.9 million in cash and 20.3 million shares of our common stock. See Note 11 for more information.

Interest expense.     Interest expense decreased by $66.2 million to $29.6 million for 2016, from $95.8 million for 2015. This decrease was primarily due to a non-cash charge of $54.9 million in 2015 related to the elimination of unamortized debt issuance costs associated with our termination of our former credit agreement with General Electric Capital Corporation, as well as a reduction of outstanding indebtedness between periods. See Note 11 for further information.

Other income (expense), net.     Other income (expense), net, decreased by $(2.9) million to $(0.3) million for 2016, from $ 2.6 million for 2015. This decrease was primarily due to a $0.8 million increase in the loss from foreign currency transactions not in our subsidiaries’ functional currency, a decrease of $0.5 million between periods as result of a litigation settlement in 2015, and a $1.6 million decrease due to a $0.9 million gain from asset disposals in 2015 compared to $0.7 million in losses from asset disposals in 2016.

Income tax expense.     Income tax expense decreased by $0.3 million to $1.3 million for 2016, from $1.6 million for 2015. The decrease in income tax expense is primarily attributable to a decrease in our tax on foreign operations between periods.

Loss from noncontrolling interest.     In 2016, we recorded a $1.6 million loss for the noncontrolling interest in the net loss of NG Advantage, compared to a $1.2 million loss for 2015. The noncontrolling interest in NG Advantage represents a 46.7% minority interest, which was held by third parties during the applicable periods.

Operating Activities. Cash used in operating activities was $5.4 million in 2017, compared to $46.1 million provided by operating activities in 2016. The $51.5 million increase in cash used in operating activities was primarily due to a reduction in operating results resulting from the absence of AFTC collected during 2017 and decreased sales of RINs and LCFS Credits as discussed under "Key Trends" and "Results of Operations" above, as well as our payment of one-time transaction fees related to the BP Transaction and CEC Combination. These operating outflows were partially offset by aggregate payments of $13.4 million received by NG Advantage related to an arrangement with one of its customers (see "Performance Overview - 2017 Developments" for more information).

Investing Activities. Cash provided by investing activities was $47.7 million in 2017, compared to $1.1 million provided by investing activities in 2016. The $46.6 million increase in cash provided by investing activities was primarily attributable to $154.5 million in cash received, net of cash transferred, in connection with the BP Transaction (see "2017 Developments" and Note 3 for more information). These investing cash inflows were partially offset by incremental purchases of short-term investments, net of maturities, of $96.6 million in 2017 compared to 2016, and a $12.7 million increase in purchases of equipment, primarily related to deposits by NG Advantage for CNG trailers and equipment.

Cash provided by investing activities was $1.1 million in 2016, an increase of $35.0 million compared to $33.9 million used in investing activities in 2015. The increase in cash provided by investing activities was primarily attributable to decreased capital expenditures of $27.8 million between periods, due to cost-cutting efforts and a decrease in construction of Company-owned fueling stations. Additionally, cash provided by our short-term investments that matured, net of purchases, increased by $10.5 million between periods. Partially offsetting this increase in cash provided by investing activities was $0.8 million used in investing activities in 2016 in connection with certain joint ventures formed in November 2016 (see Note 9) and $1.1 million received in 2015 from the sale of a former subsidiary that did not occur again in 2016.

Financing Activities. Cash used in financing activities was $43.2 million in 2017, compared to $55.6 million used in financing activities in 2016. The $12.4 million decrease in cash used in financing activities was primarily due to a $112.5 million decrease in cash used in debt repurchases, net of borrowings, offset partially by a $91.2 million decrease in cash provided by the ATM Program, net of fees, which was terminated on May 31, 2017, and $8.9 million in payments to holders of Renewables Option Awards associated with the BP Transaction in 2017.

Our business plan calls for approximately $15.0 million in capital expenditures in 2018, which primarily relates to the purchase of natural gas heavy-duty trucks, the construction of CNG fueling stations, and LNG plant maintenance costs.

In addition, NG Advantage may spend as much as $45.0 million to purchase additional CNG trailers and equipment in support of its operations and customer contracts, a majority of which will be financed by third parties.

We had total indebtedness of approximately $260.9 million in principal amount as of December 31, 2017, of which approximately $140.2 million, $54.5 million, $54.6 million, $4.2 million, $3.8 million and $3.6 million is expected to become due in 2018, 2019, 2020, 2021, 2022 and thereafter, respectively. Additionally, we expect our total interest payment obligations relating to our indebtedness to be approximately $15.6 million for the year ending December 31, 2018. As of December 31, 2017, we are permitted to issue up to 14.0 million shares of common stock to repay a portion of the principal amount of our outstanding 7.5% convertible notes. While we believe we have sufficient liquidity and capital resources to repay our debt, we may elect to pursue additional financing activities such as refinancing, debt or equity offerings to provide flexibility with our cash management.

Historically, our principal sources of liquidity have consisted of cash on hand, cash provided by our operations, including, if available, AFTC and other government credits, grants and incentives, cash provided by financing activities, and sales of assets. In addition, our PlainsCapital Bank Credit Facility (as described below) for up to $50.0 million on a revolving basis, provides us with an additional source of liquidity. As of December 31, 2017, we had total cash and cash equivalents and short-term investments of $177.5 million, compared to $109.8 million at December 31, 2016.

From the commencement of the ATM Program in November 2015 until our termination of the Sales Agreement on May 31, 2017, we received aggregate net proceeds of $117.9 million from sales of our common stock under the Sales Agreement.

See Note 11 for more information about the Credit Facility with Plains and our other outstanding debt.

On March 31, 2017, Renewables completed the BP Transaction. The net proceeds to us from the BP Transaction were approximately $142.2 million. See “2017 Developments” and Note 3 for more information.

The following represents the scheduled maturities of our contractual obligations as of December 31, 2017: