February 29, 2008
Chevron and Weyerhaeuser Form Biofuels Joint Venture
Chevron Corporation and Weyerhaeuser Company have created a 50-50 joint venture company focused on developing the next generation of renewable transportation fuels from nonfood sources. The joint venture, Catchlight Energy LLC, will research and develop technology for converting cellulose-based biomass from a variety of sources into economical, low-carbon biofuels.
The formation of Catchlight Energy is the first milestone of a biofuels alliance announced by Chevron and Weyerhaeuser in April 2007 and reflects the companies’ shared view that nonfood biofuels will play an important role in diversifying the nation’s energy supply. (Earlier post.)
Michael Burnside of Chevron has been appointed chief executive officer of Catchlight. During his 33-year career with Chevron, Burnside has held a variety of positions in manufacturing, planning and analysis and finance, and has been involved with a number of joint ventures. W. Densmore Hunter of Weyerhaeuser has been named Catchlight’s chief technology officer. Since joining Weyerhaeuser in 1980, Hunter has held key research, technology and manufacturing positions and currently leads the company’s biofuels and bioproducts research and development efforts.
Both Chevron and Weyerhaeuser will contribute resources—including funding, background technology and employees—to Catchlight Energy.
Chevron and Weyerhaeuser already have separate research partnerships under way with universities, national laboratories and technology-based companies to advance the development of nonfood biofuels. Chevron has forged alliances with the Georgia Institute of Technology, the University of California at Davis, the Colorado Center for Biorefining and Biofuels, and the US Department of Energy’s National Renewable Energy Laboratory. Weyerhaeuser is collaborating with several research universities, national laboratories and technology-based companies in research on conversion of forest products into ethanol and other biofuels.
In a presentation in 2007 at the TAAPI (Technical Association of the Pulp and Paper Industry) Renewable Energy Conference, Hunter said that Weyerhauser had examined the potential waste feedstock sources related to the forestry industry—wood products residuals, forest residuals, byproducts such as tall oil, and lignin—and concluded that a purpose-grown energy resource would be the big deal for the company.
Such a resource, he said, would be synergistic with growing high value sawtimber and consistent with the very large scale production of biofuels.
The potential for lignocellulosic-derived fuels goes well beyond ethanol, he said, and includes renewable diesel; DME; alkanes and aromatics; industrial and specialty chemicals; methanol and other alcohols; and polymers.
Biorefining Choices: How Weyerhauser looks at the biofuels opportunity” (Hunter, TAPPI Renewable Energy Conference, Atlanta, 10-11 May 2007)
Altair Nanotechnologies CEO Steps Down
Altair Nanotechnologies Inc., a provider of advanced nanomaterials technology for use in energy, life sciences and industrial applications, announced that Alan Gotcher, Ph.D., has agreed to resign as an officer of the company and that the Board of Directors has begun a search for a new Chief Executive Officer. The Board has appointed Terry Copeland, Ph.D., to serve as interim President.
Dr. Copeland joined Altair Nanotechnologies on November 13, 2007 as Vice President, Operations for Altairnano’s Power & Energy Group. Dr. Copeland’s duties have included leading global operations and supply chain management for Altairnano’s lithium titanate battery products.
Prior to joining Altair Nanotechnologies, Dr. Copeland worked as a general manufacturing and technical consultant from 2004 through the end of 2007. From 2000 through 2003, Dr. Copeland was the Vice President of Product Development at Millennium Cell, Inc., a development stage company working with alternative fuels. From 1992 through 2000, Dr. Copeland worked for Duracell, a leading consumer battery company, where he held positions as Director of Product Development (1998-2000), plant manager (1995-1998) and Director of Engineering (1992-1995).
Mitsubishi Motors Unveils New Mid-Term Business Plan; i MiEV Goes Global
|Mitsubishi Motors will emphasize its environmental strategies primarily in mature markets. Click to enlarge.|
Mitsubishi Motors Corporation has unveiled its new mid-term business plan—Step Up 2010—for fiscal years 2008 through 2010 (ending March 31, 2011). Target FY 2010 sales is 1,422,000 units, a 6.4% increase over the fiscal 2007 forecast of 1,337,000 units. For new product planning, Mitsubishi draws a distinction between financial responsibility and environmental responsibility.
In the area of financial responsibility, the company plans to expand the number of its mid-sized platform models, and to add a new SUV based on its one-ton pickup. In the area of environmental responsibility, the company will add a smaller, “lower impact” SUV, adapt its minicars for overseas markets and add a global model, and bring its i MiEV electric vehicle to mature world markets.
|Accelerating the i MiEV. Click to enlarge.|
In the area of environmental technology, the company will concentrate on the development of core technologies, including emphasis on the development of clean diesel engines and the high-efficiency automated manual transmission Twin Clutch SST (Sport Shift Transmission).
In Brazil, a focus market for the company, Mitsubishi will strive to increase sales by filling out its lineup of full-range (0%-100% gasoline / bioethanol-compatible) flexible fuel vehicles (FFV).
In North America, Mitsubishi says that it will primarily be working on improving the brand image in the mid- to long term and working with its dealers. The focus for its US-based production plant will be continued efforts at overall cost-cutting, including fixed costs, and by expanding export opportunities.
In the mature Western European market, Mitsubishi Motors will address environmental awareness and tightening CO2 emissions regulations by promoting environmental technologies and compact vehicles. At the same time, in the expanding Central European market, it will strive to increase sales with a focus on SUVs.
Mitsubishi also has identified a set of “Focus” markets for expansion: Russia and the Ukraine; the Middle East; China; and India.
VW to Show Golf Diesel Hybrid Concept, New Natural Gas Vehicle at Geneva Motor Show
Volkswagen will make seven international premiere presentations at the upcoming Geneva Motor Show (6-16 March), including a Golf TDI diesel-electric hybrid concept and the new 150-PS Passat Estate TSI EcoFuel natural gas vehicle. Other Volkswagen world premieres will be the fuel-efficient Sharan BlueMotion, the four-wheel-drive Golf Estate 4Motion, and the completely re-engineered Scirocco sports car. The Passat CC will also be on display for the first time in Europe.
The Golf TDI Hybrid concept consumes no more than 3.4 liters of diesel fuel per 100 kilometers (69 mpg US), with 89 g/km CO2 emissions. The full-hybrid supports an all-electric mode. Power transmission to the front axle is managed by 7-speed DSG technology.
The Passat Estate TSI EcoFuel features a direct-injection turbocharged engine, and delivers 110 kW (148 hp), while consuming 5.2 kg/100 km. The Passat and the Passat Estate TSI EcoFuel are due to be launched on the market around year-end.
The Sharan BlueMotion, the latest in the BlueMotion series, offers average fuel consumption of 6.0 litres of diesel per 100 kilometers (39 mpg US)—0.7 of a liter less than conventional models. CO2 emissions are reduced from 177 g/km to 159 g/km. The seven-seat van has up to 2,610 liters of cargo volume and a permissible gross vehicle weight of 2,510 kilograms. This Volkswagen is driven by a 103 kW (138 hp) TDI diesel engine complete with a diesel particulate filter (DPF). The BlueMotion option is available in combination with the Trendline and Comfortline fittings packages. Deliveries of the vehicle are scheduled to commence this summer.
Effective immediately, Volkswagen will be offering the Golf Estate in a version with permanent four-wheel drive. This automobile is designed to enable as much as 100 per cent of the vehicle’s tractive force to be transmitted to the rear wheels if so required in extreme circumstances.
The 4Motion system is coupled with a fuel-efficient, high-torque TDI engine with 77 kW (103 hp). The Golf Estate TDI 4Motion accelerates from 0 to 100 kph in 12.9 seconds, has a top speed of 185 km/h (115 mph), with fuel consumption of 6.0 L/100km (39 mpg US). The Golf Estate TDI 4Motion can tow as much as 1,500 kilograms on gradients of up to 12%—100 kilograms more than its front-wheel-drive counterpart.
China Considering Road Congestion Fees
Xinhua. China is considering charging fees on some of its traffic-congested urban roads, according to Wang Fengwu, deputy director of the Ministry of Construction’s urban construction department, speaking at a forum in Shanghai.
Fees could be charged on the basis of the frequency and intensity of vehicles road use “to reduce people’s excessive dependence on cars,” he said.
“We will learn the concept of properly allotting urban road space resource as advocated by international communities,” he said. Wang said the country would change the current vehicle-oriented practice of allotting road space into a traveler-oriented one with more space given to buses, bicycles and pedestrians.
Currently, public transport only handles less than 10 percent of journeys in most Chinese cities.
Hrein Energy Successfully Test Drives 1.2L Vehicle With Retrofitted Organic Hydride System
|The organic hydride dehydrogenation reactor is mounted inline in the exhaust system. Click to enlarge.|
Hrein Energy, in cooperation with Futaba Industrial Co., Ltd, ITO Racing Service Co. Ltd.. and Dr. Ichikawa Masaru, a professor emeritus of Hokkaido University, has successfully test-driven a 1.2-liter Nissan March retrofitted with an on-board organic hydride system (earlier post) that delivers supplemental hydrogen to the gasoline engine.
Adding several percent of hydrogen dehydrogenated from the organic hydride to the intake air supported very lean-burn combustion. Fuel efficiency was improved by 30%; CO2 emissions were cut by 30%; and concentrations of CO and NOx were “considerably reduced”, according to the company.
The test drive was conducted in the circuit of SPA Nishiura Motor Park in Gamagori, Aichi Prefecture on 21 February.
Organic hydrides are liquids under atmospheric temperature and pressure, yet offer relatively high hydrogen content: between 6-8 wt.%. The Hrein spray pulse reactor feeds the organic hydride (methylcyclohexane, C7H14) as atomized liquid to a catalyst surface heated by exhaust heat; the reactor is mounted inline in the exhaust system.
The reactor in the March has a conversion rate of 85% or higher, according to Hrein, and can produce hydrogen at the rate of approximately 3 normal m3/hour.
Because the organic hydrides are liquids (not to be confused with liquefied hydrogen), the existing fuel storage, transportation and refueling infrastructure could basically be maintained were the liquids applied to transportation.
In August 2007, Hrein tested the system on a cart with a 50cc engine. The company plans to work next on a 1.5-liter class vehicle.
Hrein Energy Hydrogen Storage and Supply System
Southern California Gas Co. to Demonstrate CNG Drayage Trucks at SoCal Ports; CNG-Hydrogen Blend to Follow
Southern California Gas Co (The Gas Company), California Cartage Co. (Cal Cartage) and Autocar will collaborate to develop and put into service the nation’s first compressed natural gas (CNG) trucks used to transport containers off-loaded from ships. Five trucks will be delivered in June 2008 and used to move containers between the San Pedro Bay ports, which include the Los Angeles and Long Beach ports, to nearby freight-consolidation yards.
The new truck engines are certified to meet the US Environmental Protection Agency’s (EPA) 2010 on-road emission standards. Cal Cartage, the largest trucking company operating at the Los Angeles and Long Beach ports, will operate the CNG-powered trucks, which are manufactured by Autocar and are powered by Cummins Westport ISLG engines.
The new drayage trucks will produce NOx emissions 80% lower than the certification level for even the cleanest heavy-duty diesel engine, performing better than the emission requirements of both the San Pedro Bay Ports Clean Air Action Plan and the California Air Resources Board. Following the initial 12- to 18-month demonstration project, The Gas Company hopes to then further reduce emissions from the CNG drayage trucks by switching the fuel from CNG to a CNG-hydrogen blend for further emissions reductions.
Autocar manufactures natural gas-powered trucks for the refuse and other industries. Many of its natural gas vehicles designed and built at its Hagerstown, Ind., factory are now operating throughout Southern California.
Kenworth Truck Company has begun production of Kenworth Class 8 T800 LNG (liquefied natural gas) trucks using Westport’s LNG HPDI fuel system technology adapted for the Cummins ISX 15-liter engine. The announcement co-incided with the Ports of Los Angeles and Long Beach announcement to approve a new $1.6 billion Clean Truck Superfund. The fund will assist replacing many of the 16,800 Class 8 trucks serving the ports with cleaner vehicles. (Earlier post.)
Israel Prime Minister Says Developing Electric Vehicles is “National Project”; Seeking Partnership with Japanese
Nikkei. On a visit to Japan, Israel Prime Minister Ehud Olmert said that the country intends to make the development of electric vehicles a national project and will seek to partner with Japanese companies.
Israel will build infrastructure by setting up 500,000 recharging devices in parking lots across the country. It will raise the tax for gasoline-powered vehicles to 72% while holding down the rate for electric vehicles to 10% to spur purchases of the eco-friendly cars.
In January, The Renault-Nissan Alliance and Project Better Place signed a Memorandum of Understanding (MoU) to catalyze the mass-market deployment of electric vehicles in Israel. The solution framework comes in response to the Israeli State’s challenge to the auto industry and its supply chain to migrate the country’s transportation infrastructure to renewable sources of energy. As part of the solution framework, the Israeli government will provide tax incentives to customers, Renault will supply the electric vehicles, and Project Better Place will construct and operate an Electric Recharge Grid across the entire country. Electric vehicles will be available for customers in 2011. (Earlier post.)
Olmert visited a Nissan factory in Kanagawa Prefecture Thursday, accompanied by Shai Agassi, president of California-based start-up Project Better Place...Noting that the electric-vehicle business will become commercially viable in Israel in 2011, Agassi said he came to Japan seeking partners.
Mercedes to Introduce BlueEFFICIENCY C-Class Models; Fuel Consumption Reduction of Up to 12%
|The C 350 CGI BlueEFFICIENCY.|
Mercedes-Benz will add three fuel-efficient variants to its C-Class range: the C 180 KOMPRESSOR, C 200 CDI and C 350 CGI BlueEFFICIENCY models. Fuel consumption of the high-volume C 180 KOMPRESSOR and C 200 CDI models will be reduced by up to 12%.
The BlueEFFICIENCY version of the 100 kW/136 hp C 200 CDI consumes 5.1 liters of diesel per 100 kilometers (46.1 mpg US), while the C 180 KOMPRESSOR BlueEFFICIENCY with 115 kW/156 hp covers 100 kilometers with 6.5 liters of premium gasoline (36.2 mpg US). This corresponds to 135 and 156 grams of carbon dioxide, respectively, per kilometer. The C 350 CGI BlueEFFICIENCY features spray-guided gasoline direct injection and burns around 10% less fuel than the model with the current V-6 engine.
Engines. For the C 180 KOMPRESSOR, Mercedes-Benz has reduced the overall displacement of the four-cylinder engine from 1,796 to 1,597 cubic centimeters, while retaining the same output (115 kW/156 hp) and torque (230 Nm/170 lb-ft). This downsizing of the engine’s displacement, combined with measures for optimizing the combustion chamber, mixture formation and engine friction, adds up to a total potential fuel saving of 0.35 liters per 100 kilometers. The 6.5 L/100km fuel consumption of the C 180 KOMPRESSOR BlueEFFICIENCY is 0.9 liters (12%) less than for the standard production model.
The displacement, output and torque of the CDI engine remain unchanged. The package of efficiency measures (below) has enabled the NEDC fuel consumption of the BlueEFFICIENCY version of the C 200 CDI to be cut by 0.6 liters (10.5%) to 5.1 L/100km.
The new C 350 CGI BlueEFFICIENCY is equipped with a 3.5-liter spray-guided gasoline direct injection engine with a compression ratio of 12.2. (Mercedes-Benz became the first car maker to put spray-guided direct gasoline injection into series production in 2006.) Despite generating a higher power output and even greater torque, the new model consumes around 10% less fuel than the C 350 saloon with the current V-6 engine.
The six-cylinder CGI engine delivers 215 kW/292 hp of power and 365 Nm/269 lb-ft of peak torque at 3,000 rpm—15 kW/20 hp and 15 Nm/11 lb-ft more respectively than the current V-6 unit with port injection. Fuel consumption of the C 350 CGI BlueEFFICIENCY has been cut to approximately 8.4 L/100km (28 mpg US), about 1 liter below the figure for the current C 350.
The C 350 CGI BlueEFFICIENCY takes 6.2 seconds to accelerate from 0 to 100 kph and is capable of an electronically limited top speed of 250 kph/155 mph (provisional figures).
The six-cylinder engine demonstrates its particular strengths during stratified-charge operation when the powerplant operates with a high excess of air and is thus very fuel-efficient. In the Mercedes direct injection engine, this favorable lean-burn operation with a stratified charge in the combustion chamber is also possible for the first time at higher engine speeds and load ranges because the engine’s combustion chambers are injected with several successive jets of fuel in fractions of a second during each power stroke, thereby substantially improving mixture formation, combustion and consumption.
Whereas stratified-charge operation was previously only feasible over a limited partial load range, the CGI six-cylinder engine can now be operated in stratified charging mode over a wider range.
High-speed, ultra-precise piezoelectric injectors are among the key components of the second-generation direct gasoline injection system. The piezoelectric valves open their injectors outwards to create an annular gap just a few microns wide, allowing the fuel jet to form with a uniform, hollow cone-shaped pattern. With millisecond switching times, the piezoelectric injectors also permit the multiple injection that promotes lean-burn operation and helps optimize conditions to deliver the engine’s consumption figures.
A high-pressure pump with downstream distributor and pressure valve supplies the fuel and regulates the amount delivered in accordance with requirements. With a pressure of up to 200 bar, the system develops around 50 times the fuel pressure of a conventional port-injection system.
Measurements show that untreated emissions (hydrocarbons) are reduced by more than half in the warm-up phase. Active control of injection and combustion also produces higher temperatures in the exhaust manifold, thereby warming up the catalytic converters faster.
Four-valve technology, variable camshaft adjustment for the intake and exhaust sides, two-stage intake manifold, balancer shaft and an intelligent heat management system with map-controlled thermostat are some of the other technical highlights that the direct injection engine has adopted from the port-injected C 350 engine. The crankcase and cylinder head are made out of aluminium; the cylinders are fitted with low-friction, dimensionally stable liners made out of a lightweight aluminium-silicon alloy.
For the new BlueEFFICIENCY models, Mercedes engineers also reduced weight, aerodynamic drag and rolling resistance and organized the onboard energy management more efficiently. Together, these measures add up to a fuel saving on the NEDC driving cycle of 0.9 liters per 100 kilometers for the C 180 KOMPRESSOR, and 0.6 liters for the C 200 CDI.
Lightweighting. Mercedes managed to shave off between 19 and 32 kilograms of weight depending on the model. This is in part due to a newly developed windshield made of laminated glass, which weighs around 1.2 kilograms less than before. This is made possible by a technology transfer from the Maybach luxury sedan: between the panes of glass is a new, acoustically effective plastic membrane which efficiently absorbs wind noise. This has enabled Mercedes engineers to reduce the thickness of the windscreen, achieving a further weight reduction without compromising noise comfort in any way.
The noise-insulating lining of the firewall has also been weight-optimized with the help of special materials and computer simulations. Mercedes-Benz recalculated the required firewall insulation and precisely redefined the material thickness of the sound-absorbing resinous foam in line with the noise input. This needs-driven redesign reduces the weight of the lining by around 20%.
Forged lightweight wheels also have a positive effect on the weight. These weigh around 1.8 kilograms less than conventional light-alloy wheels, saving a total of more than seven kilograms per vehicle. These new lightweight wheels (6 J x 16 ET 39), which have aerodynamic benefits too, are standard equipment for the new BlueEFFICIENCY variants of the C 180 KOMPRESSOR and C 200 CDI.
Aerodynamics. At 120 kph/75 mph, the aerodynamic drag of the vehicle body already accounts for around 50% of all the dynamic resistance a passenger car must overcome, according to Mercedes. With a Cd of 0.27, the C-Class is among the most aerodynamically efficient notchback saloons in its market segment. The Cd figure for the new BlueEFFICIENCY models has been reduced by 7% to 0.25. Aerodynamic enhancements include:
Smooth underbody cladding ensures that the air can flow beneath the vehicle body without turbulences. The full engine compartment and underbody panelling of the diesel models is also standard equipment in the BlueEFFICIENCY version of the C 180 KOMPRESSOR.
Partially blanking off the radiator grille reduces the airflow into the engine compartment, thereby lowering wind resistance. Adequate cooling of the four-cylinder engines is of course uncompromised by this measure.
Sealing the joins between the hood and headlamps, as well as between the bumper and headlamps, improves the airflow around the front end.
The housings of the exterior mirrors were developed in the wind tunnel, and are particularly streamlined in form.
Lowering the suspension by 15 millimeters reduces aerodynamic drag, and has a particularly noticeable effect at higher speeds.
The design of the new lightweight wheels also meets aerodynamic requirements, and improves the airflow around the vehicle flanks.
Rolling resistance. In addition to lightweight construction measures, Mercedes-Benz also collaborate with Michelin to develop lightweight tires with a particularly low rolling resistance. These are now receiving their series production premiere in the C-Class, and help to reduce fuel consumption.
Rolling resistance is primarily caused by tire deformation as the tire contacts the road surface. This has a braking effect on the car, since additional energy is required to overcome this deformation resistance. Up to around 100 kph, rolling resistance has a greater effect on fuel consumption than aerodynamic drag, according to Mercedes.
The belt of this newly developed tire for the C-Class contains a multi-layered mesh of high-strength steel for less deformation. It is also lighter in weight than conventional designs, enabling a further 1.7 kilograms or so to be saved per set of tires. The secret, however, mainly lies in the chemical composition: the rubber compound for the treads and side walls is designed to ensure that rolling resistance is reduced by 17%, while retaining the same good handling and braking characteristics.
Energy management. Intelligent control of ancillary units and the reduction of friction losses also contribute to the increased fuel economy. In the BlueEFFICIENCY models of the C-Class, the power steering system is controlled on a needs-driven basis. The standard power steering in the C-Class has an additional valve which switches off the servo pump when it is not required.
While this pump operates continuously in all driving situations in conventional steering systems, the new valve interrupts the flow of hydraulic fluid when the car has followed a straight course for a while, switching off the servo pump. This has the advantage that the engine no longer needs to provide energy to drive the servo pump, meaning that it operates more economically. Thanks to this technology, the NEDC fuel consumption is cut by 0.14 liters per 100 kilometers&mash;which equates to a reduction of 2.5% in the case of the C 200 CDI.
Drive and transmission. As a further contribution to reduced weight and friction, the BlueEFFICIENCY C 180 KOMPRESSOR and C 200 CDI saloons are equipped with a newly developed final drive featuring further-improved antifriction bearings, forged differential gears and a lightweight construction. These measures reduce the friction forces within the transmission, hence the engine expends less energy in overcoming them. The longer final-drive ratios of the BlueEFFICIENCY versions also help to reduce fuel consumption. These are as follows:
C 180 KOMPRESSOR: 2.87 : 1 (rather than 3.07 : 1)
C 200 CDI: 2.47 : 1 (rather than 2.65 : 1)
The C 180 KOMPRESSOR and C 200 CDI models are both equipped with the six-speed manual transmission with overdrive characteristics as standard. With a ratio of 0.838 : 1 and 0.828 : 1, respectively, sixth gear considerably lowers the engine speed, contributing to more fuel-efficient driving.
A newly developed gearshift display in the cockpit informs the driver when he or she should change gear to save fuel. Experience gained during the Mercedes-Benz ECO Training courses has shown that drivers are able to make average fuel savings of up to 15% with an economical and energy-conscious style of driving. In addition to gearshift recommendations, the instrument cluster features a newly developed display showing the present fuel consumption. This will appear in the centre of the speedometer as an easily legible bar chart. A brief glance at the display is sufficient to tell the driver the current fuel consumption in liters per 100 kilometers. The bar chart responds immediately when the driver changes to a higher gear or takes his foot off the accelerator to use the deceleration fuel cut-off function.
Federal Court Torpedoes California Ship Auxiliary Engine Emissions Regulations
San Francisco Chronicle. The Ninth US Circuit Court of Appeals has ruled that California must get a waiver from the US Environmental Protection Agency before it can limit sulfur emissions from ocean-going ships that enter the state’s waters.
The Ocean-Going Vessel Auxiliary Diesel Engine Regulation, which requires ships to use low-sulfur fuel instead of dirtier bunker fuel on auxiliary diesel engines within 24 miles of the coast, became effective 1 January 2007.
The California Air Resources Board had ceased enforcement of the ruling on 30 August 2007, pursuant to an injunction ordered by a federal district court. ARB appealed and requested a stay of the injunction pending its appeal. The court granted he stay 23 October 2007, and ARB resumed enforcement.
However, the court has now ruled that implementing the low-sulfur requirements will require a waiver from the EPA.
State Attorney General Jerry Brown, whose office represented the Air Resources Board, said lawyers are studying the ruling to see if the board can rewrite its regulations and implement them without EPA approval. He said he would also recommend that the board ask the full appeals court for a rehearing.
Brown has asked the EPA on the state’s behalf to regulate ships’ emissions of greenhouse gases, which contribute to global warming, and has also sued the agency over its refusal to let California enforce limits on greenhouse gases from cars and trucks.