|Illustration of the V-6 VVT DI. Click to enlarge.|
GM announced that it will make the 3.6L V-6 gasoline engine with direct injection (DI) and variable valve timing (VVT) technologies the standard engine in the 2008 Cadillac STS.
With direct injection, less fuel is required to produce the equivalent horsepower, especially at normal cruising speeds, compared to a conventional port-injection combustion system. The technology can be used to support downsizing engine applications for bigger benefits in fuel consumption, or, as in this case, to deliver more power with incremental benefits in fuel consumption.
The new standard engine for STS delivers 302 horsepower (225 kW) at 6,300 rpm (+18%) and torque of 272 lb-ft (366 Nm) at 5,200 rpm (+8%). At the same time, the engine is expected to deliver improved fuel efficiency. Official EPA fuel economy statistics for the 2008 model have not yet been announced. On the outgoing 2007 model STS, EPA fuel economy is 18 mpg city and 27 mpg hwy.
Our goal was to deliver an engine with outstanding power and improved efficiency...It’s the latest example of our strategy to continue to reduce emissions and improve fuel economy without sacrificing performance.—Tim Cyrus, chief engineer for high-feature V-6 and Northstar V-8 engines
The 3.6L V-6 engine is GM’s third engine variant offered with direct injection technology. GM offers an Ecotec 2.0-liter four-cylinder Turbo engine with direct injection on the 2007 Saturn Sky Red Line and Pontiac Solstice GXP roadsters. Since 2004, Opel has offered a naturally aspirated Ecotec 2.2-liter direct injection engine in Europe.
By the end of 2008, GM forecasts it will produce as many as 200,000 vehicles globally with direct injection technology, and projects that by 2010, one of every six GM vehicles in North America will be equipped with a direct injection engine.
With direct injection technology, fuel is sprayed directly into the engine cylinder where it is mixed with air. As the fuel vaporizes in the cylinder, the air and fuel mixture is cooled. This enables the use of a higher compression ratio in the combustion chamber, 11.3:1, which improves engine performance and efficiency.
On cold starts, direct injection can be controlled to create a richer air/fuel mixture around the spark plug, making it easier to ignite in a cold engine. This results in a smoother operation and an approximate 25% reduction in cold-start hydrocarbon emissions during the cold-start and warm-up.
Direct injection requires higher fuel pressure than conventional fuel-injected engines, and the engine-driven high-pressure fuel pump is used to supply up to 1,740 psi (120 bar) of pressure. The system regulates lower fuel pressure at idle—approximately 508 psi (35 bar) and higher pressure at wide-open throttle. The exhaust cam-driven high-pressure pump works in conjunction with a conventional fuel tank-mounted supply pump.
The 3.6-liter V-6 also features the US automotive industry’s first isolated fuel-injector system. This helps reduce the overall engine noise.
In addition to direct injection, variable valve timing (VVT) also helps reduce exhaust emissions and improve performance and fuel economy by optimizing the phasing of the intake and exhaust valves relative to the crankshaft position at all operating conditions.
The result is linear delivery of torque, with near-peak levels over a broad rpm range, and high specific output (maximum horsepower per liter of displacement) without sacrificing overall engine response and driveability.
Cam phasing helps reduce exhaust emissions by optimizing exhaust valve overlap and eliminating the need for a separate exhaust gas recirculation (EGR) system.
By closing the exhaust valves late at appropriate times, the cam phasers allow the engine to draw the desired amount of exhaust gas back into the combustion chamber, reducing unburned hydrocarbon emissions.
The return of exhaust gases also decreases peak temperatures, which contributes to the reduction of oxides of nitrogen (NOx) emissions. In tandem with the 25% reduction in cold-start hydrocarbon emissions enabled by direct injection, the 3.6-liter VVT DI V-6 surpasses all emissions mandates, and does so without emissions control systems such as EGR and air injection reaction (AIR).