Enhanced Combustion Technologies

Understanding and controlling the combustion process is the first step in reducing engine out emissions and reducing the burden on the emission control systems within the exhaust. Engine design is an important part of controlling and facilitating the combustion process.

The most dramatic approach that has been used in recent years on spark-ignited engines is the development of gasoline direct injection (GDI) engines. Early fuel injected engines used a port injection approach where fuel was injected into a port to allow it to evaporate and mix uniformly with the air. This provides little control over the air and fuel mixture entering the cylinder. GDI engines inject the fuel directly into the combustion chamber allowing varying injection strategies depending on engine load. At high load, fuel is injected into the engine early during the induction stroke giving a stoichiometric or rich air/fuel ratio. A GDI engine can also operate in an ultra lean combustion mode during cruising situations when little acceleration is required. In this case the fuel is injected in the latter stages of the compression stroke just prior to ignition. This allows a small amount of fuel to be placed near the spark plug. The effective air fuel ratio is very lean resulting in significant fuel savings. The reason that partial lean burn GDI engines have not reached broad application in the market is the difficulty in meeting NOx emissions regulation during long periods of lean operation. GDI engines can also be designed for stoichiometric operation and make use of three-way catalysts for lowering exhaust emissions.

In diesel engines, controlling combustion is the key approach to reducing engine out particulate emissions by optimizing the mixing between the fuel and air. Some common ways to increase mixing is through combustion chamber modifications to facilitate turbulent flow as well as fuel injector design to modify the spray pattern. Variable geometry turbocharging (VGT), which delivers variable quantities of pressurized air based on driving conditions, has been effective in reducing PM emissions by maintaining lean combustion in the engine. Reducing compression ratios have been shown effective in reducing combustion temperatures and in turn NOx emissions. Some common approaches to enhance air turbulence and improve fuel distribution within the cylinders include improvements to the design of fuel injectors, combustion chambers and injection ports. Some engine manufacturers have been able to achieve improvements to the combustion during cold start by making modifications to the design of intake air control valves resulting in a 40-50% reduction in HC emissions and injection ports among others.

Tags: Enhanced Combustion Technologies, Passenger Cars, SUVs, and Light-Duty Trucks