Using Multiple Injection Strategies in Diesel Combustion: Potential to Improve Emissions, Noise and Fuel Economy Trade-Off in Low CR Engines 2008-01-1329
In former high compression ratio Diesel engines a single injection was used to introduce the fuel into the combustion chamber. With actual direct injection engines which exhibit a compression ratio between 17:1 and 18:1 single or multiple early injections called “pilot injections” are also added in order to reduce the combustion noise. For after-treatment reasons a late injection during the expansion stroke named “post injection” may also be used in some operating conditions.
Investigations have been conducted on lower compression ratio Diesel engine and in high EGR rate operating conditions to evaluate the benefits of multiple injection strategies to improve the trade off between engine emissions, noise and fuel economy. These experiments have highlighted that in the thermodynamic conditions specific to low CR engines and high EGR rate interesting effects on the combustion process could be achieved via multiple injections strategies involving the following phenomena:
Fuel introduction and heat release splitting
Control of the fuel distribution in the combustion chamber
Cooling effect of the vaporizing fuel
Decreasing the peak heat release by splitting the heat release process appears to be adequate for controlling the combustion noise. It is achieved by using several injections in the appropriate thermodynamic and auto-ignition delay conditions in order to reduce the instantaneous fuel burning rate. Thanks to this resulting noise reduction, the engine settings can be re-optimized toward a new trade off.
Multiple injection strategies can also be used to better control the spatial fuel distribution to enhance the air use in the combustion chamber. Generally, this effect can lead to a reduction in particulate emissions at intermediate engine loads, allowing for potentially higher EGR rate.
The cooling effect associated with fuel vaporization lowers locally and globally the temperature of the gases contained in the combustion chamber. If well-understood, these phenomena can be applied to increase the ignition delay, allowing for a longer mixing period and thus a more homogeneous fuel/air mixture, and/or to modify the rate of heat release in the early stage of combustion. Appropriate use of the cooling effect provides a valuable degree of freedom in the optimization of the noise / emission / fuel economy trade off.