Using Multiple Injection Strategies in Diesel Combustion: Potential to Improve Emissions, Noise and Fuel Economy Trade-Off in Low CR Engines

<div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">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: <ol class="list nostyle"> <li class="list-item"><span class="li-label">1</span><div class="htmlview paragraph">Fuel introduction and heat release splitting</div></li> <li class="list-item"><span class="li-label">2</span><div class="htmlview paragraph">Control of the fuel distribution in the combustion chamber</div></li> <li class="list-item"><span class="li-label">3</span><div class="htmlview paragraph">Cooling effect of the vaporizing fuel</div></li> </ol> </div> <div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">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.</div>