Latest emissions standards impose very low NOx and particle emissions that have led to new Diesel combustion operating conditions, such as low temperature combustion (LTC). The principle of LTC is based on enhancing air fuel mixing and reducing combustion temperature, reducing raw nitrogen oxides (NOx) and particle emissions. However, new difficulties have arisen. LTC is typically achieved through high dilution rates and low CR, resulting in increased auto-ignition delay that produces significant noise and deteriorates the combustion phasing. At the same time, lower combustion temperature and reduced oxygen concentration increases hydrocarbon (HC) and carbon oxide (CO) emissions, which can be problematic at low load.
Therefore, if LTC is a promising solution to meet future emission regulations, it imposes a new emissions, fuel consumption and noise trade-off. For this, the injection strategy is the most direct mean of controlling the heat release profile and fuel air mixture. However, conventional strategies no longer apply and new injection patterns must be defined to optimise the combustion.
This publication summarises the studies that have been carried out at IFP Energies nouvelles on LTC injection strategies as a means of achieving low emissions, fuel consumption and noise, demonstrating that adapted injection patterns allow modification of heat release and fuel mixture distribution even for pre-mixed combustion conditions. This publication analyses how injection parameters impact the heat release profile under LTC conditions, and case studies will demonstrate how the injection strategies can allow optimization of the emissions, fuel consumption and noise trade-off for different operating points and engine conditions. For example, low HC and CO emissions have been obtained through enhanced air-fuel mixture and limited exhaust gas recirculation (EGR) rate, while maintaining reduced peak heat release for low NOx emissions and combustion noise. The results are achieved through closely-spaced injections with split fuel quantities.