How does the injection timing affect NOx and particulate matter formation in a CRDI diesel engine?

Injection timing in a Common Rail Direct Injection (CRDI) diesel engine significantly affects the formation of nitrogen oxides (NOx) and particulate matter (PM), which are major pollutants. NOx forms primarily at high temperatures and oxygen concentrations. Advancing the injection timing, meaning injecting fuel earlier in the compression stroke, allows more time for combustion, leading to higher peak cylinder temperatures. These higher temperatures promote NOx formation. Conversely, retarding the injection timing, injecting fuel later in the compression stroke, reduces the peak cylinder temperatures and thus reduces NOx formation. However, retarding the injection timing can lead to incomplete combustion due to less time for the fuel to mix and burn completely. This incomplete combustion increases the formation of particulate matter (PM), which consists of unburnt hydrocarbons and soot. PM formation is also influenced by local fuel-rich zones within the cylinder. Early injection timing tends to promote more homogenous mixing of fuel and air, which can reduce PM formation. Late injection timing can lead to regions where the fuel-air mixture is too rich, increasing PM formation. Modern CRDI systems use multiple injection events (pre-injection, main injection, post-injection) to optimize the trade-off between NOx and PM. Pre-injection injects a small amount of fuel before the main injection to raise the cylinder temperature and pressure, promoting more complete combustion of the main injection and reducing both NOx and PM. Post-injection injects fuel during the expansion stroke to increase exhaust gas temperature, which helps to burn off PM in the diesel particulate filter (DPF). Therefore, the optimal injection timing is a carefully calibrated balance considering engine speed, load, and emissions requirements. For example, under heavy load conditions, slightly retarded timing might be used to control NOx, while under light load, advanced timing can improve fuel efficiency and reduce PM.