In a diesel engine, what does the term 'pre-injection' refer to?

The term 'pre-injection' in a diesel engine refers to the injection of fuel before the main injection occurs. This process involves injecting a small quantity of fuel into the combustion chamber shortly before the larger main injector event. The purpose of pre-injection is to create a more uniform and complete combustion process by enhancing the mixing of fuel and air, thus reducing emissions and improving engine performance. By initiating combustion slightly earlier, it helps in controlling the combustion timing, which can lead to smoother engine operation and reduced noise.

This allows for better thermal efficiency and helps in minimizing the formation of pollutants such as nitrogen oxides and particulate matter. Pre-injection is often part of advanced fuel injection strategies that are used in modern diesel engines to meet strict emission regulations and improve overall engine efficiency.