Influence of early fuel injection timings on premixing and combustion in a diesel engine

The influence of fuel injection timing on precombustion mixing of diesel fuel and air, combustion, and emissions at early-injection conditions similar to homogeneous charge compression-ignition (HCCI) engine conditions was investigated experimentally in an automotive-size compression-ignition engine and a constant-volume vessel. The injection timing was controlled by electronic fuel injection equipment. In-cylinder pressure measurements, engine-out emission measurements, and imaging of the spray development were used to analyze the ignition delay period and fuel distribution. The ignition delay period was measured over a wide range of injection timings as well as at various compression ratios and engine speeds. With advancing fuel injection timing, the ignition delay increased and the engine-out nitrogen oxides (NOx) decreased, suggesting that increased premixing time results in a lean mixture and low flame temperature. It was also found that the ignition delay period to decrease NOx emissions to a negligible level was almost the same under any compression ratio or engine speed. From the emissions measurements, a drastic decrease in smoke, hydrocarbon (HC), and carbon monoxide (CO) emissions was observed at specific early-injection timing. To clarify the source of the observed behavior, the diesel spray impinging on the surface of a combustion chamber was visualized in a constant-volume vessel simulating in-cylinder environments under early-injection conditions. The images show that diesel spray should target the bowl-lip area to enhance precombustion mixing, which would be desirable for reducing incomplete combustion products.