Direct Gasoline Injection in the Negative Valve Overlap of a Homogeneous Charge Compression Ignition Engine

An engine with variable valve timing was operated in homogeneous charge compression ignition (HCCI) mode. In two sets of experiments, the fuel was introduced directly into the combustion chamber using a split injection strategy. In the first set, lambda was varied while the fuel flow was constant. The second set consisted of experiments during which the fuel flow was altered and lambda was fixed. The results were evaluated using an engine simulation code with integrated detailed-chemistry.

The auto-ignition temperature of the air-fuel mixture was reached when residual mass of the previous combustion cycle was captured using a negative valve overlap and compressed together with the fresh mixture charge inducted. When a pilot fuel amount was introduced in the combustion chamber before piston TDC, during the negative valve overlap, radicals were formed as well as intermediates and combustion took place during this overlap provided the mixture was lean. This combustion was observed during both the experiments and simulations. Combustion in the gas exchange phase increased the mixture temperature in the main compression stroke, hence advancing the auto ignition timing. The degree of main combustion advancement was found to be dependent on the level of oxygen available in the negative valve overlap and the pilot fuel amount.

Calculations showed that intermediates present in the mixture at the start of the compression stroke could result in a significant advancement of auto-ignition and heat release.