Combustion Chamber Geometry Effects on the Performance of an Ethanol Fueled HCCI Engine 2008-01-1656
Homogeneous Charge Compression Ignition (HCCI) combustion is limited in maximum load due to high peak pressures and excessive combustion rate. If the rate of combustion can be decreased the load range can be extended. From previous studies it has been shown that by using a deep square bowl in piston geometry the load range can be extended due to decreased heat release rates, pressure rise rates and longer combustion duration compared to a disc shaped combustion chamber. The explanation for the slower combustion was found in the turbulent flow field in the early stages of the intake stroke causing temperature stratifications throughout the charge. With larger temperature differences the combustion will be longer compared to a perfectly mixed charge with less temperature variations. The methods used for finding this explanation were high-speed cycle-resolved chemiluminescence imaging and fuel tracer planar laser induced fluorescence (PLIF), together with large eddy simulations (LES). In this paper the performance of the deep square bowl in piston, a disc shaped and a classical diesel bowl combustion chamber were compared in all-metal engine configuration. Combustion duration, pressure rise rates, rate of heat release, emissions, efficiency etcetera were evaluated at different engine loads. Also an investigation on the differences between the optical engine and the all-metal engine was performed. The engine used was a single cylinder Scania D12 Diesel engine converted to HCCI operation using port fuel injected ethanol as fuel and preheating of the inlet air to control combustion phasing. The results showed that the net indicated efficiency for a given pressure rise rate limit was similar at high loads between the deep square bowl in piston compared to a Disc shaped combustion chamber. However, for lower engine loads quenching in the squish volume results in decreased efficiency and increased emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO).