Examination of the high load limit of an HCCI engine

Abstract

The implementation of homogenous charge compression ignition (HCCI) to gasoline engines is constrained by many factors. This work examines constrains imposed by nitric oxide (NOx) emission and by the need to maintain a minimum catalyst temperature on HCCI operation. Then the nature of the approach to high load limit was examined for three fuels with very different behavior.An engine simulation was used to examine constrains imposed by NOx emission and by catalyst temperature requirement. The valve timing in a HCCI engine using NegativeValve-Overlap (NVO) was varied in the simulation to control the operating point. The engine speed and intake pressure (turbocharged mode) were varied. The High Load Limit (HLL) was attained when the NOx emission reached the regulated level for a Partial-Zero-Emissions-Vehicle (PZEV). This occurred when the engine was running at the lowest speed and the highest intake pressure. Unreasonably large residual fraction was required to achieve the NOx limit unless a three-way catalyst is used.The engine behavior in the operating trajectory to the HLL was examined by using two Primary Reference fuels (PRF60 and PRF90) and a fuel blended from refinery feed stock. The latter fuel had Extremely Low Aromatic and Olefin content and is referred to as the ELAO fuel. For PRF60 (the knock prone fuel), the Maximum Pressure Rise Rate (MPRR) increased with increase in load (by reduction of residual). The HLL was attained when the MPRR reached a pre-determined level of 5MPa/ms. For PRF90 (the knock resistant fuel), however, the MPRR decreased with increase in load, and the HLL was constrained by ignition failure. For the ELAO fuel, the MPRR first increased and then decreased with increase in load. The HLL was thus constrained by ignition failure. Thus depending on the fuel properties, there could be very different engine behaviors in the approach to the HLL of HCCI operation.