Operation strategy of a Dual Fuel HCCI Engine with VGT 2007-01-1855
HCCI combustion is well known and much results regarding its special properties have been published. Publications comparing the performance of different HCCI engines and comparing HCCI engines to conventional engines have indicated special features of HCCI engines regarding, among other things, emissions, efficiency and special feedback-control requirements. This paper attempts to contribute to the common knowledge of HCCI engines by describing an operational strategy suitable for a dual-fuel port-injected Heavy Duty HCCI engine equipped with a variable geometry turbo charger. Due to the special properties of HCCI combustion a specific operational strategy has to be adopted for the engine operation parameters (in this case combustion phasing and boost pressure). The low exhaust temperature of HCCI engines limits the benefits of turbo charging and causes pumping losses which means that “the more the merrier” principle does not apply to intake pressure for HCCI engines. It is desirable not to use more boost pressure than necessary to avoid excessively rapid combustion and/or emissions of NOx. It is also desirable to select a correct combustion phasing which, like the boost pressure, has a large influence on engine efficiency. The optimization problem that emerges between the need for boost pressure to avoid noise and emissions and, at the same time, avoiding an extensive decrease of efficiency because of pumping losses is the topic of this paper. The experiments were carried out on a 12 liter Heavy Duty Diesel engine converted to pure HCCI operation. Individually injected natural gas and n-Heptane with a nominal injection ratio of 85% natural gas and the rest n-Heptane (based on heating value) was used as fuel. The engine was under feedback combustion control during the experiments.