The Mixture Response of a Stratified Charge Gasoline Engine with Independent, Twin, Port-Fuel Injector Control

Abstract

An experimental study of the mixture response performance of novel, port-fuel injection strategies upon combustion stability in a gasoline engine was undertaken at low engine load and speed conditions in the range of 1.0 bar to 1.8 bar GIMEP and 1000 rpm to 1800 rpm. The aim was to improve the thermal efficiency of the engine, by extending the lean limit of combustion stability, through promotion of stable charge stratification. The investigation was carried out using a modified 4-valve single cylinder head, derived from a 4-cylinder, pent-roof, production, gasoline engine. The cylinder head was modified by dividing the intake tract into two, separate and isolated passages; each incorporating a production fuel injector. The fuel injection timing and duration were controlled independently for each injector. The performance effects of a single or multiple fuel injection event on a single-sided injector were compared to simultaneous and phased fuel injection for the pair of injectors, with both open valve or closed valve fuel injection timings. A model of the engine, implemented in the Ricardo WAVE software and refined using in-cylinder pressure data, was used to support the findings. The initial experimental results showed good agreement with the model’s prediction and baseline data obtained in a previous study. Analysis of the experimental results for the alternative injection strategies showed that the engine could be operated with far leaner mixtures at low speeds and loads. Combustion stability, defined for a single-cylinder engine as 10% CoVGIMEP, was improved for each engine condition tested. At 1000 rpm and 1.0 bar GIMEP, the lean combustion limit was extended from 14:1 air-to-fuel ratio (AFR) to 17.5:1. At 1500rpm and 1.5 bar GIMEP, the lean combustion limit was extended from 17.5:1 to approximately 21:1 AFR. At 1800 rpm and 1.8 bar GIMEP, the lean combustion limit was improved from 21:1 AFR to 22:1. The improved tolerance of the combustion system to charge dilution, due to the optimised injection strategies, was evaluated for high levels of trapped residuals. The relevance to conditions required for controlled auto-ignition combustion is discussed. Finally, the influence of the new strategies upon the rates of heat release and the combustion duration were evaluated and compared to cycle-resolved measurements of the concentration of unburnt hydrocarbons in the exhaust gases.