Studies on a Multi-Cylinder Direct-Injection Spark Ignition Engine

Overview

It has been reported that 80% of Unburnt Hydrocarbons (UHCs) are emitted from a Direct Injection Spark Ignition (DISI) engine during the first 100 s of a cold start. This is due to the catalyst not being lit and therefore a strategy is required in order to reduce the catalyst warm up time. A requirement of this strategy is to increase the heat flow into the exhaust while ensuring stable combustion and also reducing the engine out emissions, UHCs, PM and NOx.

A retarded ignition strategy is used in order to increase the heat flow to the exhaust, while a twin injection strategy increases the combustion stability. A combination of the highly retarded ignition and non standard valve timing reduces the engine out emissions during this warm up period. The 1st injection occurs during the intake stroke where approximately 70% of the fuel is injected and hence a well mixed but lean background mixture is therefore present in the cylinder. The 2nd injection occurs close to ignition and hence enables a locally rich mixture to be present in the spark plug gap at the time of ignition while the overall AFR in the cylinder is slightly lean. An enabler of this strategy is to develop a combustion system that is robust and misfire free while operating in the extremes of the split injection mode. This will improve the catalyst light off time avoid the risk of misfire induced hydrocarbon emissions.

It has been shown that this twin injection strategy increases the combustion stability, but in the extremes of the operation this can lead to the occasional misfiring cycles. The initialisation of combustion in the chamber is going to be sensitive to spark plug orientation, spark plug protrusion, the injector orientation and the timing of the 2nd injection in relation to the ignition.

Work conducted by Chen et al. (SAE 2009-01-1489) has shown that a spray close to the spark plug may interfere with the sparking process, either through turbulence, bulk motion or liquid fuel mechanisms. End of 2nd injection timing sweeps have been conducted in order to determine the misfiring characteristics for different spark plug orientations, spark plug protrusions, injector orientations and spark plug type.

The aim of this project is to investigate the fundamental reasons for the misfiring cycles. High speed photography will be used in order to visualise the spark, the 2nd fuel injection and also the combustion for the firing cycles. Optical diagnostics will determine the start of combustion. Bench tests with and without flow (and with liquid fuel on the earth electrode) have been used to elucidate the effect of spark plug geometry and orientation. High speed videos of the spark have been combined with the voltage and current histories.

HT measurements of the current and voltage have been taken to characterize ignition, and show how arc parameters can be related to the subsequent combustion performance. The breakdown voltage and current have been measured with sample-hold circuits since the arc parameters were only recorded at 300 kHz.

Poster

B.W.G. Twiney and C.R. Stone (2009), "Combustion Stability During the Catalyst Warm-Up Phase Of a Direct Injection Spark Ignition Engine", SET for Britain Poster Competition, House of Commons, London, March 2009, E57

Presentation

Ben Twiney, Ben Williams, Tom Smith, Richard Stone, Xiangdong Chen and Gavin Edmunds (2009), "Ignition Characteristics of a GDI Engine with Split Injection", UnICEG, Oxford, May 2009

Timescale

Start : 1st October 2006. Ends : 31st March 2010.

Involved Parties

Group : B.W.G. Twiney, C.R. Stone
Project Support : Jaguar Cars, EPSRC.