Ioannou, Marios; (2000) An investigation of the liquid fuel films within the cylinder of a spark ignition engine. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The work reported in this thesis was conducted to investigate the liquid fuel behaviour within the cylinder bore of spark ignition engines. This liquid fuel, most of which results from engine starting and the subsequent warm-up period, is one of the main contributors to unburned hydrocarbon (uHC) emissions. In modem port fuel injected (PFI) engines, fuel is injected into the port just upstream of the intake valves of each cylinder. The rate of fuel vaporisation at cold conditions is very low and more fuel than is immediately required has to be injected to form a combustible mixture at the time of ignition. It is this excess fuel that results in the high uHC emissions during the warm-up period following a start. The mixture formation process depends on several factors, the most important ones being the cylinder head (intake port and combustion chamber designs), the surface temperature (of the port, the valves and the combustion chamber), the injection timing (closed valve injection - CVI, or open valve injection - OVI), droplet size (injector type), engine load (manifold pressure) and engine speed (the higher the speed the less time for evaporation). This study was aimed at increasing the understanding of how these factors affect the liquid fuel behaviour within the cylinder, and to a lesser extent the uHC emissions. The first phase of this work involved the application of a known and reliable technique to measure liquid fuel impaction on the cylinder bore, so as to provide information on a relatively short time-scale as to how cylinder head design influences fuel impaction. The porous-liner technique, as the name implies, was based on the positioning of a porous insert in the top of the cylinder bore, through which any liquid fuel that impacted on it, was drawn off, collected and measured. However, this technique could only be applied on a pulsating flow rig and not on a running engine. Under the non-realistic operating conditions involved, the arrangement provided a worst-case indication of the liquid fuel quantity on the bore, during the first cycles of cold start. Furthermore, it can be used for comparison purposes of different operating conditions (speed, load and injection timing), or even different cylinder head configurations. The second phase of this study involved the development of a novel approach, the heat flux sensor technique, to characterise the liquid fuel within the cylinder bore of a running engine. This technique involved the installation of heat flux sensors in the cylinder bore. The face of the sensor was exposed to the in-cylinder environment and its body was mounted in the block. Heat flux readings were significantly higher when liquid fuel was present on the face, since the fuel absorbed heat to evaporate while the heat transfer coefficient between the sensor and the liquid is far higher than that between the sensor and just the gases within the cylinder. It was shown that this technique can successfully identify and measure the liquid fuel on the bore, and provide useful information on how the liquid fuel behaviour is affected by the cylinder head design, injection timing and surface temperatures.

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