Mixture preparation in automotive spark-ignition engines - With particular reference to multipoint fuel systems.

This thesis commences by reviewing current methods of fuel mixture preparation in automotive spark-ignition engines, identifying both desirable and adverse characteristics. Experimental techniques for measurement of fuel droplet spray size distributions are considered, before describing design and operation of novel test rigs incorporating a laser- diffraction drop sizing instrument. Such measurements were made upstream and downstream of the intake valve on representative multipoint fuel injected engine components. Comparisons are drawn between the mixture quality delivered by this modern form of fuel system and more traditional carburetted systems. Interpretation of such Malvern Particle Sizer results is considered critically. In-cylinder mixture preparation on one engine under test proved poor to the extent that a majority of the fuel entered the combustion chamber in liquid form. Excessive droplet size was postulated as a probable contributory factor. Motored rigs, based on two distinct types of cylinder head, were devised in order to quantify the wallfilm by measurement. A significant conclusion was that its prevalence (or otherwise) is indeed a direct function of the droplet size achieved upstream of the inlet valve. A computer model was constructed to predict in-cylinder droplet trajectories, and confirmed that relatively smaller droplets introduced at the valve seat would travel further within the cylinder before wall impaction, increasing the time available for evaporation. Rig experimentation and droplet trajectory theory is complemented by explanation of a cylinder pressure based technique for assessment of mixture preparation changes on the performance of a running engine. The benefits of a new type of port throttled fuel injection system, specifically designed to produce finer droplets at low load, are thereby demonstrated in terms of combustion stability. Reduced contamination of incoming charge by exhaust residuals, via a mechanism of port pressure recovery, is also held to be partially responsible for stability improvements and reduction in fuel consumption