The latest generation of high-efficiency gasoline direct injection (GDI) engines continues to be a significant source of dangerous ultra-fine particulate matter (PM) emissions. The forthcoming advent in the 2017–2020 timeframe of the real driving emission (RDE) standards affords little time for the identification of viable solutions. The present research work aims to contribute towards a much-needed improved understanding of the process of PM formation in theoretically-homogeneous stoichiometric spark-ignition combustion. Experimental measurements of engine-out PM have been taken from a wall-guided GDI engine operated at part-load; through parallel computational fluid dynamics (CFD) simulations of the test-engine, the process of mixture preparation was investigated. About 80% of the total particle number is emitted on average in the 5–50 nm range, with the vast majority being below the regulated lower limit of 23 nm. The results suggest that both improved charge homogeneity and lower peak combustion temperature contribute to lower particle number density (PN_Den) and larger particle size, as engine speed and load increase. The effect of engine load is stronger and results from greater injection pressure through better fuel droplet atomisation. Increases in pre-combustion homogeneity of 6% are associated with one order of magnitude reductions of PN_Den. A simplified two-equation functional model was developed, which returns satisfactory qualitative predictions of PN_Den as a function of basic engine control variables.

Controlled Auto-Ignition (CAI) combustion was investigated in a Ricardo E6 single cylinder, four-stroke gasoline engine. CAI combustion was achieved by employing positive valve overlap in combination with variable compression ratios and intake air temperatures. The combustion characteristics and emissions were studied in order to understand the major advantages and drawbacks of CAI combustion with positive valve overlap. The enlargement of the CAI operational region was obtained by boosting intake air and adding external EGR. The lean-boosted operation elevated the range of CAI combustion to the higher load region, whilst the use of external EGR allowed the engine to operate with CAI combustion in the region between boosted and N/A CAI operational ranges. The results were analyzed to investigate combustion characteristics, performance and emissions of the boosted CAI operations.

The CAI/HCCI combustion was achieved through residual gas trapping using the negative valve overlap method in a direct injection single-cylinder gasoline engine with optical access. Effects of injection timing and spark discharge on the engines performance and emission were investigated by means of simultaneous in-cylinder heat release analysis and the high-speed chemiluminescence imaging technique. The presence and magnitude of fuel reformation and heat release associated with the fuel injection into lean-burn residual gases were identified. Furthermore, the effect of quantity of directly injected air from an air-assisted injector into stoichiometric burned residual gas was studied. The in-cylinder heat release and imaging showed that spark discharge had much less effect on the heat release process in the lean-burn CAI operation than that in the stoichiometric operation because of the less ignitability of the leaner mixture. Direct injection into the hot residual gas was found to produce minor heat release and led to chemiluminescence. This process resulted in higher charge temperature and earlier autoignition followed. Furthermore, it was found that small quantity of air injection included in the air-assisted injection process during the negative valve overlap period could be used to promote the fuel reforming and minor heat release reactions in order to control the main combustion phasing.

Extensive engine experiments were carried out on controlled auto-ignition combustion, also known as homogeneous charge compression ignition, in a four-stroke gasoline engine, by internally recycling burned gases through positive valve overlap, variable compression ratio, intake temperature control and boosting. The operational range of controlled auto-ignition combustion was determined for a range of compression ratios and intake air temperatures at wide-open throttle conditions. This was followed by further engine experiments with boosted intake and external exhaust gas recirculation in order to evaluate their effect on the operational range of controlled auto-ignition combustion and the engine’s performance and emissions. It has been found that the controlled auto-ignition operational region could be extended to the higher load region by boosting and running with leaner mixtures, whilst the use of external exhaust gas recirculation allowed the engine to operate controlled auto-ignition combustion optimized in the region between naturally aspirated and lean boosted controlled auto-ignition operational regions. The highest fuel conversion efficiency was obtained when the exhaust heat could be utilized to supply the intake air heating.

This paper presents the details of a Computational Fluid Dynamics methodology to accurately model the process of mixture preparation in modern Gasoline Direct Injection engines, with particular emphasis on liquid film as one of the main causes of Particulate Matter formation. The proposed modelling protocol, centred on the Bai-Onera approach of droplets-wall interaction and on multi-component surrogate fuel blend models, is validated against relevant published data and then applied to a modern small-capacity GDI engine, featuring centrally-mounted spray-guided injection system. The work covers a range of part-load, stoichiometric and theoretically-homogeneous operating conditions, for which experimental engine data and engine-out Particle Number measurements were available. The results, based on the parametric variation of start of injection timing and injection pressure, demonstrate how both fuel mal-distribution and liquid film retained at spark timing, may contribute to PN emissions, whilst their relative importance vary depending on operating conditions and engine control strategy. Control of PN emissions and compliance with future, more stringent regulations remain large challenges for the engine industry. Renewed and disruptive approaches, which also consider the sustainability of the sector, appear to be essential. This work, developed using Siemens Simcenter CFD software as part of the Ford-led APC6 DYNAMO project, aims to contribute to the development of a reliable and cost-effective digital toolset, which supports engine development and diagnostics through a more fundamental assessment of engine operation and emissions formation.