Gautam Kalghatgi

The outlook for fuels for internal combustion engines

The demand for transport energy is increasing, but this increase is heavily skewed toward heavier fuels such as diesel and jet fuel while the demand for gasoline might decrease. As spark-ignition engines develop to become more efficient, abnormal combustion such as knock and preignition will become more likely. High antiknock quality fuels, those with high research octane number and preferably low motor octane number, will enable future spark-ignition engines to reach their full potential. Higher fuel antiknock quality is also likely to mitigate “superknock” resulting from preignition—an abnormal combustion problem in turbocharged spark-ignition engines. In many parts of the world, fuel antiknock specifications are set on the assumption that higher motor octane number contributes to increased knock resistance. Specifications for fuel antiknock quality have a great impact on fuels manufacture and will need to be revised as this mismatch between existing specifications and engine requirements widens. The primary challenge for diesel engines is to reduce emissions of soot and NOx while maintaining high efficiency, and this becomes much easier if such engines are run on fuels of extremely low cetane. Significant development is needed before such engines can be seen on practical vehicles. In the long term, compression ignition engines are likely to use fuels with research octane number in the range of 70–85 (cetane number<∼30) but with no strict requirements on volatility. Such fuels would require less processing in the refinery than today’s fuels. Such an engine/fuels system will be at least as efficient as today’s diesel engine but could be significantly cheaper and also open a path to mitigate the imbalance in demand growth between heavy and light fuels that is expected to arise otherwise. The review concludes with a possible long-term fuel scenario.

Investigation into Light Duty Dieseline Fuelled Partially-Premixed Compression Ignition Engine

Conventional diesel fuelled Partially-Premixed Compression Ignition (PPCI) engines have been investigated by many researchers previously. However, the ease of ignition and difficulty of vaporization of diesel fuel make it imperfect for PPCI combustion. In this study, Dieseline (blending of diesel and gasoline) was looked into as the Partially-Premixed Compression Ignition fuel for its combination of two fuel properties, ignition-delay-increasing characteristics and higher volatility, which make it more suitable for PPCI combustion compared to neat diesel. A series of tests were carried out on a Euro IV light-duty common-rail diesel engine, and different engine modes, from low speed/load to middle speed/load were all tested, under which fuel blend ratios, EGR rates, injection timings and quantities were varied. The emissions, fuel consumption and combustion stability of this dieseline-fuelled PPCI combustion were all investigated. The results showed that dieseline had great advantages as a PPCI combustion fuel in terms of emission reduction. This was particularly significant at high-speed engine mode. It was also found that with a blend of 50% gasoline in diesel, the particle numbers total concentration could be reduced by 90% while low NOx level and high brake fuel conversion efficiency (around 30%) were maintained at all the loads tested.

Some effects of gasoline and diesel mixtures on partially premixed combustion and comparison with the practical fuels gasoline and diesel in a compression ignition engine

If fuels that are more resistant to autoignition are injected near top dead centre in compression ignition engines, they ignite much later than diesel fuel does, and combustion occurs when the fuel and air have had more chance to mix. This helps to reduce nitrogen oxide and smoke emissions. Moreover, this can be achieved at much lower injection pressures than for a diesel fuel. However, it is preferable to have fuels with a lower research octane number than those of commonly available gasolines, because this makes low-load operation easier while retaining the advantages at higher loads. A practical approach to making such fuels is to blend the gasoline and diesel fuel available in the market. Such fuel blends have a wide volatility range since they contain high-boiling-point components from the diesel but have a lower research octane number than that of the gasoline used but have a much longer ignition delay than that of the diesel fuel. This work describes the results of running a single-cylinder diesel engine on three such fuel blends. The engine could be run on such blends with extremely low smoke and low nitrogen oxide emissions at speeds of up to 4000u2009r/min and loads (indicated mean effective pressures) of up to 10u2009bar with an injection pressure of only 400u2009bar. The smoke levels at comparable nitrogen oxide levels were extremely high with diesel fuel in these conditions, even with an injection pressure of 1100u2009bar. The engine could also be run at near-idle conditions on these blends but with higher hydrocarbon and carbon monoxide emissions but much lower nitrogen oxide emissions and maximum pressure rise rate compared with those of the diesel fuel. The wider volatility range might be of benefit in avoiding over-mixing and over-leaning, which could lead to poor combustion stability. The paper also considers the trade-offs between the nitrogen oxide, smoke, hydrocarbon and carbon monoxide emissions and the maximum pressure rise rate and discusses approaches to optimise this type of combustion.

Gasoline compression ignition approach to efficient, clean and affordable future engines

The worldwide demand for transport fuels will increase significantly but will still be met substantially (a share of around 90%) from petroleum-based fuels. This increase in demand will be significantly skewed towards commercial vehicles and hence towards diesel and jet fuels, leading to a probable surplus of lighter low-octane fuels. Current diesel engines are efficient but expensive and complicated because they try to reduce the nitrogen oxide and soot emissions simultaneously while using conventional diesel fuels which ignite very easily. Gasoline compression ignition engines can be run on gasoline-like fuels with a long ignition delay to make low-nitrogen-oxide low-soot combustion very much easier. Moreover, the research octane number of the optimum fuel for gasoline compression ignition engines is likely to be around 70 and hence the surplus low-octane components could be used without much further processing. Also, the final boiling point can be higher than those of current gasolines. The potential advantages of gasoline compression ignition engines are as follows. First, the engine is at least as efficient and clean as current diesel engines but is less complicated and hence could be cheaper (lower injection pressure and after-treatment focus on control of carbon monoxide and hydrocarbon emissions rather than on soot and nitrogen oxide emissions). Second, the optimum fuel requires less processing and hence would be easier to make in comparison with current gasoline or diesel fuel and will have a lower greenhouse-gas footprint. Third, it provides a path to mitigate the global demand imbalance between heavier fuels and lighter fuels that is otherwise projected and improve the sustainability of refineries. The concept has been well demonstrated in research engines but development work is needed to make it feasible on practical vehicles, e.g. on cold start, adequate control of exhaust carbon monoxide and hydrocarbons and control of noise at medium to high loads. Initially, gasoline compression ignition engines technology has to work with current market fuels but, in the longer term, new and simpler fuels need to be supplied to make the transport sector more sustainable.

Knock onset, knock intensity, superknock and preignition in spark ignition engines

Knock is an abnormal and stochastic combustion phenomenon which limits the efficiency of spark ignition engines. It occurs because of autoignition initiated locally in hot spots in the fuel/air mixture ahead of the advancing flame front. The onset of knock is governed by chemical kinetics and is determined by the pressure and temperature history of the hot spot and the anti-knock quality of the fuel. Knock intensity is governed by the evolution of the pressure wave set off by the initial autoignition. As engine designers seek higher efficiency through downsizing and turbocharging, occasional extremely intense knock, ‘superknock’, is found to occur, even though operating conditions are chosen to avoid knock. Superknock is a manifestation of developing detonation which can be triggered by autoignition of the fuel/air mixture at high pressures and temperatures. Another stochastic phenomenon, preignition, is necessary but not sufficient to enable the pressures and temperatures that could cause superknock. These phenomena are discussed in this review.

Fuel requirements of spark ignition engines

This paper reviews the fundamental requirements of liquid hydrocarbon fuels for spark ignition engines, namely that the fuel should vaporise satisfactorily and burn in a controlled manner. The phenomenon of knock and the development of the octane scale are discussed. The variation in the pressure–time histories for different engines is discussed, together with the reason why this leads to different fuel requirements. The difference in the octane rating tests and the way in which engine downsizing exacerbates these differences in the pressure–time histories are discussed. The applicability of the research octane number and the motor octane number to modern engines is reviewed, together with the phenomena of low-speed pre-ignition and superknock. The effects of the hydrocarbon fuel distillation characteristics on the driveability and the emissions are reviewed and discussed with respect to the historical context and the current legislative requirements. Brief mention is made of other fuel requirements such as the density, the gum content and the aromatic content.