Search Results

The auto-ignition quality of a fuel of any chemistry at a given engine condition is described by an octane index defined as, OI = (1-K) RON + K MON, where RON and MON are the Research and Motor Octane numbers respectively and K depends only on the engine design and operating conditions. The higher the OI value, the greater is the resistance to auto-ignition. A single cylinder homogeneous charge compression ignition (HCCI) engine has been run at thirty seven different operating conditions using fuels of different chemistries and different known RON and MON values. At each operating condition CA50, the crank angle for 50% of the total heat release, is established for different fuels and from this the value of K is determined. We take Tcomp15, the temperature when the pressure reaches 15 bar during the compression stroke, as a generic engine parameter. K is strongly dependent on and increases with Tcomp15 and is less strongly dependent on the mixture strength.

In this paper blends of diesel and gasoline (dieseline) fuelled Partially Premixed Compression Ignition (PPCI) combustion and the comparison to conventional diesel combustion is investigated. The tests are carried out using a light duty four cylinder Euro IV diesel engine. The engine condition is maintained at 1800 rpm, 52 Nm (equivalent IMEP around 4.3 bar). Different injection timings and different amounts of EGR are used to achieve the PPCI combustion. The results show that compared to the conventional diesel combustion, the smoke and NOx emissions can be reduced by more than 95% simultaneously with dieseline fuelled PPCI combustion. The particle number total concentration can be reduced by 90% as well as the mean diameter (from 54 nm for conventional diesel to 16 nm for G50 fuelled PPCI). The penalty is a slightly increased noise level and lower indicated efficiency, which is decreased from 40% to 38.5%.

Recent research [21] has shown that the compression ignition concept where very low cetane fuels (RON between 70 and 85) are run in compression ignition (CI) mode has several advantages. The engine will be at least as efficient and clean as the current diesel engines but will have a less complicated after-treatment system. The optimum fuel will be less processed and therefore simpler to make compared to current gasoline or diesel fuels. Naphtha, which is a product of the initial distillation of petroleum, is one such fuel. It provides a path to mitigate the global demand imbalance between heavier and lighter fuels that is otherwise projected. Since naphtha requires much less processing in the refinery than either gasoline or diesel [23], there is an additional benefit in terms of well-to-wheel CO2 emissions and overall energy consumed. Partially premixed charge compression ignition combustion with such a low cetane fuel has usually been investigated with a diesel engine base.

Predictive simulation models are needed in order to exploit the full benefits of 1-D engine simulation. Simulation model alterations such as cam phasing affect the gas composition and gas state in the cylinders and have an effect on the combustion. Modelling of these effects is particularly important when the engine is knock limited. A knock model, able to phase the combustion towards the knock limit, was previously developed by the authors. A major challenge in such knock models is to predict the pressure and temperature evolution in the end-gas accurately through an adequate combustion model. The Wiebe function is often used to model the combustion in SI engine simulations, owing to its ease of use and computational efficiency. The Wiebe function simply imposes a curve shape for the fuel burn rate and the parameters are easily determined from calculated heat release.

A single cylinder Direct Injection Spark Ignition (DISI) engine with optical access has been used for combustion studies with both early injection and late injection for stratified charge operation. Cylinder pressure records have been used for combustion analysis that has been synchronised with the imaging. A high speed cine camera has been used for imaging combustion within a cycle, while a CCD camera has been used for imaging at fixed crank angles, so as to obtain information on cycle-by-cycle variations. The CCD images have also been analysed to give information on the quantity of soot present during combustion. Tests have been conducted with a reference unleaded gasoline (ULG), and pure fuel components: iso-octane (a representative alkane), and toluene (a representative aromatic). The results show diffusion-controlled combustion occurring in so-called homogeneous combustion with early injection.