Influence of autoignition delay time characteristics of different fuels on pressure waves and knock in reciprocating engines

The functional relationship of autoignition delay time with temperature and pressure is employed to derive the propagation velocities of autoignitive reaction fronts for particular reactivity gradients, once autoignition has been initiated. In the present study of a variety of premixtures, with different functional relationships, such gradients comprise fixed initial temperature gradients. The smaller is the ratio of the acoustic speed through the mixture to the localised velocity of the autoignitive front, the greater are the amplitude and frequency of the induced pressure wave. This might lead to damaging engine knock. At higher values of the ratio, the autoignition can be benign with only small over-pressures.This approach to the effects of autoignition is confirmed by its application to a variety of experimental studies involving:(i) Imposed temperature gradients in a rapid compression and expansion machine.(ii) Onset of knock in an engine with advancing spark timing.(iii) Development of autoignition at a single hot spot in an engine.(iv) Autoignition fronts initiated by several hot spots.There is much diversity in the effects that can be produced by different fuels in different ranges of temperature and pressure. Higher values of autoignitive propagation speeds lead to increasingly severe engine knock. Such effects cannot always be predicted from the Research and Motor octane numbers

Is gasoline the best fuel for advanced diesel engines? - fuel effects in "premixed-enough" compression ignition (CI) engines

Powerpoint presentation

Is gasoline the best fuel for advanced diesel engines? - fuel effects in "premixed-enough" compression ignition (CI) engines

Powerpoint presentation

Fuel effects on knock, heat releases and CARS temperatures in a spark ignition engine

Net heat release, knock characteristics and temperature were derived from in-cylinder pressure and end-gas CARS measurements for different fuels in a single-cylinder engine. The maximum net heat release rate resulting from the final phase of autoignition is closely associated with knock intensity. Aromatic fuels have lower maximum heat release rates and lower knock intensities than expected from their octane number when compared to paraffinic fuels ; this is observed even when there is significant heating of the end-gas from pre-flame reactions. Leaner mixtures have lower combustion rates so that pressure development is slowed and hence ignition needs to be more advanced to get knock to occur as frequently as in a richer mixture. However, for a given frequency of knock occurrence, there is no significant difference in peak net heat release rates and hence in knock intensities for different mixture strengths

Prediction of the liftoff, blowout and blowoff stability limits of pure hydrogen and hydrogen/hydrocarbon mixture jet flames

The paper presents experimental studies of the liftoff and blowout stability parameters of pure hydrogen, hydrogen/propane and hydrogen/methane jet flames using a 2 mm burner. Carbon dioxide and Argon gas were also used in the study for the comparison with hydrocarbon fuel. Comparisons of the stability of H2/C3H8, H2/CH4 and H2/CO2 flames showed that H2/C3H8 produced the highest liftoff height and H2/CH4 required highest liftoff, blowoff and blowout velocities. The non-dimensional analysis of liftoff height was used to correlate liftoff data of H2, H2/C3H8, H2/CO2, C3H8 and H2/Ar jet flames tested in the 2 mm burner. The suitability of extending the empirical correlations based on hydrocarbon flames to both hydrogen and hydrogen/hydrocarbon flames was examined