Auto-ignition in Petrol Engines
Full load performance in petrol engines is limited by auto-ignition (commonly known as knock or pinking). Under normal operating conditions, the air/fuel mixture burns rapidly but regularly as the flame-front propagates from the spark-plug. At full load however, the last peak of the unburnt charge - the end-gas- can auto-ignite spontaneously and almost instantaneously. The shock waves resulting disturb the in-cylinder heat transfer and can cause structural damage, and in very extreme cases,, catastrophic engine failure and seizure. To avoid the problem, the design engineer must limit the compression ratio and retard the timing of the ignition, both of which degrade the engine performance in terms of power and economy.
In a major programme supported by engine designers and fuel manufacturers, with matching funding from the UK DTI, AEAs Combustion Centre has recently used the FACSIMILE code to model auto-ignition to help optimise future designs. Parameters such as bore, stroke, spark-plug position. and geometry of the cylinder head are included. Numerically, the model comprises a set of differential equations which describes the time-dependence of engine variables such as cylinder pressure, temperatures of the burnt and unburnt gas zones and burnt gas mass fraction. Newtons law of cooling is used to describe the heat transfer at the cylinder wall, with coefficients obtained from a standard correlation. In addition to the thermodynamics, rate equations describe the auto-ignition chemistry. FACSIMILE solves the ensemble and hence predicts how the onset of auto-ignition depends on changes in design or fuel quality.
The auto-ignition chemistry depends on a general treatment of chemical heat release and evolution of species concentrations first developed by researchers at Shel12 and refined by AEAI. A reduced mechanism has been developed for low-temperature auto-ignition, using identified classes of chemical species and reactions. The chemistry was encoded in FACSIMI LE and validated against laboratory experimental data. The model predicted clearly the observed two-stage ignition characteristics and reproduced well the observed dependence of the ignition delay period on the end-of- compression temperature and the fuel and oxygen concentrations. This modelling approach is particularly valuable because it provides an explanation for the different reactivity of fuels of different chemical structure (eg iso-octane and n-heptane) with respect to auto-ignition. With this understanding, adjustment of a small number of fuel structure-dependent rate parameters allows the same basic mechanism to be used for any type of alkane fuel.
In the engine research consortium, the theoretical predictions were further validated using a single-cylinder research engine fuelled on a range of Reference Fuels. In-cylinder pressure measurements were taken using a proprietary piezoelectric transducer. Temperature was measured using a laser-spectroscopic technique (Coherent Anti-Stokes Raman Spectroscopy CARS). The FACSIMILE model and supporting experimental data were distributed to the consortium members at the end of the project, giving both engine designers and fuel developers a valuable tool in continuing efforts to improve performance and economy.
- The Auto-ignition Modelling Study, supported by BP, Elf (UK), Ford, Jaguar, Lucas, Toyota (UK) and the UK Department of Trade and Industry
- M.P. Halstead et al, Proc Roy Soc A,.346, 515 (1975).
- R.A. Cox and J.A. Cole, Combust. Flame, 60, 109, (1985)