Autoignition models applied to the end gas

A two-zone engine simulation, which divides the cylinder volume into burned and unburned regions, each spatially uniform, provides the simplest predictive description. In early work Hirst and Kirsch [155] applied the Shell model to the unburned region and showed that autoignition in a CFR engine could be predicted, and the model used to investigate the factors that influence fuel sensitivity (Section 7.2.3). The burning rate, a 

In 1985 Leppard [156] reported engine measurements, for stoichiometric ethane-air, of pressure and end gas temperature, the latter derived from the energy equation. The occurrence of autoignition agreed closely with prediction based on an earlier chemical model of Westbrook and Dryer [52]. From their engine experiments, Cowart et al. [59] also compared, for iso-octane and -pentane, the predictions of the simplified models of Hu and Keck [75] and Chun et al. [157], and the more detailed kinetic predictions of Westbrook et al. [158]. These were found to simulate the time of knock occurrence if the kinetic data were re-calibrated. This, and the subsequent work of Brussovansky et al. [76], showed the need for accurate allowances for heat transfer and piston blow-by, because of their important effect on the derived end gas temperature. Where end gas temperature can be measured directly this problem is circumvented. 

Bradley et al. [80] report elevations of CARS temperatures, with a 90% iso-octane/10% heptane fuel prior to autoignition, of about 100 K above such values, with mean values of about 900 K in non-knocking cycles. The greater the end gas pressure, the greater was the CARS temperature and the knock intensity. The temperature elevations were in line with the computed predictions of the simplified five-reaction parrot scheme, de- 

Attempts have been made to estimate the volumetric heat release rate during autoignition, from pressure records and CARS temperature measurements [106]. It was estimated that the activation temperature for this was in the region of 20,000 K under knocking conditions. Paraffinic fuels gave a higher volumetric heat release rate than aromatic fuels and were more prone to knock severely. 

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