Experimental Investigation of Low Temperature Diesel Combustion Processes

Abstract

The work presented in this article investigates the three distinct phases of low temperature diesel combustion (LTC). Diesel LTC followed a cool flame–negative temperature coefficient (NTC)–high temperature thermal reaction (main combustion) process. The in-cylinder parameters, such as the charge temperature, pressure, and composition, had noticeable influences on these combustion stages. The NTC was strongly temperature-dependent, with higher temperatures inducing both an earlier onset of NTC and a more rapid transition from NTC to the main combustion process. An increase in the intake charge temperature led to an earlier occurrence of NTC and a reduction in the heat released during the cool flame regime. A higher fuel injection pressure improved fuel mixing and enhanced the low temperature (pre-combustion) reactions, which in turn led to an earlier appearance of the cool flame regime and more heat release during this phase. This increased the charge temperature and led to earlier onset of the NTC regime. A higher exhaust gas recirculation (EGR) rate reduced the intake charge oxygen concentration and limited the low temperature reaction rates. This reduced the heat release rate during cool flame reaction phase, leading to a slower increase in charge temperature and a longer duration of the NTC regime. This increased the ignition delay for the main combustion event. The injection timing showed a less significant influence on the cool flame reaction rates and NTC phase compared to the other parameters. However, it had a significant influence on the main combustion heat release process in terms of phasing and peak heat release rate.

Keywords:

• Cool flame
• Exhaust gas recirculation (EGR)
• Low temperature diesel combustion (LTC)
• Negative temperature coefficient (NTC)

ACKNOWLEDGMENTS

The authors acknowledge the financial support for this work provided by the UK Engineering and Physical Sciences Research Council (EPSRC Grant EP/F031351/01), the Royal Academy of Engineering, and Loughborough University. The authors thank Asish Sarangi, Adrian Broster, Steve Horner, Graham Smith, and Steve Taylor for their assistance in conducting the research.

Notes

Δ Engine speed, fuelling per cycle, control parameter (EGR, Tint, Pinj, and SoI).

*Oxygen-based equivalence ratio derived from 1D engine model results.

*The calculated end of compression temperature assumed adiabatic, polytropic compression, with a polytropic exponent of 1.35, from IVC (−134°CA ATDC) to SoI (−21°CA ATDC).