Or: How to start a fire

Fuel inside an engine is subjected to high temperatures and pressures causing it to ignite and burn, but what exactly is the process by which this occurs? A new study published in JFM combines the thermochemical conditions of a diesel engine with realistic 3D turbulence models to provide an insight into the inner workings of a combustion engine.

Diesel engines are incredibly common worldwide, from their use in personal vehicles to heavy machinery on an industrial scale. With steps being taken to help to reduce our reliance on fossil fuels they are becoming less common. However, current electric motors are not yet able to provide the high power and energy density required by heavy goods vehicles. This means that while they are still in use, understanding more about the combustion process within a diesel engine is essential if we are to improve the efficiency and reduce the pollution generated by these engines.

With this goal in mind, lead author Alex Krisman and his team wanted to simulate the engine environment at a pressure of 40 atm and a temperature of 1100 Kelvin with realistic models for both the chemistry and the turbulence generated by the ignition process. They concentrated on very small-scale structures at the sub-micron scale which exist for fractions of a millisecond as this is where they believe the key physics and chemistry occurs.

Their simulations showed that ignition begins with low temperature chemistry (LTC) which precedes the formation of spatially localised high temperature chemistry (HTC) events known as ‘kernels’. The kernels form in what Krisman described as “areas protected from intense turbulence” with the idea being that in these areas the eddies are shielded from heat and energy dissipation, thus enabling ignition to occur.

Once ignited, the initial ‘edge flames’ grow and self-propagate throughout the fuel, spreading out from their initial starting point. Another mechanism called ‘auto-ignition’ is also present, whereby the chemical substance spontaneously ignites without an external source. An example image of the instantaneous temperature field is displayed below with the areas of low temperature chemistry (LTC) and edge flames labelled.

Experimentally the ignition process is very difficult to measure accurately due to the extremely small length and time scales present. However, recent progress has been made using laser techniques, and the data available show reasonable agreement with the simulations. In particular, regions of both low and high temperature chemistry can be identified and the propagation of edge flames can also be inferred as predicted by the numerical model.

With continued improvement in both experimental measurement techniques and numerical simulations of the ignition process, Krisman hopes that such models will allow for better design of diesel engines to help reduce the amount of toxic gases and soot produced and thus comply with ever more stringent regulations. “The advantage of using DNS is that it tells us which kinds of physics and combustion are present and therefore industry-scale simulations which need to be run on a much shorter timescale can concentrate only on the most important physics relevant to the problem.”

Ultimately, Krisman believes that combustion is highly complex and is, therefore, very challenging to model. “Some parts of the process are driven by chemical effects such as auto-ignition, whereas others are driven by turbulence and flame propagation which all have different physics.” In other words, careful thought is needed going forward.

This paper is freely available through 6th August.

References:
Krisman, A., Hawkes, E., & Chen, J. (2017). Two-stage auto-ignition and edge flames in a high pressure turbulent jet. Journal of Fluid Mechanics, 824, 5-41.