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Simulating Combustion Improves Diesel Engines

Direct numerical simulations let researchers improve combustion efficiency and lower pollution in diesel engines.

“Cool flames” may sound contradictory, but they could lead to better engine designs with higher efficiency and fewer emissions. Engineers at Sandia National Laboratories recently identified a newly discovered behavior of a temperature-dependent feature of ignition, called a cool flame, in the dimethyl ether (a fuel).

The adjective “cool” is relative: Cool flames burn at less than 1,150° Kelvin (1,610°F), about half the typical temperature of a normal flame, 2,200°K. While cool flames were first observed in the early 1800s, it wasn’t until now that their usefulness for diesel engines had been investigated.

The researchers are trying to quantify the influence of cool flames in stratified turbulent jets during ignition and flame stabilization processes. The insights gleaned will contribute to more efficient, cleaner-burning engines. The goal is to understand the physics of turbulent mixing coupled with high-pressure ignition chemistry, which would help in developing predictive CFD models that can be used to improve engine designs.

Sandia National Laboratories

Direct numerical simulation shows the heat release rate of the first stage of autoignition of dimethyl ether. The first stage is initiated in fuel-lean mixtures (areas of the fuel mixture where there are low concentrations of fuel) and the cool flame that is created moves into richer mixtures as autoignition progresses (t* represents time, colored scale represents the heat release rate in W/m3).

The team has shown that during autoignition (the spontaneous ignition of fuel injected into a combustion engine), cool flames accelerate the formation of ignition kernels, tiny localized sites of high temperature that seed a fully burning flame in fuel-lean regions. The engineers used direct numerical simulations, a powerful numerical experiment that resolves all turbulence scales, during analysis.

They also performed a three-dimensional examination of n-dodecane—a diesel surrogate fuel that has been the recent focus of Sandia’s Engine Combustion Network on spray combustion in diesels—in preparation for a comprehensive study of low-temperature chemistry in autoignitive flames at different stages of ignition.

The details of starting an engine are often taken for granted. Unlike with a gasoline engine, in which the fuel-air mixture is ignited with a spark plug, the fuel in diesel engines must auto-ignite when injected into the hot, compressed air in the piston at the top of the piston stroke. As fuel goes into the engine cylinder, rapid mixing and combustion combine to burn the fuel and drive the engine. This process lasts mere fractions of a second, but the flame that starts this powerful process, and its characteristics, are crucial for improving engine efficiency and minimizing pollution.

The cool flame studies required the use of Titan, a DoE supercomputer boasting 27-petaflop, to produce an accurate and detailed calculation of the autoignition process. That’s because combustion processes are a significant challenge to study, owing to the fuel itself being quite complicated. For example, fuel oxidation chemistry consists of hundreds of species and thousands of chemical reactions. Realistic simulations of diesel combustion need to capture this complex chemistry accurately in a model that includes turbulent mixing and heat transfer.

In the future, the team would like to investigate basic questions about the speed and structure of flames at diesel engine conditions, as well as to study the relationship between spray evaporation, ignition, mixing, and soot processes associated with multicomponent fuels. These basic questions will contribute to studying the cool flame’s crucial role in engine energy production. They will also exercise the valuable capabilities of direct numerical simulations running on exascale supercomputers as a highly precise and detailed numerical simulation method.

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