Variable-Valve HCCI: A Challenge of Timing

Mechanical Engineering, Jul 2007 by Hutchinson, Harry

The phrase "homogeneous charge compression ignition" doesn't exactly roll off the tongue, but the idea is a hot topic of research. HCCI, as it's usually called, describes an advanced low-temperature combustion process that combines features of the two most common internal combustion strategies on the road-spark-ignition and diesel.

In HCCI, fuel and air are well mixed prior to compression-induced combustion. The process conserves fuel and can burn at lower temperatures. The cooler burn results in less formation of smog-generating nitrogen oxides. The fuel economy means less CO2 is emitted.

A key challenge is control of combustion timing. A research group at Purdue University's Energy Center is modifying a 6.7-liter Cummins B Series diesel engine and will use it to investigate the benefits of computer-controlled variable valve actuation. They will study HCCI combustion timing and work output control strategies under the influence of coupling through re-inducted or trapped combustion gas. They will also study other low-temperature combustion strategies besides HCCI, and will look at diesel combustion as well, with both conventional and alternative fuels, including biodiesel, coal-to-liquid, and ethanol-diesel blends.

The research engine at Purdue, located at the Ray W Herrick Labs, will have no camshaft. Instead it will use hydraulic actuation to open and close valves. According to the lead researcher, Gregory Shaver, an assistant professor of mechanical engineering at Purdue, the research engine should be ready to run as soon as next spring. Shaver, an ASME member, said the design will allow the research team to study the advantages of completely decoupled, fully flexible valve motion. The flexibility of the system will also allow the emulation of various valving approaches currently in development by industry.

Shaver's team has developed a mathematical model that will allow the team to predict the dynamics of the engine's combustion. The model and the precision control of the hydraulic actuators will let the team study various combinations that will enable engine optimization.

In the introduction to a paper published in ASME's Journal of Dynamic Systems, Measurement, and Control in 2005 (receiving the award for best paper published that year), Shaver and his research associates described their general approach this way: "Various detailed models of HCCI combustion have been developed to capture the combustion process and kinetics. These include multizone models and multidimensional CFD models using detailed chemistry. While this level of detail is necessary for accurately predicting the overall process of HCCI combustion, in particular the emissions, these models are far too detailed for controller design or validation. However, as this paper illustrates, simple models can accurately capture the properties most relevant to control-combustion phasing, in-cylinder pressure, cycle-to-cycle dynamic coupling, and work output-with comparable levels of fidelity."

According to Shaver, the research engine at Purdue will be stationary. He predicts that we may see an HCCI engine on the road as soon as five years from now. Engines with completely decoupled valve and piston motions will likely take a while longer.