Low Temperature Combustion in the Homogeneous Charge Compression Ignition Engine

The speed and load ranges of HCCI engines are extended by a temperature control strategy developed by U-M teams members K. Chang, A. Babajimopoulos, G. Lavoie, Z. Filipi and D. Assanis.

For every five gallons of gas that today’s American cars and trucks use, only one gallon becomes energy for propulsion. The rest of it vanishes as waste heat. This inefficient use of fossil fuels has far-reaching effects, including an increase in global warming, the heightening of international tension and skyrocketing fuel costs.

It’s unlikely that there will be a single solution to the transportation-energy dilemma. The answer will probably lie in a combination of solutions, one of which might be an engine that relies on Homogeneous Charge Compression Ignition (HCCI).

The characteristic that differentiates an HCCI engine from standard internal combustion and diesel engines is that HCCI engines don’t use spark plugs and flame to ignite the fuel. Instead, a pre-mixed charge of fuel and air enters the engine’s combustion chamber, and the piston compresses the mixture, creating enough heat for Low Temperature Combustion (LTC) of the fuel. This method of combustion makes it possible to use an extremely low percentage of fuel and a very high percentage of air.

Dennis Assanis, the Jon R. and Beverly S. Holt Professor of Engineering, is the chair of the Mechanical Engineering department, director of the W.E. Lay Automotive Laboratory, co-director of the General Motors Engine Systems Research Collaborative Research Laboratory and director of the Automotive Research Center.

He said that the HCCI engine concept “can improve efficiency 20 to 25 percent and, by combining the best features of gasoline and diesel engines, can reduce pollutants such as particulate matter and nitrogen oxides.”

This is significant because nitrogen oxides can react with the oxygen in the air to produce ozone and, when dissolved in atmospheric moisture, can create acid rain.

“Like gasoline engines, the HCCI engine uses a relatively uniform mixture of fuel and air to reduce soot and particulates,” Assanis said. “And like the diesel engine, it compresses the air and fuel to a high degree to improve thermodynamic efficiency.”

“The key to HCCI operation,” Assanis said, “is the LTC ignition. It’s a game-changing concept that will allow the internal combustion engine to continue dominating the automotive market for years to come. But there are significant challenges to address before the HCCI concept can be implemented successfully in the automotive market. The main challenge is to find a way to extend the useful engine operating range at high and low load.” (Load gives the engine its torque or pulling power.)

Assanis went on to say that the current experimental trials indicate “HCCI combustion can only be maintained in part of the required speed and load range of a typical engine. As a result, a practical engine would have to switch between HCCI mode and a conventional mode – and that would limit the overall potential gains in fuel economy. When engine loads are too high, the combustion is harsh and the engine knocks. When loads are too low, combustion is unstable and the engine misfires. So a key objective of our work is to extend the operating range.”

Because combustion starts by itself in a process of auto-ignition, the temperature of the air-fuel mixture is all important. But Michigan Engineering’s groundbreaking work with a single-cylinder HCCI engine has shown that the temperature of the cylinder’s internal wall also has a strong effect on stable ignition. Furthermore, the wall temperatures vary continuously throughout normal driving patterns, complicating the ignition process even more.

“Our team has modeled this effect,” Assanis said, “and we found that wall temperatures change very slowly and tend to retain memory of previous conditions. This presents a challenge and an opportunity at the same time. With what we know now, we believe we’ll be able to trick the engine into operating instantaneously in regimes where it wouldn’t normally.”

This will require using precision cooling and adjusting the residual gas, to control and compensate for the changing wall temperatures.

“In this way we’ll be able to extend both low- and high-load operation,” Assanis said.. “If you know what you’re doing you can get away with things that aren’t in the normal playbook and, we hope, eventually win the fuel efficiency game.”

Support for this program comes from the Department of Energy. Michigan Engineering leads a consortium that includes M.I.T., Berkeley and Stanford.