MIT researchers are figuring out how to allow conventional 4 stroke gasoline engines to shift into a sparkless mode similar to diesel engine operation.

Many researchers are studying a new way of operating an internal combustion engine known as "homogeneous charge compression ignition" (HCCI). Switching a spark-ignition (SI) engine to HCCI mode pushes up its fuel efficiency.

In an HCCI engine, fuel and air are mixed together and injected into the cylinder. The piston compresses the mixture until spontaneous combustion occurs. The engine thus combines fuel-and-air premixing (as in an SI engine) with spontaneous ignition (as in a diesel engine). The result is the HCCI's distinctive feature: combustion occurs simultaneously at many locations throughout the combustion chamber.

One of HCCI's big advantages over diesel is the ability to avoid oxidation of nitrogen.

That behavior has advantages. In both SI and diesel engines, the fuel must burn hot to ensure that the flame spreads rapidly through the combustion chamber before a new "charge" enters. In an HCCI engine, there is no need for a quickly spreading flame because combustion occurs throughout the combustion chamber. As a result, combustion temperatures can be lower, so emissions of nitrogen pollutants are negligible. The fuel is spread in low concentrations throughout the cylinder, so the soot emissions from fuel-rich regions in diesels are not present.

Perhaps most important, the HCCI engine is not locked into having just enough air to burn the available fuel, as is the SI engine. When the fuel coming into an SI engine is reduced to cut power, the incoming air must also be constrained--a major source of wasted energy.

This design would boost fuel efficiency by a few miles per gallon.

The researchers estimate that the increase in fuel efficiency would be a few miles per gallon. "That may not seem like an impressive improvement," said Green. "But if all the cars in the US today improved that much, it might be worth a million barrels of oil per day--and that's a lot."

The HCCI mode might eventually provide a bigger fuel efficiency boost. Engines can only run in HCCI mode in a midrange of RPMs. At first glance that limits the efficiency gain possible from HCCI. However, marrying an HCCI-capable engine to hybrid electric components might provide a way to allow an engine to work in HCCI mode more of the time that it is running. At what would otherwise be lower RPM operation periods a car could get propelled by electric motors powered by the battery. Then the engine could get started up and put directly into HCCI mode to provide power to the car at higher speed and to charge the batteries. A car with sufficiently powerful batteries and electric motors could use the gasoline engine only to charge the batteries and to supply power to the electric motors. That could allow the engine to run at a constant and HCCI-capable RPM.

Ford is funding this research just as it is funding other MIT research on ethanol turbo injection to boost gasoline engine efficiency.

This research was supported by Ford Motor Company and the Ford-MIT Alliance, with additional support from BP.

Ford's executives think they can achieve near-diesel efficiency with gasoline engines and do so more cheaply. So Ford is not following the German makers in embracing diesel for passenger cars. My guess is that the Ford folks are correct.

We need all the efficiency improvements that can get squeezed out of internal combustion engines to complement advances made in battery technology. Out of the two I'm more worried about the speed of advance of the battery technology.

Update: A couple of months ago AutoblogGreen interviewed Dr. Gary Smyth, director of powertrain research at GM, about GM's research on engines and Smyth provided a look at why HCCI is better and what problems must be solved to make HCCI workable.

GS: Well, first of all it's a homogeneous charge compression ignition. So what it really is what I would call the next generation combustion process. So it's not a technology, it's really the process, it's the combustion process that we're developing and it really is, think of it as clean, efficient combustion. Like the two-stroke, the two-stroke by the way ran very lean and by the way we could run HCCI on the old two-strokes, we're not doing the same with the four-stroke where we are running the engine extremely lean and we're not using a spark plug, it's the whole combustion process is what we call kinetically controlled. It depends on the air fuel mixture in the cylinder. So, we're now controlling the combustion without a spark plug. We're running extremely lean and we need a number of enabling technologies to help us control the combustion. One is direct injection. The other is very wide-authority cam phasing. The other is very precise control. Another one is significant residuals or exhaust recirculation gases that we would take into the cylinder. So, much more complex from an engine perspective but allows us to really get the upper bound fuel economy potential of the four-stroke engine and do that with very low emissions.

The interviewer and Smyth both know engine technologies. The whole interview is very interested and delves into other approaches for boosting internal combustion engine efficiency.