FROM the outside, the dark blue Saturn Aura accelerating to a steady 50 miles an hour on the high-bank oval here at General Motors’ proving grounds looked altogether unremarkable.

In fact, it was not much to look at under the hood either, despite an experimental engine using a method of burning gasoline that may prove to be the next major advance in fuel economy and emissions control. Only a couple of stray electrical connectors hinted at the differences distinguishing this engineering prototype from thousands of other Auras on road.

From the driving position it’s another story. A laptop computer placed between me and a G.M. engineer, Jun Mo Kang, displays a graph that plots the car’s changing engine speed against the load on the engine, just colorful enough to draw my attention away from future cars and trucks in full disguises zipping by in the faster lanes of the track.

My time behind the wheel last month was the first test drive G.M. has given to a journalist of its prototype homogeneous-charge compression-ignition engine. An H.C.C.I. engine runs on a combustion process that researchers say holds the potential for significant gains in overall engine efficiency. G.M is one of several automakers developing H.C.C.I. technology.

In principle, the H.C.C.I. engine takes advantage of the best characteristics of gasoline engines — low emissions that can be controlled effectively with available technology — and diesel engines, which offer low fuel consumption.

As I press and release the Aura’s gas pedal, a small orange dot on the laptop screen darts in and out of a wedge-shaped area on the graph. The point of the wedge begins slightly above idle speed, at about 1,000 r.p.m., and ramps up to its widest point at 3,000 r.p.m.

“Don’t push the pedal too hard or fast,” counsels Mr. Kang. “Try to keep the dot within the zone. That’s where the engine is running in H.C.C.I. combustion.”

After less than a lap of practice on the immense track, I find it easy to feather the pedal so that the orange dot constantly floats in the sweet spot on the graph, earning a smile from Mr. Kang.

When H.C.C.I. is finally ready for the road — G.M. won’t say exactly when that might be — computers will take over the task of keeping the engine in its ideal operating range. And though much work to refine the engine’s operation lies ahead, experts say the technology is worth the investment.

“I believe H.C.C.I. represents the next great advance of the internal combustion engine,” said Chris Gerdes, associate professor of mechanical engineering at Stanford University, which is one of G.M.’s partners in H.C.C.I. research.

“With minimal changes to the engine hardware, H.C.C.I. gasoline engines should be able to produce diesel-like efficiencies while simultaneously lowering emissions,” he said.

Dennis Assanis, director of the Walter E. Lay Automotive Laboratory at the University of Michigan in Ann Arbor, says the H.C.C.I. combustion process offers potential for raising gasoline engine fuel efficiency by 15 percent to 20 percent while offering reductions in oxides of nitrogen, an important contributor to smog that is difficult to control.

The 2.2-liter engine in the test Aura is based on the G.M. Ecotec four-cylinder; a 2.4-liter version of this engine is available in the Aura for 2008. G.M. rates the H.C.C.I. engine at 180 horsepower and 170 pound-feet of torque of torque; by comparison, the ’08 Aura engine makes 164 horsepower and 159 pound-feet, and is rated at 22 miles a gallon in town and 30 on the highway.

G.M. would not give fuel economy figures for the H.C.C.I engine beyond the expectation of a 15 percent improvement in mileage.

The H.C.C.I. process is something of an answer to the long quest for so-called lean-burn engines, which use less fuel in relation to the volume of air ingested. Such engines present other problems, though, especially in terms of emissions control, because it is hard to ignite the air-fuel mixture evenly.

The H.C.C.I. gasoline engine aims to solve this by igniting an almost evenly distributed (hence the term homogeneous in its name) mixture of fuel, air and captured exhaust gas in the cylinders. Combustion is spontaneous, a result of heat in the cylinder rather than a spark plug, similar to the way a diesel engine operates.

Until recently, H.C.C.I. combustion was little more than a laboratory experiment. Engineers could coax the process to work successfully on special single-cylinder test engines running at a constant speed. But when applied to multicylinder engines operating under various loads, speeds and atmospheric conditions, H.C.C.I. refused to cooperate.


To get around the difficulty of making it idle smoothly and quietly under high loads, G.M. has, for now at least, limited its H.C.C.I. mode to the 1,000- to 3,000-r.p.m. zone used in the Aura test car, a range that covers most typical driving. Below and above that range the engine uses conventional spark ignition.

Making H.C.C.I. compatible with a broad range of fuels; reducing the faint rattling noise during the transition from compression ignition to spark ignition; and reducing emissions at low loads are other issues being addressed.

“H.C.C.I. relies on a very delicate balance of chemistry happening thousands of times per minute in the cylinder,” said Paul Najt, a manager in G.M.’s powertrain research laboratory who has been working on H.C.C.I. for more than 30 years.

“Controlling the chemistry is the difficult part to implement,” he said. “If the temperature and gas composition aren’t precisely correct, either nothing happens or something very bad happens.”

Bringing H.C.C.I. to production has become a top priority among the world’s major automakers. One sign that the technology is inching closer to production is that automakers are attaching brand names to their various H.C.C.I. programs.

At next month’s Frankfurt auto show, Mercedes-Benz will announce details of its Controlled Auto Ignition system, which it has named DiesOtto in homage to diesel and gasoline engine pioneers. Volkswagen’s Combined Combustion System is under development, and Honda’s H.C.C.I. system, originally conceived for racing motorcycles, is being tested in a four-cylinder auto engine.

China’s automakers and government-backed research institutes are also intensely interested in H.C.C.I. technology, judging by a survey of technical papers presented at last year’s congress of the Society of Automotive Engineers.

G.M.’s decision to move its H.C.C.I. program out of the laboratory and into what Mr. Najt calls the advanced engineering phase, as well as beginning road tests, signals the automaker’s steady progress in critical technology areas, said Matthias Alt, manager of the company’s global H.C.C.I. program.

“In the first half of 2007 we achieved significant gains in the system’s computer controls,” he said. “This moved the program ahead faster than even we ourselves expected.”

But while Mr. Alt and his team of engineers and scientists have made great progress, they acknowledge that many challenges remain before H.C.C.I. combustion is happily percolating inside new G.M. engines.

“As our development of the enablers and computer controls continues, we’ll extend the H.C.C.I. operating range up and down the load range,” Mr. Alt said. The optimism of Mr. Alt and his engineers suggests that the H.C.C.I. Aura won’t be stuck in the slow lanes of the test track much longer.