Super-efficient engines ditch the spark

The next big step in gasoline engines burns fuel like diesel engines burn diesel. But hurdles remain.

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By Peter Valdes-Dapena, staff writer

GM's Saturn Aura HCCI test vehicle
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NEW YORK ( -- Car companies are working to perfect a new generation of gasoline engines that could get up to 20% better fuel efficiency. The system, which burns gasoline without spark plugs, relies in massive amounts of speed and power - the computer kind - to work.

It's called Homogenous Charge Compression Ignition, or HCCI. General Motors (GM, Fortune 500) has been allowing journalists to drive a test vehicle with this technology. It clearly isn't quite ready for the mass market, but GM insists that it's finally in the home stretch after years of trying.

"You're basically getting the best of gas and diesel all together," said Kelly Mundt, director of gasoline system marketing for auto parts supplier Bosch, which is working on some components needed for this new engine system.

That means more fuel efficiency, like a diesel, but with cleaner exhaust. But it's not easy to get a gasoline engine to behave this way.

This technique for burning gasoline requires loads of on-board computing power and the ability to control to the tiniest degree everything that goes on inside the engine in real-time. The ideas behind HCCI have been around for decades, but the parts have only come together in the last few years.

Big differences

In normal gasoline engines, the piston draws fuel and air into the cylinder as it comes down after all the exhast fumes have been pushed out. The fuel and air are compressed as the piston rises and, at just the right moment, a spark ignites the fuel.

Diesel engines compress air inside a cylinder until it's hot enough to ignite fuel. At just the right time, a fuel injector squirts in a spray of diesel fuel. It burns immediately in the hot air, pushing the piston down.

In an HCCI engine, the injector squirts fuel while the piston is on the way up, like a regular gasoline engine. But the fuel doesn't need a spark. It combusts from pressure and heat alone, like a diesel engine.

Now here's the tricky part: getting the fuel to burn at just the right time without a spark or fuel injection to trigger it.

To set the stage, the engine's exhaust valve closes early, trapping some hot exhaust gas in the cylinder. A series of small jets of fuel squirt into the cylinder, raising the temperature and pressure to a critical point. When the time is right, the fuel ignites. The combustion doesn't start at one point in the cylinder and move outward, like it does in regular gasoline or diesel engines. It's a key reason HCCI is so efficient.

All this is tricky enough in a stationary engine humming along at a constant rate inside. But keeping it all going under the constantly changing demands of real driving is especially hard.

HCCI engines still have spark plugs to use in situations where HCCI just isn't very efficient. At high speeds, HCCI doesn't work well, so when a car is accelerating hard to merge onto a highway, the sparks plugs take over.

Where HCCI fits

During a recent test drive in a specially prepared Saturn Aura sedan with an HCCI engine, it was clear that the technology wasn't quite ready for the consumer market.

First of all, it's noisy. For those used to a quiet gasoline engine or an equally noiseless modern diesel engine, the popping and cracking noise from the Aura's HCCI engine would be simply unacceptable.

That noise doesn't mean the system isn't working well, GM engineers insist. It's just an unpleasant byproduct that will diminish with further tweaking and more sound insulation, said GM engineer Paul Najt.

More work is also needed on "transitions." During normal driving, the engine has to switch over to regular spark-ignition then back to HCCI. In the test car, the change was noticeable.

For it to become economical, engineers still need to push HCCI's performance range farther while making engines behave and sound more like regular gasoline engines. For now, HCCI won't work at highway speeds.

But it could make a lot of sense in a gas-electric hybrid car. With the help of an electric motor, an HCCI engine could spend more time in its optimal operating range, said GM engineer Vijay Ramappan.

"It's not something you'll see next year," said Hakan Yilmaz, manager of advanced systems engineering for Bosch.

With gas prices topping $4 a gallon, though, the incentive to bring it all together has never been greater. To top of page

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