In the past, lean-burn technology has achieved lower fuel consumption.

Honda used a so-called lean-burn approach that mixes more than the usual amount of air with gasoline.

The engine that Mitsubishi introduced a year ago is a lean-burn engine.

This system surpasses many others with a wider envelope of lean-burn time, increasing overall efficiency.

Lean-burn engines have put off consumers because they have less power, which the auto companies say is not the case with direct injection.

There is a limitation on the number of qualified hybrid and advanced lean-burn technology vehicles eligible for credit.

Mitsubishi introduced an engine last year that offered a significant improvement in fuel economy by using so-called lean-burn technology.

The phase-out period begins when a manufacturer sells 60,000 qualified hybrid and advanced lean-burn technology vehicles.

Direct-injection engines use a ratio of 40 or 50 to 1, raising fuel efficiency 20 percent over lean-burn engines.

Nevertheless, catalyst-equipped engines produce cleaner exhaust than lean-burn engines.

While lean-burn gas engines offer higher theoretical thermal efficiencies, transient response and performance may be compromised in certain situations.

Near the end of the compression stroke, the layers collapse into uniform minute turbulences, which effectively promote lean-burn characteristics.

Mitsubishi's new engine uses the valve-timing approach for power but instead of lean-burn technology it employs two-cylinder operation to achieve better mileage.

Lean-burn engines, while mixing air and fuel outside the cylinder, could work with 20 to 24 parts of air to 1 part of fuel.

An engine equipped with a three-way catalyst must run at the stoichiometric point, which means more fuel is consumed than in a lean-burn engine.

For example, F Company is a manufacturer of hybrid motor vehicles, but not advanced lean-burn technology motor vehicles.

In the quest for efficiency, an approach pursued by Honda, Mitsubishi and Toyota is the lean-burn engine, which mixes more air with the fuel.

But lean-burn technology tends to increase pollution by nitrogen oxides and also tends to limit engine power, making it suitable only for the smallest cars.

The SoLoNOx system uses lean-burn technologies to reduce NOx emissions.

Last week, Honda announced the debut of the Civic VX, which is equipped with the first of those lean-burn engines.

The remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel ratio falling into the lean-burn category of up to 22:1.

Even this lean-burn engine may be short-lived in the EU as further emissions restrictions are to be introduced in the form of Euro 3.

A G.M. spokesman said the company's engineers had studied and become quite familiar with lean-burn technology, but he said it could only be used on very small engines.

Moreover, lean-burn engines emit abnormally high amounts of pollution from oxides of nitrogen, making the engine too dirty for new, stricter clean-air standards.

Ford has invested £157m to make Dagenham the sole European source for the new lean-burn 2-litre engine for the Sierra and Granada.