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Saving more fuel with direct injection

With fuel efficiency a major issue for the auto industry, a first stepis to improve the way the engine works. One solution is the increaseduse of gasoline direct injection, or GDI.

With fuel efficiency a major issue for the auto industry, a first step is to improve the way the engine works. One solution is the increased use of gasoline direct injection, or GDI.

“GDI is an enabling technology,” says Mark Shirley, senior engineer of engine design at Hyundai American Technical Center.

“The direct injection itself isn’t what makes it efficient; it only allows other changes to the engine that improve economy.”

Hyundai uses the system in its all-new Sonata and Accent.

In simplest terms, an engine works similarly to a bicycle. Just as your legs go up and down, pistons inside the engine do the same, turning a crankshaft whose spinning energy eventually turns the wheels. The pistons are powered when gasoline is injected into the engine and it explodes inside the combustion chambers.

Most modern cars use multi-port injection, in which the gasoline is injected outside the combustion chamber, mixed with air and drawn in when an intake valve opens. Once inside, it’s ignited by the spark plug to power the pistons. With GDI, the gasoline is injected directly into the combustion chamber instead.

As it moves up and down, the piston compresses the air-fuel mixture before it’s burned. The more it’s compressed, the more power the mixture can provide.

However, compressing the mixture creates heat, which comes with new problems such as pre-ignition, when the fuel spontaneously ignites before the spark plug fires. This is not only inefficient but, in extreme cases, can cause engine damage.

Injecting gasoline as a high-pressure liquid directly into the combustion chamber cools the chamber and means the air mixed with it doesn’t get as hot, Shirley says.

“Lower temperatures in the chamber allow higher-compression ratios and more spark advance, which allows the engine to be more fuel-efficient.”

Gasoline must be mixed with air before it can be burned in the combustion chamber. GDI engines run “leaner,” meaning they use less gasoline and more air.

They also burn the gas more completely and emit fewer pollutants overall compared with conventional engines.

Their main disadvantage is cost.

“Fuel pumps, fuel lines, injectors, etc. are all more expensive,” Shirley says.

“The higher pressure requires a much more robust set of materials.”

 
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