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Direct Fuel Injection

What it is, how it works

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Gasoline direct injection system

Drawing of a direct injection system -- note the injector on the far right at the top of the cylinder

Photo © General Motors

Direct fuel injection defined

Direct fuel injection is a fuel-delivery technology that allows gasoline engines to burn fuel more efficiently, resulting in more power, cleaner emissions, and increased fuel economy.

How direct fuel injection works

Gasoline engines work by sucking a mixture of gasoline and air into a cylinder, compressing it with a piston, and igniting it with a spark; the resulting explosion drives the piston downwards, producing power. Traditional (indirect) fuel injection systems pre-mix the gasoline and air in a chamber just outside the cylinder called the intake manifold. In a direct-injection system, the air and gasoline are not pre-mixed; air comes in via the intake manifold, while the gasoline is injected directly into the cylinder.

Advantages of direct fuel injection

Combined with ultra-precise computer management, direct injection allows more accurate control over fuel metering (the amount of fuel injected) and injection timing (exactly when the fuel is introduced into the cylinder). The location of the injector also allows for a more optimal spray pattern that breaks the gasoline up into smaller droplets. The result is more complete combustion -- in other words, more of the gasoline is burned, which translates to more power and less pollution from each drop of gasoline.

Disadvantages of direct fuel injection

The primary disadvantages of direct injection engines are complexity and cost. Direct injection systems are more expensive to build because their components must be more rugged -- they handle fuel at significantly higher pressures than indirect injection systems and the injectors themselves must be able to withstand the heat and pressure of combustion inside the cylinder.

How much more powerful and efficient is direct injection?

Cadillac sells the CTS with both indirect and direct injection versions of its 3.6 liter V6 engine. The indirect engine produces 263 horsepower and 253 lb-ft of torque, while the direct version develops 304 hp and 274 lb-ft. Despite the additional power, EPA fuel economy estimates for the direct injection engine are 1 MPG higher in the city (18 MPG vs 17 MPG) and equal on the highway. Another advantage: Cadillac's direct injection engine runs on regular (87 octane) gasoline. Competing cars from Infiniti and Lexus, which use 300 hp V6 engines with indirect injection, require premium fuel.

Renewed interest in direct fuel injection

Direct injection technology has been around since the mid-20th century; however, few automakers adopted it for mass-market cars. Electronically-controlled indirect fuel injection did the job nearly as well at a significantly lower cost, and offered huge advantages over the mechanical carburetor, which was the dominant fuel delivery system until the 1980s. However, recent developments such as skyrocketing fuel prices and stricter fuel economy and emissions legislation have led many automakers to begin developing direct fuel injection systems. You can expect to see more and more cars make use of direct injection in the near future.

Diesels and direct fuel injection

Virtually all diesel engines use direct fuel injection. However, because diesels use a different process to combust their fuel (gasoline engines compress a mixture of gasoline and air and ignite it with a spark; diesels compress air only, then spray in fuel which is ignited by the heat and pressure), their injection systems differ in design and operation from gasoline direct fuel injection systems.

Reviews of cars with direct fuel injection

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