Why some engines use direct and port injection

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Every new car sold in the United States today uses fuel injection, but not all fuel injection systems are the same. Some cars use port injection, while others use direct injection. Some even use both. What’s the advantage of that? Jason Fenske of Engineering Explained breaks it down in the accompanying video.

Fuel injection is a more precise way to get fuel into cylinders than its predecessor, the carburetor. It entered widespread use in the 1980s thanks to the development of electronic controls. Port injection—injecting the fuel into the intake runner—was the default from that time until after the turn of the century.

Direct injection was first used in aircraft, and a mechanical version was used in the 1950s Mercedes-Benz 300SL. But the technology did not enter mainstream use until the 2000s, when stricter fuel-economy standards forced automakers to look for new ways to boost efficiency. Ford’s EcoBoost and Mazda’s SkyActiv are just a couple of examples of engine families that use direct injection. As the name suggests, direct injection involves shooting fuel directly into a cylinder’s combustion chamber, and its done at a much higher pressure than port injection.

More recently, automakers have begun combining the two fuel-injection setups. Toyota, for example, uses its D-4S system on the Tacoma pickup truck and 86 sports car.

These systems tend to use port injection at lower engine loads and rpm, and direct injection at higher rpm, Fenske says. But it varies by automaker, he notes.

Port injection provides a better, more stable air/fuel mixture at lower engine speeds, which results in smoother operation upon startup. At higher rpm, direct injection provides a greater cooling effect, allowing for more power and less chance of knock.

Toyota’s D-4S runs in an efficiency-focused “stratified” mode and a “homogeneous” mode for greater power. Stratified mode primarily uses port injection to create different air/fuel mixtures, including leaner mixtures to quickly bring the engine and catalytic converters up to operating temperature.

Homogeneous mode uses the same, richer, air/fuel mixture at all times, using both the direct and port injectors.

Reducing carbon deposits is another reason to use both types of injection. Studies have shown that direct-injected engines tend to be more prone to carbon deposit engines than port-injected engines, especially on intake valves. By adding port injection, the fuel can wash off those valves to reduce carbon deposits.

To learn more, click on the video above.

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