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Direct Injection Gives Your Next Car More Power and Better Economy

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In general, car engine design involves a trade off between performance and efficiency. Want more performance? Then you’re going to be spending more on gasoline and putting more carbon dioxide out the tailpipe. But what if you could have both more horsepower and go farther on a gallon of gas? One new technology, direct injection, makes this possible.

What is direct injection? First, some background. In car engines, the piston travels up twice and down twice for each power stroke. During the intake stroke, the piston moves down, drawing air into the cylinder through the intake valves. During the compression stroke, the intake valves close and the piston moves upward in the cylinder, compressing the air-fuel mixture inside. During the power stroke, the spark plug ignites the mixture, and the resulting explosion forces the piston down. During the exhaust stroke, a rising piston forces the spent mixture out through the open exhaust valves.

BMW 3.0-liter DI twin-turbo six In conventional gasoline engines, fuel is injected into the intake runner that channels air to the cylinder, not directly into the cylinder, during the intake stroke. With direct injection, fuel is injected directly into the cylinder during the compression stroke, just a moment before the spark plug ignites the mixture. (The injector is located next to the larger spark plug in the accompanying photo.) This was not done earlier because it requires much higher fuel line pressures, over 2,000 pounds per square inch (psi) vs. about 40 psi with conventional fuel injection.

Direct benefits include a more even fuel-air mixture (with no fuel left behind in the runner or on the back of the valve) and a cooling effect inside the cylinder. As a result, it's possible to compress the mixture more without risking premature detonation. Compression ratios for direct injected engines tend to be about 12:1 without boost from a turbocharger or supercharger, and about 10:1 with it. In a conventional engine, these numbers would be about 10.5:1 and 8.5:1, respectively.

Why does this matter? Well, owing to the higher compression and the more even mixture, direct injection engines produce more power AND get better fuel economy. The higher the pressure inside the cylinder, the stronger the "pop" when the mixture ignites.

Taken to the extreme, we get the new Lexus LS, which earns EPA ratings of 19 city and 27 highway despite tipping the scales at over 4,200 pounds and being powered by a 380-horsepower 4.6-liter V8. The previous LS earned ratings of 18 and 25, respectively, despite weighing a couple hundred pounds less and having 102 fewer horses under the hood. Similarly, Volkswagen’s GTI and GLI with their 200-horse 2.0-liter turbo four and nifty DSG transmission earn ratings of 25 city and 31 highway. The old power train, which paired a 180-horsepower 1.8-liter turbo four with a five-speed automatic, managed only 22 city and 29 highway.

Only in the 2006 model year did direct injection become available in the United States in more than one or two models, and it remains far from common. But it should spread quickly over the next few years, and I suspect that in five years the majority of gasoline-powered cars will have it. So you can probably look forward to more power and better fuel economy in your next ride.

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About Michael Karesh

  • Direct injectin is amazing, however it disapointing that the auto manufactures have been so slow to embrace it. Mercedes benz made the technology avalliable in the first fuel injected car in the 50’s.

  • James E., Bell

    Why no reference to the GM DI engines
    such as the 3.0 & 3.6 V6’s ?

  • Its amazing technology. I’m suprised sports cars just introduced it. I would have thought BMW would have used it much earlier.

  • Katty

    Basically, it’s a method of more perfectly timing the insertion of fuel into the cylinders of vehicles, increasing ‘pop’ when the fuel pump is ignited. It’s a fuel-economy-without-sacrificing-power technology, and it’s coming to your vehicle.

  • chris

    Direct injection is a piece of shit. what you really get is extreme carbon build-up at just around every 10k plus miles. I clean my bmw intake-valves every 10k miles and these cars are doomed to give you problems down the road. these cars are focused on the outside not the inside and long term reliability.

  • rob

    I would like to see some small engines with direct injection in the 20hp range.