The Wonder of Direct Injection

By Zach Bowman

Car companies like to bombard customers with slick-sounding jargon during advertisements. It seems like each new model brings some new form of life-altering technology that’s going to save the world one horsepower or one drop of fuel at a time. The thing is, nine times out of 10, the technical-sounding phrases you’re hearing are just ad-fueled nonsense. We say 90 percent of the time because occasionally manufacturers come up with a new bit of tech that can change our threshold for what we consider efficient. Or powerful, for that matter. That’s the case with the rash of direct-injection engines that have surfaced over the past few months.


Mercedes-Benz utilized
direct-injection in the
1954 300SL.

Direct-injection engines are nothing new. In fact, they’ve been around since World War II when the technology was first implemented in the German Messerschmitt fighter plane. The fuel-injection system helped to give the planes a slight horsepower and range advantage over the Allied craft of the time. Eventually, stagnant development caught up the Axis, allowing British and American designs to overtake the Messerschmitt.

But we’re not here to talk about fighter planes. Since those terrors of London were powered by Mercedes-Benz engines, it’s no surprise the first road-going variant popped up in a Silver Arrow. What really raises some eyebrows is exactly which Mercedes received the special fuel system. Hands down one of the most easily-recognizable and gorgeous cars ever created, the 1954 Mercedes-Benz 300SL Gullwing Coupe, was the very first car to boast a direct-injection engine. From these elegant beginnings, the tech spread to the likes of Volkswagen, Mazda and Mitsubishi. Today, just about every major manufacturer has at least one direct-injection engine on the option sheet.

To really grasp what makes a DI engine so much different from the fuel-injected Accord sitting in your driveway, let’s take a look at a standard fuel-injected engine. After leaving your gas tank, fuel travels through the fuel lines until it reaches your vehicle’s fuel rail (if you have a V-6 or V-8, there may be two fuel rails). At this point, the fuel injectors measure out a specific amount of fuel based on engine load, throttle position, atmospheric pressure and temperature and inject it at a relatively low pressure into the intake manifold directly before your engine’s intake valves. Here, the fuel mixes with the air in the intake manifold and gets sucked into the combustion chamber. The whole process is controlled by your vehicle’s computer.

The result is a controlled system that saves vast amounts of fuel and generates more horsepower compared to carburetion, the other form of fuel delivery. Then there’s direct-injection. The main difference between a DI fuel system and standard fuel injection is pressure. Where a standard fuel injector pressure may hover around 40 psi, a direct-injection injector can easily reach 3,000 psi.

So why all the extra pressure? It boils down to fuel atomization, or how tiny the droplets of fuel are when they enter the combustion chamber. Lower pressure means bigger droplets, and bigger droplets mean an uneven and possibly incomplete burn. When that happens, unburned fuel gets sent right out the tail pipe and into the atmosphere.

A DI system is fairly similar to a standard fuel-injection system in that the fuel still travels down a common fuel rail to reach the injectors. From there, the fuel is pressurized inside of the injectors, though instead of releasing into the intake manifold like in a standard fuel-injection engine, the fuel gets sprayed directly into the combustion chamber. That means a unique cylinder head design that allows for the injector to be located close to the spark plug, but between the intake and exhaust valves.

The result is finely atomized fuel that generates an even, complete burn throughout the combustion chamber. An even burn means more horsepower, better fuel economy and cleaner emissions, to name a few benefits. Of course, there are drawbacks in the form of costs. Designing new heads and installing high-pressure injectors isn’t cheap, which is why most American manufacturers stuck with the good-enough fuel-injection systems for so long.

Still, with every car maker looking for more ways to squeeze the most power out of every last drop of fuel, it wouldn’t surprise me to see even more direct-injection engines pop up in the near future. That may even mean less useless ad-speak on your television. Here’s to hoping.



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