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Direct Fuel Injection - Adding Fuel To The Fire - Tech Talk

Examining Direct Fuel Injection

By David Pratte

Direct fuel injection isn't exactly a new idea. It's actually been around since World War II, when it was used on aircraft engines designed by the Germans and Russians and has been used in diesel engines for decades. But with more and more high-performance, gasoline-powered cars moving away from conventional multi-port fuel injection in favor of direct fuel injection-including sport compacts like the Mazdaspeed3 and Chevy Cobalt SS, as well as world-class sports cars like the Porsche 911 and Audi R8-it seems timely to discuss this technology in order to better understand how it may impact our ability to tune our favorite cars now and in the years to come.

So what exactly is direct fuel injection (DFI), how does it differ from multi-port fuel injection (MPFI) and how does it improve engine performance? As the name implies, direct fuel injection (sometimes referred to as gasoline direction injection), injects fuel directly into the combustion chamber, rather than into the intake manifold runners or the cylinder head ports, as is the case with conventional multi-port fuel injection. The other major difference between MPFI and DFI is that direct injection fuel pressure is as much as 50 times higher than standard port injection, with DFI pressure well over 1,000 psi being the norm while MPFI pressure is usually around 50 psi, depending on the system's design.

The advantages of injecting fuel directly into the combustion chamber at very high pressure are numerous. Combined with the sophisticated ECUs featured in today's vehicles, DFI allows for greater control of how much fuel is injected into the cylinder and when/where exactly it's being injected. As previously mentioned, with conventional port injection, fuel is injected into the intake manifold runners or cylinder head ports, where some of the fuel washes along the walls as it mixes with the incoming air charge. This makes it difficult to precisely control the exact amount and timing of the fuel entering the combustion chamber, but with DFI the injector nozzles are positioned within the combustion chamber for an ideal spray pattern and accurate targeting of the ignition spark and the air immediately around it. This gives DFI superior control of the dispersion and consistency of the fuel being injected into the combustion chamber, enabling what is known as a "stratified mixture" or "stratified charge."

A stratified charge is when the air/fuel mixture is concentrated around the spark plug and the air at the marginal areas of the combustion chamber remains pure (free of fuel). This in turn allows a leaner burn, particularly at part throttle and low engine loads, resulting in improved fuel efficiency and reduced hydrocarbon emissions. Volkswagen claims its direct injection system improves fuel economy by up to 25 percent, while Audi claims a 15 percent improvement with its DFI system. Chevys with DFI engines are seeing similarly impressive results, with a 10-15 percent increase in fuel efficiency.

With improved fuel dispersion and control, DFI offers the added benefits of improved cylinder and piston cooling, which means higher compression ratios and more aggressive ignition timing can be used. And as you already know, with high compression and more aggressive ignition timing you get more power, and more power is good! Porsche, for example, has seen horsepower and torque increases of 5 percent and 4 percent, respectively, on its LeMans V-8 race engines, which is a huge improvement to see on an engine that is already highly optimized. Cadillac sells the CTS with both a port-injected or direct-injected version of its 3.6-liter V-6 engine. The port-injected version makes 263 bhp, while the direct-injection version makes 304 bhp while getting slightly better fuel economy and doing so on regular unleaded fuel.

By David Pratte
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