Today’s direct Injection is to port fuel injection as port injection was to carbureted fuel supply back when the industry was in transition from carburetors to EFI in the 1970s. There are differences, but the comparison is valid. And as fuel economy standards continue to demand more efficient vehicles, the shift to direct injection is an important one. Just as port injection was a distinct improvement in many respects over carburetion, direct injection is clearly superior to its cousin in all of those same categories: More precise metering; cleaner burning; greater fuel efficiency; and cleaner exhaust emissions.
In recent years DI has come of age, due in large part to the combination of its superiority and the economic realities of our age. Only fairly recently has the technology been available to make DI practical. Today’s computer speeds are fast enough to get the job done, and the environmental demands calling for cleaner emissions make DI a perfect fit for an industry seeking those mandated objectives without sacrificing performance.
How do they compare?
Port injection sprays fuel into the ports behind the intake valves where it mixes with air before the valve opens to admit the mixture into the combustion chamber. The metering was a definite improvement over carburetors. But the next step up is direct injection. DI’s injectors are actually mounted with their tips inside the combustion chamber, so fuel sprays directly into the chamber–not into a “waiting room.”
While port injection represented a far higher fuel pressure than was common in carbureted systems, this also gets a big boost with DI. Port systems typically operate at between 40 and 60 psi, while DI can push the pressure range up to 15,000 psi or more.
Another advantage that is found in some direct injection systems involves the computer’s capability to reduce the amount of fuel given to the injectors during periods when the engine is not under load. When you are coasting downhill, or to a stop, for example, or at idle, the computer reduces the amount of fuel to a bare minimum. This in turn reduces power output, but at these times, power output is unimportant. The other thing that it reduces, however, is still pretty important. It cuts fuel consumption markedly.
So, the advantages are obvious. But DI brings some disadvantages to the table, as well. For one thing, it is far costlier to manufacture. That high fuel pressure requires a far more expensive fuel pump, and with the injectors mounted in the cylinder head they are exposed to far greater heat and the added pressure of the actual combustion process. This means the injectors have to be made of exceedingly high quality materials, which increases the cost even more. Even the fuel lines in direct injection systems have to be beefier to handle the extra fuel pressure. And on a more cosmetic note, DI is also noticeably noisier than port injection, especially at idle speed.
Another potential disadvantage revolves around the build-up of carbon on the valves. In port injection, the fuel is sprayed onto the back sides of the intake valves. Since gasoline contains detergents, this actually serves to keep those valves cleaner. The valves in a DI system get no such fuel bath, and so they may require service earlier. It is still early in DI’s young development, so we don’t yet know how serious a problem this may become. Early signs of carbon problems have cropped up. Mostly (but not exclusively), this has been seen on a few specific engines. The Audi V6 and the Audi/Volkswagen 2.0 liter are, so far, the most likely engines to experience this carbon problem. This is a pretty expensive problem to cure. And as mentioned, it is still early in this game, but this may prove to be a distinct advantage for port injection over DI.
The most obvious advantage of port injection is the savings in cost of manufacture, which gets passed along to the buyer in the form of lower sticker prices. Cheaper injectors, fuel pumps, and fuel lines make the shopper’s sticker shock less of a problem.
Some automakers are cleverly blending the two systems together in certain engines. Instead of the Frankenstein effect that some might suspect, this actually works out pretty well. The Subaru BRZ and Scion FR-S offer such systems, as do certain variations of Toyota’s 3.5-liter V-6 engine. These combination systems blend the benefits of each so that the port system deals with clean start-up and other low load conditions, and then the DI kicks in under the load of acceleration. You might think this is the way to go, but the biggest difficulty with DI is still the cost differential. If you have a combination system, you still have the high cost of DI with a little added cost on top of it to also support the port system.
With the undisputed superiority of DI in terms of efficiency, emissions, and performance, there is little doubt that the future of port injection is probably doomed to extinction at some point. Even if de-carbonizing the intake valves becomes a problem for many DI engines, the likelihood is that technology will find a way to overcome the problem more effectively than returning to port injection as a solution.