AUTO TECH: Injection systems will fuel your interest

Hey there, time traveller!
This article was published 21/10/2011 (5194 days ago), so information in it may no longer be current.

WITH all the auto manufacturers working to reach new fuel economy standards set by the U.S. for future vehicles, we’re seeing new technologies, materials and techniques. One of them is direct fuel injection.

Although it’s actually been around for decades, it’s only recently that materials development and computer speeds have reached the point where direct fuel injection would not only work, but work within the strict emissions regulations that are in place.

So what is direct injection? Right now there are two types of injection systems common on gasoline-powered vehicles: port injection and direct injection. Both systems use computer-controlled electric injectors to spray fuel into the engine, but the difference is where they spray the fuel.

Port injection sprays the fuel into the intake ports where it mixes with the incoming air. The injectors are often mounted in the intake manifold runners and the fuel sits in the runners until the intake valve opens and the mixture is pulled into the engine cylinder.

Direct injection has the injectors mounted in the cylinder head and the injectors spray fuel directly into the engine cylinder, where it then mixes with the air. Only air passes through the intake-manifold runners and past the intake valves with direct injection.

There are advantages and disadvantages of both systems. The biggest advantage of direct injection is its better fuel economy. Improvements of 15 per cent are not uncommon just by changing from port to direct injection. With tight fuel-economy standards, this is the main reason we’re seeing so many vehicles on the market equipped with direct injection.

Another advantage is performance. Direct injection can meter the amount of fuel exactly into each cylinder for optimum performance, and it’s sprayed in under very high pressure – up to 15,000 PSI on some vehicles — so the fuel atomizes well and ignites almost instantly. With current computer controls, the injectors can be pulsed several times for each combustion stroke so the fuel can be injected over a longer time-frame to maximize the power out of the cylinder.

Disadvantages of direct injection include cost. Because the injector tips are mounted right into the combustion chamber, the materials in the injector have to be very good quality, and that costs money. Also, the high pressure needed to inject fuel directly into the cylinders means that more expensive high-pressure fuel pumps are required. These are typically mechanically driven from the engine, so add to the complexity.

One final disadvantage is that direct injection has, in the past, make it more difficult to control emissions. Even though the fuel is injected under high pressure, there is little time for it to mix with the air. Current injector designs include precise nozzles with multiple holes in them to direct the fuel accurately. Piston and cylinder-head design that promotes swirling of the intake air also helps the air and fuel mix better — turbocharging is even better, as it causes the air to swirl rapidly as it’s forced into the cylinders. With these innovations, meeting emission standards are no problem.

Port injection systems are much cheaper to manufacture. The injectors are not exposed to the high heat and pressure of the combustion chamber and they don’t have to handle the high fuel pressures. Port injection systems typically operate in the 30-to-60 PSI range, which is dramatically lower than direct injection systems. Support systems such as fuel pumps are also cheaper because fuel pressures are lower.

Port injection systems spray the fuel behind the intake valve and the fuel has to wait until the valve opens. This does allow some fuel to drop out of the air, but the faster the engine is running, the less disadvantage this is. If your engine is running at 2,000 r.p.m. down the highway, the intake valve for a cylinder is opening at 1,000 times a minute, or nearly 16 times a second. Fuel doesn’t have to sit very long before it’s pulled into the combustion chamber.

I can see the day when port-injection systems are completely replaced by direct injection. Port injection, although much better than the old carburetor and central injection systems, just can’t match the power and economy produced by direct injection. As fuel costs increase, the difference in manufacturing cost between port and direct injection systems becomes less important.

Right now, both systems are available on new vehicles, depending on the model you select. Weighing the advantages and disadvantages of both systems may make a difference on the selection of your next vehicle.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.