Direct Injection

Multi port injection has been the mainstay of car manufacturers for decades.

The system works well and has until recently, been the most popular fuel injection design used. With the recent demands to produce cars that are even more fuel efficient, many auto makers are moving to Direct Injection. The benefit of Direct Injection is the ability to put a more precise fuel air charge into the cylinders which creates more power, while using less fuel. This is because the fuel injectors are located on the cylinder head, spraying fuel directly into the combustion chamber instead of traveling from the intake manifold, across the valves, and into the cylinders. The air fuel mixture can be controlled more precisely this way, producing more power with less fuel, than a comparable sized Multi port engine. In fact, this fuel management system could ultimately replace multi port injection in the near future.

NEW TECHNOLOGY, NEW PROBLEMS

With this improvement in power gains and fuel efficiency it would seem this method is better than the more conventional fuel injection designs. However, there is a price to be paid for this ‘break thru’. Direct injection is not actually a new design. It has been used as far back as the 1940s, being tried and abandoned by several companies. As technology has improved, so have the successes of its use. Still, there are the following drawbacks to its design.

CARBON BUILD UP

Carbon build up can happen to any engine over time. The difference with DI and MPI is that since the fuel injectors are located on the cylinder head instead of the intake manifold Direct Injection sprays straight into the combustion chamber. On the more conventional Multi port engine, the fuel air mixture flows past the intake valves which helps to keep the intake valves cleaner, longer. On the Direct Injected engine no fuel passes over the valves therefore letting carbon deposits form on the intake valves under normal use. The carbon deposits are formed by the build up of blowby gases. This build up can cause loss of power, engine misfires and can even damage catalytic convertors.

WHAT ARE BLOWBY GASES?

These gases are a combination of unburned fuel, oil and soot that slip past the piston rings, entering the engine’s crankcase and are then pulled by the crankcase ventilation system back into the combustion chambers. These gases form carbon deposits by sticking to the intake valves which lets them collect and build up.

OIL CONSUMPTION

Several factors can contribute to oil consumption but Because Direct injected engines are producing more power than an equally sized conventional engine, there are increases in engine temperature at the combustion chamber, blowby, and load on the bearings of the engine. This is harder on the motor oil and ends up causing the engine to burn more oil. Car makers have even lowered the standard of oil consumption. Where they once said using 1 quart of oil in 1000 miles was acceptable, some are now saying it’s acceptable to use 1 qt of oil in as little as 850 miles. Personally I find that excessive, as I’m sure many of you will also. It has been the recent surge of these issues coming thru our shop which made me research and type this blog.

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