Direct Injection

After last weeks post about compression ratio, I thought it would be a good time to take a look at fuel delivery again, this time direct injection.  On the most basic level, direct injection is ust injecting the fuel directly into the cylinder rather than into the manifold or the intake port.

So how does that relate to my post about compression?  Simple, the higher the compression, the air/fuel mixture is heated and that can cause pre-ignition.  If you just inject the fuel just before the spark plug starts combustion there is very little time for the fuel to overheat and start burning prematurely.

When I first read about direct injection on car engines, I thought big deal.  Diesel engines have been doing that for decades, even before they had electronics to do it.  But diesel fuel burns at a lower temperature, and has some lubricating properties, so the injectors lead a gentler life than gasoline ones do.  Plus factor in the lower RPM range so the injectors don’t have to fire as often and I started to understand why it was a bigger deal for a gas engine.

Then I though, it has also been used in two strokes for a while, to help clean up the emissions.  Since two strokes use oil mixed with the gas to help lubrication, that helps the injectors as well.

And direct injection on a gas engine isn’t exactly brand new, the idea has been around for decades, we just now have the combination of technology and need to make it workable and worth it for the manufacturers to do it.

The need stems from the ever increasing need for horsepower, increased vehicle weight due to more comfort features and more crash safety stuff, and increasing emissions and fuel economy regulations.  Any one of these is easy to accomplish on its own, but trying to do all of them at once is difficult at best, since any one of them tends to make the others harder to achieve.

So that brings us to the emergence of direct injection in gas powered cars.  It allows a higher compression ratio for more power and better fuel economy (if you don’t think that makes a difference, look at the fuel economy difference between a late 60’s car, and a mid 70’s one of the same size).  Since the fuel is even more precisely injected, it reduces emissions at the same time.

Unfortunately, it isn’t as easy as just moving the fuel injector into the cylinder instead of the port to make it work.  The need to get the fuel properly distributed means the cylinder head and piston needs to be designed to work with the injector for the whole system to work.

That means that we probably won’t be getting a direct injection kit to put on a classic car any time soon.  But once they are more available, we will be able to put the whole engine into a classic car.  How ever, it appears that there aren’t any American V8’s to swap with yet, but give it time, there will be.  So far there is just the new LT1 that will be in the new Corvette, and there is speculation the new 5.0 Ford V8 was designed for direct injection.

Unfortunately, all is not rosey with direct injection.  Let’s just go past the teething and reliability problems since they are specific to each manufacturer, and I am sure each will get it figured out.  But there is a problem that is related to injecting fuel directly into the cylinder so it doesn’t go over the intake valve.  It seems like something that isn’t a big deal, but it can be.  If it was just air going in, it probably wouldn’t be.  But since there are oil vapors from the crankcase ventilation and exhaust from the EGR flowing through there, it is possible to get buildup on the intake valves.  With regular fuel injection (or a carburetor) the fuel flowing across the intake valve helped keep it clean.  With direct injection, that doesn’t happen.

Some manufacturers use a small injector (I am not sure if it is central, or port) to periodically inject a little fuel that flows across the intake valve to keep it clean.  It seems like a simple solution to me, and simple is usually better.

The other bonus to direct injection is that not only can it run higher compression, but without the need to run higher octane fuel.

It seems to me that the ideal companion to direct injection would be forced induction.  With the reduced tendency for detonation, increasing the cylinder pressure with a turbo or a supercharger would make for an awesome combination.

Something like the Ford EcoBoost lineup, but without the fuel economy part would make for a great muscle car/performance car engine, but that is a subject for another post.


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