Fuel Injection Vs. Direct Injection; pls explain the difference

[Hendog]

The new Porsche 911 models feature Direct Injection as opposed to the older technology of Fuel Injection. I've read up on it in Wikipedia. Don't both systems inject fuel directly into the combustion chamber? Is it just a more electronically controlled version of Fuel Injection?

[Mike_Lettrich]

No, fuel injection usually sprays fuel into the intake runners, not the combustion chamber. That is the difference with direct injection.

[the]

Regular (non-direct) fuel injection does NOT inject fuel directly into the combustion chamber. It injects fuel upstream of the intake valve.

Direct injects directly into the comb. chamber.

[Zeke]

Direct=into the chamber. FI=into the intake runner. It wouldn't necessarily be more "electronically controlled." FI is pretty sophisticated.

[Hendog]

Originally Posted by Mike_Lettrich

No, fuel injection usually sprays fuel into the intake runners, not the combustion chamber. That is the difference with direct injection.

Gotcha!

Thanks

[donporfi]

And there are two different types of fuel injection
TBI Throttle body injection it is something like an electronic carburetor and
MPFI multiple port fuel injection each cylinder intake port gets an injector.

[88911coupe]

This begs the question...why is direct injection apparently better? I think that's what's used in my 2.0T A4 Audi Turbo.

[the]

Direct injection is a more precise method of metering fuel.

With indirect, you are injecting raw fuel into intake runners. You are relying on the intake turbulence to mix the fuel with air, then it is sucked past the intake valve into the combustion chamber, where it is ignited. There are a lot of variable there which cannot be fully controlled.

With direct, the injector injects directly into the combustion chamber. It can therefore be controlled more precisely.

It is similar to comparing a carb to fuel injection. With a carb, the air/fuel mixture is mixed usually a foot or more away from the combustion chamber. There are a lot of not really controllable things that can happen to that mixture in the foot that it has to travel to the combustion chamber. Regular injection mixes it a couple of inches away. The closer to the combustion chamber, the more control.

[silverc4s]

Originally Posted by the

Direct injection is a more precise method of metering fuel.

With indirect, you are injecting raw fuel into intake runners. You are relying on the intake turbulence to mix the fuel with air, then it is sucked past the intake valve into the combustion chamber, where it is ignited. There are a lot of variable there which cannot be fully controlled.

With direct, the injector injects directly into the combustion chamber. It can therefore be controlled more precisely.

It is similar to comparing a carb to fuel injection. With a carb, the air/fuel mixture is mixed usually a foot or more away from the combustion chamber. There are a lot of not really controllable things that can happen to that mixture in the foot that it has to travel to the combustion chamber. Regular injection mixes it a couple of inches away. The closer to the combustion chamber, the more control.

Food for thought:>> All of the above is correct, PLUS:
Direct injection is at much higher pressures (25 - 50 bar, i believe) using a very precise injector that is designed to live in combustion chamber environment, and have a very tight control of spray pattern and pulse timing. So, it could be looked at as the final step toward the perfect fuel system. The poor Carburetor being at the other end of that scale, full of compromises.

If you look at the FI systems on F1 engines, they do not use direct injection, but rather have very much high mounted injectors at the top of or above the intake stacks. These engines idle at 5000 rpm and peak at 19000 currently though, so the injector location decision is made using different criteria.

[Flieger]

+ 1 on the pressure.

MFI was developed long ago but remained the high performance technology throughout the era of fuel injection's introduction on street cars.

MFI is better than CIS for many performance reasons. One is the injection pressure. Similar to direct injection nowadays, MFI uses very high pressure, though nothing compared to direct injection. This atomizes the fuel very well, providing a better fuel/air mixture to the combustion chamber that lights faster and with more bang. This allows more timing options for power.

The MFI injectors are also pretty close to the combustion chamber compared to carbs (unless you have high butterfly injection). They get accurate metering, great atomization, and don't have to have an injector living in the fire of the combustion chamber. It also keeps the head from looking like swiss cheese, with 5 valves, 2 plugs, and a few injectors in the most extreme cases.

Another benefit of the atomization of the fuel is that it cools the incoming air into the combustion chamber, cooling part of the engine, providing more power, and burning more efficiently.

[Zeke]

Direct injection is no different than a diesel using electronic injection rather than mechanical.

The one parameter that is added beyond standard multi-port EFI is the timing of the injection pulse in relation to TDC, as in how the diesel works. With port injection all the fuel must be in the chamber before the valve closes. Injecting into an already compressed environment at the optimum point seems to be the reason for the technology. I would think that efficiency is better, but it's only one more control.