109 e3b against spitfire II

[Viper2000]

Originally Posted by Crumpp

Injecting into the eye of the supercharger is no more efficient than a carburetor at fuel metering.

Of course it's better than a carburettor; you're not reliant upon venturi suction to get the fuel into the airflow; you're positively chucking it in with a pump.This will provide considerably better matching across the flow range.

Because you're injecting fuel under pressure, you can positively atomise it, achieving a considerably lower Sauter mean diameter of fuel droplets than is possible with a simple carburettor, which means that it will evaporate much faster. This means that more of the temperature drop happens earlier in the compression process, which increases the overall efficiency bonus.

Once the fuel has evaporated, the mixture distribution problem goes away.

Originally Posted by Crumpp

You are still getting the compression from a supercharger and directly cooling the combustion chamber with direct fuel injection.

This is inherently less efficient. It's relatively simple thermodynamics.

Originally Posted by Crumpp

I am sorry Viper but it does not change the fact you cannot precisely meter the fuel thru an intake, either.

Fuel metering is not a particular problem. You measure the flow through the intake, and inject fuel in proportion thereto. It's much easier to get this measurement & injection process right at a single point than it is to get it right at 12 points.

Fuel distribution may be a problem at low manifold temperatures where the fuel fails to evaporate fully, but that is a separate problem.

You seem to be mostly hung up on fuel metering issues, which certainly exist for naturally aspirated engines with carburettors, especially away from their design point.

However, fuel metering is not especially important at high power if you don't care about emissions. You run rich of stoichiometric, and fuel flows say +/- 5% won't make a great deal of difference to power output. Obviously the SFC is pretty bad at that point; you can clearly see this on the SFC curve in Lovesey's paper.

Mixture distribution is not a problem at high induction manifold temperature.

The reductions in supercharger work and intercooler size are far more important than the slight increase in fuel mass fraction which you might suffer from the need to keep the leanest cylinder sufficiently rich to avoid detonation. The cost of a single point system is far lower than a multi-point system, and the fuel pressure required is lower than for true direct injection. (Port injection is a pretty horrid compromise which only makes sense if the alternative is a carburettor which would produce bad mixture distribution.)

For a supercharged spark-ignition aero-engine, operating at a fixed non-dimensional power setting, provided that you've got enough induction manifold temperature to avoid condensation, the mixture distribution will be good and the single point system wins.

Multi-point FI is an expensive solution to mixture distribution problems. It is great for naturally aspirated engines, and probably pragmatic for turbo-normalised engines, especially if the engine manufacturer isn't responsible for the turbocharger.

But if you're using a mechanical supercharger and will mostly operate the with reasonably high induction manifold temperatures, then there's no great mixture distribution problem unless your induction manifold is horrible, so multi-point injection offers limited benefit, whilst single point injection into the eye of the supercharger reduces supercharger drive power requirements. So single point injection is a pretty obvious choice.

Now, if you're designing a sports car engine, you might supercharge it to get high power, but most of the time it would operate at very low non dimensional power settings, so mixture distribution would be a major problem with single point injection, and therefore you'd probably go for multi-point FI.

But that's because the sports car engine isn't really designed for high performance. It's designed to make an expensive noise and very occasionally provide bursts of acceleration to impress the girl in the passenger seat. Most of the time it's practically ticking over, and so you're much more bothered about part-load characteristics than would be the case for an aero-engine. You're also trying to meet modern emissions regulations, which means that you're paranoid about stoichiometry so that you don't poison your catalyst. It's a totally different world, with different trades and different drivers.