Fuel injection increases volumetric efficiency?

[Circlotron]

We all know that with a carburettor setup the moving air has to impart some of it's kinetic energy to the liquid fuel entering the airstream. That is to say, the moving air gets slowed down somewhat as it encounters and begins to move the stationary liquid.

What about with port fuel injection? If the fuel droplets are entering the airstream faster than the airflow and in the same direction, would the moving fuel help speed up the airflow and thereby improve volumetric efficiency? Presumably the greater the fuel pressure for a given rate of fuel flow the greater the effect. Of course, the more highly loaded fuel pump means greater load on the alternator so greater parasitic hp drain...

[David Redszus]

Fuel emerging from a carb jet into the moving air stream, will do at least two things:
it will evaporate and thereby chill the inlet air density (depending on how much fuel has actually evaporated).
and
The fuel vapors will displace a certain volume of inlet air, reducing its volume.

So now we have a trade-off; a reduced volume of air/fuel mixture but with a greater density.
Have we increased or reduced the mass of the air/fuel mixture?

The added fuel vapor has increased mixture mass but has reduced inlet air mass.

Port injection will deliver a finer droplet size and will not displace air volume, but we will not
benefit from evaporative cooling. Direct injection would be even more efficient.

[Brian P]

I think you will find that during the relevant part of the intake stroke, the air in the port is moving considerably faster than the fuel stream.

[Truckedup]

While we are on this topic; I hear that moving a port injector upstream from the usual location close to the valve can enhance performance? If this is true, why?

[PackardV8]

FWIW, I've seen some Euro race engines with the EFI injector on a bracket across the top of the open end of the injector stack. With the air box removed, one could watch the injector firing directly down the stack into the moving air stream.

jack vines

[David Redszus]

While we are on this topic; I hear that moving a port injector upstream from the usual location close to the valve can enhance performance? If this is true, why?

Engines that run at very high rpms do not allow much time for fuel to evaporate, even from a fuel injector nozzle.

The injector is placed close to the port for low rpm throttle response and moved outboard for high rpm mixture.
It is common for road racing engines to use two injectors per inlet, one close to the port for low rpm and a second injector placed outboard for high speed; the ECU can control the transition between the two injector locations.

It is easy to see why Lambda will change with engine rpm in three stages; low, transition and high rpm.

[Brian P]

Lots of high performance motorcycle engines nowadays have the above-mentioned two injectors per cylinder arrangement.

Formula 1 (and Moto3) have in the past (not sure about now) used a single injector aimed down the intake runner, but they're not concerned about emissions or low-speed driveability or cold start driveability.

[MadBill]

Don't know about Moto GP, but I believe F1 is restricted to a single injector per cylinder.

[ClassAct]

Don't know about Moto GP, but I believe F1 is restricted to a single injector per cylinder.

Didn't one of the classes have intake runners that changed length with RPM? IIRC I think some bikes had that available to the public.

[Brian P]

Yes, Yamaha has done that.

[modok]

We all know that with a carburettor setup the moving air has to impart some of it's kinetic energy to the liquid fuel entering the airstream. That is to say, the moving air gets slowed down somewhat as it encounters and begins to move the stationary liquid.

Yes, however, that factor is SO insignificant, that IMO it can be ignored, in both cases.
For instance, CArb VS TBI, IR port injection VS IR carbs, both can be close to direct comparison, and usually the difference is in the noise floor, or due to other factors like heat.

I would say the largest factor is that port FI allows a greater design freedom, which, CAN increase VE, IF utilized well. You can do things with dry flow that you cannot do with wet flow.

[induction apprentice]

Well. I set up an efi driver set up on my flow bench a couple years back and tried some wet flow stuff with a 243 head, LS6 intake and stock LS6 injector.

With the injector tested at all different duty cycles up to 90% and varied PW. The cfm loss with injector on maxed out at a 4 cfm drop at 28" h2o. I also was hoping for some magic ram effect that would help push the air in. Even at 60 psi fuel pressure and low lifts. No luck. The air is moving to fast it seems for the fuel to influence its velocity. I lost interest and didnt test for very long. I could have got way more scientific. So its likely there is still stuff to be learned there?

Dont try this with flamables! I used a non combustable fluid and accepted a slight change in fluid properties for my testing. So make of it what you will. Did not want to convert my 1020 into a flame thrower.

[Newold1]

My dyno testing of engines convinced me at this point that in most performance engines moving from a carb setup to an EFI setup all other things being equal did not show any kind of appreciable increase or decrease in torque which is basically the best determiner of volumetric efficiency. The changes usually seen represented less than a 1% difference or so. Sometimes horsepower numbers changed by 2% or so but that was nominal. So my take is EFI does not necessarily increase volumetric efficiency.

[LoganD]

Port injection will deliver a finer droplet size and will not displace air volume, but we will not
benefit from evaporative cooling. Direct injection would be even more efficient.

Why wouldn't it benefit from evaporative cooling? The fuel coming out of a typical port injector will still evaporate, and it will do so more thoroughly and completely due to more surface area relative to fuel volume (small droplet size). I fail to see how a carb has a benefit in evaportative cooling, you just don't see it in testing because most aftermarket places aren't putting a temperature probe by the intake valve or in the cylinder.

[MadBill]

Yes, with DI the evaporative cooling benefit occurs in the closed cylinder, where it permits a higher CR than would otherwise be possible.