Best injector position

[Else]

Is it better to position the injector as near to the back of the valve as you can get it. i.e. as close to the port opening in the head as you can get it.

or

Immediately above and in the centre of the trumpet pointing down its throat so to speak?

or

Approx halfway down the inlet tract. Throttle bodies have the injector just below the butterfly in the housing which is approximately halfway for this comparison.

There are no butterflies/slides in the trumpet or manifold/throttle body, its open all the way down to the valve. The inlet tract is tipped about 10/15 degrees from vertical and virtually straight until it turns into the head.

Andy

[ridds]

What application? Ho high does it rev?

[GreenV8S]

I think it's a compromise.

At low load / low revs there can be a lot of exhaust gas reversal (especially if you have a wild cam) and you can end up with so much fuel standoff that the distribution gets uneven. Putting the fuel in a long way upstream will give it longer to evaporate so you can get more cooling and a denser charge (so potentially more power). With upstream fuelling at high torque / low revs you may find that wall wetting becomes a problem and you get poor transient response.

A setup that seems very common is with a set of injectors right down by the valves for low load work (they can be small as they don't need to cope with much power) and a second set as far upstream as you can manage. They're often put completely outside the trumpets, but pointing into them so the fuel goes into the right port.

If you use the same size injectors then you could use a simple electrical relay to change over based on revs and load. I'm going for a slightly (but only slightly) more sophisticated design with upstream and downstream fuelling independently mapped so I can control the distribution completely.

[eliot]

generally the higher the revs, the further back. Look at F1 cars, they squirt into the trumpets.

[Else]

ridds said:

What application? Ho high does it rev?

Its a RV8, V8D Stealth cam producing max BHP at 5-5500rpm if i remember correctly, certainly no benefit revving beyond 6000. Bespoke inlet manifold so injectors could go anywhere.

[Else]

GreenV8S said:

With upstream fuelling at high torque / low revs you may find that wall wetting becomes a problem and you get poor transient response.

Peter,

What do you call low revs?

What do you mean by poor transient response, would you mind explaining please.

FYI The torque starts to climb steeply at about 1400rpm, reachs 330 lbft at 1600rpm then climbs steadily to 370 at 4100rpm then drops off to 310 at 5500rpm.

Andy

[GreenV8S]

Else said:

What do you call low revs?

What do you mean by poor transient response, would you mind explaining please.

FYI The torque starts to climb steeply at about 1400rpm, reachs 330 lbft at 1600rpm then climbs steadily to 370 at 4100rpm then drops off to 310 at 5500rpm.

Andy

The running characteristics will depend on the engine but the bottom third of the rev range would typically be low enough for these effects to be an issue - getting more prominent as the revs drop.

When you have air and fuel inside a manifold, some of the fuel naturally settles out on the surface. The greater the surface area involved, the more will settle. So if you have the injectors by the ports, hardly any surface will be wet. If they're a couple of feet upstream, much much more surface area is being wetted. Eventually the manifold will reach a balance where the fuel evaporates from the surface as fast as it settles out. Steady state running can only occur when this balance has been achieved. As the engine conditions change the amount of fuel on the surface at this balance point changes. For example as you open the throttle allowing more air and fuel in, the amount of fuel on the surface increases. This means that transiently, as you open the throttle, some of the fuel doesn't reach the engine but is deposited on the manifold. Hence the engine tends to run lean for a moment as you open the throttle. To overcome this, carbs are usually fitted with some form of acceleration pump which adds a fixed quantity of extra fuel as the throttle opens to compensate for the wall wetting. EFI systems usually have an acceleratino enrichment algorithm which does the same thing.

Then when you close the throttle, the amount of fuel on the manifold at the balance point reduces again. The excess fuel evaporates, making the engine go very rich for a moment.

If you monitor the exhaust emissions as you open and close the throttle you will see the air/fuel ratio moves all over the place while the throttle is moving. This is mainly caused by wall wetting and drying.

There can also be a problem at high load and low revs where the air speed in the manifold is so low that the fuel won't stay in suspension and simply drops out (like rain) inside the manifold so the fuel can end up forming a stream along the bottom of the manifold rather than being carried as a mist. If you have a manifold with several ports splitting from a common feed this can lead to distribution problems under these conditions as the stream doesn't always divide equally. If you have an injection setup with separate flows for each port at the point the fuel is added there can't be a distribution problem but may be a timing problem as the fuel stream moves slowly relative to the air so it can cause similar transient problems to the wall wetting.

The effects from wall wetting etc usually only take a fraction of a second and would just result in a brief flat spot if the acceleration enrichment isn't added. But for example in my setup with about five feet of wet manifold when running on the upstream fuelling, I can see the AFR taking up to about 30 seconds to stabilise after a throttle transient. That's how long it takes to dry the manifold after dumping a load of fuel in. That's artificially bad because I wouldn't normally use the upstream fuelling on its own under those conditions, but it shows how much of a problem these transient effects can cause under extreme circumstances.

Edited by GreenV8S on Friday 25th January 13:22

[rev-erend]

Peter - that's very enlighening.. kind od thought that would be what happens but it's good to hear what others actually find in practice.

[Else]

Thanks Pete,

Very thorough and easy to understand as always.

So for my fairly low reving engine a position quite near the head would be a good compromise until i have the sophistication to run two sets of injectors.

Is the angle that the injector intersects the inlet tract important? I am guessing that you dont want to spray the fuel at the other side otherwise you end up wetting the wall directly with all the drawbacks that you explained earlier. ie you want to come in at as shallow an angle as possible.

Andy

[GreenV8S]

Else said:

Is the angle that the injector intersects the inlet tract important? I am guessing that you dont want to spray the fuel at the other side otherwise you end up wetting the wall directly with all the drawbacks that you explained earlier. ie you want to come in at as shallow an angle as possible.

I would guess so too, but it's only a guess. Reality has a nasty habit of thumbing its nose at theory. Looking at typical standard manifolds they usually seem to place the injector about 30 degrees away from parallel with the air flow. There may be practical reasons preventing them from reducing the angle any further. But if you visualise a 30 degree cone of spray coming out, that's going to pretty well fill the airway without directly blasting fuel on the adjacent wall. It seems to me that would be a pretty good setup if the goal is to have the fuel mixed evenly through the air without spraying onto the walls. Is that by coincidence?

I've got some results from my upstream fuelling experiments which might be interesting though.

I'm using an injector which produces a very fine spray with a cone angle of about 30 degrees. For practical reasons the injector is almost exactly perpendicular to the manifold (it's about 20 degrees away from perpendicular, pointing downstream). I've chosen to position the injector at the middle of the manifold so the fuel is coming in almost radially into the circular cross section if you're with me. At first glance the fuel is pointing straight at the opposite wall and it should be cr@p.

But just upstream from the injector is the throttle plate and air bypass. The air bypass enters the manifold tangentially and induces a massive swirl just as the air passes the injector. To put some numbers to it the air entry speed is roughly 40x the axial speed, meaning that the air travelling roughly 40" around the duct for each 1" it moves along it. I don't know how far this swirl extends down the duct, I suppose friction will stop it soon enough.

The point is that I can tell from the temperature distribution that under light load almost all the fuel is evaporating within the first few inches, and as the load is increased the 'cold' region extends further down the manifold. It doesn't ever go *very* far though which makes me think that the fuel is evaporating very quickly and not just settling on the walls.

Increasing the load further increases the amount of fuel to be evaporated and takes away the swirl and no surprise that the strong local evaporation stops. I don't have a dyno though so I can't hold it on load at low revs. As the revs pick up the air speed does too, and it seems to do a reasonable job of taking the fuel with it. There's no flat spot under acceleration, although I'm absolutely asking for it with yards and yards of wet manifold. I can see the effects of wall wetting though as it runs as rich as anything after a transient, while the manifold dries out.

[Else]

Thanks Pete, i now have a much better understanding of the theory and your experiances in practice.

Andy

[The Black Flash]

Brilliant posts GV8...well explained and informative. I knew some of this, but you've really helped put it all together in my head!