PorKen

I ran across this article by accident, it's interesting vis-a-vis the port gunk, but also the LBDS injectors, and that high atomizing injectors may reduce air volume into the cylinders.

http://www.chevytalk.org/fusionbb/sh...hp?tid/199007/

Russ Collins, RC Engineering

ATOMIZATION

High atomizing injectors are usually used in Throttle Body applications only, and have a rather wide spray pattern. A wide, finely atomized pattern is wonderful for emissions and economy but can cause problems in higher performance engines. At low RPMs, with a low air flow rate, the slow moving finely atomized fuel has enough time to get past the valve and create a close to stoichiometric mixture. (Air/Fuel mixture of 14.70 - Chemically ideal) As RPMs increase this mass can't keep up, with valve open time, and many of the fuel droplets impinge the port wall and condense. Atomized fuel can only travel at "port air speed" and in large quantities it can actually displace air in the port. With a highly atomized mix in the port, at intake valve opening, the lighter droplets of fuel will be partly blown back up the port [intake port reversion]. This is caused by the residual exhaust pressure [overlap period] still residing in the combustion chamber. Some of this reverted mixture will adhere to port walls and condense. This puddling fuel may find its way home, on the next intake cycle, but it will cause cycle-to-cycle air/fuel ratio variances. The higher inertia of the more condensed fuel will carry it to its target. "The liquid film that wets the walls represents a capacitance that greatly reduces the transient response of the engine." (SAE 950506) This problem is compounded in Gang fire and Semi-gang fire systems, but is not as troublesome in sequential fire systems. Gang fire systems fire all injectors, every rotation, at the same time, discharging half of the required fuel at each event. Semi-gang fire systems fire groups of injectors in the same fashion, half-and-half, each rotation. Sequential systems fire each injector at a pre-determined time and discharge all required fuel in one event, prior to intake valve opening. In either of the Gang fire systems there is no timing-of-event technology in operation, and as you can see it's a rather simple system.

At 8000 RPM the intake valve is opening and closing at 66 times a sec. and is only open for an average of 9 Mil/Sec. At this cyclic rate the transient time to complete the delivery of fuel from injector to valve, is critical. This is why Indy car injectors are very precisely targeted and timed to provide a solid stream of fuel with non-existent atomization, LBDS - Laser Beam Delivery System. In these engines the injectors can discharge fuel, at a "just prior to valve open position" and get it all down the hole. As the fuel impinges the hot intake valve it virtually vaporizes and mixes quite well with the incoming air forming a very homogeneous charge. This is one of the most extreme situations but it's a real interesting one. As an added benefit, the latent heat of fuel vaporized in the chamber also provides charge cooling that makes the mixture denser. A denser, heavier mixture (cold and thick) will produce more power then a thin (hot and light) charge. This is why Turbo intercoolers are so effective. Injector timing, phase angle, is altered by the ECU according to RPM in these systems and can control the delivery impact time precisely. In a Steady State Pressure Fuel System, the injector pulse is always moving at the same speed, regardless of engine speed changes. The velocity of discharged fuel is relevant to the area of the discharge port and the net operating pressure. Pressure changes activated by boost, at a 1:1 ratio, only compensate for port pressure and don't change the static pressure, flow rate or velocity. RPM adjusted fuel timing is utilized for this reason, it advances the injector timing based on engine speed, and maintains perfect impingement timing at all speeds.

It's a known fact that you can't burn fuel until it's atomized. It's also known that you can't burn fuel without air. The most important, of all known facts is that you can't burn anything, if it's not in the combustion chamber. The secret is to provide 'adequately atomized' fuel with as much air as possible. 'Adequately atomized' is the secret phrase of the day. Fuel does not have to be completely atomized at the injector tip (SMD of 10um - 20um) but it does have to get past the valve to do us any good. The more condensed the fuel delivery is the faster it will travel, (regulated by discharge area and pressure) and the more accurately it can be targeted. Recent (S.A.E.) "Injector Atomizing and Targeting" studies have provided us with one of the most prominent advances in High Performance Engine Management. These test programs have concluded that "accurate impingement onto the center of the valve head is vital for good vaporization" and "the targeting orientation of the injected fuel spray is a critical parameter in fuel evaporation" also that "fuel injected directly onto the intake valve yields a significantly better engine response" (SAE950506) What all this means is, different engine designs require a different type of injector to operate efficiently and that 100% atomization is not always required or desired. In racing situations we usually have to do the best we can with what we have or what's available. The goal, of course, is to do the best in all cases and in all situations. The best injector for your engine is the one that will yield an optimal fuel-air mixture and provide the required power output consistent with smooth and reliable operation. This is our goal, and all things considered, we feel that we provide an excellent service in this very specialized field.

WallyP

As my best friend used to say, "Not being hampered by facts, I can discuss the subject freely."

So, in our batch fire injection system, the injectors fire each revolution, presumably just after TDC-Firing on #1.

That means that for the "worst case" #8 cylinder, its injector fires twice, then the intake valve opens just before it fires again, so the injected fuel in #8 sits longer than any other cylinder. More chance to vaporize, and more chance to "plate out" on the intake walls. Which has more effect? Does position in firing order (time lag behind #1 firing) make any real difference?

Do we need a new manifold design, with the injectors spraying directly on the intake valve?

PorKen

Shhh, not too loud. Brendan or Colin may hear you!

Dual LBDS injectors per cylinder (sequential fuel management).

BC

I heard it. I am actually moving the injector UP the runner, AWAY from the valve. The reason that I was planning this was because of the REDUCED atomization possible with larger injectors, in the 120-160 range - what is required for ethanol usage. More time in the runner allows more time for vaporization and cooling of the intake charge - which seemed important for high boost situations.

The injector IS farther away from the valve in these instances, but possibly for other reasons, and if its beaming the fuel instead of "spraying it" - I'm more confused. Realistically, we are operating at up to 7000rpm - not 20,000.

CanAmJohn

It might be caused by fuel stand off at higher rpm. If you have a badly matched to the port exhaust header, restrictive exhaust flow or aggressive cam timing stand off can be severe. It also varies depending on altitude, high altitude tends to produce more stand off. I've seen race V8's in sports cars with visible fuel standoff 2' above the inlet at full chat on open trumpets on a perfectly healthy engine. (standoff is a mist of fuel/air that sits like a cloud above the inlet, caused by the air/fuel mixture in the inlet flowing out of the inlets)

If you look at cars like the GT40 and 917, they have standoff plates above the inlet trumpets, you can tune the position of these plates to change the engine torque curve. I expect the Porsche 928 inlet design is using the sideplate positioning on the 32V plenum for example, as a standoff plate to tune the torque characteristics of the engine.