How to properly turbo-supercharge the P-39 Airacobra.

[Dynasoar]

Draconis and phx1138,

To summarize my views No fuel should enter the induction system anywhere except into the intake ports in the vicintity of the valves. There are multiple reasons for this including safety - inability of a backfire detonating the highly compressed fuel-air mixture in the entire induction system between the intake valve port and the carb, and similarly, no need for Davey screens to prevent flame propagation (while reducing boost).

Not introducing fuel into a supercharger (or carburetor/supercharger assembly) results in higher supercharger outlet temperature, which allows for a substantially smaller aftercooler heat exchanger to reject a specified heat load to ambient, (or with a larger HE, cool induction air still further) since temperature difference between induction air (without fuel) and local ambient temperature is increased. The cooling effect of fuel evaporation is now applied in the engine intake ports and volumetric efficiency is increased by the cooler, denser mixture.

I hope that this description is clear enough. The major point to take with you is that where fuel enters an induction system and evaporates, substantial cooling takes place and the temperature of the flow is lower than it would be if the fuel had not been introduced.. If you want (down stream) to use a HE to further cool the induction stream by rejecting heat to the air outside the airplane, the greater the temperature difference between the stream and outside air (Delta T) the smaller or simpler or less efficient the heat exchanger needs to be.

Draconis, in the P-38 example, I wouldn't change anything after the -J. But, as an example of applying the described tech, eliminate the carburetor at the engine stage blower, introduce an air to air heat exchanger (aftercooler) at the blower outlet and in the "V" between cylinder banks and inject fuel (constant flow & relatively low pressure) into the intake ports. Air-fuel entering the cylinders would be cooler/denser so boost could be increased while maintaining detonation margin.

Dynasoar

 

[phx1138]

No fuel should enter the induction system anywhere except into the intake ports

Well said.

This post pretty well summarizes my views, too. (His reasons are better.)

Air-fuel entering the cylinders would be cooler/denser so boost could be increased while maintaining detonation margin.

It has an additional benefit: even at low or zero boost, the cooler, denser charge produces more hp.

 

[Draconis]

Draconis and phx1138,
Dynasoar

Now I can see the advantages this approach would have. And it's not a complication as long as the fuel injection technology is adequate in the early 1940s. It is a clever approach as it does two things at once. Injecting the fuel directly into the compressed air at the intakes ports would also do the same function as water/methanol injection. I wonder if there would be sufficient mixing of the fuel spray and compressed air to produce an uniform density in the charge mixture as it's a very brief interval in time and distance between the fuel injection and the intake valves opening.

How much of an increase in horsepower would this method produce?

 

[EverKing]

I wonder if there would be sufficient mixing of the fuel spray and compressed air to produce an uniform density in the charge mixture...

I wonder if this isn't at the crux of the issue: that injector technology wasn't sufficiently advanced when Allison designed the V-1710 to produce a sufficiently homogeneous charge so they placed the injectors on the S/C so that the action of compressing the air with the fuel would improve the distribution of the mixture (and by injecting the fuel directly on the S/C impeller it would help cool the S/C itself). Of course, we are talking about a mid-1930's design...how much had fuel injector quality improved in the intervening years and what would it take to alter the V-1710 to use the port fuel injectors?

EDIT: as an aside, this discussion is beginning to sound more like it belongs in the Better Allison V-1710 thread.

 

[Dynasoar]

Draconis,

The proposed fuel injection system (a simple elaboration of the Hilborn approach to cover altitude compensation etc) would constantly spray fuel at the intake valve stem. With the valve closed the fuel would be expected to largely evaporate in the relatively hot port cavity, with some pooling. As the valve begins to open the transient near-sonic air velocity would very thoroughly vaporize any residual liquid over the valve edge.

Key to the entire process is the provision of an efficient aftercooler at the outlet of the engine mounted supercharger to reject blower heat input to ambient. More about the proposed "heat pipe" aftercooler later.


Dynasoar

 

[Dynasoar]

I wonder if this isn't at the crux of the issue: that injector technology wasn't sufficiently advanced when Allison designed the V-1710 to produce a sufficiently homogeneous charge so they placed the injectors on the S/C so that the action of compressing the air with the fuel would improve the distribution of the mixture (and by injecting the fuel directly on the S/C impeller it would help cool the S/C itself). Of course, we are talking about a mid-1930's design...how much had fuel injector quality improved in the intervening years and what would it take to alter the V-1710 to use the port fuel injectors?

EDIT: as an aside, this discussion is beginning to sound more like it belongs in the Better Allison V-1710 thread.

 

[Dynasoar]

Everking,

Much of the fuel injection problem (not considering the assignment of priorities) was the assumption that we had to follow the German approach of elaborate timed (Diesel like) cylinder injection. Constant flow port injection was well within our capabilities and may well have been experimented with at the Indianapolis Speedway in 1935 or 6, ten years before its practicality was established by Hilborn.

Present general aviation fuel injection, which is similar to that discussed in my posts, often uses variation in injector port outlet jet size to address differences in airflow at intake ports.

The relatively elaborate FI system adopted for late war (Silverplate B-29) R-3350 engines was replace by a CW developed commercial engine supercharger impeller fuel dispenser which, after much induction manifold tinkering, contributed to the success of the essentially redesigned engine.

Frankly, I'm amazed that we haven't seen port injection on some of the reno racers by now.

Dynasoar

 

[phx1138]

I wonder if this isn't at the crux of the issue: that injector technology wasn't sufficiently advanced when Allison designed the V-1710 to produce a sufficiently homogeneous charge so they placed the injectors on the S/C so that the action of compressing the air with the fuel would improve the distribution of the mixture (and by injecting the fuel directly on the S/C impeller it would help cool the S/C itself). Of course, we are talking about a mid-1930's design...how much had fuel injector quality improved in the intervening years and what would it take to alter the V-1710 to use the port fuel injectors?

As I understand what Dynasoar is proposing, it's a bit like the nozzle on a garden hose: constant flow with an "on/off" control. It's not real sophisticated. The German timed system is more complicated, & less fuel-wasteful (constant flow tends to run over-rich), but also harder to get to. (The hot wire flow meter actually suits the German system better.) Similar systems would be found on diesel engines of the period, so not beyond the pale.

EDIT: as an aside, this discussion is beginning to sound more like it belongs in the Better Allison V-1710 thread.

Anybody object if I copy & repost these remarks there?

 

[EverKing]

Constant flow port injection was well within our capabilities

As I understand what Dynasoar is proposing, it's a bit like the nozzle on a garden hose: constant flow with an "on/off" control. It's not real sophisticated.

It wasn't the timing or complexity of the injection I was trying to comment on but the extent of atomization of fuel in the fuel/air mixture. I was wondering if they weren't perhaps relying on the pressure and induction velocity from the S/C to better homgonize the mixture and distribute the fuel more uniformly?

 

[Dynasoar]

phx1138,

Flow thru the nozzle would be continuous. There is no reason that this system should to run rich (or lean). This largely depends on settings in the inherently simple fuel control methodology

Dynasoar

 

[GunsCarsGuitars]

It has an additional benefit: even at low or zero boost, the cooler, denser charge produces more hp.

Which is why cool, damp weather is known in auto racing circles as 'turbo weather'.

 

[phx1138]

It wasn't the timing or complexity of the injection I was trying to comment on but the extent of atomization of fuel in the fuel/air mixture. I was wondering if they weren't perhaps relying on the pressure and induction velocity from the S/C to better homgonize the mixture and distribute the fuel more uniformly?

I wouldn't expect the jets to be worse than contemporary carb jets, so... It's just a matter of fuel flow in (or, put another way, no starvation).

Flow thru the nozzle would be continuous. There is no reason that this system should to run rich (or lean). This largely depends on settings in the inherently simple fuel control methodology

It's because fuel flow is constant that you tend to get over-rich (because over-lean is a bad idea generally, it tends to be avoided by default): flow isn't metered by demand, as it is in the German systems.

 

[Dynasoar]

phx1138,

You are correct that a basic port fuel injection system based entirely on the Hilborn system would be set to run rich on a race car application. Early on in other posts I referred to mass flow sensing. I'd certainly expect throttle position data, automatic altitude compensation, pilot mixture adjustment and engine RPM all to be integrated into a simple controller (probably less complex than most then contemporary pressure carbs or a Hydramatic transmission control). Certainly well within 1939-40 technology.

Dynasoar

 

[phx1138]

You are correct that a basic port fuel injection system based entirely on the Hilborn system would be set to run rich on a race car application. Early on in other posts I referred to mass flow sensing. I'd certainly expect throttle position data, automatic altitude compensation, pilot mixture adjustment and engine RPM all to be integrated into a simple controller (probably less complex than most then contemporary pressure carbs or a Hydramatic transmission control). Certainly well within 1939-40 technology.

Somehow I missed that.... I agree, it's within the tech capabilities of the era.

 

[Draconis]

phx1138,

You are correct that a basic port fuel injection system based entirely on the Hilborn system would be set to run rich on a race car application. Early on in other posts I referred to mass flow sensing. I'd certainly expect throttle position data, automatic altitude compensation, pilot mixture adjustment and engine RPM all to be integrated into a simple controller (probably less complex than most then contemporary pressure carbs or a Hydramatic transmission control). Certainly well within 1939-40 technology.

Dynasoar

I do like the concept. But the one part I'm having trouble understanding is how in that small volume where the intake manifold meets the intake valves where the fuel nozzle would be located that an even and uniform density in the mixture can be achieved.

The interval between intake strokes at 3000 RPM is I think about 80 milliseconds. So the compressed air is constantly sprayed with fuel and every 80 milliseconds the intake valves open. What is the action or mechanism that ensures a uniform charge density at the intake valves just before they open? Does some of that mixing have to occur in the cylinder?

 

[phx1138]

I do like the concept. But the one part I'm having trouble understanding is how in that small volume where the intake manifold meets the intake valves where the fuel nozzle would be located that an even and uniform density in the mixture can be achieved.

The interval between intake strokes at 3000 RPM is I think about 80 milliseconds. So the compressed air is constantly sprayed with fuel and every 80 milliseconds the intake valves open. What is the action or mechanism that ensures a uniform charge density at the intake valves just before they open? Does some of that mixing have to occur in the cylinder?

Working on a very incomplete understanding of how constant flow FI works, I'd say you design your nozzle to spray as much fuel as you need (or can) in an 80ms window. Since the amount of fuel needed & the amount of air that will flow past the valve when it's open would be known (unless Allison has never heard of a flowbench...)... How does a carb do it? Don't forget, there's vacuum behind the valve: it's sucking air (and fuel), not just letting it pass.

That said, let me repeat something I mentioned on the V1710 thread: changing the valve seat angle (or base angle) from 30 deg to 22 would improve flow & hp at all throttle settings & boost levels. (How you get somebody to realize this, IDK; a "bungled" repair?)

 

[Dynasoar]

Draconis,

The interval between intake valve opening events at 3000 RPM in a four stroke engine cylinder is 1/25 sec (or 0.040 sec). If I had the cam timing diagram of the Allison available at the moment we could address the subject relative to the time the the intake valve is open or closed. Let's assume the valve spends one intake stroke plus a little overlap open, say 33% of the time to open, reach full lift then close. As I said earlier, fuel injection into a relatively hot intake port, including valve guide and stem times two can readily evaporate (under most operating conditions) all the fuel arriving there. So I would expect the intake ports to contain essentially saturated fuel vapor, diffusing back into the static air in the intake runners. If any late arriving fuel puddles in the valve cavity, imagine the process as the intake valve begins to open. At the delta P between the manifold and the intaking cylinder, early flow over the relatively sharp valve and seat edges approaches sonic velocity, so any liquid involved is at the very least well dispursed. with the valve fully open or closing, fuel is still being sprayed into the entering airstream, to absorb heat and evaporate. The volume of fuel required by the cylinder is rationed to it by the injector (hit or miss in a carb system)

The distribution of fuel within the now compressing cylinder is to some extent beyond my pay grade (first time saying that-), but I would not expect it to be worse than any other method of fuel introduction, including timed direct injection.

Dynasoar

 

[phx1138]

The distribution of fuel within the now compressing cylinder is to some extent beyond my pay grade (first time saying that-), but I would not expect it to be worse than any other method of fuel introduction, including timed direct injection.

That's getting into issues of head design: is it a wedge, hemi, or something else? Where are the valves? How big are the valves? Where's the spark plug(s)? All I can say with any confidence is, GM around 1988 introduced a new combustion chamber shape for its 2v 60deg V6 that improved combustion by introducing "swirl" in the chamber; does anyone think Allison (or 1940s engine designers) would even conceive of it? I really doubt it...

That said, Allison's head design, a 4v hemi, is about as good as it gets before you come to the "swirl" idea, so... And the idea of introducing even more valves (done in the '80s by Kawasaki, IIRC) would not occur to anyone, either, I don't think... (FYI, IIRC, Kwacker ended up with an oval-piston 7v {4in/3ex} revving around 20K.)

 

[marathag]

Somehow I missed that.... I agree, it's within the tech capabilities of the era.

I'm not aware of any OTL electrical feedback systems like that on something as small on an Aeroengine. I know of temperature based, but nothing beyond vane systems, too course and too slow for mixture

 

[phx1138]

I'm not aware of any OTL electrical feedback systems like that on something as small on an Aeroengine. I know of temperature based, but nothing beyond vane systems, too course and too slow for mixture

No, AFAIK the first hot wire system was in the '80s--but it wasn't impossible, had somebody thought of it, in the '40s.