Do Injectors "Inject"?

[jhausch]

Does the mechanical fuel injection system in AC "inject" or just provide a point near the intake valve where a more precisely metered portion of fuel can be "sucked into" the cylinder?

If that is the case (the "sucked in" point), does the shearing action of the air by the nozzle cause vaporization?

Just curious.

 

[gismo]

Does the mechanical fuel injection system in AC "inject" or just provide a point near the intake valve where a more precisely metered portion of fuel can be "sucked into" the cylinder?

If that is the case (the "sucked in" point), does the shearing action of the air by the nozzle cause vaporization?

Just curious.

The purpose of what's called the "injector is to atomize the fuel and control the volume of the fuel precisely for a given pressure. As part of the atomization process, air from outside the injector is added to the fuel as it passes through. AFaIK the atomization is less complete at high power / high fuel flow rates.

The fuel from the injector passes into a small chamber outside the cylinder (but inside the cylinder head) adjacent to and "behind" the intake valve. The fuel is injected continuously, building up a nearly full charge prior to the intake valve opening. One problem with this setup, especially on Continental engines with a common intake runner for each side of the engine is that some of the fuel delivered to the rear cylinders makes it's way to the more forward ones which causes the front ones to have a richer mixture than the rears.

 

[Tom-D]

Does the mechanical fuel injection system in AC "inject" or just provide a point near the intake valve where a more precisely metered portion of fuel can be "sucked into" the cylinder?

If that is the case (the "sucked in" point), does the shearing action of the air by the nozzle cause vaporization?

Just curious.

It is known as "Indirect fuel injection" it does not inject fuel directly into the cylinder as the diesel does. Throttle body fuel injection is also a " indirect" type of fuel injection.

Fuel is not sucked into the airstream, it is under pressure and is squirted into the intake pipe as directed by the injection pump. Then the whole air/fuel mass is sucked into the cylinder on the intake stroke.

 

[Henning]

Does the mechanical fuel injection system in AC "inject" or just provide a point near the intake valve where a more precisely metered portion of fuel can be "sucked into" the cylinder?

If that is the case (the "sucked in" point), does the shearing action of the air by the nozzle cause vaporization?

Just curious.

To answer simply, yes, it is just a metered continuous spray pointed into the intake runner.

 

[jhausch]

Thanks, all.

I figured it was not "sequential port" injection and the fuel metering was likely continuous. I was just unsure of how vigorous the injection was.

I also knew it was not direct into the cylinder.

What is typical PSI between the metering (distribution?) block and the individual injector nozzle?

 

[Henning]

Thanks, all.

I figured it was not "sequential port" injection and the fuel metering was likely continuous. I was just unsure of how vigorous the injection was.

I also knew it was not direct into the cylinder.

What is typical PSI between the metering (distribution?) block and the individual injector nozzle?

It varies, that is how it controls the flow. That's why the factory "Fuel Flow" is actually a pressure gauge calibrated to represent fuel flow. The nozzle is the last resistor, it relies on that for the spray pattern.

 

[jhausch]

I'd guess the drop in pressure in the "active" runner likely helps create a pressure delta greater than the pressure supplied by the pump...

 

[Skylane81E]

Many folks call what we have on airplanes "Continuos (sp) flow" fuel injection. All the injectors are is a calibrated hole.

 

[Dan Thomas]

Thanks, all.

I figured it was not "sequential port" injection and the fuel metering was likely continuous. I was just unsure of how vigorous the injection was.

I also knew it was not direct into the cylinder.

What is typical PSI between the metering (distribution?) block and the individual injector nozzle?

Continental's is around 30 PSI at full throttle, IIRC. Not much.

Dan

 

[Ted]

I'd guess the drop in pressure in the "active" runner likely helps create a pressure delta greater than the pressure supplied by the pump...

That is correct. Some turbocharged installations have a boost reference to the fuel pump so as to provide more fuel pressure with more boost.

The other answers were all correct - injectors are just calibrates holes that provide a continuous flow of fuel. Mechanical fuel injection systems are not like the injectors that you have in your car. However, they work quite well for aircraft operation I find. Plus the failure modes are typically nicer.

If I was building an experimental and could get the safety to my liking, I'd use electronic fuel injection like on a car. Probably with an automotive engine. However for traditional aircraft engines, I like the mechanical setups.

 

[jhausch]

Continental's is around 30 PSI at full throttle, IIRC. Not much.

Dan

30 PSI + maybe the negative pressure in the runner of, say, -10psi....I'd think 30-40psi is pretty good pressure for atomization. Better than I thought.

 

[ebacon]

When does the injector inject?

Old car engines with port injection used closed valve injection scheduling. The fuel injector was aimed at the top of the intake valve. When the engine was hot the fuel shot hit the top of the closed intake valve, boiled, and then got sucked into the combustion chamber when the valve opened.

 

[Capt. Geoffrey Thorpe]

When does the injector inject?

Old car engines with port injection used closed valve injection scheduling. The fuel injector was aimed at the top of the intake valve. When the engine was hot the fuel shot hit the top of the closed intake valve, boiled, and then got sucked into the combustion chamber when the valve opened.

Aircraft injectors (and the old mechanical stuff on cars) blow fuel all the time.

Cars with port injection ranged from just trigger all the cylinders at one time to synchronize injections with valve events. With synchronized injection, closed valve injection is more common (open valve injection always seems attractive, but has some issues with bore washing).

Typically, some of the fuel evaporates, but a large part of the fuel ends up on the port walls and/or the back of the intake valve - the air flow during the intake stroke shears liquid the fuel off the edge of the valve / port.

The mass of fuel in the puddle on the walls changes with manifold pressure / engine speed - when you open the throttle the manifold pressure drops and more of the injected fuel tends to get stuck in the puddle and not make it into the cylinder - that's why engines can stumble when you open the throttle (and why there is ususally a way to add extra fuel on throttle transients). That's one of the things that makes my job entertaining...

 

[Skylane81E]

When does the injector inject?

Always

 

[ebacon]

. . . . That's one of the things that makes my job entertaining...

I also did an engine control software/hardware gig in the D. 1992-2000 hoes.

 

[dmac5356]

The metered nozzle pressure of a TCM engine at full power and full rich ranges from 12.5 psi to 22.5 psi (per TCM specs), depending on the model of the engine and which end of the allowable range is used. This pressure is the differential between the fuel pressure applied to the fuel distributor, and ultimately the injectors, compared to ambient air pressure (ambient air is supplied through the injector air screens for atomization).

In the case of turbocharged engines the pressure differential is relative to the upper deck pressure, which is supplied to the injectors through tubes for atomization.

TCM SID 97-3E describes this in some detail. Some people run the full power full rich metered pressure just a little on the high side for better cooling, but I'm not aware of any TCM engines that are at 30 psi.

 

[Henning]

The metered nozzle pressure of a TCM engine at full power and full rich ranges from 12.5 psi to 22.5 psi (per TCM specs), depending on the model of the engine and which end of the allowable range is used. This pressure is the differential between the fuel pressure applied to the fuel distributor, and ultimately the injectors, compared to ambient air pressure (ambient air is supplied through the injector air screens for atomization).

In the case of turbocharged engines the pressure differential is relative to the upper deck pressure, which is supplied to the injectors through tubes for atomization.

TCM SID 97-3E describes this in some detail. Some people run the full power full rich metered pressure just a little on the high side for better cooling, but I'm not aware of any TCM engines that are at 30 psi.

Only fools do that, especially at these fuel prices. Now you waste money two ways. The excuse used to always be "fuel is cheaper than cylinders", (The whole leaning argument has been around a long time, somehow institutional knowledge of WWII was lost) well, now it's not. Besides, LOP is what saves your cylinders and valves. Rich is what causes all the coking and leading on the stem/guide and seat/faces. You can look at the ends of my augmentors when I get done with a flight, just a light dust of light grey powder and she's happy. For the record, I lean the old fashioned way, I pull it back until the power falls off, usually around 9.8gph with throttles full 22" turning 2400. 10.2 she starts pulling and 10.5 (this is reason 32 why real fuel flow and totalizer are good) she's come up on the pipe and that's where I run her fat dumb and happy. Just so happens I have an engine analyzer that allows me to confirm, yes, I'm about 20*LOP. That leaves me doing 176KTAS at 7500 and 180KTAS at 10,500.

 

[Ted]

Only fools do that, especially at these fuel prices. Now you waste money two ways. The excuse used to always be "fuel is cheaper than cylinders", (The whole leaning argument has been around a long time, somehow institutional knowledge of WWII was lost) well, now it's not. Besides, LOP is what saves your cylinders and valves. Rich is what causes all the coking and leading on the stem/guide and seat/faces.

No, not only fools do that. The Continentals tend to be a bit leaner at full power full rich, and people do that to be able to not get their cylinders too hot on takeoff and climb. The change in fuel consumption for the trip is imperceptible, but not getting your cylinders too hot is a very good thing.

You can always lean out a mixture as much as you want.

 

[Henning]

No, not only fools do that. The Continentals tend to be a bit leaner at full power full rich, and people do that to be able to not get their cylinders too hot on takeoff and climb. The change in fuel consumption for the trip is imperceptible, but not getting your cylinders too hot is a very good thing.

You can always lean out a mixture as much as you want.

In my experience observing these peoples operations, it's that they want to keep the cylinders below some arbitrarily cold number. CHT is fine at redline in a climb if you want to climb ROP, 320 is not required. If it is sitting at or just under redline CHT, that is just fine until you get to altitude. It actually is made to run there. Thing is, if you're LOP, the CHT stays down because you aren't producing as much waste heat so your CHT never gets up to redline. Normally in a LOP climb at 75-80% power my CHT will be 350 or so in an extended Vy climb, that's way cool. There is no need to "Cool with Fuel", it's a foolish and detrimental way to operate.

 

[Ted]

In my experience observing these peoples operations, it's that they want to keep the cylinders below some arbitrarily cold number. CHT is fine at redline in a climb if you want to climb ROP, 320 is not required. If it is sitting at or just under redline CHT, that is just fine until you get to altitude. It actually is made to run there. Thing is, if you're LOP, the CHT stays down because you aren't producing as much waste heat so your CHT never gets up to redline. Normally in a LOP climb at 75-80% power my CHT will be 350 or so in an extended Vy climb, that's way cool. There is no need to "Cool with Fuel", it's a foolish and detrimental way to operate.

The experience you have with your 310 does not apply to all aircraft (it doesn't even apply to the 310 I fly). In my Aztec, if you tried that, you would run the cylinders very hot, same for the 310. Can they take it? Sure. But then people wonder why their cylinders have low compressions and are burning oil. Meanwhile, the people who keep their heads cool don't have those issues. Hmm... wonder why. Meanwhile, the 310 has two engines that are 250 over TBO, and I haven't had to change cylinders on it (yet). The Aztec only needed a top overhaul when I was running the heads really hot prior to having an engine monitor and knowing just how I was running it.

The engines are not intended to run at redline temperatures for extended periods. They are tested to demonstrate that they can, but limits are not goals, and to expect that they will just happily run along at limits forever is a foolish and detrimental way to operate that will cost you money. I agree that 320-350 is much cooler than you need to keep your heads, but I shoot for 380-400. Even in the 310 that I fly, the only way to do that in takeoff and climb is keeping it rich, if you lean it out you'll redline your CHTs in no time. Putting that fuel in is neither detrimental to the engine nor my wallet. Again, the time you spend in takeoff and climb on a naturally aspirated plane isn't significant enough to have a big impact on a long trip.

 

[Henning]

The experience you have with your 310 does not apply to all aircraft (it doesn't even apply to the 310 I fly). In my Aztec, if you tried that, you would run the cylinders very hot, same for the 310. Can they take it? Sure. But then people wonder why their cylinders have low compressions and are burning oil. Meanwhile, the people who keep their heads cool don't have those issues. Hmm... wonder why. Meanwhile, the 310 has two engines that are 250 over TBO, and I haven't had to change cylinders on it (yet). The Aztec only needed a top overhaul when I was running the heads really hot prior to having an engine monitor and knowing just how I was running it.

The engines are not intended to run at redline temperatures for extended periods. They are tested to demonstrate that they can, but limits are not goals, and to expect that they will just happily run along at limits forever is a foolish and detrimental way to operate that will cost you money. I agree that 320-350 is much cooler than you need to keep your heads, but I shoot for 380-400. Even in the 310 that I fly, the only way to do that in takeoff and climb is keeping it rich, if you lean it out you'll redline your CHTs in no time. Putting that fuel in is neither detrimental to the engine nor my wallet. Again, the time you spend in takeoff and climb on a naturally aspirated plane isn't significant enough to have a big impact on a long trip.

You miss my point, you can, and I do, keep my cylinders and heads cool all the time by restricting fuel flow rather than augmenting it. You can do it with the Aztec, you can do it with the 310 you fly. By doing so I prevent valve contamination and cylinder glazing as well as saving fuel. I also produce less sulfuric acid to erode my freshly overhauled augmenters. I also did not say to run redline CHT continuous, I said it would be ok in the climb. In the long run it will be less detrimental than pouring excess fuel through the motor which is what leads to stuck and burnt valves and worn guides. Running cold is worse on an engine that running at peak. Using excess fuel to cool an engine is foolish. If you want to restrict heat restrict fuel. You don't need every last horsepower all the way up.

 

[Ted]

You clearly miss my point, Henning. Your 310 is not how all aircraft work. And furthermore, since you typically fly very lightweight, your horsepower requirements are different than the rest of us who fly heavier.

I run LOP in cruise. There are planes I would not fly that way. And the planes that I fly LOP in cruise in I fly ROP in climb. You operate your plane the way you want, and if I fly it I'll operate it the way you tell me to. The ones I fly and operate the way that I choose to are very happy that way, and I have the maintenance bills to prove it.

 

[kgruber]

You clearly miss my point, Henning. Your 310 is not how all aircraft work. And furthermore, since you typically fly very lightweight, your horsepower requirements are different than the rest of us who fly heavier.

.

Henning is wonderful, however I fail to see how his 310 operates mechanically different than any other airplane.

Also, his requirement for any % power is identical for any weight. %power is independant of weight.

There is a class, advanced pilot techniques (or something like that) from which you would benefit.
http://www.advancedpilot.com/

 

[Skylane81E]

Also, his requirement for any % power is identical for any weight. %power is independant of weight.

True, but more weight means longer, slower climbs so when introducing temperatures to the equation weight is very much relevent.

Plus if you are heavy how much climb performance will you see at the % powers noted?

The engines are tuned to run richer WOT than at cruise because they need to be. However I wouldn't set it any richer (or leaner) than the TCM spec, the power enrichment is already there.

 

[Dan Thomas]

TCM SID 97-3E describes this in some detail. Some people run the full power full rich metered pressure just a little on the high side for better cooling, but I'm not aware of any TCM engines that are at 30 psi.

You're right.


Dan

 

[Ted]

Henning is wonderful, however I fail to see how his 310 operates mechanically different than any other airplane.

Every airplane has different cooling characteristics, even two different types with the same engine. Your ability to run particular combinations of manifold pressure, RPM, and mixture setting in one aircraft without running high temperatures will vary depending on these points. As an example, the Turbo Trinidad has effectively the same engine has my Aztec does, yet the Aztec's head temperatures are much hotter in all phases of flight. As such, I have to be more careful with my mixture settings to keep the engine happy than a Trinidad pilot does.

Then there's the other aspect of how lean you can run each engine practically. This is primarily a function how well balanced the fuel system is. The better balanced it is, the leaner you can run it, and then that will allow you to keep the head temperatures in check. So if you want to run a lean climb, then can you run it lean enough to keep the head temperatures in check? Of course, this can be a losing battle because the more you lean out the engine, the less power you have, and thus the worse climb performance you have.

The 310 I fly is an N model with a Colemill conversion having IO-520-E engines, whereas Henning's is earlier and has IO-470s in it. The Colemill conversion did not have any additional cooling modifications that I am aware of or have observed. So it should be expected that the one I fly will be prone to having warmer cylinders if it makes more power with equal cooling.

While general techniques do apply to all aircraft, the specifics of how you apply them to each aircraft are not always identical. For an example, even George Braly has admitted that flying a Duke lean of peak is possible, but "there are issues."

Also, his requirement for any % power is identical for any weight. %power is independant of weight.

You're missing the statements. Henning said you don't need every ounce of power for climb. When you have a lightweight aircraft with lots of power, that is true, especially on a cold day.

Now, take a heavy plane that is slightly underpowered on a hot day. The Aztec with a heavy load at a ROP climb on a hot day will still sometimes be lucky to see 500-700 FPM. Now you want to talk about doing a LOP climb with less power? How long do you really want to spend climbing? That's a choice that you can make. I personally do not want to do that, especially since I want to get up to the colder air faster.