Exhaust Thrust

Shortround6 · Apr 8, 2018

I would think a good many of the V-12s, but obviously an engine running at 18lbs boost (66-67in) of manifold pressure going into the cylinders is going to have higher pressure leaving the cylinders than an engine running at 40-44inches (5-7lb boost).

The only radials that I know of in WW II to use individual stacks were the R-2600s on some A-20s and B-25s. I could well have over looked some.
The later Zeros used paired stacks, 4 on one side and 3 on the other? WHich leads to difficulties on the top and bottom cylinders having much longer paths than the side cylinders. The FW 190 used side exhausts with most paired

but since 8 doesn't go into 14 evenly two cylinders had their own pipes? two cylinders shared two pipes?

F4Us used 6 exhaust pipes but the pre-4 had some rather long pipes.

and then

We can cover basics but when it comes to getting specific about any one installation/benefits it gets a lot harder as we are guessing at many things that they knew from measurements. By late in the war they were not designing exhaust systems by guessing or by what looked good or trying to fit it into space left over.

DarrenW · Apr 9, 2018

Shortround6 said:
I would think a good many of the V-12s, but obviously an engine running at 18lbs boost (66-67in) of manifold pressure going into the cylinders is going to have higher pressure leaving the cylinders than an engine running at 40-44inches (5-7lb boost).

The only radials that I know of in WW II to use individual stacks were the R-2600s on some A-20s and B-25s. I could well have over looked some.
The later Zeros used paired stacks, 4 on one side and 3 on the other? WHich leads to difficulties on the top and bottom cylinders having much longer paths than the side cylinders. The FW 190 used side exhausts with most paired
View attachment 489017
but since 8 doesn't go into 14 evenly two cylinders had their own pipes? two cylinders shared two pipes?

F4Us used 6 exhaust pipes but the pre-4 had some rather long pipes.

and then
View attachment 489018

We can cover basics but when it comes to getting specific about any one installation/benefits it gets a lot harder as we are guessing at many things that they knew from measurements. By late in the war they were not designing exhaust systems by guessing or by what looked good or trying to fit it into space left over.

I can appreciate the more difficult task of designing an efficient exhaust system for a radial engine and how by it's nature the V-12 intrinsically allowed for a much simpler and straightforward exhaust arrangement. Thanks for explaining the basics to me.

swampyankee · Apr 9, 2018

One other issue is that some radial engines had the exhaust valves on the front side of the cylinders.

I think it also can't be mentioned enough: using excess pressure from the exhaust system to provide thrust is much less efficient than using that pressure to increase the power passed to the propeller; the reason it was not done was because to do so required blow-down turbines, which added a level of complexity, weight, and volume to the engine installation. Turbocompounding is used in large engines today; it's just not used on engines where fuel economy is not a driving consideration.

Zipper730 · Apr 9, 2018

Swampyankee said:
Possibly because getting thrust from engine exhaust is only really possible with highly supercharged engines.

Meaning the pressure of the exhaust is higher, and as the air gets thinner, more thrust can be produced which helps out?

pbehn said:
It is a sort of all encompassing word used to describe the replacing of burnt fuel/air with fresh fuel air mixture.

Looking at the wikipedia entry, it seems to be the ability to properly breathe out all the air in the engine before breathing in new air for combustion.

there are other things to consider like the exhaust and inlet valves being open at the same time

That sounds like it wouldn't expand all the air out right...

or on a two stroke back pressure and harmonic shock waves stop all the charge shooting straight out of the exhaust increasing actual compression ratio and decreasing consumption.

I'd have figured stopping the charge shooting out would just make the engine perform badly, though I'm surprised you'd see shockwaves form in the engine.

Shortround6 said:
There are all sorts of tricks used on non supercharged engines to increase the gas flow.

Thrust?

For one thing an engine at 20,000 ft has a LOT less air pressure trying to keep exhaust gases inside the exhaust pipe/cylinder than an engine at sea level

Which produces an effect similar to a rocket...

another thing is that whatever "pulses" you get in an intake manifold between the inlet to the carb and the cylinder/piston

Does this have to do with the pistons moving?

It is also a LOT easier to set-up and work with those pulses when you used one carburetor throat per cylinder.

So basically the carburetor splits up to feed each cylinder?

Best design for exhaust thrust is a "pipe" as short as possible with one 90 degree bend and an outlet sized to let out the most gas as fast as possible consistent with keeping the exhaust gas velocity high.

The 90-degree bend is to avoid an excessively long duct?

When P-40s were allowed to use WEP power settings they had to hacksaw off part of the exhaust pipe nozzle in order to accommodate the increased mass flow. Too small a nozzle prevented all the exhaust gas from exiting the cylinder.[/QUOTE

Benjdragon said:
The one aircraft that I know was designed to use exhaust thrust was the XP-67.

That's not exactly right: It was a turbocharged engine that was supposed to get more thrust than usual turbochargers.

As I understand it, turbochargers get little thrust out of their exhaust because of the turbine and the fact that in many cases the exhaust tends to go downward not straight back. This system had the exhaust go right out piston, through the turbo, and right out the back.

Shortround6 · Apr 9, 2018

Well, there is efficiency and there is efficiency.
For a fighter (or small high performance plane) the exhaust manifolds/stacks on a V-12 engine can weigh 30-40lbs and provide, under optimum conditions around 10-11% more power over and above what is going to the prop.
less than optimum conditions means a lot less.
However the blown down turbine and drive set-up are going to add several hundred pounds and several cubic feet of space (per engine).

Granted this engine has the blowdown turbine grafted onto the auxiliary stage supercharger which adds some bulk.
It also doesn't have any exhaust duct for the gases after they leave the turbine.

At what point does the extra power/fuel economy balance out over the engine without the blowdown turbine?
As in just add 20-30 gallons to the fuel capacity of the single engine fighter and get another 30 minutes at cruising speed?
The Wright turbo compound

Please notice that the supercharger and accessories section has been moved back away from the power section (cylinders/crankcase) to allow room for the turbines and drive system. And system it is as there has to be a variable speed drive from the turbines to the crankshaft to match the relative speeds at different power settings/altitudes.

pbehn · Apr 9, 2018

Zipper730 said:
That sounds like it wouldn't expand all the air out right...
I'd have figured stopping the charge shooting out would just make the engine perform badly, though I'm surprised you'd see shockwaves form in the engine.
.

In a supercharged engine I would think it is more likely to blow unburned fuel straight out of the exhaust. In order to have a longer duration with the valves fully open the inlet starts opening before the exhaust is closed.

I was talking about a two stroke, if ever you have run one without the exhaust on it is a very painful and loud experience, here is a schematic.
www.instructables.com/id/Tuning-Two-Stroke-Engines/

Zipper730 · Apr 9, 2018

Shortround6 said:
Well, there is efficiency and there is efficiency.
For a fighter (or small high performance plane) the exhaust manifolds/stacks on a V-12 engine can weigh 30-40lbs and provide, under optimum conditions around 10-11% more power over and above what is going to the prop.

So, if the prop is producing 3000 pounds of thrust, the exhaust produces 300-330?

pbehn said:
In a supercharged engine I would think it is more likely to blow unburned fuel straight out of the exhaust.

You mean the inlet valve opening before the outlet valve?

In order to have a longer duration with the valves fully open the inlet starts opening before the exhaust is closed.

Do you mean more time for everything to burn right?

I was talking about a two stroke, if ever you have run one without the exhaust on it is a very painful and loud experience, here is a schematic.
www.instructables.com/id/Tuning-Two-Stroke-Engines/

So the back pressure is used to deliberately hold the exploding fuel in the combustion chamber to make it burn all the way?

Zipper730 · Apr 9, 2018

pbehn said:
I was thinking that the early exhausts on Merlins were to do with having twin blade fixed pitch props and later variable coarse/fine pitch props.

So the Spitfire went from fixed pitch, to twin-pitch, to variable pitch?

Shortround6 said:
Just going from the tests at Spitfire performance it appears that the fixed pitch prop planes were flown off the ground and climbed at very much reduced RPM until aircraft speed built up to keep from stalling the props.

Why would spinning the blades slower reduce the odds of stalling the props? I figure spinning them faster would move more air over them and keep them lifting...

I just thought the climb performance on the Spitfire was poor early on because the blade was geared for higher true airspeeds.

swampyankee said:
Most constant speed propellers have a significant portion of their blades stalled during the takeoff run

I never knew that

With a fixed pitch propeller, the problem is that engine power is (roughly) linear with RPM, but the propeller power load is roughly proportional to the cube of the RPM

Power load? Do you mean wing-loading?

it's not that the propeller will stall, it's that the engine can't turn the prop any faster.

I'm wondering if you're talking about a dynamic similar to a feathering prop, whereby air on either side of the prop tend to hold it in place? The high pitch prop geared for high speed would be unable to spin fast because of this?

Zipper730 · Apr 9, 2018

Shortround6 said:
For some reason the Air Ministry thought variable pitch or constant speed propellers were a passing fad
They had tested variable pitch props back 20s and found them wanting

Which was because earlier planes had more drag and a narrower range of speeds to operate across

pbehn said:
I read here years ago that civilian airliners used variable pitch props, but they were "commercial chaps" only interested on making money, the RAF was not a commercial enterprise so why would they need new and expensive propellers?

Well efficiency has use for military enterprises too, more thrust per horsepower means they can climb, accelerate, and hold power in turns...

With a fixed prop. the Spitfire satisfied the specification so why spend more money, (even more money in civil service speak) on a toy for a war that probably wont happen.

And since they were older people who lived in an area prior to powered aircraft...

It's interesting how the U.K. and U.S. both had ways they were slow to adopt newer ideas

The U.K. with variable pitch props
The U.S. with jet-engines

Companies saying "you must buy this, it is a new wonder" were commonplace.

The issue is to figure out how to identify which is nonsense and ignore it, then identify things that might work and test them out to see if they do.

pbehn · Apr 9, 2018

Zipper730 said:
1 Do you mean more time for everything to burn right?
2 So the back pressure is used to deliberately hold the exploding fuel in the combustion chamber to make it burn all the way?

1 No, the valves are working at their best when opened fully, but they are opened by a cam. To get to the fully opened position and back to fully closed takes time so they sometimes overlap, the exhaust is still closing when the inlet starts opening, but behind the inlet valve the air is pressurised by the supercharger.

2 No, this is a two stroke engine, the power stroke provides power until just after the piston moves down and uncovers the exhaust port, slightly further down the stroke the inlet ports are opened so the cylinder is emptying the spent charge and filling with the new charge at the same time. These shock waves are not the same as back pressure, they stop the charge shooting straight from the inlet and out of the exhaust as you can see on the schematic.

This is harmonics, a highly tuned two stroke runs and sounds like a bag of hammers at low revs, when it gets into the power band, torque and power rise sharply, then just as quickly it runs out of the power band, there is no point at all in over revving a two stroke, nothing happens because the engine cant breathe at all.

I once saw Barry Sheene practicing at Brands Hatch and he was the only one with an engine running, you could hear the bike go into the power band as he left the pits, like a musician hitting the right notes after hitting a lot of wrong ones.

Shortround6 · Apr 9, 2018

Zipper730 said:
So, if the prop is producing 3000 pounds of thrust, the exhaust produces 300-330?

For the example I have, the Merlin XX in a Hurricane, the engine had 1126hp going to the prop at 20,000ft, the exhaust HP (not thrust) was calculated at 126.8. This is with an aircraft speed of 340mph and using 50.67in of manifold pressure.
At the same altitude but using 48.24in of manifold pressure aircraft speed dropped to 335mph, prop hp was 1073. Exhaust HP dropped to 113.0

Three reasons.
Mass flow (air plus gasoline) had dropped from 151lb/min to 144.0lb/min
exhaust gas velocity had dropped from 1788fpm to 1695fpm.
The aircraft was moving slightly slower.

At 15,000ft the Exhaust HP was only 86.5 compared to the prop hp of 1048. Mass flow was 140.5lb/min but exhaust velocity had dropped to 1395fpm and the plane was doing 325mph.

Explanations of car/motorcycle intake exhaust for none supercharged engines will have to wait.
or google like
Intake Tuning | Intake Runner Lengths | Engine Tuning

Please note that such tricks are useless on supercharged engines and also useless at the rpms that aircraft engines operate at.

Zipper730 · Apr 9, 2018

Shortround6 said:
For the example I have, the Merlin XX in a Hurricane, the engine had 1126hp going to the prop at 20,000ft, the exhaust HP (not thrust) was calculated at 126.8. This is with an aircraft speed of 340mph and using 50.67in of manifold pressure.

Exhaust horsepower is 11.26% of the engine horsepower at 50.67"

At the same altitude but using 48.24in of manifold pressure aircraft speed dropped to 335mph, prop hp was 1073. Exhaust HP dropped to 113.0

10.53% of engine horsepower, a reduction of 0.73% of engine horsepower with a manifold pressure decrease of 4.796%

Three reasons.
Mass flow (air plus gasoline) had dropped from 151lb/min to 144.0lb/min
exhaust gas velocity had dropped from 1788fpm to 1695fpm.

Sucking in and blowing out less

The aircraft was moving slightly slower.

Ram compression effects?

At 15,000ft the Exhaust HP was only 86.5 compared to the prop hp of 1048. Mass flow was 140.5lb/min but exhaust velocity had dropped to 1395fpm and the plane was doing 325mph.

8.25% of engine horsepower.

wuzak · Apr 9, 2018

Zipper730 said:
Meaning the pressure of the exhaust is higher, and as the air gets thinner, more thrust can be produced which helps out?

It is mass flow rate which gives the thrust.

I read an article some time ago explaining that in some cases the Merlins used by Reno racers had pushed the boost pressure up so high that they were no longer getting any extra power at the prop (more power in the main section was cancelled by the extra power required to drive the supercharger). The reason that they would do such a thing is that the extra mass flow creates greater thrust effect. And though they are at relatively low altitude, they are flying at 500mph or above.

wuzak · Apr 9, 2018

Interestingly, airframe manufacturers in the US were often responsible for the exhaust systems.

Allison, for example, supplied the engine with flanges, to which the exhaust pipes or stubs would be welded.

In the UK a lot of the exhaust was done by the manufacturer, such as Rolls-Royce. That is why the Hurricane, Spitfire and some other Merlin powered aircraft had the same style ejector stubs early on.

Shortround6 · Apr 9, 2018

Zipper730 said:
Exhaust horsepower is 11.26% of the engine horsepower at 50.67"
10.53% of engine horsepower, a reduction of 0.73% of engine horsepower with a manifold pressure decrease of 4.796%
Sucking in and blowing out less
Ram compression effects?
8.25% of engine horsepower.

The formula for figuring thrust to horsepower is force X speed of the aircraft divided by 375. Just make sure you get all the measurements sorted out as to seconds, minutes etc.
But obviously the HP changes with speed. At 375mph the force equals HP. At 340mph the same force is multiplied by 0.9066.
At 335mph it is 0.8933.
At 200mph (say climbing) it is 0.5333 of the force.

We know it exists, but trying to figure out the actual benefit for individual aircraft and different flight conditions needs a lot of information.

Zipper730 · Apr 10, 2018

Shortround6 said:
The formula for figuring thrust to horsepower is force X speed of the aircraft divided by 375.

I assume this doesn't factor in ram compression?

Just make sure you get all the measurements sorted out as to seconds, minutes etc.

You mean airflow requirements?

But obviously the HP changes with speed.

So this comes out to

150 mph: 1 lb/thrust = 0.4 HP
200 mph: 1 lb/thrust = 0.5333 HP
250 mph: 1 lb/thrust = 0.6667 HP
300 mph: 1 lb/thrust = 0.8 HP
325 mph: 1 lb/thrust = 0.8333 HP
350 mph: 1 lb/thrust = 0.9333 HP
375 mph: 1 lb/thrust = 1 HP
400 mph: 1 lb/thrust = 1.0667 HP
425 mph: 1 lb/thrust = 1.1333 HP
450 mph: 1 lb/thrust = 1.2 HP
475 mph: 1 lb/thrust = 1.2667 HP
500 mph: 1 lb/thrust = 1.3333 HP
525 mph: 1 lb/thrust = 1.4 HP
550 mph: 1 lb/thrust = 1.4667 HP
575 mph: 1 lb/thrust = 1.5333 HP
600 mph: 1 lb/thrust = 1.6 HP

When was his realized?

wuzak · Apr 10, 2018

Zipper730 said:
When was his realized?

When Newton published his laws of motion?

swampyankee · Apr 12, 2018

Zipper730 said:
I assume this doesn't factor in ram compression?
You mean airflow requirements?

It does; ram compression is buried in the thrust calculation: thrust is mass flow rate times change in velocity plus pressure times area at the nozzle exit; the latter is almost certainly negligible. Exhaust mass flow is going to be intake air flow plus fuel flow minus any system leakages, which one would assume to be negligible.

Zipper730 said:
So this comes out to

150 mph: 1 lb/thrust = 0.4 HP

200 mph: 1 lb/thrust = 0.5333 HP

250 mph: 1 lb/thrust = 0.6667 HP

300 mph: 1 lb/thrust = 0.8 HP

325 mph: 1 lb/thrust = 0.8333 HP

350 mph: 1 lb/thrust = 0.9333 HP

375 mph: 1 lb/thrust = 1 HP

400 mph: 1 lb/thrust = 1.0667 HP

425 mph: 1 lb/thrust = 1.1333 HP

450 mph: 1 lb/thrust = 1.2 HP

475 mph: 1 lb/thrust = 1.2667 HP

500 mph: 1 lb/thrust = 1.3333 HP

525 mph: 1 lb/thrust = 1.4 HP

550 mph: 1 lb/thrust = 1.4667 HP

575 mph: 1 lb/thrust = 1.5333 HP

600 mph: 1 lb/thrust = 1.6 HP

When was his realized?

No later than the mid-19th Century, when people started designing locomotives for pulling stuff and including specifications for tractive force at a given speed.

Zipper730 · Apr 13, 2018

wuzak said:
When Newton published his laws of motion?

Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).

I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that

Most early engines didn't produce all that much horsepower, so the amount of thrust to be gained by a creative exhaust pipe set-up wouldn't have been worth it
Most early engines didn't have particularly high manifold pressures, so it would have interfered with scavenging
Most early engines were naturally aspirated or used minimal supercharging, and were not capable of propelling an aircraft high enough to make use of it
Most early aircraft were not capable of flying fast enough to make use of such a thing

swampyankee said:
It does

Okay

thrust is mass flow rate times change in velocity plus pressure times area at the nozzle exit

I'm confused about something. Some things increase to the square of velocity, others are proportional.

I keep mixing things up

swampyankee · Apr 13, 2018

Zipper730 said:
Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).

I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that

Most early engines didn't produce all that much horsepower, so the amount of thrust to be gained by a creative exhaust pipe set-up wouldn't have been worth it

Most early engines didn't have particularly high manifold pressures, so it would have interfered with scavenging

Most early engines were naturally aspirated or used minimal supercharging, and were not capable of propelling an aircraft high enough to make use of it

Most early aircraft were not capable of flying fast enough to make use of such a thing

Okay
I'm confused about something. Some things increase to the square of velocity, others are proportional.

I keep mixing things up

One way to sort out whether velocity is squared or not is to sort out the units.

Mass flow is mass per unit time, velocity is distance per unit time; multiplying these gives (mass times distance) divided by (time squared), which is the mix of units for force. This is easier to explain in SI, partly because the US Customary system uses the same term, pound, for mass and force.

Work and energy are force times distance (torque is a vector, and torque and energy are not interchangeable!); both have the units of joules, or (mass times [distance squared]) divided by (time squared). It's measured in joules. Power is energy per unit time, so its units are joules per second, or watts. Dimensional analysis is a useful technical skill.

Exhaust Thrust

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