Exhaust thrust

[SchmidtMotorWorks]

I talked to a few more guys today about it, there seems to be agreement that there is a lot of force but not on how it happens.
It seems that either the gas in the pipe has to be super sonic or the pressure is developed from fuel burning outside the pipe.
This might be a good question for some of my friends that work at Spacex, I'll ask on Monday.

My best guess is that the force is not form continuous flow like an acceleration computation can answer.
Instead a very high pressure wave expands as it exists the pipe and releases what would feel like an explosion each cycle.

In 2005 the cylinder pressure was about 6,000 psi.

[BrazilianZ28Camaro]

I talked to a few more guys today about it, there seems to be agreement that there is a lot of force but not on how it happens.
It seems that either the gas in the pipe has to be super sonic or the pressure is developed from fuel burning outside the pipe.
This might be a good question for some of my friends that work at Spacex, I'll ask on Monday.

My best guess is that the force is not form continuous flow like an acceleration computation can answer.
Instead a very high pressure wave expands as it exists the pipe and releases what would feel like an explosion each cycle.

In 2005 the cylinder pressure was about 6,000 psi.

AFAIK, Nitro engines dump raw fuel into the zoomies, so the exaust force may be actually gasses in expansion due combustion combined with the pumping work made by the piston?

"Lets increase the exaust duration to improve donwforce and traction"

Imagine that!!!

[SchmidtMotorWorks]

I talked to a few more guys today about it, there seems to be agreement that there is a lot of force but not on how it happens.
It seems that either the gas in the pipe has to be super sonic or the pressure is developed from fuel burning outside the pipe.
This might be a good question for some of my friends that work at Spacex, I'll ask on Monday.

My best guess is that the force is not form continuous flow like an acceleration computation can answer.
Instead a very high pressure wave expands as it exists the pipe and releases what would feel like an explosion each cycle.

In 2005 the cylinder pressure was about 6,000 psi.

AFAIK, Nitro engines dump raw fuel into the zoomies, so the exaust force may be actually gasses in expansion due combustion combined with the pumping work made by the piston?

"Lets increase the exaust duration to improve donwforce and traction"

Imagine that!!!

All engines do to some extent but nitro has an additional factor that it is self oxidizing so it might be building waves of pressure as it travels down the pipe.
That has to happen and keep the pressure at the exhaust valve well below manifold pressure during overlap.

In regards to increasing exhaust duration to get more down force, I think the opposite might be true. If the down-force comes from the expansion of the waves as they exit the pipe then a longer duration would have lower amplitude waves and less force from expansion.

[BrazilianZ28Camaro]

That has to happen and keep the pressure at the exhaust valve well below manifold pressure during overlap.

You're saying a TF engine will excavenge the cylinders with the gobs of exaust gasses @ EVO at unreal pressures with that short of pipe?

In regards to increasing exhaust duration to get more down force, I think the opposite might be true. If the down-force comes from the expansion of the waves as they exit the pipe then a longer duration would have lower amplitude waves and less force from expansion.

Might be , but if you have a earlier EVO more cylinder pressure is being bleed off through the pipe pushing the car down.

Damn! my head hurts....

[SchmidtMotorWorks]

You're saying a TF engine will excavenge the cylinders with the gobs of exaust gasses @ EVO at unreal pressures with that short of pipe?

Every nitro crew chief that I have asked about changing the tuned length of the exhaust pipe to fit the change in RPM over the years has told me that they don't think it matters much.
They have so few opportunities to test things they are mostly focused on keeping a predictable tune-up. You can bring them the greatest idea in the world to increase power but you would be surprised how difficult it is to get it tested.
Let's say you figure out a sure way to get 100hp, you might think that would be a great thing right? Not really if your clutch, tires, crankshaft, rods are all on the edge. That 100 hp would be chaos.

Might be , but if you have a earlier EVO more cylinder pressure is being bleed off through the pipe pushing the car down.

EVO on a nitro engine isn't as easily adjusted as it is with a gasoline engine, they already run solid push-rods to get the valve open under pressure.

[140Air]

Exhaust gas thrust depends on exhaust gas momentum which is the mass of exhaust gasses times the first order velocity (not v^2). This velocity is maximized for jet and rocket propulsion by maximizing the back-pressure in front of the nozzle. The amount of exhaust in a piston engine is very small compared to a jet, so this is not a good way to move the car (or to provide downforce). The car is best moved by shaft horsepower which itself can be maximized by good breathing. Downforce is provided by aerodynamic effects from the forward motion, provided by shaft hp. Back-pressure is bad for breathing (even a turbo installation is best with lowest back pressure after the turbo). The exhaust velocity for NA engines is invariably chosen to help scavenge the cylinders and draw intake flow. Whatever velocity it ends up with does provide some thrust, but not much. You would lose out if you tried to increase back pressure to make more exhaust thrust at the cost of breathing and hp.

F1 cars' use of exhaust velocity is to make a fast flow stream over the top of the diffuser (or under the wing), augmenting diffuser effectiveness (this is a v^2 effect). They probably(?) do restrict the exhaust very slightly to increase the velocity, but how much is their secret compromise.

140Air

[digger]

work out what the ave mass flow rate through the engine is and work out the velocity of gases from the pipe exit multiply them together and there you get an indication of the change in momentum of the fluid (i.e Force)

note it is mass flow rate and velocity

if the engine is stationary and the inlet to the engine is large enough you can neglect the velocity into the engine and the exit velocity is all that matters as above

[MadBill]

You're saying a TF engine will excavenge the cylinders with the gobs of exaust gasses @ EVO at unreal pressures with that short of pipe?

Every nitro crew chief that I have asked about changing the tuned length of the exhaust pipe to fit the change in RPM over the years has told me that they don't think it matters much.
They have so few opportunities to test things they are mostly focused on keeping a predictable tune-up. You can bring them the greatest idea in the world to increase power but you would be surprised how difficult it is to get it tested.
Let's say you figure out a sure way to get 100hp, you might think that would be a great thing right? Not really if your clutch, tires, crankshaft, rods are all on the edge. That 100 hp would be chaos.

Might be , but if you have a earlier EVO more cylinder pressure is being bleed off through the pipe pushing the car down.

EVO on a nitro engine isn't as easily adjusted as it is with a gasoline engine, they already run solid push-rods to get the valve open under pressure.

It's been claimed that even with solid 5/8" pushrods, the exhaust valves on a TF/TC engine open not when the cam dictates, but rather when the cylinder pressure drops enough for the pushrod to straighten out. Despite the weight, maybe they need a higher modulus material like carbide. 1,000 psi on the (what, 2.0"?) valve = 3,140 lb. plus the spring force..

[srq]

OP here, heck, all I was trying to figure out was what kind of force might come out of a 600 or 700 hp gas motor so I could put it to good use by either venting it through one of the steps on the boat bottom or a diffuser of some sorts on the transom to clean some of the turbulance right at the sterndrive . On 30 ft plus hulls, acceleration out of the turns is always so mello dramtic and boring compared to you car and bike guys. Anyway to break the waters surface tension on that large of wetted surface is a plus for us. Large amounts of thrust coming from under our steps could really screw our handling or help if not done right. Looks like some trial and error is in order.

I will say posts coming from all the various forms of racing have given me some lucrative avenues to research and I want to thank everybody for participating with their thoughts and experiances. The outlaw and circle track guys have given me some neat ideas. You TF guys if I had your exhausts to play with I wouldn't need propellors !

[SchmidtMotorWorks]

It's been claimed that even with solid 5/8" pushrods, the exhaust valves on a TF/TC engine open not when the cam dictates, but rather when the cylinder pressure drops enough for the pushrod to straighten out. Despite the weight, maybe they need a higher modulus material like carbide. 1,000 psi on the (what, 2.0"?) valve = 3,140 lb. plus the spring force..

That's right but there are lot's of other very flexible links in the chain. The rocker stands and shafts for example.
The long exhaust rocker, that same old BAE piece has been around forever.

[digger]

OP here, heck, all I was trying to figure out was what kind of force might come out of a 600 or 700 hp gas motor so I could put it to good use by either venting it through one of the steps on the boat bottom or a diffuser of some sorts on the transom to clean some of the turbulance right at the sterndrive . On 30 ft plus hulls, acceleration out of the turns is always so mello dramtic and boring compared to you car and bike guys. Anyway to break the waters surface tension on that large of wetted surface is a plus for us. Large amounts of thrust coming from under our steps could really screw our handling or help if not done right. Looks like some trial and error is in order.

I will say posts coming from all the various forms of racing have given me some lucrative avenues to research and I want to thank everybody for participating with their thoughts and experiances. The outlaw and circle track guys have given me some neat ideas. You TF guys if I had your exhausts to play with I wouldn't need propellors !

a 400CI engine at 10000rpm 100% VE flows 0.65 kg/s of air. the air exit two 3" pipes with a velocity of 62m/s. this creates a force of 0.65*62 = 40N (10lb) someone check my math PLS

[srq]

Digger, your math seems in line what Schmidt had to say in one of his earlier posts and there was a you tube video of a dyno run with some felxible 4 inch pipes lifting up on a BBC that also looks like it confirms both of yours math so this 10 to 20 lb area of thrust sounds spot on. Not really alot of useable thrust to work with. (at least with the engines I deal with) But I can really see where even this amount can help the circle track guys since they're running right on the edge as far as traction goes.

But seriously I guess I was confusing thrust with pressure/velocity out of the exhaust pipes. With a 600 hp engine if you wre to put a one square foot plate with a scale attached 2 inches behind the collector I bet it would certantly wind the scale up a bit more once you ran the engine up full load to 6 G.

But still many many interesting posts and reads. I'll be working and researching this idea for awhile !
WB

[SchmidtMotorWorks]

Digger, your math seems in line what Schmidt had to say in one of his earlier posts and there was a you tube video of a dyno run with some felxible 4 inch pipes lifting up on a BBC that also looks like it confirms both of yours math so this 10 to 20 lb area of thrust sounds spot on. Not really alot of useable thrust to work with. (at least with the engines I deal with) But I can really see where even this amount can help the circle track guys since they're running right on the edge as far as traction goes.

But seriously I guess I was confusing thrust with pressure/velocity out of the exhaust pipes. With a 600 hp engine if you wre to put a one square foot plate with a scale attached 2 inches behind the collector I bet it would certantly wind the scale up a bit more once you ran the engine up full load to 6 G.

But still many many interesting posts and reads. I'll be working and researching this idea for awhile !
WB

If Digger is right then, I didn't do it right, I have never thought much about rocket thrust so he is probably right.
If I have time tomorrow, I'll collect up all the numbers.

If someone wants to do it, use 500cubic inches at about 60psi going in and 8,200 rpm.
I'll measure the exhaust pipe ID tomorrow.

[MadBill]

If we're still talking TF engines here, don't forget that nitro can be a mono-propellant and I believe may be injected in near-hydraulicing proportions..

[swatson454]

If we're still talking TF engines here, don't forget that nitro can be a mono-propellant and I believe may be injected in near-hydraulicing proportions..

This is pretty insane. http://www.youtube.com/watch?v=xGTbQuhhluY


Shawn