I've always wondered if this was possible..

[aiasib]

"The Rolls Royce Merlin engine's exhaust was so powerful it could actually provide useful thrust. Tests increased the top speed of the Spitfire by 10mph." Link

Anyone know of any other examples of this? I've also just read that the exhaust on top fuel dragsters can produce up to 1000lbs down-force.

That's ridiculous.

[pimento]

Well, if it says so on the internet it must be true.

I guess a supercharged 27l engine delivering up to an over 2000hp would pass through a lot of air, so I'm not surprised by the idea.

[RacingManiac]

The amount of energy in the exhaust is not insignificant....considering a typical efficiency of a internal combustion engine is <30%, most of that energy goes into heat, and a good chunk of that I'd think is in the exhaust flow....

The current F1 shouting war on "blown diffuser" is basically a way to scavenge(and actually using the exhaust by actually not just scavenge, but actually using the hot exhaust) to drive the diffuser flow....

[f6fhellcat13]

Makes you wonder why the Merlin was only supercharged; seems like there's a whole lot of exhaust energy for a turbo to work with.

[Magnum9987]

The merlin wasn't supercharged because the technology wasn't developed enough at the time. Lag was huge, reliability was crap, and it was difficult to lubricate in a high G turn. And when that happens the turbo blows up, and with it the motor. Theirs modern turbo Merlins in competition Mustangs pumping 4000HP.

But, I don't doubt that even with a turbo, the exhaust could produce alot of thrust. We are talking a 27 liter V12. It wouldn't produce as much as a top fuel car though.

[MilesR]

It certainly is possible. That was the reason for the fishtail-shaped exhausts tips, on the later variants of Merlin Spitfires and Hurricanes. The principle is the same as for a turbocharger. Where a turbo would use the hot, high-pressure gas flow to turn a compressor, the Merlin's exhaust tips used it to directly drive the aeroplane. Another trick to boost performance was to fill in any surface features, like panel gaps and seams, and polish the skin of the aeroplane. De Havilland Mosquitos could gain an extra 12-15km/h like this.

An even more extreme example of exhaust thrust was the Rolls-Royce Crecy. This was an engine that was developed during the second world war, but never produced, as a sprint engine for interception. It was a supercharged two-stroke, 26L engine. The flow from the exhausts provided an additional 30% more thrust than the propeller alone generated. Later experimental variants used the exhaust flow to drive a turbine attached to the engine's accessory shaft. This served as a power recovery device, to reduce fuel use, rather than to increase power, but it is a very good example of the half-way point between using the exhaust directly for added thrust, and using the exhaust to drive a turbocharger.

Magnum, I believe that the Merlin was supercharged, but not turbocharged (I expect that that was a typo). Turbos came later, but were used in world war two aircraft, such as the Lockheed P-38 Lightning, which used Allison V-1710 engines:

Allison V-1710 - Wikipedia, the free encyclopedia

I would have expected that lag should not have been a problem for these engines, as they ran at an effectively constant speed, and used variations in propeller pitch, in conjunction with throttle control, to control speed and thrust. They did have trouble with reliability, though, until they got the hang of heat management and lubrication.

The reason that supercharged exhausts can produce so much thrust is that the higher the charge pressure in a piston engine, the more it behaves like a jet engine. A jet uses a fan to force air in the front, and a turbine at the back, recovering energy from the exhaust to power the fan. The engine delivers its useful power as thrust, rather than shaft horsepower. Likewise, if you put a really big supercharger on a small engine, you have a very big fan forcing air in the front, and a set of pistons recovering energy from the exhaust to drive the supercharger. If the supercharger is big enough, all of the engine shaft power will go into driving the supercharger, and all of the useful power will come out of the exhaust as thrust, like a jet.

If you are interested in other odd uses of engine exhaust thrust, take a look at the Napier Nomad:

[ame="http://en.wikipedia.org/wiki/Napier_Nomad"]Napier Nomad - Wikipedia, the free encyclopedia[/ame]

[culver]

Top fuel dragsters get downforce from their engines to the tune of about 1000lbs. No surprise given the TF engines produce around 8000hp.

BTW, the P-51 (and likely other aircraft) got positive thrust from the radiators heating air as well.

[MilesR]

Yep. I had forgotten that. Both water- and air-cooled aero-engines could be designed to generate thrust from their cooling systems. It was common practice throughout world war two. It meant that air-cooled radial engines were more efficient than the smoother-looking in-line engines. They could generate thrust from their cooling ducts, and they did not have to carry around the weight, and extra cross-sectional area, of the radiators, coolant, and associated channels in the engine castings.

Another configuration that I missed out before is the pressure recovery turbine, where an exhaust turbine is used to deliver power to the crankshaft, increasing shaft power directly. This was used on variants of the Wright Cyclone engine, and resulted in a reputation for unreliability, as the idea was a bit ahead of the engineers' ability to realise it, at the time.

Wright R-3350 Duplex-Cyclone - Wikipedia, the free encyclopedia

They claimed to recover about 20% of the otherwise wasted energy from the exhaust, boosting the shaft power by about 500hp (about 20-25% increase over the power of a conventional configuration).

[SonyAD]

That just goes to show how horribly inefficient aviation piston engines were. The petrol ones, anyway. Something to be expected with a 6 to 1 compression ratio.

[f6fhellcat13]

I thought that they were inefficient, not only because they were made in the '30s and '40s, but because they were designed to operate in an environment with a deficit of oxygen, so they usually ran quite rich.
Also, 6:1 doesn't sound too bad for a forced-induction engine, especially considering the era it was built in.