What thrust is produced by a motorcycle exhaust?

[dw230]

LSR rules point out that the exhaust CANNOT be pointed at the ground(course).

DW

[rouse]

This is over all a interesting question, and by the answers, not an easy one at that.

So if you take a look at the pressure in the exhaust with lets say 2" outlet, you really don't have much to work with.
There will be a big surge up to just over 2 bars for 20 degrees of crank rotation out of 720 degrees, then the pressure drops to about .5 (1.5 absolute) for the next 180 degrees of crank rotation. If the pipe is close to the correct size the velocity will be close to Mach .4 to .5. Now the next 520 degrees of rotation the pressure and velocity are at "0" are less ( so no help during that time ). You are moving your exhaust in pulses that are working for you about 27 % of the time are less so your net for 150 hp will be somewhere around 3 to 4 pound of thrust max at WOT.

You really can't compare what's going on with a gasoline engine with a Nitro-methane engine. Nitro engines have a major chemical reaction that converts the Air/ Nitro mixture into Hydrogen. The Hydrogen then ignites in the pipes and adds tremendously to the thrust in the pipes, and looks cool at night.


Rouse  

[Interested Observer]

IG,
Any thrust generated is due to the change in momentum of the exhaust gasses and is in a direction opposite the direction of flow at the exit.  The passing airstream has nothing to do with it and, since the pressure at the exit is atmospheric, pressure has nothing to do with it.

Rex,
If we had taken into account the inlet air flow, it would have pretty well clobbered the net thrust and your 3.75 hp. But a rearward outlet is probably still the best arrangement.

[IG]

Got it! Thank you, guys!

[Robin UK]

F1 designers have been utilising exhaust gasses for years- a practice now effectively banned by a change in the technical regulations. Hot and cold blown diffusers were used primarily to increase rear down force even when the driver was off the throttle (and as this article also mentions) sometimes to clean up turbulent air and thus reduce drag around the rear wheels. I know bike exhaust gasses cannot be compared with those from an F1 engine, but depending on your point of view about the importance of weight reduction, an exhaust blown diffuser on a 4 wheel liner could be used to create enough down force to minimise wheelspin without having to add weight. It could also be used as this article suggests to reduce drag around open wheel vehicles such as lakesters. Not suggesting that this is easy or even affordable but an interesting topic nevertheless.

http://www.formula1-dictionary.net/diffuser_blown.html

Cheers

Robin

Robin

[Rex Schimmer]

Robin,
Thank you so very much for the great article on "blown diffusers"! It illustrates the advantage that can be made by controlling the exhaust outlet. And I certainly agree that a blown diffuser on a Bonneville car could certainly be a big advantage if you have a traction problem. Just a thought for teams with large amounts of money laying around looking for some aero advantage (Not many of those around) Adrian Newey, the designer of the Red Bull F1 cars is going to be in semi retirement next year and might really be interested in a land racing car. Would not be cheap!!

If you want to see a very innovative use of exhaust find a picture of George Johnson's "Saltasaurious"  C unblown gas lakester. He held the C gas record at 273.4 for a year or two.

Rex

[Robin UK]

Rex - cant find a pic of George's lakester - only extinct four-legged beasties with the same name. Even the newspaper article I found didn't have a pic. Can anybody point me to one of the four wheeled version please.

Rumour is that Newey will join Sir Ben Ainslie on his project to finally take a serious shot at getting the Americas Cup back from the various colonies after all these years  If the Swiss can do it without even having a coastline, then it would be rude not to try. Newey also does rather well racing his own classic cars these days. His E-Type Jaguar (co-driven by Bobby Rahal on occasions) and his GT40 are usually front runners at the Goodwood Revival.

Not exactly a blown diffuser but along the same lines was the Blue Star project from the late 60's. Using turbines that were originally fuel pumps for the Blue Streak stand off atomic cruise missile to drive the wheels, air was pumped out of wheel pods to suck the whole thing down just like the Chapparal and Brabham fan cars. David Gossling was killed in a bike accident and that was that although a mock up was mounted on a bridge over Donington Racetrack for years before being removed a couple of years back.

Robin

[4-barrel Mike]

From Sum's site..

Mike

[wobblywalrus]

IG, it is good tha you are looking at things in a quantitative way.  In other words, how much?  A lot of us look at things from a qualitative viewpoint and worry a lot about cause and effect that does not mean much, or the opposite, we ignore important things.

[IG]

Yes, would make no sense to pursue something which ends up being ineffective. However, in these types of discussions, there is always something to learn - almost 100% of the time.