Author Topic: What thrust is produced by a motorcycle exhaust?  (Read 14035 times)

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Offline wobblywalrus

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #15 on: November 02, 2014, 12:35:16 AM »
It can be figured out using simple calculations.  We had it as a textbook problem in engineering skool decades ago.  The class was dynamics, if I remember correctly.  All I remember was it was real confusing.

Offline WhizzbangK.C.

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #16 on: November 02, 2014, 12:58:12 AM »
It can be figured out using simple calculations.  We had it as a textbook problem in engineering skool decades ago.  The class was dynamics, if I remember correctly.  All I remember was it was real confusing.

Define "simple"  :evil: and "confusing"  :?

And then, even when you have a number derived from knowns, known unknowns, and swag, you still need to test to verify if you crunched the numbers right. Sometimes it's just as well to jump straight to the test and get an actual number.  :cheers:

On the subject of exhaust thrust or exhaust gas volume filling in the low pressure areas behind a vehicle, it seems that most consider this a negligible consideration, due to the relatively low volume of exhaust gas generated compared to the mass of air displaced by the vehicle. What if one were build a "muffler" that had plates in it heated by the exhaust? And what if water were to be misted onto these hot plates to be vaporized and exhausted out the back of the vehicle, giving extra "exhaust volume"? Probably not legal in LSR per the rules, but might be something the O.P. would find beneficial in his application.  :-)
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Offline WOODY@DDLLC

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #17 on: November 02, 2014, 08:16:40 AM »
You need lots of exhaust mass and/or velocity to generate any usable thrust: https://www.grc.nasa.gov/www/k-12/airplane/thrsteq.html

Put a bathroom scale on your garage wall at the center line of the front wheel and plot us an rpm vs thrust curve. Repeat test with FC!  :-o
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Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #18 on: November 02, 2014, 02:11:02 PM »
WhizzbangK.C., this might work, but this is pretty involved, and that's exactly what I am trying to avoid. Just not practical to accomplish this in my garage.

Woody, what you are suggesting won't work because...

1) I would  have to suspend the bike in the air to remove rolling resistance of the tires, and it is significant - at least 20 lb based on my seat of the pants assessment after rolling the bike around;

2) without the load I can reach red line with barely open throttle - there is no thrust in such case, and the engine probably generates a few hp only. The bike needs to be loaded to generate 100 hp and 200 hp respectfully, and this can only be accomplished on a dyno. So, I would have to suspend the entire dyno with the bike on it - in the air and press it against the scale on the wall.

I am really hoping someone with knowledge/experience could figure it the ballpark number.

What about my logic/thinking mentioned in my previous post (Reply #12)?

Offline tortoise

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #19 on: November 02, 2014, 06:07:20 PM »
Let me ask something concrete. If I use the ratio from FC case (8000 hp -> 800 lb of thrust), and try to apply similar ratio of 10:1 to my bike's hp, how should I correct the result?
Fuel drag motors run so over-rich that they burn a whole bunch of fuel in the exhaust system, so they would likely produce much more thrust/horsepower than road vehicle motors which do not belch huge flames.
Quote
My thinking is that whatever fuel mixture goes in FC vehicle, it must have more energy per unit of weight than gasoline
Not necessarily so, in fact I believe not. The FC air/fuel mixture does have more energy per unit volume.

Offline jlmccuan

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #20 on: November 02, 2014, 06:09:44 PM »
I have some actual real world experience with the water in the exhaust creating thrust.  My grandfather had a 4 wheeler he rode 50 yards to my Mom's for breakfast every day for a couple of years.  I borrowed it once.  All those short runs collected water vapor in the muffler.  As I rode long enough to heat the muffler water to boiling, the engine would choke and die due to the exhaust pressure being generated by the steam.  It took about an hour to boil off enough water to allow me to continue my ride for the day.  My conclusion is that the pressure of the water expanding into steam would stop the engine and no thrust would result.
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Offline Interested Observer

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #21 on: November 02, 2014, 06:57:41 PM »
Taking a crack at the original question--
From “Mechanics and Thermodynamics of Propulsion” by Hill & Peterson, for air-breathing engines,

Equation 6-6   T = me * ve 

where T = thrust, me = mass flow rate at the exhaust, and ve = exhaust velocity
and neglecting the terms having to do with the incoming air velocity which, if considered, would reduce the thrust, also neglecting the mass of the fuel component and assuming the flow is subsonic:

On the basis of  9 hp per lb/min of air consumed, at 100 hp mass flow rate would be 11.11 lb/min

Exhaust air density at an assumed 200 F   Rho = 2.7(P/T) = 2.7 ( 14.7/(460+200)) = 0.060 lb/cuft

Volume flow rate = mass rate/density = me/rho = 185 cuft/min

Assuming 1.75” diameter tailpipe, flow area = 0.0167 sqft

Flow velocity, ve = V/A = 185/.0167 = 11,062 ft/min = 184 ft/sec

Then,   T = me*ve = 11.11* 11,062 = 122,906 lb-ft/min^2  =  1.06 lbf

So, it would appear there’s not much thrust to be had there.

Offline datsaxman

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #22 on: November 02, 2014, 09:46:50 PM »
Okay...I am a mathematical physicist at work...so I cannot really let this one go without comment.

OP, and a few others...I mean this kindly, but you are flailing. 

I did the calculation two different ways.  First, by thinking of the fuel consumption.  I assumed 9mpg @ 180 MPH.  Yes, it made the numbers easier to work with.  There are worse things.  My own Kawasaki, 220HP or thereabouts, @12,000RPM.

I got 6.5Newtons.  Pound anna half in English units. 

I did the calculation again using a thermodynamics approach.  Mass of the air, stoichiometry, guessed at the exhaust temperature, etc.  Got 7.1 Newtons. 

Given the very sloppy calculation, done in my head, I would call that terrific agreement with the previous post.  In fact, I defer to the previous calculation.  1 Pound Thrust.  I am sure that is ballpark, which is to say within a factor or two.  Or so.





Offline wobblywalrus

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #23 on: November 02, 2014, 10:04:18 PM »
Maybe this will muffle dissent on this exhausting topic.

Offline DaveL

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #24 on: November 02, 2014, 10:30:55 PM »
That would be 1lb Gross thrust as well. As the bike would be able to outrun its exhaust velocity Net thrust would be a negative amount.

Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #25 on: November 02, 2014, 11:08:40 PM »
Thank you very much for figuring this out guys, and for all your input. Especially, Interested Observer, and datsaxman.

I am also relieved as we finally could muffle dissent on this exhausting topic.  :roll:

Offline Rex Schimmer

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #26 on: November 03, 2014, 03:36:35 PM »
Back before airplanes were mostly powered by "weed burners" (gas turbines) Supermarine, the designers and builders of the Spitfire, along with Rolls Royce found out that buy building special exhaust outlets for the big V12 they could get an extra 20 mph. If you do a little rough math and use 380 mph as the base speed and 400 mph as the final speed, the hp required for that additional speed (assuming the engine is  a later model Merlin and rated at 2000 hp) would be about 330 hp. This is one of the reasons that Rolls Royce never developed the Merlin or Griffon with turbo chargers. Their two stage and two speed super charger system along with the exhaust thrust augmentation provide equal or better performance without all of the turbo head aches!

One more thing, if you do the math one pound of thrust at 100 mph is equal to 3.75 horse power. It is basically free hp so I think I would always point the exhaust to the rear.

Rex
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Offline 55chevr

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #27 on: November 03, 2014, 03:53:40 PM »
Was the exhaust design for scavenging or thrust.   Combustion efficiency would be a stronger argument for 20 mph than "zoomie" pipes.

Offline Rex Schimmer

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #28 on: November 03, 2014, 04:05:56 PM »
They were actually designed for thrust as the actual "header pipe" was only about 5-6 inches long. I think they were more intent on lots of blower pressure rather than exhaust efficiency.

Rex
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Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #29 on: November 03, 2014, 06:44:46 PM »
Rex, that's very insightful. Thank you!

I actually have an additional question... So, we established that the exhaust gasses will be pushing the exhaust pipe with approx. 1 lb of force.

Now, please keep in mind while reading the question, this is not what I intend to do, but is only to provide the circumstances...

If I point the exhaust pipe straight up, or straight down, in other words in a way that the exhaust pipe orientation is at 90 degrees to the stream of passing air, how would this affect the 1 lb force on the pipe?