Landracing Forum
Tech Information => Technical Discussion => Topic started by: IG on October 31, 2014, 11:55:21 PM
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Hey guys! I am trying to find an answer to this seemingly simple question, but have no luck. Even called Two Brothers Racing and spoke with their tech... Nope, they don't know and never measure. Did searches online - nothing... except, there were some discussions here, but of course not for bikes, but for something a bit more powerful. For instance, it was mentioned something along the lines that a Funny Car with 8000 hp produces around 800 lb of exhaust thrust.
So, here is what I am looking for...
1. What thrust is produced by a sportbike's exhaust at let's say 100 hp and 200 hp respectfully?
2. Does the exhaust pressure add anything to the thrust, and if so, how it is affected by different scenarious? For instance, when my bike idles and I place the palm of my hand a few inches from the exhaust, I clearly sense decent pressure which moves my hand, although I know the thrust at this point is minuscule. I would imagine the pressure is a lot more when engine is under load and RPM is a lot higher.
Here are scenarios I am interested in...
a) the bike is on a dyno (i.e stationary).
b) the bike is moving.
c) the bike is moving, but the exhaust is directed vertically.
Thanks,
Igor
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That number you mentioned for a dragster seems like it's "in the ballpark". Having said
that, my guess is that it's not likely to be worth all the effort to accurately measure the
thrust from a motorcycle exhaust, unless it's extremely important.
For land speed racing purposes, the exhaust thrust could be used either to push the
vehicle forward, or to push the rear tire down for added traction, or a combination of
both. The decision in this case would probably be yours to make, based on where the
maximum benefit would be achieved. You probably know that wheelspin is an issue for
fast, powerful vehicles such as Speed Demon (400+ mph), that have enough power to
spin the wheels even at top speed.
If your top speed will be, say, 200 mph or less, then you probably don't have enough
power to spin the wheel at top speed, and having the thrust push the bike forward makes
more sense than using the thrust to gain traction. This is assuming that traction is not
the limiting factor in how fast you can go. In this case, the race track is long enough
to achieve top speed, even with limited traction. At a short venue, like El Mirage,
traction may indeed be a limiting issue.
Providing a little more info about what you are trying to achieve here could be helpful.
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Igor, while I don't know of anyone having measured or tried to use the thrust, I do know that a number of our brethren have used the exhaust output stream to fill in (at least part of) the low pressure area behind the tail of the bike. That that is successful is pretty-well documented and understood. As for measuring the thrust -- maybe it's something that Woody or another CFD guy could model for us and come up with a number that's at least within an order of magnitude or so. Let's see if anyone picks up on it. There's maybe even a formula that'd bring a bit of information to the table -- not only engine hp but also stuff like exhaust pipe outlet diameter, restrictions all along the pipe from exhaust valve to the outlet, heat loss in the entire ex. system, and so on. But that kind of math isn't my tea of cup. :evil:
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Your bike is traction limited on the salt... but not so much on pavement at speed. I doubt you would see any change with stock bodywork.
If you decide to race, go like it is to get a baseline, then make changes and have fun.
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IMHO, and mind you this is still early.
Thrust is usually a result of restricting down to accelerate the gasses.
That being said, I would think to reduce back pressure on motor to reduce "thrust" would be more beneficial to HP produced and driven into ground.
The creative use of exhaust to help fill in the displaced air behind the bike is genius.
OK, heading back to coffee pot.
John
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Thank you guys! I should've clarified that I am looking for a ballpark number, and what's more important I need to know it for something other than LSR venue. I just have some ideas to try, and whether I want to even pursue them greatly depends on the amounts of thrust/pressure produced.
If I take hp/thrust ratio for FC numbers I mentioned (8000 hp -> 800 lb), that would be great for my purpose as 100 hp would produce 10 lb of thrust, and 200 hp would produce 20 lb of thrust. I realize that I can't just assume such ratio, and the numbers I speculated about are just that - speculative assumptions.
I searched the net, and this forum is the only place I found that had some discussions about thrust, although it was within the context of ultra high hp vehicles.
The closest scenario which would answer my question would be the exhaust pointed UP. While the amount of thrust wouldn't change, I would imagine the pressure effect would be quite different vs. if the exhaust was pointed back. Obviously, in the latter case, the exhaust gasses fill the vacuum behind the bike reducing the drag, but NOT generating much of a pressure which would push the bike. However, I would think that when the exhaust is pointed UP, the exhaust gasses have to push against the air and might meet more resistance, therefore, pushing on the bike to some degree... I could be completely off with my logic.
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Exhaust gases don't push on the air, or anything. (else rockets in space wouldn't work) Newton's whichever law of motion applies. Reducing your drag I think would be far more useful than the thrust from the exhaust per se but the TF/FC guys do use the exhaust for downforce as well as thrust- note the angle of the headers and what happens to the car when they lose a cylinder- big turn toward the dead one. It is important to them
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Jack, obviously in vacuum the pressure of exhaust gasses contributes nothing to the propulsion. However, when my bike idles and I place my hand a few inches from the exhaust, my hand gets pushed by the exhaust gasses with noticeable force - which according to one of Newton's Laws applies the same force to the bike, but in the opposite direction, hence pushing the bike forward. I recognize that it is a lot easier to move air than my hand, and such force may not contribute much to the propulsion, but it would be nice to know for sure.
If the exhaust is pointed back and simply fills the vacuum, it is reasonable to assume the exhaust gasses meet no resistance and therefore exhaust gas pressure contributes nothing to the propulsion. However, when pointed upward, there may be some resistance, and in such case I would imagine the exhaust gas pressure contributes to the propulsion.
Think of another example. Let's say I install my bike on a boat, and stick exhaust in water, pointing it backwards. Water being a reasonably dense substance would be pushed back by the pressure from the exhaust, and the boat would be pushed forward - mainly thanks to the exhaust pressure, but with little effect from the exhaust thrust.
But aside from the exhaust gas pressure related propulsion which is probably very small, the main question remains - what thrust is being generated?
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If rockets can accelerate in space, your exhaust can still push your bike when dumped into a low pressure area. Thrust is a result of pressure differential inside the pipe. Not pushing the air or lack of air at the outlet.
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Sorry, it doesn't work that way :?. The exhaust gases are pushing on the air, yes, and accelerating the air, or your hand which YOU are resisting. But the force that accelerated the exhaust gases ( the engine etc that is attached to the bike) is trying to accelerate the BIKE. If the Bike was on a frictionless surface, it would be accelerated in a vector opposite to the vector of the exhaust gases and the acceleration would be proportional to the mass of the bike and the mass of the exhaust gases. If YOU were on same friction less surface, your hand (attached to you etc) would be similarly accelerated again proportional to the relative masses. The bike sitting where it is has more friction and overcomes the acceleration of the exhaust so doesn't move in the real world. However, the force is being applied none the less and you want it added to the thrust of the tire etc etc. If that force of the exhaust is directed down, it is applying it to the tire to help traction. What the exhaust gases do to the flow of air around the back of the bike is a different story. Ask the wind tunnel guys what this might do. As I said before, relatively speaking, improving the Cd by those gases may be more a help than the direct force of the exhaust pushing you forward. Ask the F1 guys, and the rule makers thereof, what they are doing running the exhaust over their wings and such,
http://en.wikipedia.org/wiki/Newton's_laws_of_motion
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The mass of the exhaust gas can be calculated (air plus fuel etc- conservation of mass). Velocity of the gas coming out the pipe might be harder to measure but likely there are ways.( somebody step up here) The exhaust is accelerated from 0 at the port to whatever that velocity is at the end of the tail pipe in x distance (length of the pipe) From that you can calculate the acceleration of the exhaust gases and that with the mass of gasses will give you the force (F=MA). Force tries to accelerate the bike in the opposite direction of the exhaust flow. This will answer your primary question. Just don't ask me to do the math....... :cheers: :cheers:
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Filling the wake....really..?...or pissing in the wind..?
Posted by A2 Windtunnel here years ago...:
will tell you that exiting exhaust in the wake will have little to no effect unless it is something in the realm of a top fuel (i.e. high HP engine). Imagine whatever you race in a wind tunnel like AeroDyn (or driving through a tunnel with same cross sectional area for those who don’t like to think that way) at 130MPH there would be 2.5 MILLION (2,500,000) CFM going by the vehicle. Now tell me how much CFM your engine puts out and what the percentage difference is? (that is only at 130mph) Do you kind of understand what I’m getting at? I’m not saying don’t make the simulation as close to the real world as you can. If you think that for your test it would be important then go right ahead, but I’m just telling you (my opinion) what I wouldn’t waste my time/$$ on. For a first time customer visiting any wind tunnel I would not worry about CFM in or out of the motor under most circumstances. Now, if you have tested a dozen times and now starting to look at things through a microscope then things like this might be worth perusing. The reason, like I said before, lets say it changes the absolute values 1% but at the end of the day you helped the drag 15%. Now let’s tape it off and run the test over and you have improved the drag 16%. Was it worth the effort?
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I tried to do the math, but I am not proficient with this subject although I looked at the formulas and understand how to calculate. However, I do not have the data needed to do the calculations. That's why I am asking.
As I mentioned earlier, I need to know this to figure out whether it makes sense to pursue some ideas I have, but this is not for LSR or for going fast, i.e. not for your typical application where the exhaust thrust of a low hp vehicle is minuscule. Even a few lb of thrust might be useful to me.
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? Should I increase the ratio or decrease it, and if so by how much?
My thinking is that whatever fuel mixture goes in FC vehicle, it must have more energy per unit of weight than gasoline. So, to produce 1 hp in FC, would require less fuel mixture mass than to produce 1 hp from gasoline. Therefore, when my bike is generating 100 hp running on gasoline, the exhaust throws out more mass than in the case if my bike was powered by FC fuel mixture. So, the ratio should be decreased to maybe 9:1, or even 8:1 - which works to my advantage. If I assume the ratio of 8:1, this means 100 hp -> 12.5 lb of thrust, and 200 hp -> 25 lb of thrust - not a joke. Sure, if the bike's engine generates 500 lb pull at 200 hp in a certain gear, the extra 25 lb of thrust doesn't seem like much. However, for my purpose that would be very useful.
Does it make any sense?
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When I was younger I worked in the piston engine overhaul shop for UAL. The Convair 240 aircraft they had were powered by P&W R2800 engines with exhaust augmentation. That meant they had cool headers that mixed outside air and exhaust to two openings on the trailing edge of the nacelle. It was supposed to provide some thrust, but i have no idea of how much. You might try to find out something about that. But i guess your engine will be smaller than 2800 cubic inches.
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Only one way to get the actual numbers that you are looking for. Find one of these,
http://www.land-and-sea.com/absorber/dynamometer_water-brake_absorber.htm
Mount it in place of the rear wheel on your bike, anchored to the bike itself so that it does not impart any force except to the bike frame.
Now hang the bike from the ceiling, attach a scale to the front of the bike and fix the other end, and run it full throttle against the water brake. The reading on the scale will tell you exactly how much thrust the exhaust is generating.
:-D
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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.
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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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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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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)?
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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.
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.
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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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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.
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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.
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Maybe this will muffle dissent on this exhausting topic.
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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.
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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:
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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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Was the exhaust design for scavenging or thrust. Combustion efficiency would be a stronger argument for 20 mph than "zoomie" pipes.
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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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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?
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LSR rules point out that the exhaust CANNOT be pointed at the ground(course).
DW
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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
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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.
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Got it! Thank you, guys!
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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
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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
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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 :-D :-D 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
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(http://purplesagetradingpost.com/sumner/bville-cars-1/91-1-08.jpg)
From Sum's site..
Mike
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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.
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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.