Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Toque v's Horsepower

  1. Mar 2, 2004 #1
    The bigger the bore and stroke lenth means more toque, but the engine will rev less due to the bigger bore and stroke lenth, partly because it takes energy to change a pistons direction. the bigger the piston, the heavyer it is, the more energy is used making it go back up.

    but what about wankle engines, since they only ever go one way, could you have a 5 litre wankle engine reving like a 2 stroke motorbike? (as in really high)

    the x2 power output aside, high torque AND high revs would make one sweet engine. right?
  2. jcsd
  3. Mar 2, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    The amount of torque that an engine produces varies depending on the speed that the engine is going at.

    At 100% efficiency of, [tex]Power=Torque*RPM[/tex]. For electric motors, where efficiency is high - often greater than 0.9, this means that the performace of the engine can be approximated by that formula. At low RPM's the engine produces a large amount of torque. As the engine speeds up the torque decreases.

    Gasoline powered engines have lower efficiency, (I think around 0.3) and the efficiency can vary significantly depending on the speed that the engine is running at. Typically, engines are rated at peak torque and peak power for torque and power respectively.

    It's a physical reality that maximum power is limited by fuel consumption rates (effectively displacement) in internal combustion engines, and volume and magnetic permeability in electric motors. In general, there is a power density (power per volume) for a particular technology that is effectively constant from the scale of a model airplane engine, to a large industrial engine the size of a house.

    So, to get high torque at high rpm, you need a big engine, regardless of the technology that you're using electric, steam, or gasoline.

    Two stroke motors consume fuel about twice as fast per displacement because they fire each cylinder twice as frequently. As a result peak power is higher about double for a particular displacement. However, technical limitations make them less efficient and they tend to burn less cleanly. For applications where high power density is desired - models, hand-held IC motors like chainsaws, and motorcycles - they're quite popular.

    Wankel motors have different efficiency characteristics than other motor designs, and may have some theoretical efficiency advantages, but they have technial limitations as well.

    Increasing the number of cylinders in an engine does not increase the power density, but it allows for a higher maximum displacement, and provides for more consistant torque.
  4. Mar 3, 2004 #3


    User Avatar
    Science Advisor

    The main advantage of wankels is that they do rev really high. Their torque is unimpressive, but they rev high enough to make up for it with gearing (if you don't mind the noise). As was written above, power increases with higher rpm if you can maintain torque. That formula is always true btw. It has nothing to do with efficiencies.

    As I understand it, there are lots of technical issues with making wankels work at all, yet alone in the way you're describing. GM spent a lot of money back in the 70's trying to build a big one to put into Corvettes, but they could never make it reliable.

    Nate, increasing cylinders usually allows a shorter stroke, which implies a higher rev limit, and therefore higher power. Decreasing the bore a little can sometimes improve combustion efficiency too. If you look at exotic cars, you can find 3-4 liter V12's. A 6 cylinder can function perfectly well at 3.x liters, but it won't rev to 7500+ rpm.
  5. Mar 3, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

    That really depends on how you get the figures that you use with it. If you try to use it to predict the performance of an automotive engine by using the Horsepower rating, and the RPM to predict the torque you might not do so well.

    Big problems include the traveling seal between the spinning piston and the 'cylinder' wall and gas flow/combustion control issues. A secondary problem is that they tend to be gas guzzlers. I haven't seen any references to Corvettes with Wankel motors. Cummings-Wright was doing research on a 31.5 liter version, but that's a bit big, even for a US car.
  6. Mar 3, 2004 #5
    I read that the last generation Mazda RX7 had a better HP/Weight ratio than any production Ferrari.

    Although they have had some problems with seals in the past, the latest models are surprisingly reliable (with the possible exception of turbo issues that are unrelated to a rotary engine design).
  7. Mar 6, 2004 #6
    Yea the new mazda that uses the wankel engine is very reliable. Im not sure what material it is that they are using for the seals but whatever it is, its very durable.
  8. Mar 8, 2004 #7


    User Avatar
    Science Advisor

    The wankel rotor doesn't spin very fast at all - the crankshaft spins 3x faster than the actual rotor. From what I've read, the biggest difficulty with the wankels is their combustion byproducts since the sealing made it difficult to get high-compression (for power and efficiency) and large amounts of unburned fuel. It seemed Mazda did an outstanding job with the RX7 to use two spark plugs and clever chamber design to minimize these issues and turbocharging to aid in compressing the mixture.

    Two-cycles may be free to rev quite high, but are generally low-torque and very high rpm motors. Having low torque output allows usage of very light componets. But the wear of the piston/cylinder bore and emissions of unburned fuel pretty much leave them out of the running for an automotive engine. In motocross, the 4-cycle or 'thumper' as they call it has nearly equaled the 2-cycle in terms of performance on a racetrack from the same engine displacement, leading a few in that industry to predict the demise of the 2-cycle by the end of the decade.

    For high revs to make high HP, it would seem Formula 1 is at the forefront but the typical performance motorcycle is really impressive from a HP/liter ratio at an affordable price. 900 BHP from 3.0L at 18,000 RPM sounds great until you factor in the $1mil pricetag and 3hr lifetime before rebuild. So you can get almost 200BHP from 1.3L for $15k instead and only run a mid 9 second 1/4 mile at 150MPH.

    In the late John Ligenfelter's book, he explains the importance of good low to mid RPM torque for cars designed for street use by a simple observation that I'll turn into a question: How much time do you spend at 5000-6000RPM on the street?

  9. Mar 8, 2004 #8
    Almost never, my car is quite old now about 13 years has a 1.1 litre engine. I dont like to push it up to that kind of rev range spends most of its time between 2-3000rpm.

    But the Smart car that i quite often drive for work purposes does rev much higher than that and to get the most of its tiny 698cc engine keeping it between 5-6000 rpms works best, this is also helped by the small turbo the engine uses, i think this cuts in at about 3500-4000rpm but am not sure about that.
  10. Mar 8, 2004 #9


    User Avatar
    Science Advisor

    That's with a turbo, right?
  11. Mar 8, 2004 #10


    User Avatar
    Science Advisor

    Oh, sorry, that's a Hayabusa and not a car. The local track in Commerce, GA is at about 1100 ft and there was 3 Hayabusa's at the track last Sat. Don't have exact times/speeds but from memory IIRC one ran 9.60@150, another 9.2@153 and the last one at 10.1@151 with a slow lauch. None of them even sounded like they had a pipe on them. I've ridden a ZX600, a couple 750s, and the biggest at a modded VMax, but these things looks nuts! Some very impressive OEM equipment with a warranty and enough civility for daily usage.

    I grew up in the midwest, muscle cars are still pretty common. A new Z28/Corvette/Mustang Cobra might be faster in the quarter, but they don't feel as fast as 525 ft lbs of torque from a 454 Chevelle. I've always wanted to do some data capture to try to figure out why the intuitive feel didn't match external numbers. I think a lot might just have to do with the mental RPM derived from the exhaust note compared to the acceleration experienced.

    Back to my question: No, I don't spend much time over 3000 RPM either. That's why I'd love to have a monster big-block Chevy in my car with gobs of torque, then I could accelerate swiftly without too much noise & attention or without feeling like I'm running my engine hard. Then at the track, monster burnouts and hopefully good timeslips. C'mon lotto...

    I've never experienced a Smart car, but an engine that small has many advantages to exceed at high revs/low torque that would never scale to 5L. Looking at the performance street bikes, most 1L have much lower redlines like 10-11K instead of 13K for a 600cc bike so it would seem the economies of scale are already taking effect. I know the connecting rod stresses increase by the square of RPM change, so it does make sense.

  12. Mar 18, 2004 #11
    I don't think its the weight so much as the rotating mass. You can only spin a huge object so fast before it vibrates itself apart. When you lengthen the stroke, or the bore or create more weight farther away from the center of the crankshaft. Which turns your engine into a train, slower to rev up and slower to slow down. Anways, the wankel engine basically has an internal gear to change "piston" speed compared to crankshaft speed. Since the piston is spinning 1/3 as fast as the crank, it is losing torque, like when you are in high gear on your bicycle. The advantage of the rotory engine is that if you can get it to not leak, and be emmissions compliant, you can make it bigger, which would increase the torque. Personally i would like a bigger engine, cause when you make your power lower in the rpm range, your engine lasts longer, and you are way better off the line. This is the reason why the HD v-rod will kick any other stock bike with the same displacement in a drag race.
  13. Mar 18, 2004 #12
    No it won't. Not even close.
    Last edited by a moderator: Mar 18, 2004
  14. Mar 18, 2004 #13
    my bad i ment to say that the v-rod would beat any bike with the same horsepower rating, not displacement
  15. Mar 19, 2004 #14
    I don't have the data, so I don't know. You may be right.
  16. Mar 19, 2004 #15
    I disagree with this statement. Brake horsepower is only half of the story because it doen't describe how much power is applied to the road, or the vehicles ability to use said power. My Yamaha R6 with about 110 Hp (these things come stock with about 95 BHP, but judicious use of a flow bench, and racing cams imroves things quite a bit) will beat a V-Rod any day of the week because the peak Hp is produced at 15,000'ish RPM. This allows me to use the transmission and front/rear sprocket combination to develope my useable speed/power band. Additionally, you have to factor in the power to weight ratio. My Yamaha with me on and 1/2 a tank of gas tips the scale at about 800 lbs. The V-Rod weighs significantly more.

    When all is said and done, My little R6 will run the 1/4 at Milan dragway in Michigan with a best time of 10.988s. That was a fluke, because I typically runs are in the low 11's (11.1'ish).

    My bike's ability to run low 11's, and hit speeds approaching 150 MPH is not strictly due to displacement, bore to stroke ratio, power to weight ratio, or brake horsepower. My bikes performance is a combination of all of the above as well as drive train design.

    The V will probably have better 60 foot trap times, but at the end of the 1/4....

    Finally you made a comment about weight being further from the crank... I wouldn't say that this is the aspect you should be analyzing. The weight of the piston/rod are inconsequential at peak power; however you are correct in that heavier rotating assemblies will be slower to react. At peak power, or any constant RPM, the additional weight usually cancels out in that the weight of one piston going down is countered by another piston going up. Analyze the moment about the crankshaft center axis, and you'll see that different masses will have about the same net moment. What you should be looking at is the rod length/bore ratio.

    Rod/Bore ratio dictates the levels of engine vibration, cylinder wear, timing, when in the intake stroke vacuum begines to rise... There are a lot of aspects of the engine power band related to this rod/bore ratio that have less to do with the actual length of the rod or the distance the piston is from the crankshaft axis.

    Rod length can't be fully discounted because different rod lengths have the following charactersitics:

    Long Rods:

    Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.

    The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.

    For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

    They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.

    Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.

    Short Rods:

    Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.

    The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure.

    Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.

    Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

    The mass of the rotating assembly affects the rod and the piston (specifically area directly below wrist pin) more than the crank so you should analyze the stresses in these two assemblies rather than the actual length of the pistoon/rod from crank axis.

    What I'm trying to get at with all the above is you can't say "my engine is such-and-such with 100 HP thus it will outperform all other engine designes with 100Hp." There are a 1000 factors that are involved in getting the power to the ground as well as the the 1000's of factors going on within the engine. Peak Hp, is a lot less important than the area under the power curve.

    One last thing, big-blocks can be made to spin at high RPM's. the Ford 427 is one of those engines know for its ability to turn at high RPM's.
  17. Mar 19, 2004 #16
    This isn't a good question IMO. If your engine was designed to operate at high rpm's you'd probably spend more time at those higher rpm's. I like to mention modernday superbikes (I know they are different than cars) because superbikes are designed with peak Hp's at or well above 10K. On my R6 I cruise at 6-7K rpm. The reason you don't see these types of engines employed more often than not is because they require a better educated, more attentive user to operate them.

    To address another topic here, Wankels havn't seen a wider deployment because along with some of the mechanical their volumetric effeciency is much lower than that of reciprocating engines. This is due in large part to the lack of valve overlap in the wankel design. Wankels perform very well with power adders but are not as attractive as N/A engines.
  18. Mar 19, 2004 #17


    User Avatar
    Science Advisor

    I've driven a ZX6, granted this is 1994 so its ancient in superbike terms but still representative of the engine you describe. I found it very annoying to drive as prepped by Kawasaki, it made no real power until about 7K where it came alive, but in first gear this was nearly 30MPH. So unless I would rev it up and slip the clutch like a two-stroke dirtbike, a Hyundai or Honda econobox car would pull a car length at a stop light if I let the clutch out and used the motor's torque. At WOT it accelerated very well (as long as you slipped the clutch on the launch) but everyone knew you were doing it since its spinning 10k and is plenty loud. Sure a shorter gear would help, but even the smallest front/largest rear might get you 2x the final drive, but that still leaves you dead until 15MPH. Are we talking highway or city street driving, and how far above the suggested speed? :wink:

    Its opinion for sure, but I've read plenty of car reviews that use the phrase 'drive it like you stole it' to describe the concentrated upper-rpm HP engine. IMO, I prefer a low-mid torquer for the 95% of the time that I am not at WOT.


    P.S. As fast as a V-Rod might be compared to standard HD bikes, I think a more fitting race would be the nearly two decade old Yamaha VMax. Anyone got 60ft and 1/4 mile times for the bikes stock?
  19. Mar 19, 2004 #18
    This statement is not true, the 250 4-strokes race in the 125 2-stroke class. The 2-strokes still produce roughly twice the hp of a 4-stroke at the same displacement. The reason 2-strokes might disappear is entirely due to the "greenies"

    The 4-strokes definately have a torque and traction advantage though.

    lol, My Yamaha R1 has a 0 to 60ft time of 1.8sec and a 0 to 60mph time of around 3sec, do that in your Honda.
  20. Mar 19, 2004 #19


    User Avatar
    Science Advisor

    The annoucers of the last race I watched on TV claimed they were the same displacement. It was some outdoor venue, not a supercross. Of course, some of those announcers....lol

    I drive a TransAm. :smile: So your R1 will pull off those times with how much clutch slip? Even the Hayabusas I see launch at the track are slipping the clutch like mad...

  21. Mar 19, 2004 #20
    Since they do the 0-60 in 3 secs or less, and bottom gear is only good for about 70~80mph, I suppose they only slip the clutch for maybe the first second and a half or so.

    Dumping the clutch and spinning the back wheel doesn't ususally give a good launch on a road bike - better to use the clutch (IMO of course).

    I have a Honda Blackbird. Not quite as fast as the Hayabusa, or ZX12. [zz)]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook