"TheOnelectronic" (theoneelectronic)
06/23/2015 at 18:57 • Filed to: Oppocation
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"TheOnelectronic" (theoneelectronic)
06/23/2015 at 18:57 • Filed to: Oppocation | 3
| 18 |
So, a thought occurred to me today. HP is directly related to RPM. That is, theoretically, if you can double your RPM, and still make the same torque, you double your HP. Thing is, many “performance” engines today only rev to seven thousand or so. Why?
Basically, what is it that limits how fast an engine can operate? My first guess would be that as the crank spins faster, it needs to be more and more precisely balanced to avoid tearing the block apart. Secondly I would guess that standard spring-operated valves can’t cycle that fast, leading to “Valve float.” Hence technologies like Koenigsegg’s pneumatic valves or Ducati’s Desmodromic valves. Third guess is that above a certain speed, the friction on bearings and other contact surfaces builds up more heat than can be easily carried away. Fourth guess that just occurred to me is that it’s limited by the mass of the pistons/rods.
Are there elements I’m missing? What are the sort of things that more exotic engines do to deal with these issues that allow them to rev so high?
*preceding statements have no basis in practical knowledge and may be quite false.
SLK
> TheOnelectronic
06/23/2015 at 19:06 |
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From what I know, in car engines your limiting factors are precision and emissions. Its very hard to make an engine that can be properly timed and balanced to reach much higher rpms than 8-9000 which is where we see most high performance cars top out. Add to that having to meet emission standards as the faster an engine spins the more fuel it needs and you can see why torque and horse power lower in the rev range has become more common.
RamblinRover Luxury-Yacht
> TheOnelectronic
06/23/2015 at 19:08 |
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Not just valve float, but resonance issues, and spring failures, and pushrod failures (on pushrod engines), and timing arrangement strain... basically anything doing any reciprocating in an engine runs into more and more trouble, because the acceleration forces get higher and higher. You are correct that the heat transferred to friction surfaces is proportional to speed - anything with liquid undergoing shear, anyway. Past all that, you run into issues with pumping - it gets harder and harder to get air in and exhaust out. Even further past, it becomes harder to ensure good combustion and being able to harness it effectively. In short, revs are hard.
HammerheadFistpunch
> TheOnelectronic
06/23/2015 at 19:13 |
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You have lots of the right ideas. Piston speed is a big factor as well, the other problems are interrelated too in that if you want a fast engine you need light internals to maximize pistons speed and reduce vibration...light internals don’t take well to large torque applications. Its relatively easy to get an engine to spin to 7000 rpm, but much harder after that and it requires sacrifices.
camaroboy68ss
> TheOnelectronic
06/23/2015 at 19:14 |
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You also have to look at the material used and the production process. Forged parts are much stronger over cast parts so they can handle the load higher RPM’s. In an example of actual material an early 327 small block chevy block is much stronger than say a 1978 350 because the early blocks had a higher nickel content in them thus making them stronger.
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HammerheadFistpunch
> RamblinRover Luxury-Yacht
06/23/2015 at 19:14 |
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I think there was a great interview with a honda engineer when the s2000 came out explaining that 7000 rmp is easy, but 8000 is really hard.
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RamblinRover Luxury-Yacht
> HammerheadFistpunch
06/23/2015 at 19:18 |
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I’m shooting for 6k capability with my Rover V8, possibly a little more... and even that is considered ambitious by some. It’ll need the improved pushrods for the strain - just how it is geometrically speaking - and better valve springs, not to mention some oiling “help”, probably. Light pistons, forged rods. Smaller mains on my version are actually an asset in some respects.
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TheOnelectronic
> HammerheadFistpunch
06/23/2015 at 19:39 |
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Ah, so that’s why engines with a long stroke can’t rev as fast; the piston’s are travelling much faster for a given RPM. Cool.
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TheOnelectronic
> RamblinRover Luxury-Yacht
06/23/2015 at 19:42 |
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That was very informative. Thanks!
FastIndy
> TheOnelectronic
06/23/2015 at 19:51 |
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I’m in the process of building a 2.6L 4cyl that I’m designing to hit 10,000 RPM. You really nailed all of the major issues.
One is that engine friction losses increase with something like the 3rd order of engine RPM.
Another is that piston rings for emissions-compliant vehicles are largely incompatible with the running requirements for a high RPM engine with a correspondingly high piston speed. High piston speeds generally require extremely thin rings, which are susceptible to wear, and eventual blow-by into the crankcase.
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TheOnelectronic
> FastIndy
06/23/2015 at 19:57 |
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Interesting. I didn’t know the rings were involved in emissions, but it makes sense.
Keep us posted on that engine build. That sounds awesome.
deekster_caddy
> TheOnelectronic
06/23/2015 at 20:06 |
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Combustion, fuel delivery, air delivery all change a lot as RPMs increase. Tiny motorcycle engines were designed to rev, make HP and RPMs, but don’t have the low end torque required to move something as heavy as a car off the line. So compromises are made - larger bore and stroke for more torque, but lower overall RPM. Nascar V8s run at a gajillion RPM, but racecar - not a very streetable engine. To rev like that in a large engine you need air, and lots of it.
Opposite Locksmith
> FastIndy
06/23/2015 at 20:14 |
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How can 2.6 rev so high??? That's awesome. I gotta know
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HammerheadFistpunch
> TheOnelectronic
06/23/2015 at 20:30 |
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exactly plus a longer stroke means more volume per cycle which is harder to pump in and out at high engine speeds
DoYouEvenShift
> TheOnelectronic
06/23/2015 at 20:58 |
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Ive never been caught up in the high rpm thing. Im more of a torque and average power over peak rpm power.
One of my favorites there. 8.4 liter, 640hp 600lbft, redline around 6200 I think.
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TheOnelectronic
> HammerheadFistpunch
06/23/2015 at 21:05 |
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Ah, yeah, that makes sense too.
You can tell a Finn but you can't tell him much
> TheOnelectronic
06/24/2015 at 11:59 |
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It pretty much comes down to F=ma. The linear acceleration of a reciprocating part is proportional to the square of the rpm’s. As rpm increases the inertial forces acting on the pistons, connecting rods, valve springs and all the other reciprocating parts goes up as a power law function. To take an engine from 5000 rpm to 10,000 rpm means that things like your connecting rods need to become four times as strong. Even to increase from 7500 rpm to 10,000 rpm means you are subjecting the reciprocating parts to a 1.8x increase in inertial forces.
There is also an increase in friction that is directly proportional to the velocity of the moving parts relative to each other. This friction increase is directly proportional to the rpm, but I’m not sure on the relationship between friction force and heat generated. Obviously the friction is going to generate a lot of heat that you need to dump out of the system.
Besides the mechanical forces to consider you also need to get air, fuel and exhaust in and out of the cylinder which becomes more difficult as speeds go up. The faster the engine spins the less time you have to mix your charge in the cylinder and the less time you have to complete the combustion cycle and get the exhaust out of there. From what I’ve seen it appears that the inertial loads on reciprocating parts are the limiting factor in increasing engine rpm’s. The other things come into play, but being able to build light enough and strong enough reciprocating parts seems to be the difficult part of the problem.
You are definitely on the right track as far as your thinking as to why rpm’s are difficult to increase. Here is a good read talking about piston motion .
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TheOnelectronic
> You can tell a Finn but you can't tell him much
06/24/2015 at 17:04 |
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Another very informative post. Thank you!
samssun
> TheOnelectronic
06/26/2015 at 03:01 |
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That’s why high-rev engines are oversquare: you shrink the stroke as much as possible while making displacement with a bigger bore. And long stroke engines don’t make more torque because of a lever arm effect, it’s just that their rpm and therefore power are limited relative to their displacement/output at lower revs.