Piston speed

[arnebret]

I was just thinking about something i hear quite a lot of people say who think they are into cars and know everything about the mech side of it. One discussion is always: how fast will a piston go at say about 6500 (factory rev limit) on a 944 engine.

I heard people saying over 300mph or some even claimed over the sound barrier. The guys that claimed that a piston goes over mach1 where them guys who put big open exhaust under a 120bhp car and think it will result in 500bhp, because the sound is so heavy... yep..

I started calculating:

Take the 944T engine: 100mmx78.9mm
At a rpm of: 6500

6500rpm=108.33rp/second One rpm indicates TWO strokes of the piston. So in one rpm the piston travels 78.9mmx2=157.8mm
It does this 108.33 times a second, which means 17094.474mm every second. This means 17.1m/s=61.54kmh (38.24mph).

Of course this is the AVERAGE speed over two strokes (or 1 rotation). The point is the piston comes to a halt now and then (top dead and bottom dead). As i don't know how the acceleration of a piston is, i assume the top speed of piston might be significantly higher, though mach1 will NEVER be reached and things like 300mph seem out of the question as well.

Still, it's nice to know your piston will up and down for over 200 times a second when you push it to the edge.

How about an f1 car which will do a stunning 18500 rpm

Is there a mech in the house who can clear things up a bit?

[timsirk]

i have don't have much experience with specific engines, but i do know that high pistons speeds are avoided in road racing (especially motorcycles) by using a shorter stroke. anything w/ a longer stroke has a higher piston speed than an engine w/ a shorter stroke at the same rpm. i think thats how the F1 cars get away w/ such high rpm (besides all the great technology like pneumatic valves).

HP=Torque*rpm, so i figure the F1 cars don't need as much torque as it would seem to make those big horsepower numbers.

of course alot of times torque is more important than rpm (like a tight curvy track) and a torquer, longer stroke motor (running less rpm, but the same piston speed) will work better since it would be easier to manage mid-range or bottom-end horsepower.

As for the max piston speed, if we assume it happens halfway down the stroke (this estimate is ideal, but maybe not accurate??) then

velocity = (speed in radians/min)*(radius of circle or stroke)
=(2*3.1416*6500)*(78.9 mm)=3222331 mm/m=
193.3 km/h

i've never calculated max piston speed, so hopefully i'm not forgeting something important.

Tim

[Lawrence Coppari]

The 78.9 is the stroke, therefore the diameter or radius of the crank circle?

[arnebret]

F1 cars have an engine that has all it's power on the top range. The problem they have is that they have to get as much hp from 3l block. So they push every bit to the edge and make a V10-3litre run at over 18000rpm. At this speeds you don't need a lot of torque to get high hp numbers. One thing the drivers have to keep in mind is that the rpm stays above 12000rpm or so. They of course adjust their gearboxes to that. To be honest, i have no clue what kinda stroke an f1-engine has, but it must be way shorter than ours.

If you look at a motorbike for instance the new Ducati999 that has a stroke of 63.5mm. Which is much less than our engine. The engine puts out 136hp at 9750rpm and has a rev limit of 12500rpm. There are motorbikes out there with the max bhp at rpm's like 11000 and a revlimit of 14000rpm. The same problem here: how to get as much bhp from a small block as possible (and making a VERY good engine design to avoid blow-up).

It's all a complex thing, but let's just hope the designers know what they are doing and are giving us the best 'solution' of high-rpm versus high-displacement blocks.

Now, what would happen if you make an 8litre V12 go at 18000rpm... pretty high torque at a very high rpm = extreme hp, or is this not true?

[overkill]

V12, 8L, 18,000RPM? Is that even possible? F1 are BIG engines with small insides, isn't that how they move so fast? I mean if you have a BIG with BIG insides moving at 18,000rpms,... wouldn't that blow up?

Ok, here's one,... how fast does the drive shaft on a Wankle spin? Keep in mind that it spins 3 times for every 1 rotation of the rotor. What is the max REV or street set max REV for a Wankle engine? I know its alot faster then a piston engine. What about turbine engines? HELL, my airgrinder does 25,000rpms

[dmraves]

Stroke (in ft) = 78.9mm / 304.8mm/ft = 0.259 ft

The stroke is the diameter of a circle traced out by the crank, and therefore, the crank velocity is,

6500 rot/min * (3.1416 * 0.259) ft/rot = 5289 ft/min

At the 90deg and 270deg positions of the crank, the vertical velocity component of the velocity vector is 100%, and the instantaneous speed of the piston is the same as the instantaneous speed of the crank, 5289 ft/min.

Converting to MPH,

5289ft/min * (1/5280)miles/ft * 60min/hr = 60.1 MPH

[arnebret]

Hmm, i know that the jet-engine on my airplane does over 100.000rpm, but i'd like to see someone build a piston engine doing that

Though, very interesting on that piston speed. It seems we are talking in the order of 50, maybe 100mph as max speed. Which is pretty high, but not as much as many people claim. Interesting. Any specialist that designes stuff like this that can tell us even more REALLY (uhm?) interesting about this?

[divezic]

arnebret, what airplane do you fly? The engine in my airplane (turboprop) runs at about 13,000 rpm.

I think the more astounding fact in a car engine is that the piston accelerates from zero to 60mph back to zero in a matter of 3 inches or so. F=ma and a is the derivative of the speed function blah, blah, blah. The forces are tremendous! Now do that for 250,000 miles.

Last week I went to the Smithsonian Air and Space Museum and saw a 1920's 500 cubic airplane engine that put out a whopping 100 hp at it's redline of 2000 rpm. The heck with computer technology. I am 100 times more impressed with the advances in metallurgy technology over the past 100 years.

[timsirk]

dmraves is right, i was using diameter instead of radius so my number was twice too big..

Does anyone if the actual peak piston velocity happens where it should?? remember that the rpm being used above is an average. btw, i've heard that a motorcycle with a heavier flywheel can actually gain two or three mph in top speed testing over a lighter flywheel, since more energy can be 'stored' when its not on the power stroke...of course this would really hurt accelation!!!!

Tim