What is it about an engine that allows it to run at a high rpm for long periods of time compared to one that doesn't. A lot of times with motorcycles, especially with small displacement engines, they run at or around 10k with no problems at all. What allows them to do this safely? I believe OHC engines can run faster than pushrod engines but there are a lot of OHC that still only run at about 3k.
What makes an engine "built to run at high RPM?"
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lighter pistons and rods,and shorter stroke engines like motorcycles. A higher compression ratio cushions a pistons upward travel before tdc.
better balance, more precise tolerances...
JHZR2
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There is a basis for maximum velocity that parts can run at. I do not know how the determination is made (windage, stresses, etc.).
Originally Posted By: spasm3
lighter pistons and rods,and shorter stroke engines like motorcycles. A higher compression ratio cushions a pistons upward travel before tdc.
Correct.
In addition: Low tension piston rings, extremely fine balance, tight tolerances (NOT clearances), highly rigid internals and construction, efficient cooling, and some other stuff I'm sure I'm forgetting...
lighter pistons and rods,and shorter stroke engines like motorcycles. A higher compression ratio cushions a pistons upward travel before tdc.
Correct.
In addition: Low tension piston rings, extremely fine balance, tight tolerances (NOT clearances), highly rigid internals and construction, efficient cooling, and some other stuff I'm sure I'm forgetting...
all of the above!
A lot of the time I think it's the valve train that isn't rated to take the high RPM, not the lower end or the pistons.
If the valve springs aren't strong enough, you can get valve float. Additionally, a valve train with standard rockers and not 'roller' components won't handle as much. Correct me if I'm wrong, but engines with mechanical lifters will handle higher RPMs than hydraulic ones too. And the engine may just not be designed to breathe (flow) very well above a certain point.
If the valve springs aren't strong enough, you can get valve float. Additionally, a valve train with standard rockers and not 'roller' components won't handle as much. Correct me if I'm wrong, but engines with mechanical lifters will handle higher RPMs than hydraulic ones too. And the engine may just not be designed to breathe (flow) very well above a certain point.
valve springs...
short stroke...
even the way it flows, length of intake runners, etc.
short stroke...
even the way it flows, length of intake runners, etc.
A higher compression ratio does not cushions things better.
Or, do you spin the engine over with a machine, and not run it under it's own power?
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
Small and light parts allow higher RPMS, with proper cams. Larger engines can be made to do this, but require very careful parts machining, balancing, and assembly.
Or, do you spin the engine over with a machine, and not run it under it's own power?
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
Small and light parts allow higher RPMS, with proper cams. Larger engines can be made to do this, but require very careful parts machining, balancing, and assembly.
Originally Posted By: mechtech2
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
I'm curious why that is?
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
I'm curious why that is?
Originally Posted By: spasm3
Originally Posted By: mechtech2
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
I'm curious why that is?
I'd think it would be the sudden reversal of direction and the energy required to overcome all that inertia.
Originally Posted By: mechtech2
The worst load on a rod/piston is the UNLOADED TDC of the overlap/exhaust stroke.
I'm curious why that is?
I'd think it would be the sudden reversal of direction and the energy required to overcome all that inertia.
I think it's because
A) The piston is actually moving faster at TDC than it is at BDC, so it is a faster reversal of direction. This is why even though they are in primary balance, straight-4 engines have a secondary imabalance. There is an up-down vibration at twice crankshaft speed. This is why balance shafts are generally needed to get a smooth 4 cylinder engine. And it would probably occur at TDC between exhaust/intake because on compression-Combustion, spark ignition occurs generally a few degrees before TDC, so it has already started expanding as the piston hits TDC, pushing it back, whereas at E-I there is nothing.
A) The piston is actually moving faster at TDC than it is at BDC, so it is a faster reversal of direction. This is why even though they are in primary balance, straight-4 engines have a secondary imabalance. There is an up-down vibration at twice crankshaft speed. This is why balance shafts are generally needed to get a smooth 4 cylinder engine. And it would probably occur at TDC between exhaust/intake because on compression-Combustion, spark ignition occurs generally a few degrees before TDC, so it has already started expanding as the piston hits TDC, pushing it back, whereas at E-I there is nothing.
More rpm requires less friction too.
FRM cylinder walls & roller cam followers.
The engine needs to be able to breath at all rpm's otherwise it turns into a "race engine" so some type of variable valve timing/lifting is needed.
In short: one needs a Honda F20C2 engine
FRM cylinder walls & roller cam followers.
The engine needs to be able to breath at all rpm's otherwise it turns into a "race engine" so some type of variable valve timing/lifting is needed.
In short: one needs a Honda F20C2 engine
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I'll always remember this tidbit, myth or not I can't comment on.
VW TDI's(ALH) have a bottom end capable of 10k rpms but past 5.2k there isn't enough time to combust the diesel quick enough. There was no mention of the top end being able to spin faster.
I wager even 5.2k is pretty fast for a diesel engine, 10k...
VW TDI's(ALH) have a bottom end capable of 10k rpms but past 5.2k there isn't enough time to combust the diesel quick enough. There was no mention of the top end being able to spin faster.
I wager even 5.2k is pretty fast for a diesel engine, 10k...
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