what limits an engines ability to rev?

Jun 14, 2003
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what factors of an engine affect the maximum speed at which it can rotate?

i understand that big pistons with long strokes will rev slower.

thats why the veyron has 16 cylinders, they wanted 8litres displacement but also wanted 8000+ RPM out of it.

F1 cars have small pistons and super short strokes less than 40mm... helps them scream up to 19000rpm

are there other factors? rate of compression? lubrication? timing issues? also what affects how quickly an engine can rev up? (ie jumps from 2000-6000 quickly or does the same range slower and more lathargically)

 

dug777

Lifer
Oct 13, 2004
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bearings i guess too.

The stresses would be incredible on certain parts at high rpm as well, and i assume those stresses get greater with larger bores/strokes...
 

Specop 007

Diamond Member
Jan 31, 2005
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Moving mass mainly. But theres a whole bunch to it, and you really cant say any 1 part is the weakest link in general as its usually specific to the engine.

Sometimes the limit is due to the valvetrain, sometimes due to the pistons and crank etc etc. And of course that can be changed by the materials used, how its designed etc etc.

Simplest answer is weight of components.
 

Vic

Elite Member
Jun 12, 2001
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Balance (vibration), weight, friction. Pistons don't move in circles.
 

Adul

Elite Member
Oct 9, 1999
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danny.tangtam.com
its a variety of factors that need to be considered. Ultimately you set a target and design the engine to meet that target.
 

Umberger

Golden Member
Apr 13, 2005
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The amount of mass that has to change direction. Larger displacement engines have pistons and rods that weigh more. They have to change directions. That generates a lot of force.
 
Jun 14, 2003
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Originally posted by: dug777
bearings i guess too.

The stresses would be incredible on certain parts at high rpm as well, and i assume those stresses get greater with larger bores/strokes...


well lots of bearings can be had that are capable of well beyond what most engines can make.

but i guess with IL4's and 6's which are inherently unbalanced the bearings cant take too much. they do balance the crank of course to make it as smooth as possible but its never perfect, and if it is it doesnt last forever
 

foghorn67

Lifer
Jan 3, 2006
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Originally posted by: otispunkmeyer
what factors of an engine affect the maximum speed at which it can rotate?

i understand that big pistons with long strokes will rev slower.

thats why the veyron has 16 cylinders, they wanted 8litres displacement but also wanted 8000+ RPM out of it.

F1 cars have small pistons and super short strokes less than 40mm... helps them scream up to 19000rpm

are there other factors? rate of compression? lubrication? timing issues? also what affects how quickly an engine can rev up? (ie jumps from 2000-6000 quickly or does the same range slower and more lathargically)

It's not the size, really. It's the weight of the rotating assembley.
 

Demon-Xanth

Lifer
Feb 15, 2000
20,551
2
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There's alot of things, what limits a specific engine varies from engine to engine.

Example:
My oil pump will cavitate above 6000RPM
Many engines have to worry about valves floating
Physical strength of the connecting rods swinging around
The cam timing and intake design make taking my engine over 5500RPM stupid (power drops off sharply)

DOHC _CAN_ allow an engine to rev higher, but that doesn't mean it will. It reduces the valvetrain weight and reduces the likelyhood for valve floating. But there are DOHC engines with 6000RPM redlines and pushrod engines with 7000. So that's not a sure fire way.
Shorter strokes reduce the forces on connecting rod pins, as the piston has to accellerate to a higher speed if it has a longer stroke. But a shorter stroke means a wider piston, which means more weight. So there's a tradeoff there. You can go to a smaller piston, but that means smaller displacement, again, a tradeoff.
 
Jun 14, 2003
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Originally posted by: Umberger
The amount of mass that has to change direction. Larger displacement engines have pistons and rods that weigh more. They have to change directions. That generates a lot of force.



oh yeah change in momentum, the bigger the change the bigger the force
 
Jun 14, 2003
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Originally posted by: Demon-Xanth
There's alot of things, what limits a specific engine varies from engine to engine.

Example:
My oil pump will cavitate above 6000RPM
Many engines have to worry about valves floating
Physical strength of the connecting rods swinging around
The cam timing and intake design make taking my engine over 5500RPM stupid (power drops off sharply)

DOHC _CAN_ allow an engine to rev higher, but that doesn't mean it will. It reduces the valvetrain weight and reduces the likelyhood for valve floating. But there are DOHC engines with 6000RPM redlines and pushrod engines with 7000. So that's not a sure fire way.
Shorter strokes reduce the forces on connecting rod pins, as the piston has to accellerate to a higher speed if it has a longer stroke. But a shorter stroke means a wider piston, which means more weight. So there's a tradeoff there. You can go to a smaller piston, but that means smaller displacement, again, a tradeoff.


i take floating valves are when they move so fast they dont close properly>?
 

Demon-Xanth

Lifer
Feb 15, 2000
20,551
2
81
Originally posted by: otispunkmeyer
i take floating valves are when they move so fast they dont close properly>?

Place something in the palm of your hand, move it up and down faster and faster. Eventually, you will get to a point where that object is no longer in your hand because your hand is moving down faster than gravity can pull the object. That is effectively what happens with the valve and the cam. Higher spring pressures can help, but even the springs have a certain amount of inertia, and higher pressures cause more stress on the actuating pieces.
 

lyssword

Diamond Member
Dec 15, 2005
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f1 cars don't have "floating" problems because they have pneumatic valvetrain. They don't use cams, also hybrid/rare materials are used to make engine parts light (titanium n'sh1t)
 

Demon-Xanth

Lifer
Feb 15, 2000
20,551
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Originally posted by: lyssword
f1 cars don't have "floating" problems because they have pneumatic valvetrain. They don't use cams, also hybrid/rare materials are used to make engine parts light (titanium n'sh1t)

They also only have to last about 135 miles and cost about on par with houses in some areas of the US.
 

fitzov

Platinum Member
Jan 3, 2004
2,477
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What do you mean by "ability to rev"? Past a certain RPM, or rate of change of RPM? Each engine is made to have a maximum torque and HP at a particular RPM, and obviously, there is a limit as to how much fuel the engine can burn instantaneously.
 

Vic

Elite Member
Jun 12, 2001
50,422
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Originally posted by: Demon-Xanth
Originally posted by: lyssword
f1 cars don't have "floating" problems because they have pneumatic valvetrain. They don't use cams, also hybrid/rare materials are used to make engine parts light (titanium n'sh1t)

They also only have to last about 135 miles and cost about on par with houses in some areas of the US.

Yep. Racing engines are built with very loose tolerances compared with regular production engines.
 

Demon-Xanth

Lifer
Feb 15, 2000
20,551
2
81
Originally posted by: Vic
Originally posted by: Demon-Xanth
Originally posted by: lyssword
f1 cars don't have "floating" problems because they have pneumatic valvetrain. They don't use cams, also hybrid/rare materials are used to make engine parts light (titanium n'sh1t)

They also only have to last about 135 miles and cost about on par with houses in some areas of the US.

Yep. Racing engines are built with very loose tolerances compared with regular production engines.

My point was that cost and durability are also limiting factors :)
 

sao123

Lifer
May 27, 2002
12,653
205
106
Originally posted by: otispunkmeyer
what factors of an engine affect the maximum speed at which it can rotate?

i understand that big pistons with long strokes will rev slower.

thats why the veyron has 16 cylinders, they wanted 8litres displacement but also wanted 8000+ RPM out of it.

F1 cars have small pistons and super short strokes less than 40mm... helps them scream up to 19000rpm

are there other factors? rate of compression? lubrication? timing issues? also what affects how quickly an engine can rev up? (ie jumps from 2000-6000 quickly or does the same range slower and more lathargically)


turbine based jet engines have much higher RPM's than traditional piston/cylinder engines.
 

Nyati13

Senior member
Jan 2, 2003
785
1
76
The largest factor is the amount of reciprocating mass, and the distance that mass travels during the 'reciprocation'.
Rotating mass is a smaller factor, mostly due to the friction/bearings limit, past a certain RPM.
 

skyking

Lifer
Nov 21, 2001
22,550
5,597
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for large displacement engines ( greater than .01 liters displacement ) it is the same thing that keeps airplanes in the air:













the wallet:laugh: