How is engine redline determined?

[ledslinger]

It doesn't seem it could all be done with math or stress analysis. Even if they know what a part can handle, they know that some parts will be weaker due to defects.

It could be they give a redline that instructs the operator that going over this RPM will reduce power.

Use formulas or computer models, or also factor in designers or engineers intution?

 

[JHZR2]

I would think it has to do with forces on or velocities of certain moving parts.

 

[tinmanSC]

The speeds of sensors on the engine can play a part (not on most new engines, however), but from what I understand it's most often governed by 1) how fast the valves can be opened and closed (valve springs), 2) how the crank shaft is built (to avoid crank flex), and 3) the size and speed of fuel injectors (when at very high RPM, the injectors only get a few milliseconds to deliver all fuel needed for that combustion cycle. It's inversely proportional. That is, more power = less time is available for fuel delivery).

 

[Lethal1ty17]

it has to do with the internal components' limits. if you have one engine at 6000rpm and one engine at 7200 rpm the forces on the second engine are something like double.

ive heard that diesel engines are governed due to their exhaust gas temperature but im not totally sure on that.

 

[JimPghPA]

The 225 slant straight 6 in my 1976 Volare 3 speed manual at first, and 3 speed auto later, would limit the engine top RPM to get 79 MPH while in gear. I think the valves would not move fast enough due to the limit of the spring speed. It did not matter if you were climbing up a hill, or going down a very steep hill. The top speed in gear was 79 MPH, and the car had too much air drag to go faster than that down hill in neutral.

So it looks like some engines will self limit the maximum RPMs, and others are capable of achieving RPMs that will cause self damage.

 

[Lethal1ty17]

Originally Posted By: JimPghPA
others are capable of achieving RPMs that will cause self damage.


modern fuel injected engines have built in rev limiters that will either cut the fuel, ignition, or both in hopes of saving catatrophic failure and undue stress on the internals. this is why if you floor it your tachometer needle will bounce off the redline, the fuel/spark will be cut, it will drop down, then the fuel and spark will be triggered, rpms raise, hits the limiter, fuel/spark cut, and so on.

if you remove the reve limiter then you're in dangerous uncharted waters and run a much higher risk of throwing a rod or something else catastrophic and exciting.

 

[Tempest]

Energy goes up by the square of the speed (as I recall) so if you double the speed, you have 4 times the force, 3 times the speed = 9 times the force....

So it doesn't take much to greatly increase the amount of force on components.

I'm sure there are tables for given materials that show how much they can handle and I'm also sure there is computer software that will do it as well.

 

[Lethal1ty17]

Originally Posted By: Tempest
Energy goes up by the square of the speed (as I recall) so if you double the speed, you have 4 times the force, 3 times the speed = 9 times the force....

So it doesn't take much to greatly increase the amount of force on components.

I'm sure there are tables for given materials that show how much they can handle and I'm also sure there is computer software that will do it as well.


correct. i have a book on turbocharging fundamentals by corcky bell and he discusses this in great detail, however after 20 minutes of looking i cant find the specific page but i did do some calculations to show how much stress increases proptortionally with the square as rpms go up.

5000rpm vs 6000rpm redline:
- a 20% increase in rpm results in 44% more stress on internal engine parts (rods, crank, pistons) at 6000rpm vs 5000rpm. even with just a 1000rpm increase.

6000rpm vs 9000rpm redline:
-a 50% increase in rpm results in 225% more stress on the internal parts.

if i remember correctly, mean (average) piston speed does not like to exceed around 4000-5000fpm, when they do, they like to go boom.

 

[Dualie]

A diesels governed RPM has to do with valve timing and piston speed/velocity.

big diesels are built to go a million miles, to do that you need BEEFY parts that need to operate at lower rpm's. (under 3000)


For longevity MAX rpm is factored around piston speed and inertia. that's a very heavy piston to flying around the bore faster than 2800 RPM

second is with the side and mass of the valves if they don't retract quick enough the piston will kiss them and make a holy mess outta things.

third is how fast the injectors can fire with repeatability and refill and fire again.

 

[Lethal1ty17]

Originally Posted By: Dualie
A diesels governed RPM has to do with valve timing and piston speed/velocity.

big diesels are built to go a million miles, to do that you need BEEFY parts that need to operate at lower rpm's. (under 3000)


For longevity MAX rpm is factored around piston speed and inertia. that's a very heavy piston to flying around the bore faster than 2800 RPM

second is with the side and mass of the valves if they don't retract quick enough the piston will kiss them and make a holy mess outta things.

third is how fast the injectors can fire with repeatability and refill and fire again.


thanks. i read somewhere that it was determined by exhaust gas temperature which is a critical factor in turbocharged engines.

so basically same rules apply for diesels as with gasoline engines.

no one likes expensive fast moving bits meeting other expensive fast moving bits.

 

[Eddie]

Mercedes would take a "pre-production" engine and run it up the rpm scale until it blew up. They set the red linne at 20%? lower.

 

[mechtech2]

A failure would occur with a faulty part anyway.

The redline has a bunch of reasons.
First is the valve train.
Even in a non interference engine, the valves can float and kiss a piston if over revved. And this can also happen if you downshift to the wrong gear - the limiter won't work in this situation.

Other things that can go wrong at too high RPM:
The oil pump may cavitate . Ring control/sealing may be lost.
Rod or main bolts may be overstressed and break.
The engine may simply be 'out of breath', and is way over making useful power.
Actually all sorts of things are coming to mind.
But the engineers really do know how strong the parts are, and what forces it will take to break them.

 

[KrisZ]

Originally Posted By: Dualie
A diesels governed RPM has to do with valve timing and piston speed/velocity.

big diesels are built to go a million miles, to do that you need BEEFY parts that need to operate at lower rpm's. (under 3000)


For longevity MAX rpm is factored around piston speed and inertia. that's a very heavy piston to flying around the bore faster than 2800 RPM

second is with the side and mass of the valves if they don't retract quick enough the piston will kiss them and make a holy mess outta things.

third is how fast the injectors can fire with repeatability and refill and fire again.


Another limit of the diesel is that diesel fuel burns a lot slower that gasoline. So inherently diesels have to rev less to allow complete combustion.

 

[Nick R]

I thought diesel redline was lower than gasoline engines because there is no spark, the combustion cannot be controlled as precisely, or something like that.

 

[steve20]

I avoid hitting the limiter on the Vettes-quite an unpleasant experience, if you ask me.
I used to shift by sound, but when you've got bikes running flat out next to you, it is impossible to catch it in time. A shift light would make perfect sense on these cars, wonder why it is not offered?


steve

 

[Lethal1ty17]

Originally Posted By: mechtech2
A failure would occur with a faulty part anyway.

The redline has a bunch of reasons.
First is the valve train.
Even in a non interference engine, the valves can float and kiss a piston if over revved. And this can also happen if you downshift to the wrong gear - the limiter won't work in this situation.

Other things that can go wrong at too high RPM:
The oil pump may cavitate . Ring control/sealing may be lost.
Rod or main bolts may be overstressed and break.
The engine may simply be 'out of breath', and is way over making useful power.
Actually all sorts of things are coming to mind.
But the engineers really do know how strong the parts are, and what forces it will take to break them.


absolutely excellent point. things can only work so fast. at 6000rpm that piston travels the length of the cylinder about 200 times per second.

Originally Posted By: Nick R
I thought diesel redline was lower than gasoline engines because there is no spark, the combustion cannot be controlled as precisely, or something like that.


diesel engines have a much longer stroke than normal gasoline engines do, a gas engine might have a 3 inch stroke, and a diesel could have a 6 inch stroke. going back to what i said earlier, mean piston speed doesnt like to exceed XXXX feet per minute, in high performance engines this is around 4000-5000fpm but to keep reliability up it has to be a bit lower than that.

a diesel engine at 3000rpm could be equivalent to a gas engine at 6000rpm in terms of mean piston speed.

 

[DreamerGT]

Originally Posted By: mechtech2
A failure would occur with a faulty part anyway.

The redline has a bunch of reasons.
First is the valve train.
Even in a non interference engine, the valves can float and kiss a piston if over revved. And this can also happen if you downshift to the wrong gear - the limiter won't work in this situation.

Other things that can go wrong at too high RPM:
The oil pump may cavitate . Ring control/sealing may be lost.
Rod or main bolts may be overstressed and break.
The engine may simply be 'out of breath', and is way over making useful power.
Actually all sorts of things are coming to mind.
But the engineers really do know how strong the parts are, and what forces it will take to break them.


I agree there no since in letting the car rev to the moon when, it stopped making power.

 

[Scott_Tucker]

Redlines are set based on piston speed (in feet per minute) and the strength of internal components. The formula for piston speed is:

Piston Speed = stroke length (in inches) x RPM (of redline) x .166

For example a 350 cid engine with a 3.48" stroke that redlines at 5000 rpm would be:

Piston speed = 3.48" x 5000 rpm x .166 = 2888 feet per minute.

Around 3000 fpm would be about average for a passenger car.

The reason some cars have a higher redline is because they have stronger internal components combined with a short stroke. The shorter the stroke the lower the piston speed will be for a given rpm.

For example, if the above engine only had only a 3.27" stroke it would be able to withstand approx. 5500 rpm without any increase in piston speed. Today's high reving engines can withstand it because the bore to stroke ratio is really high. A Formula 1 engine spins to 18,000 rpm. Part of the reason it can do this is that while it has a 4" bore, it's only got a 1.7" stroke. This keeps piston speed down for a given rpm.

 

[Spazdog]

Yeah....not so much.

Maybe in my old '85 LX 5.0. Redline in that car was 1000 rpm higher than useable power.
3.00 x 6000 x .166 = 2988
Okay, that works there.

But lets take a look at Honda's D16Z6:
3.50 x 7200 x .166 = 4,183.2
Holy cow! that engine must have titanium connecting rods and a forged crank!!!
It doesn't have anything special internally. in fact, the parts are rather slight even considering the displacement of the engine. thin connecting rods, small journals... In fact if you compare the D16's rods to the B16A2 (with only 800 rpm higher redline) they are toylike.
It's so undersquare that it actually has a longer stroke than the Ford 302.
So it has a sigificantly higher redline and a longer stroke than the venerable 4.9L Ford 302. Why?
an extra set of intake lobes. At 4800 it switches over to a higher lift longer duration intake lobe. The engine can breathe.

 

[SteveSRT8]

everybody take a breath.

The redline in your motor is surprisingly arbitrary.

My favorite example is the IS-F Lexus.

The engine redline was reduced just prior to sale for LONG TERM durability concerns.

Just depends where the company's focus is.