What factors limit the max rpms on an engine?

[grrl]

What causes an engine's rpms to max out? I'm assuming the fuel can be fed fast enough, so is it simply a matter of the balancing of the components? Or the particular harmonics of the whole assembly? Something else?

 

[blahblah99]

The valves, rockers, pistons, rods, and crank are a good place to start. The pistons and rods move up and down very fast at high rpms and not only do they move fast, but they have to CHANGE directions very fast and because of their momentum, it limits the rpm at which the engine can max out.

Also, the valves are moving up and down very rapidly, the spring retainer is moving up and down (don't want that to pop loose). The rockers are bouncing up and down. The cam has to work harder, more heat is produced, the crank is rotating faster so any minor off-balance in the crank that wasn't as noticeable before becomes more noticeable, etc etc.

Engines that have higher redlines usually have either a shorter stroker, or a smaller displacement, or both.

 

[jungle]

-don't forget school bus yellow racing strips....cuz yellow is the color of speed!!!
-wings/spoilers that can pass as park benches
-body kits
-rims/tires that are at least 2" taller than stock

 

[DarkManX]

yep its mostly the springs check this out my dads idea

 

[capybara]

1. balance
2. gas flow
3. air flow
4. speed of the detonation wave
5. electronic ignition with built in governors
6. the cops!!!
7. the inherent limitation of piston engines with energy wasting reversal of direction of pistons, as compared
to a turbine, where combusion is continuous, and all masses move unidirectionally.
8. money can correct items 1-6, but item 7 is an inherent flaw

 

[Evadman]

In most engines it is not the reciprocating assembly that has a problem. the crank, pistons, and con rods are usually not the limiting factor. The limit is usually the valve train. Valves not closing fast enough because the lifter floats off the cam or because the valve spring is not strong enough, or even because the valve bounced off the seat. Then the valve hits the piston, and things break. Like connecting rods out the side of the block. Those are nice.

I own a 99 dodge with a v-10. Original RPM limit was 4250 RPM. Yes, that low. stainless competition valves and triple valve springs ( one spring inside the other, all 3 pulling on one valve ) made it possable to hit 6500 RPM where my limiter is currently set. Stainless is stronger and lighter, which makes it easier to return to rest once the valve is open.

There is also a phenomenon known as "valve flutter" which happens when engine rpm hits a perportion of the natural frequency of the material the valve is made out of. Basicly, it makes the whole valve shake and bounce, sometimes high enough to hit the piston. IIRC this is between 7800 and 8200 RPM for most steel valves. That is one of the reasons there are so few "stock" engines that can hit this RPM range. Hi revving engines take care of this by using Titanium valves, which flutter would not happen too until 25k rpm. Then the limiting factgor is your recip assembly

Honestly, I do not know whatt he valves in the s2000 are made of. Probably an alloy that gets around this limitation.

 

[dmcowen674]

A Wankel engine AKA Rotary engine has no valve train. Mazda RX-7 was last production car to use the design.

 

[RemyCanad]

The wankel will be back in the RX-8. Supposed to have fixed the gas guzzuling/oil burning problems with the old wankels.

 

[Killrose]

Valve train components and piston skirts are two of the main reasons for an RPM limit.
As an example an engine with a long stroke has a lower RPM limit than one with a shorter stroke. The longer the stroke the higher the piston speed and the possibility of the skirt shattering, the valve train is not even taxed.

The exact opposite is true with the shorter stroked motor, where the valve train becomes the limitting factor. The Springs and other valve train components begin to "float" or are not able to return the valves to their seated position fast enough, and might get rammed by the piston crown.

 

[Peter]

That's why formula one cars have pneumatically operated valves. No valve train, and active steering of both directions - rather than having the cam hammer it down and hoping for the spring to push it back up. BMW broke the 19000 rpm barrier recently - in the car, not on the lab bench.

That said, I want my everyday car to have PUNCH not REVs. I want newtonmeters, not kilowatts. (And I have that, my beaten old turbodiesel Citroen puts out 120 horsepowers at 3900 rpm, and 260 Nm at 2000. Hits 215 km/h (reving 4500 then), does 180 with the throttle not even half down - and I'm allowed to, 'cause I'm in Germany )

Consumption at 180 km/h (110 mph) is at 8.5 liters per 100 km, 28 miles per gallon if you please. And that thing is 15 years old, 246k km (153k miles) on the clock.

 

[Howard]

Formula 1 cars do indeed use a valvetrain to actuate the opening of valves. The pneumatic system you speak of is only for valve return. Pressurized N2 is used, BTW.

 

[Howard]

Allow me to be the first to add port area (of the valves).

Reason being, if the ports are too small, at higher revs the engine will have problems breathing, therefore output\RPMs will decrease. And this is a given, but fuel type is also an answer.

 

[geno]

One factor that limits some cars as well: accessories

The 1st and 2nd generation Ford SHO's had motors/internals that were good up to 9k, the only reason the rev limiter was set to 7k (I believe it's a flat 7000 redline) is to save on the wear and tear of the belt driven accessories. You have to remember, every car that comes out of a plant has got to be warrenty worthy, so corners have to be cut

 

[merlocka]

Originally posted by: geno
One factor that limits some cars as well: accessories

The 1st and 2nd generation Ford SHO's had motors/internals that were good up to 9k, the only reason the rev limiter was set to 7k (I believe it's a flat 7000 redline) is to save on the wear and tear of the belt driven accessories. You have to remember, every car that comes out of a plant has got to be warrenty worthy, so corners have to be cut

Why isn't there some type of rev limiter on the accessories? Or would the rev limiter break before the accessories?

 

[Peter]

... ummm ... because belt driven accessories rotate at a speed proportional to the motor driving the belt?

Now that (at least in European cars) most of the accessories are moved from belt driven to electrical - mainly to improve operation of things like power steering and brakes while the engine is revving low - things might improve in this area.

 

[dkozloski]

The idea of high revving engines stems from the fact that all things being equal, the engine of a given displacement that peaks at the highest RPM developes the most power. For years the two items that seemed to get the most attention from design engineers were piston speed in feet/minute and bearing surface speed in feet/minute. Piston speed reflects the effects of long strokes on both reciprocating loads and the speed of the piston rings against the cylinder walls. Reciprocating loads for the most part are pretty small compared with the loads both from normal cylinder combustion and very small compared to the loads of destructive dentonation. Getting the piston rings to follow the cylinder walls without flutter and thus provide sealing and without breakage due to the impact with the sides of the ring grooves at the ends of the stroke are prime considerations. The engineer would like to have large bearing diameters for strength and stiffness in the crankshaft but the larger the bearing diameter the faster the relative speed between the crankshaft and the bearing shell. High bearing speeds bring lubrication problems and unwanted hydrodynamic effects in oil flow. Once you get beyond the effects of piston and bearing speed you have to address engine breathing. The problems here are not all mechanical. The proof of that is that in most every case a two-stroke engine will not rev as high as a comparable four stroke even though it could have no moving valve parts at all. Valve opening rates and flow characteristics rule. Valve action can be much more radical in a four-stroke. Here we encounter a whole host of problems with primary and harmonic vibrations that are both torsional and reciprocating for afterall, valve action is itself a vibration. An example of torsional vibration is that the crankshat is elastic and twists with the firing of each cylinder. Because the crankshaft has a natural vibration frequency it must be designed for harmonic vibrations to avoid normal operating ranges or provide damping. I had an engineer tell me that he saw a crankshaft that weighs over 100 lbs. winding and unwinding over a quarter of a turn on a vibration torsional shaker table. The sight to him was so startling that he turned and ran. The early Ford Indy engine was showing timing errors under running conditions of more that 45 deg. and was thought to be the result of badly cut gears but was it fact due to driving the gear train from the end of the engine opposite the flywheel. This induced torsional vibrations in the drive shafts and cams that required an enourmous design effort to cure. On the other hand the Chevrolet Ilmore engine drove the cams from the flywheel end of the engine and avoided most of that hassle. Getting the valves open and closed has seen some pretty ingenious schemes over the years. Old F1 mercedes and Ducati motorcycles used desmodromic valve gear that had two cam lobes per valve; one with a valve opening mechanism and the other lobe rigged to positively close the valve. Some of this stuff produced revs over 20,000RPM. Pushrod engines tend to be looked down on but we had small block Chevys turning 10-12,000RPM almost forty years ago with magnesium rocker arms and auxiliary valve springs operating on the lifters in the block. Additional problems are flexing of pushrods, rocker arms, rocker arm stands and anything else involved. Ed Iskendarian of racing cam fame told a story about a very high revving engine that produced almost no power at all. With the aid of a synchronized strobe light he could see that almost every part in the valve train was flexing in one way or another resulting in very little valve action. Modern electronics has just about eliminated ignition problems. Lately engineers have rediscovered the benefits of advancing and retarding cam timng dynamically. Mercedes did this 65 years ago.

 

[Rabbyroo]

Having been a mechanic for 20 odd years i never could understand why ayone would want to rev there engine beyond its max as something is going to give (expensive) what you want is a high compression engine which will do faster speeds without blowing the thing to bits.

how would anybody feel if they had no choice and were forced to run flat out all day? (wouldn't work). Best advice i can offer you is invest in a high performance race tuned engine.

 

[dkozloski]

My Buick Regal GS is an interesting case. The 3.8L V-6 has a belt driven Rootes type blower that produces prodigious amounts of low end torque. When the blower drive pulley is reduced in size by only a few tenths of an inch all the power improvement is on the low end. The stock rapid accelleration becomes lurid tire smoking burnouts. Quarter mile times are in the mid thirteens. Not too bad for an old man's car. The whole purpose of the exercise is to efficiently burn the most stoichiometrically correct mixture per unit time. This can be done with normally aspirated high reving engines or lower revving force fed motors. You pays your money and you makes your choice.

 

[Peter]

I did - me wan punch

 

[Bleep]

That's why formula one cars have pneumatically operated valves.

I sure dont know where this piece of bad information came from but this is FALSE.
Having built many many racing engines in my life there are so many limiting factors that it is nearly impossible to pick out one thing. Camshaft windup is one of the things not even mentioned in any of the answers. anyone ever listen to a Indy engine or a forumla 1 engine they idle really funny because of the way the cam is ground to take up the change due to cam wind up when getting up to about 10K rpm. Just remember the piston stops twice every stroke, Once the bearings actually touch the crankshaft you are done in a short time.
Now relating to the 2 stroke statements, I know of no 2 stroke engines on the market except for really small ones that do not have valves.

Bleep

 

[Evadman]

Originally posted by: Bleep

That's why formula one cars have pneumatically operated valves.

I sure dont know where this piece of bad information came from but this is FALSE.
Having built many many racing engines in my life there are so many limiting factors that it is nearly impossible to pick out one thing. Camshaft windup is one of the things not even mentioned in any of the answers. anyone ever listen to a Indy engine or a forumla 1 engine they idle really funny because of the way the cam is ground to take up the change due to cam wind up when getting up to about 10K rpm. Just remember the piston stops twice every stroke, Once the bearings actually touch the crankshaft you are done in a short time.
Now relating to the 2 stroke statements, I know of no 2 stroke engines on the market except for really small ones that do not have valves.

Bleep

How small is really small? I have a 3 hp in my snowblower that only uses a reed valve (oooold). or is that what you are refering to as "valves"? They are not the standard type of tulip valve.

 

[Howard]

Poppet valves are the more common name.

 

[dkozloski]

Bleep, almost all racing two-stroke motorcycles and snow machines are piston port.

 

[earthman]

What's the point of all this? Engines that rev really really fast tend not to last all that long, regardless of how advanced they are...how long do you think racing engines last? One race, if they are lucky. My old van can't rev over 4800 probably, but its run for 300,000 miles. In the real world, you need to balance performance with longevity.

 

[Howard]

Any evidence to prove that statement?

There are many factors influencing engine life. Most racing engines do not use air filters, anyway.