[julupani]

@vina

All the reason A-F that you have listed are very well valid, but remember each and every one of those factors effect each other, so what factor is effecting the torque/power curves and by how much will vary from engine to engine. Thus we cant say that this one issue is limiting rpms in a very general way.

[SPIKE ARRESTOR]

Originally Posted by vina

I don't see anybody taking about it, but I guess the following are other reasons (Correct me if I'm wrong):


(E) ------
(F) ------

Hi,

Very, very important and valid point. If we look closely into the combustion process of Gasoline & Diesel engines we would be able to comment better, as to what exactly results in this different behavior.

Spike

[vina]

Originally Posted by SPIKE ARRESTOR

Hi,

Very, very important and valid point. If we look closely into the combustion process of Gasoline & Diesel engines we would be able to comment better, as to what exactly results in this different behavior.

Spike

well, are they true? can anybody comment?

Also I was thinking about what sg sir wrote earlier on this thread (and what has been written elsewhere too) that high-rpm engines are usually oversquare.

While high piston speed (with respect to flame velocity) may limit the rpm, I was thinking whether that is a VE issue too (I guess Born2slow already mentioned something like that - do other bhpians have any opinions?) because oversquare allows larger cyl. head and hence large valves are possible - allowing better air flow, both into and out of the engine.

[Sutripta]

Originally Posted by SPIKE ARRESTOR

Hi,

Very, very important and valid point. If we look closely into the combustion process of Gasoline & Diesel engines we would be able to comment better, as to what exactly results in this different behavior.

Spike

The OP had said

Originally Posted by vina

May be the gurus can give their feedback on this one (please don't make this a diesel/petrol thread)

Originally Posted by vina

Thanks to everyone for great and passionate responses. I have always wondered some things said in several places:

1. high rpm usually means oversquare design
2. diesels, and specially turbodiesels have lower rev-limits than petrols (and even turbo petrols)
3. high rpm engines do not produce good torque at low rpms

thanks to everyone, I'm beginning to get some idea of the conventional wisodm. (1) above may be due to (C) (D) and (E) below (keep reading).

(2) above may be because first of all most applications of diesel do not require high rpm is the first place and also because high compression requires long strokes leading to (1) above.
Heavier construction. More inertial loading. Nature of diesel combustion. Necessacity for swirl. There are square/ oversquare diesels.

I can't particularly see any reason for (3) above so far except that an engine that revs very high and hence generates high power, generates (relatively) low torque all through its rev range - it is just that people notice at low rpm and say "what, no torque?" .
Optimised breathing at higher rpm will mean poorer breathing at lower rpm.
....

(B) Stresses will largely rise linearly, since as long as torque remains constant power also rises largely linearly, this should not be a problem. Though it can limit the engine rpm by compromising long term reliability.
Not be a problem for what?



I don't see anybody taking about it, but I guess the following are other reasons (Correct me if I'm wrong):


(E) Time needed to burn the fuel completely will limit the minimum time in ignition/combustion stroke; flame velocity for a given fuel will limit max. piston speed for long-strok engines.
For petrols not the limiting factor.
(F) At the end of the ignition/combustion stroke the gas still has some energy left - not extracting it will lose FE, while extracting it will take some more time and hence reduce rpm (and peak power and well as torque - the last part of the extraction is from relatively low pressure gas). Since race cars don't care too much about FE and emissions their engines can have a higher rpm.

Don't get what you are saying in (F). How do you want to extract more work? When do you want to open the exhaust valve?

Regards
Sutripta

[vina]

Originally Posted by Sutripta

The OP had said

Don't get what you are saying in (F). How do you want to extract more work? When do you want to open the exhaust valve?

Regards
Sutripta

I was wondering why you hadn't replied at all !

I don't know how to break down quotes within quotes further, so will write about them here:

Heavier construction. More inertial loading. Nature of diesel combustion. Necessacity for swirl. There are square/ oversquare diesels.

Can you tell us more about the diesel combustion process inside the engine (in relatively plainer english - Heywood's book bounced clear of my head)

Also are their any small diesels that are square/oversquare? I thought some space (volume) will be required on top for opening/closing valves - leading to some length of the cylinder reserved for that, and hence limiting how short a stroke can be had for a given compression ratio.

Optimised breathing at higher rpm will mean poorer breathing at lower rpm.

Thisis interesting. In turbocharged engines I know the turbocharger doesn't function at low rpm, so this can definitely happen, however why would this happen in NA engines? do the high rpm engines use some special mechanisms to improve breathing at high rpms? Is this some sort of an "air ram" effect (similar to "water hammer" seen when valves on long pipelines are closed)


Not be a problem for what?

Well I hope you didn't take my comment too seriously - as you know I hardly know anything. What I meant was that while with increase with speed, forces will have to increase and so will the stress, most likely it will not cause outright failure immediately (though long term reliability may be a problem). So for engines designed for short duration operation (i.e. race cars and most other high-rpm applications) may not have an issue with this particular one.

Also better materials can probably take care of this, however with passenger car engines even though we know that better (or more accurately different) materials are being used for pistons, cyl etc, the rpm limits are roughly same. So may be existing materials can already take the extra stresses.



For petrols not the limiting factor.

Good to know. I did some computation last week based on stroke length, max rpm and the flame velocity, and it didn't seem to be the limiting factor. I did it for passenger cars only - can this be a limiting factor for race cars?

also on the engine control books, (in the knock control sections) they mentioned the spark happens slightly before the TDC, I guess fuel may partially burn out even before the piston reaches TDC


Don't get what you are saying in (F). How do you want to extract more work? When do you want to open the exhaust valve?


What I meant was - if we allow the flue gases to expand more before opening the exhaust valve - they will do more work thus increasing FE. However if the exhaust valve opens before BDC then while the FE will take a hit, exhaust gases will have more time to escape - possibly helping the engine designer improve breathing (e.g. more time for exhaust => less area dedicated to exhaust valve, more to inlet valve).

So if VE is limiting rpm the engine designer could have some valve lead and improve rpm this way (may be).

[Sutripta]

^^^
Hi,
Re: rpm limits, I actually have nothing further to add to what I've said in post #4. Also, I normally log in at night, and sometimes during lunchtime.

I thought we were not discussing diesels. Square/ Oversquare diesels which immediately come to mind are the Peugot/ Mahindra engines, and Cummins V series.

Breathing: You are on the right track. The air column needs a certain velocity (momentum) so that it continues filling in the cylinders even when the piston is moving up. Which is why the inlet valve has to be kept open beyond BDC. So two things- a) if you make your ports/ runners too big (a common rookie modifying mistake) you loose velocity, and breathing suffers. You make it too small, and it acts as a restriction, and breathing again suffers. b) Valve has to be kept open after BDC. Longer (in terms of crank degrees) if you want to optimise breathing at higher rpms. Similar, (but more complex) reasoning for exhaust. So breathing can only be optimised for a small band of rpm, and will have a detrimental effect for other rpms.

Turbos a different can of worms. Not just rpm, but there is a time function (delay) as well. Will wait for others to chip in.

You are right. Failure/ reliability is statistical. With a spread.

For a thread on rpm, lets leave it at that. For the nature of diesel combustion, maybe it will be discussed in another thread which is also active right now.

Do check out the Miller/ Atkinson cycles. Again, not rpm related.

Regards
Sutripta

[vina]

Originally Posted by Sutripta

^^^
Hi,
Re: rpm limits, I actually have nothing further to add to what I've said in post #4. Also, I normally log in at night, and sometimes during lunchtime.

I thought we were not discussing diesels. Square/ Oversquare diesels which immediately come to mind are the Peugot/ Mahindra engines, and Cummins V series.

Breathing: You are on the right track. The air column needs a certain velocity (momentum) so that it continues filling in the cylinders even when the piston is moving up. Which is why the inlet valve has to be kept open beyond BDC. So two things- a) if you make your ports/ runners too big (a common rookie modifying mistake) you loose velocity, and breathing suffers. You make it too small, and it acts as a restriction, and breathing again suffers. b) Valve has to be kept open after BDC. Longer (in terms of crank degrees) if you want to optimise breathing at higher rpms. Similar, (but more complex) reasoning for exhaust. So breathing can only be optimised for a small band of rpm, and will have a detrimental effect for other rpms.

Turbos a different can of worms. Not just rpm, but there is a time function (delay) as well. Will wait for others to chip in.

You are right. Failure/ reliability is statistical. With a spread.

For a thread on rpm, lets leave it at that. For the nature of diesel combustion, maybe it will be discussed in another thread which is also active right now.

Do check out the Miller/ Atkinson cycles. Again, not rpm related.

Regards
Sutripta

As it happens when you do decide to losen up: wealth of information ! thanks. Missed your post #4 earlier - that had a huge wealth of (very condensed) information too !

On the diesel thing - I don't want it to be a diesel vs. petrol thread - there is one active and plenty of inactive ones for that PLUS people start venting their preferences. Let us leave it to be a technical thread.

If this one has to be X vs. Y, it should be race-car (rally, F1, drag) vs. passenger car vs. truck vs. aeroplane ...

as long as it remains technical ...

From that point of view, discussion on diesel can be (in a very limited context) sometimes relevant.

But I agree with you, it is better to keep out of that at least for now.

[vina]

Was going through Heywood's book today. On page 45 he mentions:

"Resistance to gas flow into the engine or stresses due to the inertia of the moving parts limit the maximum mean piston speed to within the range 8 to 15 m/s"

For an engine with a stroke length of 100mm (=0.1m) this would limit the rpm to between 2400 to 4500.

For the old swift engine (75.5mm stroke) the max rpm this gives is roughly 6000 - and that is where the old swift produced its max power !

For the K12B engine has slightly shorter stroke (74.2mm), and sure enough it generates max power at slightly higher rpm (6200).

[pjbiju]

Why there is a limit on engine RPM for an internal combustion engine?

While there are many factors collectively contributing to this, the main factor is that the engine cannot get enough time to feed itself as the rpm increases. This means even if you keep on increasing the rpm (by some means), you will not get any additional power/torque out of it, which is the whole point of increasing the rpm.

Let me illustrate with a layman's example.

You are at the airport, picking up your luggage from the luggage conveyor belt. Let us assume you have 10 bags to pick up which are kept one behind the other on the conveyor belt. Now compare the speed of the conveyor belt to the rpm of an engine, and the bags to the air/fuel mixture that gets into the cylinder and yourself to the inlet valve. When the conveyor belt is moving slowly, you have ample amount of time to leisurely pick up your bags. As the speed of the conveyor belt increases, you will have less time to pick up the bags and instead of picking up 10 bags you may succeed only in picking up 8 bags. Keep increasing the speed of the conveyor belt and your ability to pick up bags will reduce and beyond some point you will not be fast enough to pick up any bag. In this case an external motor is running the conveyor belt, but not so in the case of an engine. Part of the power produced by the engine is used for running itself.

An engine produces its power from its feed of air/fuel. As the rpm increases, the ability of the engine to get enough of the feed reduces and frictional losses increase. So beyond a point, increasing rpm without proportionately increasing the air/fuel mixture (and you have lesser time to do it) will not provide any benefit.

Hope this makes sense to a non-technical person in very simple terms.

-Biju

[sgiitk]

Let me add another joker to the pack. With large valves when at low speeds you may not get sufficient 'swirl' in the air sucked in to ensure optimal mixing and combustion of the fuel. More of an issue in petrol engines I guess since the diesel engine does not constrict the air intake, this may not be that much of an issue.

While the earlier multi-valve engines had the same size for the valves modern ones do not necessarily subscribe to this.

[pranavt]

Are we talking about mechanical limits? If so, IMO it would mainly depend on the breathing and the cam. Think about it. Let's say, an engine is spinning at 6000 rpm. To accelerate it beyond that, more force is required than the amount required to keep it running at 6k. There is only a finite amount of air and fuel that can be pumped in per cylinder at those RPMs before the valve closes. Unless you are inducing the charge by way of force (turbocharging, supercharging, both), you are relying on the atmosphere, plus any resonance or ram effect for induction. Beyond a point, the energy required to accelerate further would be more than what is produced in each cylinder unless you changed some part of the engine (cams, valves, ports, etc).

[DKG]

Here's what I can think of in terms of what affects rev-ability of an engine:

1. Type of fuel used- start looking at fuels other than petrol for higher burn characteristics
2. Engine's volumetric efficiency(intake geometry, valve size, chamber design)
3. Piston Stroke (shorter stroke, more bore)
4. Crank size (small short cranks with short throws will rev more, low weight cranks)
5. Flywheel weight (lighter the better) actually lighter everything helps mass has a mind of its own, lighter changes direction quicker
6. Valve springs limit revs. Alternative valve system will enable quicked opening/closing
7. Overall lowering of mass of anything revolving or reciprocating will improve revs
8. Frictional losses (metallurgy etc to lower losses and increase rev, superior bearing mfg)
9. Ability to vary valve timing
10. Smaller displacement engine. Large lungs make the process long and slow. Small lungs allow for quicker inhale exhale My 800 cc VFR revs to 12000 rpm

[vina]

Thank you sir, that is a long list.

Originally Posted by DKG

...
4. Crank size (small short cranks with short throws will rev more, low weight cranks)

Isn't this related to short stroke (already mentioned in one of your points)?

Originally Posted by DKG

5. Flywheel weight (lighter the better) actually lighter everything helps mass has a mind of its own, lighter changes direction quicker

while lighter flywheel will definitely help when a change in rpm is needed, how would it help increase the top rpm? Can you elaborate more?

Originally Posted by DKG

9. Ability to vary valve timing

Will it help via improved breathing at higher speeds or is it something else?


Also which one of these factors is the most important? or is it that all become important roughly at the same rpm?

[dhanushs]

Originally Posted by vina

while lighter flywheel will definitely help when a change in rpm is needed, how would it help increase the top rpm? Can you elaborate more?

To increase the rpm, the engine has to produce enough power to accelerate the crank, flywheel ect.. So, the lighter the parts, the easier to accelerate and hence a more higher rpm can be achieved.

Or.. there is point beyond which the engine cannot produce enough power to accelerate a given mass of flywheel. So lighter the mass, the point will be higher.

Originally Posted by vina

Also which one of these factors is the most important? or is it that all become important roughly at the same rpm?

IMO, the major thing that limits rpm is the engines breathing capabilities. If there is a brake through in high rpm breathing technologies, then I'm sure the average mass production engine rpm will definitely increase atleast 50%.


P.S - Some cosworth engines (CA2006?) are known to revv till 20,000 rpm.

[sgiitk]

Rev limit of a modern ECU engine is programmed in. You will not be allowed to exceed it. Only the limiter comes on like a brick wall.

Earlier you had to be careful. In my Escort 1300 in my student days (mildly modded - electronic ignition + thermostatic fan) the rev limit was 5800 continuous and 6100 intermittent. At 16mph / 1000 rpm it added up to 93 and 98 mph. I once touched 102 mph on an Autoroute. This was before I put a Tacho in. I have many times used to 6100 limit. This was the famous Ford Kent series engine with pushrods!