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Old 20th May 2011, 16:32   #31
DKG
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Default Re: What limits rpm for an internal combustion engine?

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Originally Posted by vina View Post
Isn't this related to short stroke (already mentioned in one of your points)?
Long heavy cranks (here I am not talking about the throw but the crank length itself) simply cannot rev higher. I have an inline eight engine with a crank that is almost a mile long and weighs a ton (well literally ) All this rotating mass is great for low end torque but dramatically limits the engine's ability to rev higher.

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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?
I am not an engineer so you must excuse if my experience has taught me things incorrectly. Basically a flywheel only smoothens out the power impulses and is counter productive to an engine's ability to keep revving higher. An engine with a very light flywheel will rev higher as very little of the engine's power stroke is facing resistance from a rotating heavy flywheel. Its kind of acting like a damper or a brake on impusles and hence limiting them in a way. Lesser the resistance on a power stroke, higher its ability to move onto the next power stroke? Don't you think? Logically I believe this is so but I should be corrected if I assume incorrectly.

A long stroke engine with heavy flywheel is good for low end torque, but limits revs.


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Will it help via improved breathing at higher speeds or is it something else?
Valve timing is basically the opening and closing of the valve relative to an engine crank's position ie the TDC. At what point the inlet opens and closes or the exhaust does affects the power output. When you can vary this as the engine spins higher and higher just as you need to advance the timing you will get higher rpm. Basically as you spin higher you want the air entering earlier as the air mass characteristics and the burn characteristics for a fixed engine chamber design and fuel cannot be altered much

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Also which one of these factors is the most important? or is it that all become important roughly at the same rpm?
I would say a small light weight engine with a short stroke setup and light fllywheel and improved air intake and hahaha you get the picture ? I think they are all needed to be tweaked to get one that can go ballistic.

I change gears on my VFR at 11000 rpm regularly and am amazed at how robust a Honda engine is. My humble opinion is they make the world's greatest engines and the most rev happy engines

Last edited by DKG : 20th May 2011 at 16:40.
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Old 20th May 2011, 16:51   #32
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Default Re: What limits rpm for an internal combustion engine?

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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)
.
Reasons for low torque at low rpm's
1. At lower rpms due to the increased time available, more and more heat from combustion chamber is lost to the body.
2. Compression leakage across valves and piston become more pronounced.
3. Valve overlaps are good for higer rpms only, at low rpms it will result in exhaust gases being sucked into the cylinder.
4. Delayed closing of intake valve after BDC intake stroke reduces the VE.
5. Early opening of exhaust valve before BDC exhaust stroke is a good thing at higer rpm as the gas pressure has less time to escape but at lower rpms it results in loss of useful work.
6. At lower rpm the air fuel mixture burns poorly due to lack of swirl etc. so hydrocarbon emissions go up as we have to make the mixture richer to compensate.
7. Engines run efficiently when the peak cylinder pressure is closer to about 16- 20 after TDC ignition stroke.At low rpms even though there is more time available for combustion, the mixture inside cylinder is very thin due to mostly closed throttle, as a thin mixture burns more slowly engines still run a minimum spark advance of about 10-15 (btdc ignition). so if you open the throttle suddenly then a denser mixture needs less time to burn resulting in peak pressure reaching before TDC ignition and lugging the engine.

Any ways no manufacturer of passenger cars are interested in torque so low down the rpm because of emissions and nor are the race engine enthusiasts. Also if you are somehow able to take care of all the problems above even then a car which idles at 1000 will only idle at 100 rpm if you at least increase the angular mass of the crank and flywheel by 10 times.
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Old 20th May 2011, 16:51   #33
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Originally Posted by DKG View Post
...


I am not an engineer so you must excuse if my experience has taught me things incorrectly. Basically a flywheel only smoothens out the power impulses and is counter productive to an engine's ability to keep revving higher. An engine with a very light flywheel will rev higher as very little of the engine's power stroke is facing resistance from a rotating heavy flywheel. Its kind of acting like a damper or a brake on impusles and hence limiting them in a way. Lesser the resistance on a power stroke, higher its ability to move onto the next power stroke? Don't you think? Logically I believe this is so but I should be corrected if I assume incorrectly.

...
I'm an engineer and I still don't know , I don't think one's degrees matter much, I know plenty of engineers who can not add 2+2 - they invariably do 10 + 10 = 20 (even after you tell them it is binary )

Flywheel adds rotational inertia - via Newton's laws it is harder to make it spin, and harder to make it stop once it is spinning. For a given torque, a heavier flywheel will take longer to achieve an rpm.

However at top rpm, flywheel by itself will not eat any more torque/power.

Now that is the theory - and I'm not sure whether I missed something. In practice a heavier flywheel would probably increase the friction in the bearings supporting it (this is true by theory too, the difference between practice and theory is usually the complexity where people throw their hands in the air and say "let's get rid of computation and just experiment") - and that will limit max rpm.

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7. Engines run efficiently when the peak cylinder pressure is closer to about 16- 20 after TDC ignition stroke.At low rpms even though there is more time available for combustion, the mixture inside cylinder is very thin due to mostly closed throttle, as a thin mixture burns more slowly engines still run a minimum spark advance of about 10-15 (btdc ignition). so if you open the throttle suddenly then a denser mixture needs less time to burn resulting in peak pressure reaching before TDC ignition and lugging the engine.
one question - can sudden closing of the throttle cause similar problems in a diesel, especially at low rpm? My Figo gets jerks at relatively low rpm (1500-1700, this is beyond Figo's turbo lag range) if I release the pedal suddenly (e.g. if I anticipate I need to brake or just trying to slow down). Happens in the first and second gear only.




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Any ways no manufacturer of passenger cars are interested in torque so low down the rpm because of emissions and nor are the race engine enthusiasts. Also if you are somehow able to take care of all the problems above even then a car which idles at 1000 will only idle at 100 rpm if you at least increase the angular mass of the crank and flywheel by 10 times.
I understand the point you are trying to make is that on the lower side, the idle rpm is limited by the time the flywheel carries enough energy to make the compression stroke work - is that correct?

Last edited by benbsb29 : 20th May 2011 at 19:18. Reason: Please use Multi-quote function when replying to more than one post.
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Old 20th May 2011, 17:12   #34
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Default Re: What limits rpm for an internal combustion engine?

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However at top rpm, flywheel by itself will not eat any more torque/power.
Once again I'll try my layman's logic and you play the informed engineer devil's advocate to counter the arguement till we see it clearly

Lets say a particular engine given its volumetric efficiency and characteristics is generating max power X which allows the engine to rev to a max of Y. Surely one of the factors limiting Y is the mass of components power X has to encounter to spin higher and higher. The heavy components are acting like a brake on X. I see weight as a limiter because power is wasted to counter its inertial forces.

So if we were to improve on any of the factors that inhibit or waste power X you only end up increasing Y does it not?

Quite obviously the speed with which an engine revs up and slows down is also dramatically enhanced on a lightened flywheel. What we are further stating is that it will also allow a engine to rev higher. But this is to be whetted out in terms of the physics of it all

I am just applying logic like will a runner run faster if he wears lighter shoes compared to lead shoes ! Here we can say that a man wearing lead shoes has the benefit of stored energy by way of momentum aiding him as he runs faster and faster, But eventually he's also wasting energy lugging the lead shoes? So lighter clothes and lighter shoes must allow a runner to run faster.

Last edited by DKG : 20th May 2011 at 17:34.
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Old 20th May 2011, 17:28   #35
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Default Re: What limits rpm for an internal combustion engine?

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Originally Posted by DKG View Post
Once again I'll try my layman's logic and you play the informed engineer devil's advocate to counter the arguement till we see it clearly

Lets say a particular engine given its volumetric efficiency and characteristics is generating max power X which allows the engine to rev to a max of Y. Surely one of the factors limiting Y is the mass of components power X has to encounter to spin higher and higher. The heavy components are acting like a brake on X. I see weight as a limiter because power is wasted to counter its inertial forces.

So if we were to improve on any of the factors that inhibit or waste power X you only end up increasing Y does it not?

Well, let me put it this way -

Let's think about force and mass (this is a valid physical analogy to torque and moment of inertia).

Let us say there are two balls, same diameter, same surface finish same colour (though it doesn't matter), but one made of lead and the other made of wood. Let us say their mass numbers are m1 and m2

Let us say both are travelling through air horizontally (gravity is not a factor). Both are being pulled by a constant force F. At rest they will encounter no resistance and the acceleration will be F/m1 and F/m2 Clearly the lighter one will have a faster acceleration (or "pickup" in our lingo).

As their velocity increases, the drag will increase on each of the balls. At any point of time the acceleration will be (F-F')/m1 and (F-F")/m2 where F' is the drag on the m1 ball and F" is the drag on the m2 ball.

The m1 ball will stop accelerating, and would have reached its peak speed once F=F' , m1 has nothing to do with it, though it will decide how much time it takes to reach that peak speed. Similarly m2 ball will stop accelerating once F=F".

Basically once the balls have reached their max speed, the drag on each ball is the same, irrespective of their mass.

Now here's the twist: in the example I took, for similar surface smoothness and same diameters, if both balls have same drag then the velocity of each has to be the same (the balls can be hollow, and the drag wouldn't change as long as speed, surface and diameter don't).

Extend this to torque/rpm - the rpm will keep increasing till the driving torque is more than the resisting torque (due to friction, viscosity ...). It may increase slowly or it may increase fast, but rpm will increase all the way till the driving torque matches the resisting torque. And the latter usually doesn't depend on moment of inertia.


Now, as I wrote earlier, in practice a heavier crank will increase the friction - and that is increased resisting torque.
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Old 20th May 2011, 17:38   #36
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Default Re: What limits rpm for an internal combustion engine?

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Extend this to torque/rpm - the rpm will keep increasing till the driving torque is more than the resisting torque (due to friction, viscosity ...).
Vina, I think the engine rpm is limited not due to the resisting torque, but due to the fact that the power developed at higher rpm's is not enough to accelerate the moving parts more.

Think of it this way. You have an engine with a flywheel of given mass revving at its max rpm. So.. the question is will the rpm increase if you replace the flywheel by another one with lesser mass?

Yes. Because, A=F/M. here at a given rpm (max), F of an engine is constant. So if we reduce M, then A will increase.
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Old 20th May 2011, 17:41   #37
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Default Re: What limits rpm for an internal combustion engine?

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one question - can sudden closing of the throttle cause similar problems in a diesel, especially at low rpm? My Figo gets jerks at relatively low rpm (1500-1700, this is beyond Figo's turbo lag range) if I release the pedal suddenly (e.g. if I anticipate I need to brake or just trying to slow down). Happens in the first and second gear only.
This happens because when you release the throttle instead of the engine driving the vehicle, the vehicle drives the engine. Again why it happens will need some explanation. You can avoid this by keeping the clutch slightly depressed in the lower gears while accelerating and decelerating.

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I understand the point you are trying to make is that on the lower side, the idle rpm is limited by the time the flywheel carries enough energy to make the compression stroke work - is that correct?

Correct. The energy produced in the power stroke is conserved by the crank,flywheel combination. This energy should be enough to take it through the other three strokes and overcome friction. Ignoring all other factors I had mentioned in the earlier post if we can make the crank heavier (angular mass) then we can make an engine idle lower and lower.
May be even 1 rpm. why not.

Last edited by Born2Slow : 20th May 2011 at 17:42.
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Old 20th May 2011, 17:55   #38
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Default Re: What limits rpm for an internal combustion engine?

Ok now I am going back to school days

is it okay to say the rpm of an engine is the kinetic energy and the flywheel mass rotating is storing the potential energy right? with some energy getting converted into heat?

So when heat is constant and potential energy storage is reduced the kinetic motive power must increase increasing rpm right? considering energy overall has to remain a constant?
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Old 20th May 2011, 17:58   #39
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Default Re: What limits rpm for an internal combustion engine?

I am reminded of what Dilip Bam said (not sure if it is his original quote). Well actually not quoting him but paraphrasing him.

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A high rpm engine is like a cheetah and a low rpm engine is like a tortoise. The cheetah lives for about 14 years and the tortoise for about 200 years.
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...
is it okay to say the rpm of an engine is the kinetic energy and the flywheel mass rotating is storing the potential energy right? with some energy getting converted into heat?
...
I think the flywheel cannot be made lighter beyond a certain point, if you want a smooth engine. The speed at which the crankshaft will move during the power stroke will be very high and it will slow down during the compression stroke. So you need a heavy flywheel to absorb and release energy during these cycles. But theoretically yes, the lighter the flywheel, the higher the RPM an engine can achieve. But I guess it will not be very useful. With a very light flywheel, the engine will constantly need to be revved high to get enough torque to sustain speeds.

Last edited by pjbiju : 20th May 2011 at 18:08.
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Old 20th May 2011, 21:24   #40
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Default Re: What limits rpm for an internal combustion engine?

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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.
Thought swirl was even more important for diesels.

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Originally Posted by Born2Slow View Post
7. Engines run efficiently when the peak cylinder pressure is closer to about 16- 20 after TDC ignition stroke.At low rpms even though there is more time available for combustion, the mixture inside cylinder is very thin due to mostly closed throttle, as a thin mixture burns more slowly engines still run a minimum spark advance of about 10-15 (btdc ignition).
I thought we were discussing WOT scenarios.

so if you open the throttle suddenly then a denser mixture needs less time to burn resulting in peak pressure reaching before TDC ignition and lugging the engine.
Don't get it. Ignition timing is controlled almost cycle by cycle.
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Old 20th May 2011, 21:33   #41
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With a very light flywheel, the engine will constantly need to be revved high to get enough torque to sustain speeds.
Just think of the brutal acceleration it will allow as the engine will rev up so quickly!! F1 cars usually stall on take off for this reason. You have to get things right or it will stall easily. I can just visualise a stiff clutch with a lightened flywheel, all so snatchy and brutal ! But once on the go it must be such a delight for a super sensitive throttle response
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Old 20th May 2011, 22:24   #42
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Default Re: What limits rpm for an internal combustion engine?

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I am not an engineer so you must excuse if my experience has taught me things incorrectly. Basically a flywheel only smoothens out the power impulses and is counter productive to an engine's ability to keep revving higher.
DKG, you are 100% correct! A flywheel is like a store house of energy (similar to Surge Tanks in Hydroelectric Power stations). It stores excess energy and feeds back when needed in the system.

One very important point :- today, engines usually have their RPM limited due to a very important reason - EMISSION . It has been scientifically proved that, smaller the cylinder dia, higher is the oil consumption, more so for cylinder bores having dia less than 100 mm. So, piston speeds / RPM needs to be controlled.

Let the discussion on other factors continue.

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Old 21st May 2011, 00:49   #43
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Default Re: What limits rpm for an internal combustion engine?

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One very important point :- today, engines usually have their RPM limited due to a very important reason - EMISSION . It has been scientifically proved that, smaller the cylinder dia, higher is the oil consumption, more so for cylinder bores having dia less than 100 mm. So, piston speeds / RPM needs to be controlled.

Let the discussion on other factors continue.

Spike
I didnt know that, Can you elaborate or give the source, is it because of the volume/surface area ratio.
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Old 21st May 2011, 08:59   #44
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^^ You can find more details in SAE papers, refer paper no. SAE 930955. It has more to do with the ring geometry and associated characteristics.

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Old 21st May 2011, 18:07   #45
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Default Re: What limits rpm for an internal combustion engine?

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^^ You can find more details in SAE papers, refer paper no. SAE 930955. It has more to do with the ring geometry and associated characteristics.

Spike

while we are going off topic, can you send the link and/or other information (authors, title, month/year of publication, journal name)
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