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Old 27th April 2011, 16:54   #61
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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And to bring the focus back to the topic, let me reiterate the question.
"Is it possible to design a short stroke engine with a torque curve similar to a long stroke engine."
All things being equal, the long stroke might have the edge.

But if the short stroke engine is in a higher state of tune, it can have a torque curve which is as strong as, or even stronger than a mildly tuned long stroke.

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Old 27th April 2011, 21:48   #62
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by pranavt View Post
Should be doable pretty easily if you have a good engine simulator and some time.
That's an interesting way of going about it. But I was wanting some theoretical foundation to this 'everyone knows' thing.

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Originally Posted by Shan2nu View Post
All things being equal, the long stroke might have the edge.
Why? (I mean we know why the long stroke will not ultimately match the power of the short stroke. )

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Old 28th April 2011, 00:44   #63
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Well here's my thought (torque only) -

Let us assume two engines with similar displacement but different stroke, L1 > L2

Let us assume they have similar frictional losses over a cycle, similar thermal losses (as a percentage of heat lost) similar thermal efficiency (via clever use of newest technology available) etc. and similar compression ratio and fuel-air mix.

Then it is reasonable to assume that for a given amount of fuel both will do similar amount of work (losses are similar both mechanical and thermal).

If that is the case then for 4 rotations (or 2 for a 2-stroke or 6 for a 6-stroke, etc. however let's limit ourselves to 4stroke here, others can be extended similarly) of the shaft - both have to do similar amount of work (efficiency is similar and same displacement, fuel-air ratio and compression ratio means you put same amount fuel per 4-stroke cycle into each engine).

Now over one complete rotation:

Work = Avg-torque * 2*pi where pi = 3.1416 (roughly).

For four complete rotations it'll be:

work = 4* Avg-torque * 2*pi


Now if work over one 4-stroke cycle is W1 for engine 1 and W2 for engine 2, and torque generate by the two engines are T1 for engine 1 and T2 for engine 2 then using the above work-torque equations and the fact that W1=W2 (deduced above) we get:

T1=T2

you don't even have to look for bore, stroke etc.


If two engines have similar fuel to work conversion efficiency then no matter what you do at the same rate of fuel consumption they'll generate the same brake power. If it so happen that the same rate of fuel consumption means similar rpm (i.e. same fuel-air ratio, similar compression and same displacement) then no matter what your bore/stroke are same amount of torque will be generated (Torque = Power/rpm, both quantities on the right are same for both engines so the quantity of the left must be equal too).

Last edited by vina : 28th April 2011 at 01:07.
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Old 28th April 2011, 01:00   #64
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

On top of what I wrote previously -

Thermal (conduction) losses via cylinder and piston walls will be minimised if for a given displacement the area of all such walls together is minimised. This is complicated because highest temperatures will be obtained, especially under low rpm conditions, right at the time when the piston is still near TDC, and by the time the piston reaches lower down the temperature of the gases themselves has lowered considerably - heat transfer to walls will be less of a problem (even though sidewall area is now considerably more).

Over the whole length of the stroke however the area of the piston and the cylinder head is exposed throughout the stroke - minimising this area (as against sidewall area - most of which is not exposed to hot gases in the initial portion of the combustion stroke) may help improve thermal efficiency at low rpm. Undoubtedly it'll help power and hence torque at low rpm (this can explain some portion of the long-stroke's better low-rpm power and torque).

At high rpm though the thermal (conduction) loss will still predominantly happen from these surfaces, given the lesser time available the conduction losses may not be important (expansion is almost adiabatic) - in fact at high rpm thermal losses will predominantly be via exhaust gases (this component will be present at low rpm too)

Friction losses are more complicated (sidewall forces, shaft friction elements, viscosity of lubricants leading to piston-speed dependent friction coefficients ...), however suffice it to say that long-stroke will have faster moving piston at the same rpm than a short stroke leading to higher viscous friction. So it is actually at a (slight) disadvantage here.

Last edited by vina : 28th April 2011 at 01:09.
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Old 28th April 2011, 13:57   #65
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Why? (I mean we know why the long stroke will not ultimately match the power of the short stroke. )
Coz if both engines create the same amount of downward force on the piston, then the one with the longer stroke will have better leverage and thus result in a higher torque figure.

For example, 1kg of force acting on a 1mtr lever creates 1kgm of turning force. But 1kg of force acting on a 3mtr lever will produce 3kgm of turning force.

But if you can make the short stroke create 3kgs of force over 1mtr then the resulting torque can be similar to the long stroke.

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Old 28th April 2011, 14:49   #66
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^
Hi,
The increase in piston area compensates.

@Vina,
For practical reasons, it is best if peak pressure is generated slightly after TDC. Theoretically peak pressure at TDC is optimal. There is too much to discuss, and I hate typing!

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Old 28th April 2011, 14:57   #67
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Hi,
The increase in piston area compensates.
I've already mentioned that a short stroke with a bigger force can match a long stroke.

BTW, I've read that an engine achieves peak torque when the exhaust velocity is between 240-260ft/s. So could the stroke length have an effect on this?

Shan2nu

Last edited by Shan2nu : 28th April 2011 at 15:07.
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Old 28th April 2011, 15:28   #68
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^^
Shan2nu[/quote]
Should be a function of the design of the exhaust system. (Which to me is as much art as science.)

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Old 28th April 2011, 16:15   #69
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Should be a function of the design of the exhaust system. (Which to me is as much art as science.)
Im talking about the velocity at the exhaust port. Lets say we disconnect the exhaust unit, let a short stroke and long stroke engine run at 4,000rpm. Would they have the same exhaust velocity?

I guess it would again depend on the size of the exhaust valves/port.

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Old 28th April 2011, 16:18   #70
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Shan2nu View Post
Im talking about the velocity at the exhaust port. Lets say we disconnect the exhaust unit, let a short stroke and long stroke engine run at 4,000rpm. Would they have the same exhaust velocity?

I guess it would again depend on the size of the exhaust valves/port.

Shan2nu
No they would not. Longer stroke motor has more volume, so keeping everything else equal, the one with more displacement will have more exhaust velocity since it has more air to push out the exhaust.
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Old 28th April 2011, 16:22   #71
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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No they would not. Longer stroke motor has more volume, so keeping everything else equal, the one with more displacement will have more exhaust velocity since it has more air to push out the exhaust.
We're talking about short stroke vs long stroke for the same engine capacity.

So the exhaust volume remains the same. But what happens to the exhaust velocity, is what i wanna know.

Couldn't find any articles on this topic.

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Old 28th April 2011, 16:23   #72
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Sutripta View Post
^^^
Hi,
The increase in piston area compensates.

@Vina,
For practical reasons, it is best if peak pressure is generated slightly after TDC. Theoretically peak pressure at TDC is optimal. There is too much to discuss, and I hate typing!

Regards
Sutripta
Peak pressure at 16 degrees atdc is regarded as optimal. Less shock on the bearings and piston pin, more power transferred to the crank. Having peak pressure at TDC would destroy bearings like nobody's business.
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Old 28th April 2011, 21:15   #73
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Shan2nu View Post
So the exhaust volume remains the same. But what happens to the exhaust velocity, is what i wanna know.

Couldn't find any articles on this topic.

Shan2nu
Think of the gas volumes generated after combustion. Most of the exhaust will be done because when the exhaust valve opens, the exhaust gases are still under pressure. The piston essentially pushes out the last of it. Essentially a question for experts in CFD/ gas dynamics.

@pranav
Heat losses equally significant reason.

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Old 28th April 2011, 22:44   #74
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Quote:
Originally Posted by Sutripta View Post
^^^
Hi,
The increase in piston area compensates.

@Vina,
For practical reasons, it is best if peak pressure is generated slightly after TDC. Theoretically peak pressure at TDC is optimal. There is too much to discuss, and I hate typing!

Regards
Sutripta

I know that already. did I mention it otherwise (may have done in a simplified model somewhere but that was not the intention)
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Old 28th April 2011, 22:48   #75
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Quote:
Originally Posted by Shan2nu View Post
I've already mentioned that a short stroke with a bigger force can match a long stroke.

BTW, I've read that an engine achieves peak torque when the exhaust velocity is between 240-260ft/s. So could the stroke length have an effect on this?

Shan2nu

Hi Shan2nu

I think what sutripta meant was that the piston area compensates the stroke exactly (if displacement is equal) - so once displacement is same stroke should be irrelevant.

What is trying to ask (and so am I) is that why would for the same displacement two engines with different stroke generate different torque at a given rpm (i.e. the conventional wisdom)
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