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Old 30th April 2011, 01:22   #91
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Mpower View Post
OK so how is pressure the same if if are igniting a charge in a smaller bore vs larger bore.

Back to square 1 then? LOL


I wasn't talking about engine strokes when I wrote about high school physics (sorry about that comment by the way)

@Born2Stroke explained exactly what I meant - and pressure depends on things known as gas equation (PV=nRT) and thermodynamics (heat capacity of gases ...) .

Gas doesn't care about area - it cares about volume (i.e. displacement when you talk about engine) and if it starts from a certain pressure, volume and temperature (depend on input pressure, input temperature and compression ratio) then for a given amount of fuel burnt (i.e. heat added) final pressure will be the same even if your "cylinder" doesn't even look like a cyclider (e.g. Wankel engines).

Work done is W= integ{d(P*V)} where P = Pressure at any time and V = volume at a given time. (please don't write the proof - I know it, and I'm sure you know it too, and this is all Newton's mechanics though a little more finely used than you seem to be doing).

This leads essentially to W=Pi*Vi - Pf*Vf where "i" means initial and "f" means final.

Going with PV=nRT, and assuming adiabatic process (a reasonable assumption for first order rough calculation - neglects thermal losses -accuracy improves for larges cylinders and higher rpms)

using the gas equation and the above,

W=nR(Ti - Tf) = Pi*Vi(1 - Tf/Ti)

Now Pi, Vi, Ti and even Tf depend on the compression ratio, initial pressure (of the intake stroke), displacement and initial temperature of the intake gas (since W depends on Tf/Ti and reducing both by the same amount will increase W - that should tell you why cool air intake improves power), and on amount of heat added (i.e. fuel added and burnt - that is why I mention fuel-air ratio).

None of these depend on bore/stroke.


I sincerely hope this clears the maths, if it doesn't then I request you to have a look at
A question about long-stroke or undersquare engines . It is thread suggested by another BHPian (@Badri) on another thread. If you read through it (post #17 to #32) you'll find work done by a guy @mender who supports the maths here - and he makes engines so he has experimental verification too.

Last edited by vina : 30th April 2011 at 01:38. Reason: Rude language used - sorry -changed
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Old 30th April 2011, 01:25   #92
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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...


360. Why discriminate against the other 10 degs.

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Sutripta

I think he is leaving some angle to prevent pre-ignition
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Old 2nd June 2012, 15:53   #93
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Because of high revving of the short stroke engines (diesel), do such engines require higher maintenance over a period of time, say, five years?
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Old 3rd June 2012, 21:16   #94
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^
Wear:- quite a large number of factors come into play. For a private owner running a modern car, don't worry about engine wear. Just don't neglect the manufacturers oil/ filter recommendations.

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Old 4th June 2012, 19:22   #95
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by nitinbose View Post
Because of high revving of the short stroke engines (diesel), do such engines require higher maintenance over a period of time, say, five years?
Theoretically yes, because

1. Higher speed means more distance covered by each wear surface.
2. Higher speed increases wear faster.

So on the whole there will be higher wear and tear of components, but then the modern high speed diesels have taken this into account and provide better materials to reduce the wear and tear.

One component which the older low speed diesels did not have is the turbo charger, and this is the component which requires a lot of maintenance relative to the rest of the engine.

With CRDI you have higher pressures in the diesel fuel line, hence the pipes will requires higher maintenance - faster replacements.

But unless you are clocking significant milage (say > 40k/year), modern diesel engine will last you the whole ownership period, especially if you do 10-20K/year.
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Old 4th June 2012, 20:31   #96
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Aroy View Post
With CRDI you have higher pressures in the diesel fuel line, hence the pipes will requires higher maintenance - faster replacements.
Hi,
What exactly wears out and is replaced, and why?

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Old 5th June 2012, 20:23   #97
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Hi,
What exactly wears out and is replaced, and why?

Regards
Sutripta
Basically higher pressure in the fuel line requires better materials and more robust construction of components in the system.

. Flexible hoses deteriorate faster at higher pressure.
. Joints, due to vibration, will leak faster at higher pressure.

Due to higher pressures, most of the components will be more expensive simply because they have to be more robust, amongst other items -
. Fuel filter
. Injectors
. Fuel pump
. Fuel lines, both metallic and flexible.
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Old 5th June 2012, 20:33   #98
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^
The question was not about initial cost, but
Quote:
Originally Posted by Aroy View Post
With CRDI you have higher pressures in the diesel fuel line, hence the pipes will requires higher maintenance - faster replacements.
Does that actually happen?

Regards
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Old 5th June 2012, 20:50   #99
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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
^^^
The question was not about initial cost, but

Does that actually happen?

Regards
Sutripta
As stated in my post the Hoses, joints and maybe the injectors will have a shorter life, hence more frequent replacement.

The damage increases much faster than linearly with pressure, and any small shock or pinprick affects the integrity of pipes and hoses much more.
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Old 5th June 2012, 20:57   #100
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by Aroy View Post
As stated in my post the Hoses, joints and maybe the injectors will have a shorter life, hence more frequent replacement.
What replacement cycles are we looking at?

Regards
Sutripta
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Old 5th June 2012, 21:18   #101
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

@Aroy, although you are directionally correct, there are no flexible hoses in a diesel FI system's hi pressure plumbing and none of the parts need replacement. Pump is bolted directly to the engine and only hard lines and banjo fittings are used.

One more characteristic about short stroke (large bore rather) engines is that they have a poor(er) surface to volume ratio and thats not good for thermal efficiency.

Last edited by Mpower : 6th June 2012 at 07:17.
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Old 6th June 2012, 18:43   #102
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Quote:
Originally Posted by Sutripta View Post
^^^
Wear:- quite a large number of factors come into play. For a private owner running a modern car, don't worry about engine wear. Just don't neglect the manufacturers oil/ filter recommendations.

Regards
Sutripta

Are short-stroke diesel engines very common? wouldn't it be harder to get a high compression ratio?

Also, shorter stroke means more force on the pistons and connecting rods, diesel means the same thing - wouldn't this be a double whammy?

And why do you need short stroke on diesels? The rpm usually doesn't go very high anyway.
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Old 6th June 2012, 20:43   #103
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Are short-stroke diesel engines very common? wouldn't it be harder to get a high compression ratio?
Let alone Diesels, even in petrols, undersquare predominates.
CR = (swept volume + dead volume)/(dead volume)
Stroke does not come into the picture. And DIs have lower CR than IDI's.

Quote:
Also, shorter stroke means more force on the pistons and connecting rods, diesel means the same thing - wouldn't this be a double whammy?
Shorter strokes also mean lower velocities/ forces for same rpm.

Quote:
And why do you need short stroke on diesels? The rpm usually doesn't go very high anyway.
Designers choice.

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Old 6th June 2012, 22:10   #104
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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
Let alone Diesels, even in petrols, undersquare predominates.
CR = (swept volume + dead volume)/(dead volume)
Stroke does not come into the picture. And DIs have lower CR than IDI's.
what is DI/IDI? is it direct/indirect injection?

wouldn't (bore area)*(clearance required for valves in engine head) limit how small a dead volume you can have? If so, then for a given dislacement of the engine, wouldn't the CR be limited?


Quote:
Originally Posted by Sutripta View Post
Shorter strokes also mean lower velocities/ forces for same rpm.
I'm confused. If we compare two engines with similar displacements, one long the other short, then at a given rpm they should produce roughly same power and hence torque.

This in turn means that the engine that has a shorter stroke has to generate greater compressive force on the connecting rods during ignition/expansion - because Torque=Force*stroke/2

what am I missing?

Quote:
Originally Posted by Sutripta View Post

Designers choice.

Regards
Sutripta

Is it entirely? don't the burn characteristics of the fuel play a role?
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Old 6th June 2012, 22:30   #105
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by vina View Post
what is DI/IDI? is it direct/indirect injection?
Yes.

Quote:
wouldn't (bore area)*(clearance required for valves in engine head) limit how small a dead volume you can have? If so, then for a given dislacement of the engine, wouldn't the CR be limited?
Clearance pockets in the piston, if necessary. Piston top need not be flat.




Quote:
I'm confused. If we compare two engines with similar displacements, one long the other short, then at a given rpm they should produce roughly same power and hence torque.

This in turn means that the engine that has a shorter stroke has to generate greater compressive force on the connecting rods during ignition/expansion - because Torque=Force*stroke/2

what am I missing?
Inertial forces. (Mechanical) reason why short strokes can rev higher.

Regards
Sutripta




Is it entirely? don't the burn characteristics of the fuel play a role?
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