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Old 17th June 2012, 12:09   #121
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

For over square engines
- The pistons are lighter as they are shorter in height
- The connecting rods are lighter as they are designed for lower torque
- The crank shaft is lighter as the strokes are shorter, hence extension from centre is shorter.
- The CAM shaft is lighter, as the lobes are smaller because the valves extend less into the combustion chamber.

The above factors mean that on the whole the rotating components are lighter and have less angular momentum, hence the engine can sustain higher RPM for same strength of components.

The design for over square engines emphasizes higher RPM so that the peak power is more despite the torque being less. In contrast the longer stroke engines have higher torque but due to lower upper band less peak power.
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Old 17th June 2012, 16:14   #122
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Quote:
Originally Posted by Mpower View Post
If the throw is shorter, then the lever arm of the crank throw is shorter and therefore the bending moment and the resulting stresses are lower!

Secondly since the displacement is the same, the force on the piston is the same so even though the dia is larger the sections can be made smaller minimizing the weight gain.

I think you are getting it all wrong.


Since the displacement is the same the torque (i.e. bending moment) is going to be the same, not the net force on the piston. Force on the piston will depend on piston area and gas pressure inside the engine - for an apples to apples comparison the gas pressure has to be assumed roughly same (both engines NA, similar CR, similar fuel/air mix ...) - larger bore=larger piston area = more force.


Even otherwise, same displacement implies similar fuel burn rate, which implies similar power output which means same torque at a given rpm. Now for the same amount of torque, shorter lever arm means more force.
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Old 18th June 2012, 11:04   #123
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Even otherwise, same displacement implies similar fuel burn rate, which implies similar power output which means same torque at a given rpm. Now for tbetter and faster combustion.he same amount of torque, shorter lever arm means more force.
Gives me an idea - shorter distance for flame to travel, which may lead to
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Old 18th June 2012, 20:57   #124
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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shorter distance for flame to travel, which may lead to
will lead to 4 valve heads with central plug/ injector predominating in new designs. (Now go figure out triple plug technology!)

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Old 19th June 2012, 12:14   #125
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Sorry the post got truncated. What I wanted to say was lead to --- shorter distance for flame to travel. Sutripta essentially completed the thing.
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Old 20th June 2012, 16:22   #126
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by sgiitk View Post
Sorry the post got truncated. What I wanted to say was lead to --- shorter distance for flame to travel. Sutripta essentially completed the thing.
Which means you can essentially have lesser time for the ignition stroke and hence higher rpm.

That part I had understood. However this reasoning has nothing to do with the mechanical properties and inertial forces, this is a property of fuel/air burn rate.
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Old 20th June 2012, 20:47   #127
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

For petrols, combustion nature/ process/ time not the limiting factor for RPM.

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Old 21st June 2012, 09:27   #128
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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
For petrols, combustion nature/ process/ time not the limiting factor for RPM.

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In a way the direct in-cylinder-injection engine's combustion also behaves like petrol. The fuel is injected directly into the cylinder at the appropriate time so that there is no lag in combustion.
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Old 21st June 2012, 20:25   #129
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^
Don't get you. Referring to Diesels, or GDI? Or something else?

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Old 23rd June 2012, 16:15   #130
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

Referring to diesels. With computer control, diesel is injected once the compression reaches optimum value, hence the fuel ignites immediately (and at the appropriate time), just like petrol engine.
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Old 23rd June 2012, 23:24   #131
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Default Re: Power characteristics of long and short stroke engines: Cast in stone?

^^^
OT for this thread. I'm sure some other thread in this forum discusses this.
Similar? No, not really. Indicator diagrams would be illuminating.

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Old 19th January 2014, 01:09   #132
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Post Re: Power characteristics of long and short stroke engines: Cast in stone?

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Originally Posted by thamban View Post
The full write-up is attached as a Word Document.

Oversquare engine Torque is theoretically equal to Undersquare engine Torque but practically Oversquare engines are designed so that it has a stroke above minimum So for mechanical reliability. We practically see that most high revving engines typically have low loethod is to drive at higher RPMs to get the most out of the machine.

Now coming to Diesel engines, the important characteristics of a NA Diesel engine is its high Torque at lower RPM. This makes it desired for applications having high inertial load, for e.g trucks, heavy machinery etc. These are not high speed application but applications with higher initial....
Thank you for this excellent article, I've had three to five PDFs all giving me dyno charts explaining, torque vs horsepower but though I got the point, this post explained it to me clearly once again.

So forgive me if I am beating a dead horse, but I understand the NA part, but how does a turbocharger make a difference in linear power delivery?

Cheers!
VJ

Last edited by VijayAnand1 : 19th January 2014 at 01:11.
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