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Lets say you have two single cylinder engines. Each one is 500cc. The first one has a big bore and a short stroke. The second one has a small bore and a long stroke.

They both have the same compression ratio. They are both running at the same RPM. Volumetric efficiency will not taken into account.

Which engine will have better mechanical efficiency and why?
 

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All other things being equal I believe that the longer stroke would be the more efficient of the two. I think it would be because of more leverage from the ability to use a larger crank throw, and that the fuel charge has a longer oportunity to act on the piston converting more of the heat into work. Diesel engines are pretty efficient and they have a ridiculously long stroke.
 

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titomike said:


Is that 2000rpm or 9000rpm ?

I think the short stroke would be more efficient at 9000rpm as the long stroke would have detonated :p


:)
No reason why the long stroke would detonate at 9000 rpm.
Detonation usually occurs at low rpm, and the rpm is not one of the variables that affect it.
Let's discuss this in another thread :)

Aris
 

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Andy said:
Lets say you have two single cylinder engines. Each one is 500cc. The first one has a big bore and a short stroke. The second one has a small bore and a long stroke.

They both have the same compression ratio. They are both running at the same RPM. Volumetric efficiency will not taken into account.

Which engine will have better mechanical efficiency and why?
Can I play, or should I wait?

I'll just give a hint. The only way to improve Mechanical efficiency is to reduce friction.

Aris
 

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Volumetric efficiency will not taken into account.

volumetric efficiency: the ratio of the actual mass of gas compressed to the mass of gas occupying piston displacement at the inlet pressure and temperature; used as a measure of engine efficiency; Or how much air the piston can suck in while the intake valve is open.

Are we also ignoring thermal efficiency as well?

I guess you weren't looking for diesels, but some fun with simple math judging by the diagram.

my math sucks but what I imagine you want to find is which cylinder displacing half a liter has the least surface area on its sides,

a 6x18cm cylinder will have 340cm square area for the rings

while a 8.5x9cm cylinder will have 240cm square area

I am ignoring the difference required to find out the combustion chamber size to keep the CRs the same.
 

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Actually the bore/stroke ratio is a variable that affects many aspects in the performance of any engine.
Volumetric efficiency and Thermodynamic efficiency included.
Generally a slightly oversquare design gives the best results.

Since Andy's question is only referring to Mechanical Efficiency, which refers to the difference between Brake Horsepower and actual output horsepower due to friction losses in the piston rings, bearings,oil pump etc.
Actually most of the friction losses occur between the cylinder and the rings.
The answer is that the short stroke is more efficient in this aspect as piston friction depends mainly on the contact area and the piston speed.
The contact area depends on the number of rings and the circumference of the rings (piston diameter * 3.14).
The piston speed depends on the stroke, the shorter stroke for given RPM has less piston speed and thus less friction.
Finally, the friction is much less for the short stroke, although the piston ring area is larger.
In fact the piston speed is the limiting factor for engine rpm, as there is a critical piston speed above which the rings will disintegrate. This limit changes with time, as metallurgical science is advancing, and the rings can stand higher speeds.
We should be talking about Average piston speed really, because the piston speed is changing from 0 at TDC, to max in the middle of the stroke to 0 again at BDC.

Aris
 

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Discussion Starter #8
Hi Aris,

You are 100% correct :). The answer I was looking for was:

"The piston speed depends on the stroke, the shorter stroke for given RPM has less piston speed and thus less friction.
Finally, the friction is much less for the short stroke, although the piston ring area is larger."

Andy
 
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