DorsetStrider
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| posted on 25/4/05 at 01:49 PM |
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Engine Power
I know this might seem an odd question. But does anyone have any idea how much force the piston is forced down as combustion takes place? Obviously it
depends on the mixture and size of the engine but if anyone has even a ball park figure I'd love to know.
Who the f**K tightened this up!
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nick205
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| posted on 25/4/05 at 01:56 PM |
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probably quite a fair little bit I would imagine - sorry I can't be more precise 
Nick
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flak monkey
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| posted on 25/4/05 at 02:00 PM |
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This is basically what determines the engine torque, and therefore the horsepower of the engine. (The horsepower equation is related to the
torque).
The piston is forced down and this force is translated to a rotational motion via the crank shaft. It is the offset of the con rod journals to the
centre line of the crankshaft and the force with which the piston is pushed down that determines the torque of the engine. (Very simplistically anyway
as there are many factors involved).
Some simple maths with some random numbers:
Say the torque is 150Nm, and the offset on the crank is 30mm (0.03m) Then the force on the piston to create this torque must be:
150/0.03 = 5000N
Which is give or take half a tonne (500kg).
David
[Edited on 25/4/05 by flak monkey]
Sera
http://www.motosera.com
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DorsetStrider
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| posted on 25/4/05 at 02:05 PM |
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I know that but what I'm after is some sort of rough figure (i.e. lb's per square inch) by which the piston is pushed down with.
I'm not explaining myself very well today am I? blame it on the north london derby later on.
Who the f**K tightened this up!
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flak monkey
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| posted on 25/4/05 at 02:07 PM |
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See my edited post above. Thats just an average force, and simplified at that. As the force at the top of the stroke straight after the ignition will
be higher than than. Imagine that as the force halfway down the cylinder....
Sera
http://www.motosera.com
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smart51
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| posted on 25/4/05 at 02:29 PM |
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Perhaps BMEP - Brake Mean Effective Pressure "The work accomplished during one engine cycle divided by the engine swept volume. It is
essentially the engine torque normalized by the engine displacement. The word “brake” denotes the actual torque/power available at the engine flywheel
as measured on a dynamometer. Thus, BMEP is a measure of the useful power output of the engine."
150 PSI is thought of as poor and 200 is V good, according to good ole google
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Fred W B
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| posted on 25/4/05 at 02:53 PM |
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I was taught in Mechanics of Machines when I studied Engineering years ago, that for an IC engine (and If I recall correctly)
HP= PLANK
where
HP=power
P=bmp (as explained above)
L=length of crank arm
A=piston area (for all pistons in a multi)
N=rpm
K= a constant for engine type and measuring brake
So, to increase your HP, just increase any of the above!
Apologies to all those to which this is common sense, I find it helps to go back to the basics when you are trying to understand things
Cheers
Fred WB
[Edited on 25/4/05 by Fred W B]
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Donners90
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| posted on 25/4/05 at 03:04 PM |
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If it helps, at full load (peak torque) on some 2.2 turbo diesel engines the maximum cylinder pressure is optimised at 150bar, and with a bore of
86mm:-
150bar = 15.2 kg/cm^2
bore 8.6cm
Area = 58cm^2
F = Pressure x Area
F = 15.2 x 58
F = 881.6kg
Force on piston = 8816N approx
someone better check these calcs, but the 150bar and 86mm bore are correct 
HTH
James
[Edited on 25-4-2005 by Donners90]
[Edited on 25-4-2005 by Donners90]
[Edited on 25-4-2005 by Donners90]
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britishtrident
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| posted on 25/4/05 at 03:15 PM |
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BMEP as already mentioned but that is an average figure the peak figure which occurs a few degrees after TDC is much higher this is given by an
indicator diagram (so beloved of motor ship chief enginers) which graphs the pressure at all stages of the engines cycle.
Do a google for Otto Cycle indicator diagram
or to save time see http://hypertextbook.com/physics/thermal/engines/
[Edited on 25/4/05 by britishtrident]
[Edited on 25/4/05 by britishtrident]
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