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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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equations to calculate power and torque?
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Tue, 17 June 2003 12:58
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Heya Ppl,
was just musing over some numbers and thought i'd share.
thinking of a way of guesstimating what power and torque an engine _can_ produce (any engine) and how to tell when ppls numbers are over the top...
i was thinking that a good estimate the max torque of an engine is to take a figure 100Nm/L (since bikes get around that and are damn efficient, close to 100%volumetric efficiency) then multiply by your capacity, multiply by 1+ number of atmospheres above boost (ie 1atm boost is x2, 1.5bar boost is x2.5), or by the expected efficiency like 0.9 for 90%, and since power is calculated from torque, then plug ya rpm numbers into
Nm = (9402 x kw) / rpm
or
kw = (Nm x rpm) / 9402
so then
kw = (engine size x 100 x (1+ boost) x rpm) / 9402
where engine size is in litres, boost is in Bar or atm)
and Nm = engine size x 100 x (1+ boost)
or Nm = engine size x 100 x efficiency (in %)
you can then divide by your expected losses to get expected RWkw
note that this assumes 100% efficiency unless you factor in lower than 100%VE.
thoughts?
Cya, Stewart
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Location:
Brisbane
Registered:
February 2003
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Re: equations to calculate power and torque?
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Tue, 17 June 2003 14:31
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hi stewart,
i suppose the next thing to do is draw up a chart of Ve values for different standard engines - at the least, it would throw up some baseline figures?
Maybe there's some Ve guestimates buried away in the Megasquirt archives? 
And maybe some standard % losses from known drive systems (from g/box & diffs) to make the RWkw more real-life?
at least i could then guess what my RA40 should be producing with some degree of accuracy...
jeez it's late to be thinking about this stuff
ta,
thechuckster.
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Location:
Perth
Registered:
July 2002
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Re: equations to calculate power and torque?
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Tue, 17 June 2003 17:54
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Interesting theory.
One thing though, would you be able to tell me how you calculated the divisor value of 9402 for the power/torque equation? (opposed to 5252, or 16500/pi for imperial values)
When i converted HP and lb-ft to kW and Nm, i came up with a conversion factor for the divisor of 20/11, yielding 9549 (or 30000/pi), so i'm interested where the discrepancy between the two lies.
ie:
HP = (lb-ft x RPM)/5252
and since
HP = 1.34 kW
lb-ft = 0.737 Nm ... 3.28/(0.454 x 9.807)
we have
1.34 kW = (0.737 Nm x RPM)/5252
Therefore
kW = (0.55 Nm x RPM)/5252
kW = (Nm x RPM)/9549
Regards,
Liam.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: equations to calculate power and torque?
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Wed, 18 June 2003 00:22
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or if you know your MPH at the end of a quarter mile.
HP = (MPH/234)^3 * weight in pounds
do that and then multiply by .8 because that is for perfect aerodynamics unless you have a f1 car or the likes  eg. supras are around .8. this calculates HP at the wheels.
also, you can twist it around and see what is the best possible MPH you can achieve if you know how much power you have.
later
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Location:
Brisbane
Registered:
May 2002
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Re: equations to calculate power and torque?
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Wed, 18 June 2003 00:49
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Note that determining power from a torque figure will only give the power figgure for that specific rpm. So if you start with the engine's peak torque (which may be at 3000rpm), you will end up with the engine's power at 3000rpm. The peak power figure is always at higher rpm than the peak torque.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: equations to calculate power and torque?
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Wed, 18 June 2003 03:23
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Chuck, well, if we can get a bunch of data for different engines and their torque and power numbers for stock motors (at specific rpm) then we can get a good idea of what efficiency/ losses they have..
Liam,
well starting with kw, multiply by 1.34 to get HP.
then ftlbs = (HPx5252)/rpm
then Nm = 1.336 ftlbs and 1 ftlb = 0.748Nm
(since 1ft lb = (1/2.2)x9.8x0.3)
0.748Nm = (1.34kwx5252)/rpm
0.748Nm = (7037 kw) /rpm
so
Nm = 9408.66 kw / rpm
that's where i got the 9402 from (carried numbers this time tho
bassaholic, thanks for that, was wondering about that too
hmm so 92mph for my rolla then... got any ET estimate calcs?
i think an F1 car has more drag actually, since they produce like a few hundred kilos of downforce (can't they run upside down above a certain speed?)
Norbie, yup, understood.. i used these eqns with my dyno day 5 graph, picked numbers along the graph and plugged into excel, so i could generate a torque curve.. as you said, these Eqns are _only_ to calculate torque/power at a specific rpm.
hopefully someone wil bring a laptop to the dyno day so we can do some number plugging!!!!
Cya, Stewart
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