# Calculating engine power.

## Main Question or Discussion Point

hello!
i have calculated an engine power of a prototype car by calculating the power losses against road gradients, air drag and rolling resistance. i have calculated these losses for max. car speed,lets say 50 kmph. what i need to know is do i need to consider acceleration allowance even when i have calculated the power of an engine for max. speed??if yes, then why calculating acceleration allowance for an engine when car cannot accelerate beyond 50 kmph as all my calculations are based on maximum speed.
plzz do reply, im confused..........

## Answers and Replies

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Capitalize "I" when you use it in sentences.

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It would also help if you could state your question clearly and in 1 line preferably.

If you ever plan to stop then yes you should consider it (in the form of negative acceleration).

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jack action
Gold Member
what i need to know is do i need to consider acceleration allowance even when i have calculated the power of an engine for max. speed??if yes, then why calculating acceleration allowance for an engine when car cannot accelerate beyond 50 kmph as all my calculations are based on maximum speed.
plzz do reply, im confused..........
Think of it this way, if you size your engine power according to your max speed of 50 km/h, then it would mean you have no power left for acceleration at that speed (terminal velocity). So it would also mean that from, say, 45 to 50 km/h you will experience very little acceleration and it will take you forever to break that 5 km/h gap.

To get an approximation of the acceleration of your vehicle you can use the simple Pw - Pl = Fvavg = (ma) (vf + vo) / 2, where Pw is the power at your wheel, Pl is the power losses @ vavg, m is vehicle mass, vavg is the average speed of the vehicle during acceleration, vf and vo are final and initial speeds and a is the average acceleration from vo to vf.

Of course, the time taken to go from vo to vf will be approximately t = (vf - vo) / a. This will give an optimistic value, but it will give you a good idea.

Furthermore, no matter what is the engine power, the maximum traction force you can get is dependent of the friction coefficient of the tire. So Fmax = µmtg, where mt is the mass of the vehicle on the powered wheels, g = 9,81 m/s² and µ is the tire coeff of friction (tourism tire = 0.75-1.0, racing tire = 1.25-1.5, bicycle tire = 0.6-0.75 (that last one is a guess))