I'm a bit confused about power output and gear ratios. I think it's best to explain by going by my example (imperical units, sorry Americas & Brits):(adsbygoogle = window.adsbygoogle || []).push({});

Mass: 1300kg

max. power: 128 kW @ 5500 rpm

max. torque: 245 Nm

Gear ratios (from a slightly different car type, but should be similar):

1st 3.55

2nd 2.11

3rd 1.30

4th 0.94

5th 0.79

Reverse 3.50

Final Drive 4.11

Torque = F.s (F and s perpendicular)

Now, in general, power = work / time = F*s / t = 2*Pi*Torque*(rpm / 60)

In this case, filling in that formula with the torque value gives: power P = 2*Pi* 240 * 5500 / 60 = 138 kW

=>Not exactly 128kW, but okay, that's not my problem.

Officially, the car goes from 0 to 100 km/h in 8.0 seconds.

The kinetic energy difference between 0 and 100 km/h is:

.5 * 1300 * 27.8^2 =~ 500 kJ. Of course, this doesn't include roll and air friction. As a rough estimation, let's add 20% to get the total energy, required to 100 km/h:

500 * 1.2 = 600 kJ

So, divide this by 8 seconds, and you get an average power output of 75 kW - 59% of the maximum.

This number seems to be too high?

It is my understanding that power output at the wheel = engine power / gear ratio. So, in the first gear, you have a max. power of 128 / 3.55 = 36 kW. In second gear, this is 60 kW. Which doesn't make sense to me, because in going from 0-100, even if you constantly drive in 2nd gear at maximum power output, you'd accelerate according to 60kW (that is not even taking into account shifting, being outside of the power band, transmission losses, etc), while the 8.0s requires an average of 75kW.

Where do i go wrong?

Also, what is the "final drive" number from the gear ratios? An additional transmission that is always there?

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# Gear ratio and power output in a car

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