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Frequently asked questions

If a body (it could be a car) moves with a certain speed V and it weighs m, it has a kinetic energy of:

Energy = 1/2mVV

If we accelerate the car from one speed to another, we need energy

Formula 1

Energy = 1/2mV1V1- 1/2mV0V0

The energy shown is in joule, but since joules are commonly used by women on a diet, we will convert it to kW:

We know that:

w = j/s

so therefore:

kW = j/(1000s)

This we put together with formula 1 and get

formula 2:

kW = m(V1 - V0)(V1+V0)) / 2000s

So if we can measure m by weighing the car (and driver, helper, computer etc.), if we can get the speed of the car at two points in time, s seconds apart; we can measure the power. We are often interested in the power loss due to wind, air friction and road friction. We can measure this by observing the decceleration while the engine is decoupled (i.e. the clutch is depressed). The power loss due to friction, air friction etc. is measured
using the same formula. According to DIN 70020 norm, power should be measured at 20C and at an air pressure of 1013 mb. We therefore adjust for varying temperatures and air pressures:

kWkor = kW(1013/air pressure) x sqr((273+temp)/293)

If we want to make that into Nm (torque) we multiply by 9550 and have now

formula 3:

Nm = kW 9550 / Rpm.


Dynomet Aps, Mølleskovvej 112, 4370 Store Merløse, Denmark. Tlf +45 57891166 +45 40571592 email: jba (at) dynomet.dk