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.
