You should by now have the feeling that we know how to interpret the performance figures given in `Mega Power' magazine. We know how to interpret the power and torque figures for the engine, and how these translate to the force accelerating the car. If we understand it, the we should be able to make a reasonable estimate of the 0-60 time that the car is capable of. From equation 5
Notice the factor of 0.7 that has appeared? This is to allow for the fact only a part of the torque produced by the engine becomes useable force for accelerating the car. Approximately 30% of the power of the engine is `lost' due to friction in the gear box and in spinning up the engine and gearbox components to make them rotate faster. It's a pity its such a large fraction is used up, but there you go.
If we stay in a particular gear, the acceleration is roughly constant (for our idealised engine) until we reach the maximum power rpm. We can therefore use the standard equation of motion:
to calculate the time between two speeds (Tipler, p. 30).
Figure 3: Speed as a function of time during a 0 to 60 dash. The idealised engine gives a constant rate of acceleration in each gear.
To reach 60 in the Alfa Romeo, we will have to use 1st, second and third gears, so the speed will increase with time as shown in Figure 3. It takes about 1 sec to change gears. The estimated 0--60 time is 8.0 sec. That's in reasonable agreement with the manufacturer's quoted time of 8.4 sec. We've so far made no allowance for the limited grip between the tyres and the road.
Notice that we can make a faster 0-60 time in three ways.