HOME OF -VIRTUAL STOPWATCH- THE VEHICLE ACCELERATION AND LAP TIME CALCULATOR

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WHAT ARE POWER AND TORQUE?

Power and Torque are two different ways of expressing the output of an engine.  Below is a graph showing the output of a 5.0 litre V8 Mustang engine:

 The purpose of an engine is to convert chemical energy in the fuel into kinetic energy to move and accelerate our vehicle.

Torque is proportional to the amount of kinetic energy produced per crankshaft revolution.

And,

Power is proportional to the amount of kinetic energy produced by the engine per second.

Therefore

Power is calculated simply by multiplying the energy produced per crankshaft revolution by the number of crankshaft revolutions per second.

 

The Maths

1 Nm  =  1 J /radian  =  2πJ / Rev

1 Nm @ 1 radian /sec = 1 J/sec = 1 watt

100lbft = 0.852 kJ/revolution

100Nm = 0.628 kJ/revolution

100bhp = 74.6 kJ/sec

100kW = 100 kJ/sec

 

It's clear from the above definitions that whilst Torque is important, what is really important is the power output; to move and accelerate our car as fast as possible we need the engine to produce as much kinetic energy per second as possible.

Since we can express both Power and Torque as quantities of kinetic energy output we can show their relative values on the chart (below is the energy output chart for the Mustang engine above):

You can see that the amount of kinetic energy produced per crank revolution by a typical internal combustion engine is very low; what makes them good for moving vehicles is that they can rotate at very high speeds, so they produce a lot of kinetic energy per second.

It's also clear that if we want to increase power output we have two options; we either increase the amount of energy produced per revolution (i.e. increaes the torque; which is typically achieved using a bigger engine or turbocharging), or we increase the number of crank revolutions per second.  This second strategy is the one we employ when we choose a low gear coming out of a corner; we increase the revs to maximise power output.  It also explains why the high RPM of motorbike engines provides high performance despite having low torque outputs.

Autocar magazine tested the 5.0 litre Mustang and showed that acceleration was always highest when you operate the engine in the rev range with the highest power output, not the rev range with the highest torque:

 

  

MID-RANGE POWER

Car manufacturers and car magazines often quote peak power and peak torque, but it's clear from the above that rather than peak torque it would be be more useful to know the mid-range power; this would indicate the performance available from the engine when the driver is being a bit lazy with his gear selection.  For example the 5.0 litre Mustang engine above generates nearly 190bhp at 3000rpm: About the same as a hot-hatch engine at high RPM.  Sure enough, the Autocar road tests tell us that the Mustang accelerates 30-70mph in 4th gear about as fast as a Peugeot 208 GTI driven flat out.  At high RPM the Mustang generates around 400bhp and it therefore accelerates much faster than the GTI when it is also driven flat out.

You can use the converter below to help you quickly determine the mid-range power from the flywheel torque and engine rpm:-

Flywheel Torque: lbft, at an engine speed of rpm

Equivalent power

 

SUMMARY

Torque is proportional to the amount of kinetic energy produced per crankshaft revolution.

 

Power is proportional to the amount of kinetic energy produced by the engine per second.

 

An engine which produces more power (regardless of whether that power comes from more

flywheel torque, a higher engine speed, or a combination of the two) will be able to accelerate a car quicker.

 

Peak Torque figures sell diesel cars, a good Power Curve wins races!

 

You can see the affect on acceleration of various engines with different Power and Torque characteristics using the Virtual Stopwatch calculator (click Image below):-

 

 

 

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