Power and Torque.
So why do these power and torque figures that are quoted in technical specifications matter?
Torque is the rotational version of force. The more torque an engine produces, the more force it can exert at the rim of a flywheel of a given radius.
Power is force multiplied by speed. The more power an engine generates, the more work it can do in a given time.
An engine will run evenly from somewhere around idle speed (about 800 revolutions per minute or rpm) to its "red line" which might be anywhere from 4000rpm for an older engine to 12000rpm for a Formula One engine. The power and torque will vary through this rev range.
Torque increases as revs increase from idle to a certain figure and then falls as the revs increase above this figure. Acceleration is proportional to the amount of force pushing the vehicle forward, so maximum acceleration in a given gear is attained when maximum torque is attained.
Power is force (~torque) multiplied by speed (~revs) so power increases with revs up to and past the point of maximum torque. However, at still higher revs the engine starts to be limited by the amount of air that it can draw in (4 valves per cylinder help) and torque then decreases more rapidly than the revs increase and therefore power also decreases.
Power is force multiplied by speed, and maximum acceleration is attained by having maximum propulsive force at the wheels. Use of a low gear-ratio multiplies the engine torque at the wheel (at the price of having the engine rotate more quickly). Maximum acceleration at a given speed is attained by having the engine operating at maximum power.
Driving a car is easier and more relaxed if it has a flexible engine. Flexibility even becomes a safety issue in four wheel drives. Maximum torque is given at certain rpm and maximum power at higher rpm; an engine is flexible if these rpm figures are widely different, say in the ratio 1:2 or more.
A vehicle operating above its maximum-torque rpm is in a "stable" speed regime: If it slows down by a small amount (incline, head-wind, etc.) torque will tend to increase and resist the slowing. Conversely, if it speeds up by a small amount, torque will tend to decrease and discourage further increase in speed.
A vehicle operating below its maximum-torque rpm is in an unstable speed regime: If speed falls (decline, tail-wind, etc.), torque decreases so speed falls further. The driver can compensate by opening the throttle. Conversely, if the speed increases then the torque increases so the speed increases more. The driver can compensate by closing the throttle (or risk a speeding ticket). The driver has to actively compensate for these variations so the car is less relaxing to drive.
The driver cannot correct for a fall in engine revs and loss of torque if the throttle is already wide open, except by changing into a lower gear. This can be a safety matter on a steep trail. If you start up a steep hill in too high a gear, or have to slow down due to rocks, for example, the engine may fall below maximum-torque rpm and be unable to recover from a downwards spiral. Changing gear looses more speed and traction may be lost as the clutch engages. Inaction leads to a stalled engine and a restart on a dangerous slope. These risks are minimized if maximum torque is attained at low engine revs.
So, if you are into racing, particularly Formula One racing, you want an engine with the highest possible power output and this is best achieved by generating maximum power at very high rpm. Such engines also tend to generate maximum torque at very high rpm and are inflexible and difficult to drive - but that is what the aces are paid for.
If you are into four wheel driving, you want a very flexible engine that generates maximum torque, and lots of it, at low rpm and generates maximum power at high rpm - a ratio of 1:2 is good. A ratio of 1:3 e.g. 1500rpm:4500rpm is excellent. As a side benefit, engine wear is largely determined by piston speeds and producing high torque at low rpm allows a high "over-drive" top gear for quiet highway cruising and for long engine life.
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