skar19
Retired
- Joined
- Feb 26, 2005
- Messages
- 2,667
- Points
- 38
found this on the netTorque and powerWhat is the difference between torque and power? The unit for torque is Nm, and the unit for power is horsepower or kiloWatt). We know that engines have it, but what is it?Imagine a bicycle. You are going uphill in the top gear. You will probably not even manage to get going from a standing start. If you are a fully-grown man with the feet locked into the pedals, you will produce about as much torque as a BMW V8 engine, around 400 Nm. That is an indication of the force you apply to the crank. It can be compared to how an engine pushes on its crankshaft with its pistons.How much power do you put out, then? The answer is zero! You can't have power as long as you don't get the bike moving. Your weight is one tenth of a ton, and the car could be two tons. What is it doing differently since it accelerates so well? You have the same torque, but you are stuck.The secret is the gear ratio. If you pick a lower gear, you will get up to speed too. If you have a mountain bike, you will notice that you can hardly get past walking pace in first gear. With such a low gear, you produce a lot of torque on the rear wheel. The same thing goes for cars. Even though it has 400 Nm at the flywheel, it has something like 6000 Nm on the driven wheels in first gear. The torque is multiplied in the transmission. First it is multiplied with approximately four in the gearbox, then something similar in the differential. (Yes, these are the numbers from the specs, like 3.73:1)In the top gears the torque is a lot lower, as the fifth gear doesn't increase the torque at all. That could leave you with only 1500 Nm on the driven wheels. If we had that low gears on the bicycle, we would be spinning the crank around without getting anywhere. On the bike you have approximately 400 Nm on the rear wheel in first gear, and then it gets less and less in the higher gears. You are perhaps left with 100 Nm in the top gear. In addition to that, the diameter of the rear wheel is larger. That will reduce the forward thrust even further. (Torque is force times arm length, and when the arm is long, the force gets weaker.)So how can the car cope with those low gears? The answer is high RPM. The pistons in an engine are pumping a lot quicker on the crankshaft than we can dream of doing on the pedals of a bicycle. When we are spinning the crank around as fast as we can, we are hardly keeping up with the pedals. The force is close to zero. That is the torque. We produce a lot of torque at standstill, but it drops off badly the faster we are pushing the pedals around. At one point the torque equals zero, and we reach our limit.The torque produced by an engine will actually increase with RPM up until a point. On BMWs this is usually around 4000 RPM. Then the torque will drop off gradually until the engine runs into the limiter, where the torque is zero. An engine will produce the same torque when pumping the crankshaft around at an insane rate as we do at standstill. This is where the power comes in, to express this difference. Power equals force times speed. If we are talking about a rotational motion, power equals torque times rotational speed (RPM). That power is what pushes a car forwards. The faster you can push the pedals around with the same force, the lower gears you can use. This is where the engine is so much better than us, and that is why it will beat us in a quarter mile race. The torque peak is the same, but the RPM is a lot higher. The product of these is power, and that is a good indicator of performance. The torque peak doesn't really say much about the performance potential. If it is achieved at a very low RPM, it can't use low gears. A BMW 320d has a lot more torque than a Honda S2000, but the Honda engine will make the car a lot quicker. The reason is that it has the torque peak at a very high RPM, so it can go fast in the low gears. That does of course make it very strong. The power reflects it too, as the Honda indeed has more power than the 320d. Since the torque on the driven wheels pushes the car forwards, you get the best acceleration by picking the lowest gear possible. That will multiply the torque more than higher gears would. As a biker you can risk getting too high RPM so that you have no more force left on the pedals. To get the optimum acceleration, you should change to a higher gear when the power there is the same as in the current gear. The RPM will drop, but the torque will be increase (since we, like electric motors, are stronger at low RPM). The peak power of a well-trained bicycle rider is approximately one horsepower at 90 RPM, and it is the same in all gears. The torque on the rear wheel does however vary a lot from gear to gear. Modern Car engines will produce so much torque at high RPM that you don't have to change up to the next gear until you run into the rev limiter. Not until that point does the torque drop enough to counter the RPM change to the next gear. Those who have seen Ketill "Varvtalsmatadoren" Jacobsen drive his BMW M518i have seen this principle implemented. He knows that when he changes up one gear, he will not get as much help from the gearbox multiplication as he does in the current gear. That is why he is so reluctant to change up.Ove Kvam
k:
