Quote:
Originally Posted by bfi2906
How does one convert Newtons to Nm's?
What does 6000 Newtons equate to?
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You need alot of extra info to do this.
Now I have been working with cars on dynos for a long time. I am an engineer and trained in physics. I just want to say that the vast majority of people including workshops owning dynos do not understand how dynos work. I am going to give you some info here in an attempt to help you to understand.
Firstly... a dyno only measures one thing (plus roller speed). It does not seperately measure power and torque. It measures tractive effort and the speed of the rollers. Tractive effort is the amount of force pushed on the outer circumference of the roller. This is what is displayed on the graph above in Newtons (N). So naturally, car will have more force in 1st gear than 6th even if it makes the same power. Therefore the tractive effort measurement is next to useless for comparing cars unless they have identical gearing including diff ratio and tyre size.
The software in the dyno can convert the tractive effort to torque of the rollers (which is totally different to torque of the engine). Torque = Force * radius. The software knows the radius of the rollers, so it calculates torque at the rollers. Once it knows Torque of the rollers, it can then calculate power at the rollers. Power = Torque * RPM. Therefore if you have a constant Torque with RPM, the power graph will be rising linearly in a straight line, as power will increase with increaseing RPM. Notice how in my graph the tractive effort is almost constant between 3000 and 4500 on the blue line, and the resulting power is straight increasing in a straight line. When tractive effort starts to drop, the power flattens. So basically torque is the slope of the power graph. Torque and power are connected... so at a given RPM point, a certain power implies a certain torque. it is impossible (at a given RPM point) for a car to make more torque than another car if they have the same power, as power = torque * RPM, so if power and RPM are the same, torque is also the same.
Now, torque is multiplied by gearboxes, but power is not. If a gearbox has no losses, then the power in equals the power out, but the same cannot be said for torque (which is related to radius of gears as torque = force * radius). Therefore unless you know the radius of the rollers, and every gear ratio in the system, you cannot calculate torque using just the tractive effort chart. The tractive effort graph shape will be the same as the torque graph shape (since Torque = tractive effort * radius), but the actualy units are meaningless without the other data.
Another observation is that you can 1000Nm of torque with 0 power... it is possible for say an electric car to be pushing with heaps of tractive effort but if the retarders on the dyno stop the rollers from moving (so RPM is zero), power is 0!!.
Anyway, I will now tell you the most accurate way to determine the peak torque that I am aware of using the info we have available. Firstly, the graph shape for tractive effort is the same as torque. So when the tractive effort is at a max, so will torque be at a max. This occurs at approx 4500RPM.
So we know peak torque is 4500 (although it is pretty flat 3000-4500). At 4500, the car is making approx 220kW ATW. Now we need to estimate the power at the flywheel (engine). A standard car has 225 kW and dynos 190kW ATW, which is 35kW loss in the drivetrain. So lets add the 35kW to the 220. So at the flywheel at 4500RPM, we are making around 220+35=255kW. Now power = torque * RPM, so torque = power / RPM. Now there is actually a constant in this calculation that is a result of the units change (2pi radians per revolution and 60 seconds per minute etc.), so the exact formula is torque = power * 9549 / RPM. So torque is 255 * 9549 / 4500 = 541Nm.
Now there are some assumptions made to get to this calculation (like losses from flywheel to wheels), but I can assure you this is pretty close.
For instance, the engine makes similar tractive effort at 3300, where is makes about 160kWATW. So that is 195kw at engine. Torque = 195 * 9549 / 3300 = 564Nm. Now this is 20Nm higher (but still close), and I will tell you why... the drive train losses are actually lower at lower speed of the wheels (as tyres are spinning slower and they are the main source of losses). So losses are probably closer to 25kW instead of 35kW. If you recalculate for that, you get Torque is 185 * 9549 /3300 = 535Nm.
Anyway, I would say the torque is in the 520 to 550Nm range.
So that is a long rambling, but it is a pet hate of mine the way people ramble on the internet about dyno curves. Especially how people who do not make the power figure they hoped for tend to rave on about how its OK, because they have heaps of torque.
Moral of my post... Power and torque are directly related via RPM. You can't magiacally have heaps of torque but poor power at a certain RPM. What can happen however is that an engine has poor flow, which mainly hits at high RPM, so an engine may have good torque at low RPM, but poor torque at high RPM which results in low power since power = torque * RPM.
So I hope you guys can better analyse and compare dyno graphs now!!
Adrian