You've seen it a bunch of times when you're reading the specs on a new car: the number of foot-pounds of torque that it puts out. Well, the 2009 New York Auto Show just happened, and I was reading the synopsis of a new Mercedes that just came out:
369 foot-pounds of torque, it says. (That's 500 Newton-meters, for you mks/SI folks.) Torque is the amount of "turning power" you have, much in the same way you turn a wrench. 369 foot-pounds means that if you had a wrench that was 1 foot long, and applied a force of 369 pounds directly perpendicular to that wrench, you would get 369 foot-pounds of torque.
Well, what can this do to a car? The answer is: cause it to accelerate! The torque specification they give is the maximum torque of the internal-combustion engine, which is usually a higher value than the actual torque on the wheels. (See wikipedia for more details.)
But this torque can tell you a lot about how fast the car can accelerate. Let's turn it into a physics problem. We'll assume that this "500 Newton-meters" is an actual, legit value for how much torque the tires experience. We can estimate that the mass of a typical car is about 1500 kg, and that the typical distance between the center of mass of the car and the wheel's rotational axis is about 20 cm; this gives us a moment of inertia for the car of 60 kg m^2. The car's wheel size plus the sidewall radius of the tire is about 20", or 51 cm.
The acceleration of this car? 4.25 m/s^2, or (more commonly), it can do 0-60 miles-per-hour in about 6.3 seconds. Want a car that can accelerate faster? Here are the things that can help:
- more torque (duh),
- a lighter car,
- a lower center-of-mass (closer to the wheel axle in height),
- larger diameter wheels & tires,
- and an engine that can output this large amount of torque over a wide range of engine speeds.
So yes, it's just a prototype, and in theory it can do it in 2.3 seconds, but that's pretty good for some real-world physics, and now hopefully when someone brags to you about how much torque their car has, you'll actually know what they're talking about!
Physical Science
Comments
A post on how engine torque relates to the acceleration of a car is really missing something if you fail the mention the gear ratio.
Even if we ignore internal friction and such, the engine torque is related to the torque on the wheel by the gear ratio: if the total (transmission times differential) gear ratio is 10:1, for example, so that 10 revolutions of the engine results in one revolution of the wheels, then the torque exerted by the wheels is "geared up" by a factor of 10 over the torque exerted by the engine. For the Corvette referred to in the linked wiki article, this ratio ranges from 10:1 in 1st gear down to about 2:1 in 6th gear.
In low gear, even with losses, the torque available at the wheel should be quite a bit higher than the torque at the output of the engine. Nothing comes for free, of course: the lower the gear (and therefore the higher the ratio as I've expressed it), the faster the engine must spin to make the car move the same speed, so the more power the engine must generate. If we had a perfectly lossless, infinitely variable transmission and drivetrain, it wouldn't matter one bit how much torque the engine had at the output -- the only thing that would matter would be power, which is the product of torque and angular velocity.
I've hear car and motorcycle enthusiasts use "engine torque" as a shorthand for how much acceleration is on tap without downshifting to change the gear ratio...
Posted by: Alex R | April 21, 2009 1:34 PM
Only the axle he1ght is relevant to the resulting acceleration. The c of g height has only small secondary effects at normal car power levels. Powerful vehicles would experience pitching moments in response to differences between axle height and c of g height. In extreme cases, like rear wheel drive drag cars, maximum acceleration would be limited by the vehicle becoming unstable in pitch and flipping over backwards. (Unstable as any pitch-up puts the c of g even further above the thrust line, and eventually horizontally closer to the axle.
Posted by: David L | April 22, 2009 3:26 AM
Car manufacturers: all torque and no action! It's a pity they talk torque and turn green when you ask them how green their vehicle is....
Posted by: Ian | April 22, 2009 4:05 AM
If I understand it at least partly correctly, 'torque' is engine power at low rpms, while 'horsepower' is the same measurement, but taken at at a specific higher rpm.
Some engines don't develop their maximum power until higher rpms are achieved, while others, like diesels, achieve most of the power quickly at the lower rpms, and actually suffer power losses as rpms get higher.
To accelerate quickly from a standstill, then, requires power at low rpms, thus, torque foot pounds is a much more reliable indicator of car acceleration performance than horsepower foot pounds would be.
The whole discussion gets mixed up in most people's minds because somehow over the years the concept of 'torque' and that of some kind of 'twist' have become entangled.
Posted by: Gingerbaker | April 24, 2009 9:50 AM
Torque in an engine and horsepower are related, but are not the same. Horsepower is the total power output of an engine, while torque measures the "turning/torquing ability" of the engine.
Practically, torque is a better measurement of how quickly your car will accelerate, while horsepower (relative to weight) is a better measure of your car's top speed. I have two vehicles, a 1997 toyota with a top speed of ~110 mph, and a 16,000 pound school bus with an 8 L diesel engine, with a top speed of ~60 mph, going downhill on the highway.
Posted by: Ethan Siegel | April 24, 2009 10:07 AM
here is a great explanation :) :
http://www.4x4abc.com/jeep101/torque.html
Posted by: Gingerbaker | April 25, 2009 2:53 PM
That Awsome Sunbeam Tiger has awsome accelaration twice as the new 2010 chevy challenger.
Posted by: Betto | July 8, 2009 11:38 AM
I believe the car "Red Victor" actually holds the record for the street legal car, doing 0-60 in about 1 second.
Correct me if I'm wrong, please.
Posted by: Kevin | October 7, 2009 12:42 AM
^^^^ Red victor , yes. its knot to 60 in 1 second flat. theres a video as well on youtube. check it out.
Posted by: rahul | October 9, 2009 10:50 AM
really nicely explained torque vs hp. I like the wrench example. Thx
Posted by: sandy s | April 4, 2010 9:03 PM
Sorry but the fastest street legal car in the world is actually a 1970's Vauxhall Victor thats running over 2000bhp. Has MOT and everything, Just look for it on Youtube, but it does 0-60 in about 1 second lol.
Posted by: Andy | April 19, 2010 9:59 AM
Some engines don't develop their maximum power until higher rpms are achieved, while others, like diesels, achieve most of the power quickly at the lower rpms, and actually suffer power losses as rpms get higher.I would like to know if the engine of 190Nm @2000rpm is better than the engine 140Nm @3000rpm?
Posted by: rakshpal singh salathia | September 9, 2010 6:38 AM
Dear sir,
i would like to know the cc,torque,displacement & bhp terms used in car.If you give me details with examples,i shall be thankful to you.
Eagarly waiting your reply
With regards
Sunil Kumar(improvement team)
Posted by: sunil | September 14, 2010 2:53 AM
Hmm,,interesting article about the torque.I guess greater the torque ,lesser the force you need to use to complete the task.That is a bottom line i guess.
Posted by: Hand tools | January 5, 2011 4:32 AM
Hi, so if there are more torque available at the low end, does the engine consume more fuel?
Posted by: alvin | February 9, 2011 8:15 AM
Hi, so if there are more torque available at the low end, does the engine consume more fuel?
Posted by: alvin | February 9, 2011 8:15 AM
Torque isn't a better indicator of a car's acceleration power. Diesel engines have massive amounts of torque but this doesn't guarantee exceptional acceleration. E.g.
Audi A3 2.0 TDI's make 320nm@1750rpm where Corsa GSi 1.6's make only 150nm@3200rpm. Despite the 400kg difference in weight, according to this torque theory the A3 should have far more acceleration from standstill. But this simply isn't the case, as even with its turbo boost the A3 only overtakes at 60mph, with the corsa quite far ahead during 1st and 2nd.
"Horsepower is another measure of engine power, but it is based on torque. Horsepower is the abiltiy to do work over a period of time. Other things are taken into account, but it is based on torque so the two will follow each other to some degree. Torque comes on first because it is the instant twisting force. Horsepower is built as you get going and things like momentum of engine components come into play. "
Posted by: Degan | February 15, 2011 4:17 PM
Hi there i have a query about torque and kilowatts.
if i car for example has 3oo torque yet 200 kilowatts and another car has 175 kilowatts and 500 torque what would be faster or more quicker acceleration wise.
cause one car has more kilowatts but alot less torque yet the other one has less kilowatts but more torque?
Posted by: richard | April 19, 2011 3:45 AM
Max speed depends on power, acceleration at low speed set by torque. Acceleration at high speed needs torque to kick the car faster but you need excess energy to allow work to be done.
But, since maximum legal speed is ~70mph and you can maintain that on the flat with less than 50bhp with modern efficient bodyshapes, you don't really need much power to drive legally.
Posted by: Wow | April 19, 2011 7:31 AM
Sorry people, you are mixing units left and right, confusing power with force or torque. For acceleration it is the torque AT THE WHEELS per unit mass of the vehicle that counts. The amount of torque at the engine output shaft does not matter a diddly squat as the torque can be increased by gears. The engine power is proportional to the product of the torque and RPM. So the initial acceleration is limited by the grip of the wheels, the acceleration at higher speed is limited by the engine power (per unit weight) as at higher wheel RPM the more powerful (HP)engine can deliver higher torque. From the car operation point of view a flat torque / RPM engine characteristic as it is more important: it is more forgiving of gear shifting.
Posted by: George | October 5, 2011 5:30 PM
ya i want some detail of torque....in deep...plz help me
Posted by: Harshal | January 11, 2012 1:56 AM