Tech Talk: How Power Torques.
When car makers advertise their products, apart from price you’ll probably notice or hear xxx kilowatts. Great. Wonderful. Fantastic.
Huh?
A kilowatt is, unsurprisingly, one thousand watts. You’re probably familiar with the term via your home theatre system or perhaps in kilowatt-hours for your power bill. But what does it mean in car talk, and, how does it relate to the more important yet ignored part of an engine output, torque?
Kilowatts and torque are produced by an engine spinning, be it electrical, petrol, or diesel powered. Kilowatts or horsepower are a measure of power, as defined here: It is the amount of energy consumed per unit time. Having no direction, it is a scalar quantity. In the SI system, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the steam engine.
A petrol and diesel engine work by ingesting fuel into cylinders and either igniting (petrol) or compressing (diesel) those fuels in the cylinders. Those explosions rotate a crankshaft which spins at so many times per second. By their very nature, petrol engines will spin to a higher rpm (revolutions per minute) than diesel, and it’s a high revolutions that petrol powered engines make their peak amount of kilowatts. Motorbike engines, in particular, make their power at well over ten thousand rpm, but are limited, in a sense, as to the outright capacity of the cylinders.
As a rule, bigger capacity engines are able to make more power however some aren’t physically able to rev as high as some smaller capacity engines. A great example is a car from Honda in the early part of the 21st century. The S2000 was initially powered by a two litre capacity engine, which was extended to a two point two litre size. Its peak power in Japan was quoted as 184 kilowatts. However, in order to produce that amount it had ro rev to 8300 rpm. Holden’s Chevrolet sourced V8, with a capacity in excess of six litres, produces 304 kilowatts, at between 5500 and 6000 rpm, somewhat less that the peak rpm of the smaller engine.
Torque is the forgotten part of the equation and is actually the part of driving that’s initially and constantly more important. To go back to the initial part of this, about how makers quote a kilowatt figure, it’s simple marketing to have those numbers in your headspace, but it’s torque that gets your car going and, especially in towing, becomes vital. Here’s the balance: torque is always produced at a lower rpm than power and it’s here that its useability is what you’ll notice.
It’s been said that torque is what gets you going and power is what keeps you going. In acceleration tests as seen in a certain British car oriented TV program, it’s the torque that will launch the cars off the line, but it’s the power (leaving out the weight of cars and the gears in their gearboxes) that garners the attention as they cross the finish line.
One of the characteristics of diesel engines is where, in their rev range, the peak torque is made. Because the crank is spun by the reaction of fuel being compressed to explosion, there’s torque being produced far lower in the rev range than petrol. Torque is also a measure of force, a twisting force Think of loosening a stuck screw; by twisting the screwdriver you’re exerting force or torque to (hopefully) start twisting the screw, before power takes over to finish the job. 
Torque’s also visible in a physical form. We’ll presume you’ve seen a car do a “burnout”, where the tyres are spun to a point that they produce smoke. It’s torque that will eventually break the traction of the tyres.
Power is also a gradual climb before fading off, but torque can be found within a rev range as a constant number between two points on a rev range. Measured in either foot-pounds or Newton-metres, a flat torque delivery will make the sheer driveability of a car easier and safer. This graph shows one example of a “table top flat” toque delivery.
So when shopping for your next car, consider HOW the car will be used. Will you be towing, will it be a tradie’s ute, are you driving around town more than driving on freeways, are you driving the under 8’s netball team around? Although a peak power of 200 kilowatts might sound attractive, consider that in order to have that figure you’ll need to have your engine constantly at 6000 rpm…everywhere you go. Torque is what will get you going and is a real world more usable figure. Check out the information available on company websites for the car you’re looking at.
(Burnout figure thanks to Street Machine, info sourced from online sources).
Level 2, 9 Help Street
Darren Bennetts says:
Thanks kindly for the stuck screw requiring Torque (twist) to get it moving and Power to unscrew the screw once it is moving.
As a kid Inasked my dad to explain the difference between power and Torque and he struggled to get across the distinction to me as a 12 year old.
In the end he said Torque is what you feel as the car accelerates and power is what keeps it going. I Was confused because there was only one driveshaft and it has to be twisted to make he car roll and so struggles to see that Power and Torque are different from each other
I am 48 now and your description may have helped a little!
February 27th, 2017 at 5:45 pm
David Alley says:
Similar to Darren Bennetts, I also asked my Dad, an electrical engineer, to distinguish between kilowatts and torque. He simply stated that kilowatts give you speed and torque gives you muscle or strength. Is that analogy a reasonable simplification?
February 27th, 2017 at 7:57 pm
Tom Mulligan says:
As a matter of record the only measurable output of an engine is Torque. Horse Power (or Kilowatts) is derived by using a mathematical function. which relates a number given RPM and torque at those Revs.
The formula is:
Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488
February 28th, 2017 at 1:03 pm
Alexander McClintock says:
Why are the gears not fully considered in these statistics?
I say to myself that if automatic gears were changing optimally then there would be little difference between Torque and Kw.
I’ve not seen this stated but surely this must be the case except in 1st gear?
If there were gearboxes with 20 speeds surely kW would be the best predictor of 0-100 KPH?
The typical, simple discussion assumes there is only one gear!
February 28th, 2017 at 1:06 pm
Dave Conole says:
Find a manual car with a small and less torquey engine than a car fitted with a V8 or turbo engine. Start off in fourth and see which is most effective in getting the car going. Kilowatts have nothing to do with getting a car under way, torque does. Gear ratios are slected to suit the characteristics of the engine, not the other way around. We’ll even go so far as to offer that’s why some racing categories have changeable ratios allowed for different circuits, due, again, to the torque, with kW coming into play at speed.
February 28th, 2017 at 1:35 pm
CW says:
“Because the crank is spun by the reaction of fuel being compressed to explosion”,
this is completely wrong, air is in the cylinder and compressed becoming very hot, fuel is injected into the hot compressed air and it is a controlled burn not an explosion, diesel manufactures go to great lengths to prevent explosion,because it destroys the engine.
March 1st, 2017 at 12:27 pm