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.
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).