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Tech Talk: Kilowatts/Horsepower and Torque

When you’re shopping for a new car, there should be a list of things that are important to you. Nowadays it’s how many USB ports or bottleholders but in the past it was about the “donk” and how many “neddies” under the bonnet. Before Australia went metric and still how the U.S. measures power, there’s horsepower. Metrification uses kilowatts and then there’s this mysterious thing called “torque”…

Horsepower, by its very name, measures output or work done in comparison to horses. Scottish engineer, James Watts (whose name also gives us part of the other measure) adoptedthe unit’s measure in the late 18th century with: Horsepower (hp) is a unit of measurement of power (the rate at which work is done) in order to compare the output of steam engines to draft horses. That was subsequently expanded to compare to other types of engines like turbines and electric engines.

Watt, being an engineer, put forward this equation:  P = \frac{W}{t} = \frac{F\cdot d}{t} = \frac{180\,\mathrm{lbf}\cdot 2.4 \cdot 2\, \pi \cdot 12\, \mathrm{ft}}{1\,\mathrm{min}} = 32,572 \cdot \frac{\mathrm{ft} \cdot \mathrm{lbf}}{\mathrm{min}}.

In short, Watt had calculated that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute), with the wheel measuring 12 feet (3.6576 meters) in radius; therefore, the horse travelled 2.4·2π·12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. What does all of this mean? Horses would perform a certain task with a certain amount of work performed over a given time period, which boils down, without all of the physics involved, to be calculated as horsepower.

Kilowatts is the other measure of what an engine can provide and is, simply, an amount of thousands of watts. For example, a car engine may be quoted as 200 kilowatts, being: The unit is defined as joule per second and can be used to express the rate of energy conversion or transfer with respect to time. The unit was named after the aforementioned James Watt.

When it comes to kilowatts versus horsepower, one kilowatt is equivalent to 1.34 horsepower, with neddies or ponies considered as suitable replacement words. In reverse, there’s about 0.75 horsepower per kilowatt.

Engines found in cars will quote x amount of kilowatts in their marketing material, with the higher the number seemingly the better. The “problem” with this approach is that PEAK kilowatts and horsepower are generated at high engine revs. That’s fine for applications where that approach is needed, say Formula 1 racing or powerboat racing but then fuel efficiency for the common man, not to mention the sheer driveability,becomes an issue…

power graph

Torque, the forgotten part of what a car engine does, is, in simplest terms, the amount of grunt or the measure of twisting force an engine can generate. A great way to think of what torque and kilowatts can do is by imagining a screwdriver and a stuck screw. The twist that you use to move the screw is torque and the continued turning of the screwdriver to get the screw out is the kilowatts or horsepower. Should you see a skilled driver perform a burnout, let’s say a motorsport driver that’s won a race and is celebrating, they will use both torque and kilowatts,with torque coming into play to break the traction between rubber and road then power (kilowatts) to continue to spin those tyres against the gripping force of both road and hot rubber, which then produces the spectacle of heaps of smoke.

Torque is made by an engine at a lower rev range than horsepower or kilowatts; a diesel engine will produce more torque than a petrol powered engine simply because of the way a diesel engine works. Petrol engines use electricty and spark plus to ignite the fuel vapour inside the cylinders, which pushes the piston down and turns the crankshaft. Diesels, on the other hand, use compression (squeeze a balloon until it pops) by injecting diesel fuel into an air filled combustion chamber (or cylinder, in the case of a car engine) and pushing the piston against that until it “explodes” and forces that piston back down. Diesel torque

Or in tech terms: The diesel engine (also known as a compression-ignition or CI engine) is an internal combustion engine in which ignition of the fuel that has been injected into the combustion chamber is initiated by the high temperature which a gas achieves when greatly compressed (adiabatic compression). This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to petrol), which use a spark plug to ignite an air-fuel mixture.

Torque is also the motive force that gets your car going in the first place. Using the transmission, with a set of differing ratios or gears, that torque is sent from the engine to the driving wheels via that transmission and that torque is then what starts moving the tyres against the road. As the revs rise, kilowatts then take over to keep your car going, before the transmission changes ratio and drops the revs back down.

And, as we all now know, the less revs the engine is doing, the less fuel it’s consuming, hence the growth of gearboxes with more ratios, the most common being a six speed but with some luxury brands having eight, perhaps nine….and an engine with enough torque to utilise those ratios.

(Information sourced from Wikipedia. This article is not intended to be an in-depth explanation of kilowatts, horsepower and torque, but an overview to suit its intended audience.)