: How is the Horsepower of a car measured? And how did they measure it for the firsts motored cars?
In: Engineering
A dynamometer is a torque meter and speedometer in one. It is attached to the wheel of a car while the car is suspended in the air. Then, the car starts to spin the wheel. The dynamometer measures the torque and the speed. Finally, power = torque * speed
How is it measured today? They measure how much power the engine puts out in kW, then convert it into the archaic unit of “horsepower”.
How was it first measured? Apparently they lifted weights with a specific speed to a specific height. For each 33000 American Pounds lifted by 1 foot of height over the timespan of one minute, the car was declared to have 1 horsepower. I assume that is what their “standard horse” was capable of lifting, hence the strange numbers.
Nowadays a horsepower is simply defined as 746 Nm/s or 746 W
We measure rotational power output with dynamometers. They vary in form but the general idea is to accelerate a mass with known characteristics over a set length of time.
For example, if we have a tire in contact with a steel drum (and we know the drum’s rotational inertia – it’s resistance to accelerating) we can measure the change in spinning speed of the drum over a period of time. Since w can calculate how much energy it takes to speed up the drum that much and since we measured the time, we can calculate power (energy/time)! The math for ths method had been around longer than motor vehicles.
Other dynamometers test engine (or shaft power) by accelerating a fluid (like water or oil) with a pump.
You don’t measure horsepower. You measure torque, the turning force on the crank shaft. Then you put that into an equation with current rotations per minute the engine is running to get horsepower:
horsepower = torque x rpm/5252
So all you need is something to measure torque and something to measure the rpm of the engine, and you get horsepower. This is why horsepower on a car is a curve, with it only achieving that highest horsepower for a small part of it.
Now that would be shaft horsepower. To measure at the wheels you put the car on a dynamometer, which is like a treadmill for cars, then measure the torque the tires apply to the rollers while the car is at various rpms. This will always be lower than shaft horsepower due to power losses in the transmission and the rest of the drivetrain.
Horse power was first used to sell steam engines to mines so a guy watched horses and saw how much they pulled, on average throughout the day. I believe it’s 10,000 pounds one foot in a minute, which you could do, with some leverage, but probably for five minutes, not hours.
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