We toss the words "horsepower" and "torque" around pretty often in the automotive industry, but perhaps the meaning of both gets lost along the way. Never fear, Engineering Explained is here. In a new video, EE host Jason Fenske helps explain in the simplest of ways what power and torque are , and why the old saying of " Horsepower is how fast you hit the wall, torque is how far you move the wall" is incorrect.
It helps to have a basic understanding of what both things actually are. Torque is a force multiplied by distance, and the easiest way to understand it is by using a wrench. As an individual provides force to the wrench, it travels a distance and provides torque to tighten a bolt. Combustion provides the force in a cylinder to press a piston down, which then presses down on a crankshaft at a specific distance.
This is where the phrase "units of twist" comes from, since the piston and crankshaft provide a twisting force. Horsepower, on the other hand, is the rate at which work is done. Torque multiplied by rpm returns horsepower. Basically, the faster the crankshaft spins with the same amount of force, the more power an engine will make.
A car with more hp than torque will always be quicker since this gives a car acceleration and speed. Jason uses two hypothetical cars to illustrate all of this. Both have the exact same gear ratio, but one uses a diesel engine with pound-feet of torque, and the other uses a gasoline engine with lb-ft of torque. The diesel engine with twice the torque will accelerate quicker initially because it has a greater sustained force to make more power.
However, it only revs to 2, rpm. Actually, that simplification is partially correct. Torque and power are what engines produce when you turn the key and press the accelerator. Air and fuel ignited in the combustion chambers cause the crankshaft, transmission, and drive axles to do the twist.
This is the miracle of energy conversion: the potential energy contained in a gallon of recycled dinosaur efficiently changed to the kinetic energy needed for driving. Energy is the capacity for doing work. In this instance, engines perform the drudgery work formerly done by horses. Work is the result of a force acting over some distance. The U. In the International System SI , work is measured in joules and, in rare instances, newton-meters.
The more torque an engine produces, the greater its ability to perform work. The measurement is the same as work, but slightly different. In this case the distinction is static torque, the kind you apply with a wrench to tighten head bolts. To avoid confusion, the units for static torque are traditionally foot-pounds. Just to be contrary, SI sticks with newton-meters for both static and dynamic torque measurements.
Power is how rapidly work is accomplished. Eighteenth-century Scottish inventor James Watt gave us this handy equivalency: one horsepower is the power required to lift 33, pounds exactly one foot in one minute. In vehicles, the engines rotate around an axis, thus creating torque. It is the force that rockets a sports car from in seconds and pushes you back in the seat. It is also what powers big trucks hauling heavy loads into motion.
These are the basics of horsepower and torque, but how are these concepts measured and how are they interrelated? Mathematically speaking, horsepower is the force needed to move pounds one foot in a second, or 33, pounds one foot in a minute. Horsepower is found by measuring torque because torque is easier to calculate. Torque, as previously mentioned, is the expression of a twisting force and is measured in units of force times distance from the axis of rotation. So for instance, if you use a 1-foot-long wrench to exert a force of 10 pounds on the end of a bolt, then you are applying a torque of 10 pound-feet 10 lb-ft.
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