When an piston is forced down the cylinder during the power stroke it applies the force to the connecting rod. The connecting rod then causes the crankshaft to turn. The force that makes the crankshaft turn is called torque. The metric unit for the measurement of torque is newton meters, the imperial measurement is pounds feet.
If we assume that a shaft has a lever attached perpendicular to its axis, and that lever is 1 meter long. If a force of 100 newton were applied to the end of the lever, the torque applied to the shaft is 100 newton per meter or 100 newton meters. Similarly if a force of 100 pounds were applied to the end of the lever that was 1 foot long, the torque applied to the shaft is 100 pounds er foot or 100 pounds feet.
Power is a term used to describe how much work is done in a period of time. An engine produces POWER by applying TORQUE to a ROTATING shaft. So the measurement of engine power is calculated from the amount of torque applied to the crankshaft and the speed at which it is turning. When expressing engine power it is necessary to express not only the power value, but to include the engine speed at which it occurs.
The metric measurement of power is the Kilowatt. The imperial measurement is the Horsepower.
The watt is the metric system measurement of power, however engine power is expressed in kilowatts, as the watt has such a small value. A kilowatt is equivalent to 1000 newton per meter per second.
The term horsepower was expressed by James Watt, who determined that a horse could lift 330 pounds, 100 feet in one minute. Therefore 1 horsepower is equal to 33,000 pounds per foot per minute. 1 horsepower is more than a person can produce.
There are different standards of power measurement. These are ECE, SAE and DIN
The ECE standard is European. Engine power is measured at 99 kPa of dry air and 25°C (77 F). Friction torque is not taken into consideration at all.
The SAE standard American. Engine power is measured at 99 kPa of dry air and 25°C (77 F) and applies a friction correction and uses a default Mechanical Efficiency (ME) value of 85%. This is approximately correct at peak torque but not at other engine operating speeds.
The DIN standard is determined by the German automotive industry. engine power is calculated at 101.3 kPa of dry air and 20°C (68 F). With the advent of ECE standards, the DIN is rarely used.
Source: CDX Global
