## Engine Power | Torque and Horsepower | Automotive Engines

Definition of Torque:

Torque is the term used to describe a rotating force that may or may not result in motion. Torque is measured as the amount of force multiplied by the length of the lever through which it acts. If you use a 1 feet long wrench to apply 10 pounds (lb) of force to the end of the wrench to turn a bolt, then you are exerting 10 pound-feet (lb-ft) of torque. ● See Figure 1.

Fig.1 Torque is a twisting force equal to the distance from the pivot point times the force applied expressed in units called pound-feet (lb-ft) or newton-meters (N-m).

Torque is the twisting force measured at the end of the crankshaft and measured on a dynamometer. Engine torque is always expressed at a specific engine speed (RPM) or range of engine speeds where the torque is at the maximum. For example, an engine may be listed as producing 275 pound-feet @ 2,400 RPM.

The metric unit for torque is newton-meters, because the newton is the metric unit for force and the distance is expressed in meters.

1 pound-foot = 1.3558 newton-meters

1 newton-meter = 0.7376 pound-foot

Definition of Power:

The term power means the rate of doing work. Power equals work divided by time. Work is achieved when a certain amount of mass (weight) is moved a certain distance by a force. If the object is moved in 10 seconds or 10 minutes does not make a difference in the amount of work accomplished, but it does affect the amount of power needed. Power is expressed in units of foot-pounds per minute and power also includes the engine speed (RPM) where the maximum power is achieved. For example, an engine may be listed as producing 280 hp @ 4,400 RPM.

Fig.2 Engine Power Testing Unit

Horsepower and Altitude Because the density of the air is lower at high altitude, the power that a normal engine can develop is greatly reduced at high altitude. According to SAE conversion factors, a non-supercharged or non-turbocharged engine loses about 3% of its power for every 1,000 feet (300 m) of altitude.

Therefore, an engine that develops 200 brake horsepower at sea level will only produce about 116 brake horsepower at the top of Pike’s Peak in Colorado at 14,110 feet (4,300 m) (3%. 14 – 42%). Supercharged and turbocharged engines are not as greatly affected by altitude as normally aspirated engines, which are those engines that breathe air at normal atmospheric pressure.

Is Torque ft-lb or lb-ft?

The definition of torque is a force (lb) applied to an object times the distance from that object (ft). Therefore, based on the definition of the term, torque should be: lb-ft (a force times a distance) Newton-meter (N-m) (a force times a distance) However, torque is commonly labeled, even on some torque wrenches as ft-lb.

What’s with these Kilowatts?

A watt is the electrical unit for power, the capacity to do work. It is named after a Scottish inventor, James Watt (1736–1819). The symbol for power is P. Electrical power is calculated as amperes times volts: P (power) = I (amperes) ∙ E (volts) Engine power is commonly rated in watts or kilowatts (1,000 watts equal 1 kilowatt), because 1 horsepower is equal to 746 watts. For example, a 200 horsepower engine can be rated in the metric system, as having the power equal to 149,200 watts or 149.2 kilowatts (kW).