Engine power is the
power that an
engine
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...
can develop. It can be expressed in power units, most commonly
kilowatt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
, metric horsepower (often abbreviated
PS), or
horsepower
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...
. In terms of internal combustion engines, the engine power usually describes the ''rated power'', which is a power output that the engine can maintain over a long period of time according to a certain testing method, for example ISO 1585. In general though, an internal combustion engine has a power take-off shaft (the crankshaft), therefore, the rule for shaft power applies to internal combustion engines: Engine power is the
product of the engine
torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
and the crankshaft's
angular velocity
In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...
.
Definition
Power is the
product of
torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
and
angular velocity
In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...
:
[. p 233]
Let:
*
Power in
Watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
(W)
*
Torque in
Newton-metre
The newton-metre or newton-meter (also non-hyphenated, newton metre or newton meter; symbol N⋅m or N m) is the unit of torque (also called ) in the International System of Units (SI). One newton-metre is equal to the torque resulting fr ...
(N·m)
*
Crankshaft speed per
Second
The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
(s
−1)
*
Angular velocity
In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...
=
Power is then:
:
In internal combustion engines, the crankshaft speed
is a more common figure than
, so we can use
instead, which is equivalent to
:
[. p. 21]
:
Note that
is per Second (s
−1). If we want to use the common per Minute (min
−1) instead, we have to divide
by 60:
:
Usage
Numerical value equations
The approximate
numerical value equations for engine power from torque and crankshaft speed are:
[. p 154][. p 259]
International unit system (SI)
Let:
*
Power in
Kilowatt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
(kW)
*
Torque in Newton-metre (N·m)
*
Crankshaft speed per Minute (min
−1)
Then:
:
Technical unit system (MKS)
*
Power in
Metric horsepower (hp, PS)
*
Torque in
Kilopondmetre (kp·m)
*
Crankshaft speed per Minute (min
−1)
Then:
:
Imperial/U.S. Customary unit system
*
Power in
Horsepower
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...
(hp, bhp)
*
Torque in
Pound-force foot (lbf·ft)
*
Crankshaft speed in Revolutions per Minute (rpm)
Then:
:
Example
A diesel engine produces a torque
of 234 N·m at
4200 min
−1, which is the engine's rated speed.
Let:
*
*
Then:
:
or using the numerical value equation:
:
The engine's rated power output is 103 kW.
Units
See also
*
List of production cars by power output
Bibliography
*
*
*
*
References
{{reflist, colwidth=30em
Mechanics
Power (physics)
Engines