Engines are commonly rated by the power, torque and speed.
Power output of an engine is commonly rated in units of horsepower. Recall that power is the amount of work done per unit time or the rate of doing work, and that:
Torque describes the twisting force produced by the engine. Torque is commonly given in units of lb-ft.
Speed is the rotational speed of the shaft, commonly measured in revolutions per minute (rpm). Torque and speed are commonly presented as functions of speed in a torque-speed curve.
Figure6.3.7.Engine Specs, typical
Indicated horsepower is the power transmitted to the pistons by the gas in the cylinders and is mathematically calculated.
Brake horsepower refers to the amount of usable power delivered by the engine to the end crankshaft. Indicated horsepower can be as much as 15% higher than brake horsepower. The difference is due to internal engine friction, combustion inefficiencies, and parasitic losses, for example, oil pump, blower, water pump, etc.
The brake power produced by an engine can be calculated from using the formula:
\begin{equation}
P = \frac{T n}{5252}\tag{6.3.1}
\end{equation}
Where:
\(P\) is the shaft power in horsepower (hp).
\(T\) is the torque generated by the engine, measured in ft lb.
\(n\) is the rotational speed of the engine in rpm.
5252 is the conversion factor \(\left[5252 \dfrac{\text{ft lb rpm}}{\text{hp}}\right]\)
The ratio of an engine’s brake horsepower and its indicated horsepower is called the mechanical efficiency of the engine. The mechanical efficiency of a four-cycle diesel is about 82 to 90 percent. This is slightly lower than the efficiency of the two-cycle diesel engine. The lower mechanical efficiency is due to the additional friction losses and power needed to drive the piston through the extra two strokes.
Example6.3.8.Engine Power.
Determine the brake horsepower of an engine which produces 330 ft lb of torque at 2600 rpm.
Answer.
\begin{equation*}
P = 163.4 \text{ hp}
\end{equation*}
Solution.
\begin{align*}
P \amp = T n\\
\amp = (330 \text{ ft lb})(2600 \text{ rpm})\\
\amp = 858,000 \text{ ft lb rpm}\\
\amp = 858,000 \text{ ft lb rpm} \div \left[5252 \dfrac{\text{ft lb rpm}}{\text{hp}}\right]\\
\amp = 163.4 \text{ hp}
\end{align*}
