Calculate steam engine horsepower from bore, stroke, RPM and mean effective pressure, or solve for piston area, piston speed or pressure from known power.

Average pressure over the stroke, lower than boiler pressure. The suggestions are examples, not measured values.

Most steam engines are double-acting. Piston-rod area is ignored, so double-acting power is slightly overstated.

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Steam Engine Power Formula

The following formula calculates indicated power before mechanical losses from mean effective pressure, effective piston area and equivalent working-stroke speed. Mean effective pressure is not boiler pressure. Sum contributions separately when cylinder ends have unequal areas or pressures. 

Pโ‚›โ‚‘ = (P ร— A ร— S) / (33,000)
  • Where Pse is the steam engine power (HP)
  • P is the mean effective pressure (psi)
  • A is the area of the piston (in^2) 
  • S is the equivalent working-stroke speed (ft/min): use stroke ร— RPM for one working stroke per revolution, or 2 ร— stroke ร— RPM for equal-area double-acting operation. 

To calculate steam engine power (HP), multiply the mean effective pressure by the piston area and equivalent piston speed, then divide by 33,000.

How to Calculate Steam Engine Power?

The following two example problems outline how to calculate the Steam Engine Power.

Example Problem #1:

  1. First, determine the area of the piston (in^2). In this example, the area of the piston (in^2) is measured to be 16.
  2. Next, determine the equivalent piston speed (ft/min). For this problem, the equivalent piston speed (ft/min) is calculated to be 71.
  3. Next, determine the mean effective pressure (psi). For this example, assume a mean effective pressure of 60 psi.
  4. Finally, calculate the Steam Engine Power using the formula above: 

Pse = (P*A*S) /33,000

Inserting the values from above and solving the equation with the input values gives: 

Pse = 60*16*71 /33,000 = 2.065 (HP)

Example Problem #2: 

Using the same process as example problem 1, we first define the variables outlined by the formula. In this case, the values are:

area of piston (in^2) = 3

equivalent piston speed (ft/min) = 1000

mean effective pressure (psi) = 60

Entering these values into the formula or calculator above gives us: 

Pse = 60*3*1000 /33,000 = 5.455 (HP)