With this piston speed calculator, you can calculate the mean speed of a piston with just 2 inputs, namely strokes and revolution per minute (RPM). The results will be the average piston speed. You can use this article to understand what piston speed is, learn how to calculate it, and interpret the results produced by the piston speed equation.

What is piston speed?

Piston speed, also known as the mean or average piston speed, is the average speed that a piston moves from the Top Dead Center (TDC) and Bottom Dead Center (BDC) and back again, completing a full cycle. It can be displayed in different units, with feet per minute or meters per second being the most common ones.

The piston comes to a complete stop at two points in a full cycle, namely the TDC, which is the top of the stroke, and the BDC, which is the bottom of the stroke. Between its movement from TDC to BDC or the other way around, the piston's speed changes constantly. This makes it hard to determine the actual speed of a piston at any given time. Therefore the mean piston speed is generally taken as a proxy of the piston's speed for the purposes of calculations.

How to calculate piston speed, also known as the average piston speed?

Our piston speed calculator allows you to calculate the piston speed of every piston in 3 simple steps.

Before we dive into an example and shown you how to perform the piston speed calculation, we first need to define the variables involved. Let's look at the piston speed equation.

Piston speed = 2 * Stroke * RPM

  • Piston speed - Mean or average piston speed that a piston completes a full cycle within the cylinder;

  • Stroke - Full distance that the piston travels in one cycle within the cylinder; and

  • RPM - Stands for revolution per minute which is the number of revolutions, or full cycles, that a piston can perform in 1 minute.

As a full-cycle involves 2 strokes, where the piston moves up and down in one revolution, we need to multiply the equation by a factor of 2.

Alright, let's look at the example now. Imagine you now want to calculate the piston speed for a piston that has a stroke of 5 inches and performs 1,000 revolutions per minute.

  1. Determine the stroke of the piston

As defined, a stroke is the distance that the piston travels in a full cycle. Hence, in our example, stroke = 5 inches.

  1. Determine the RPM of the piston

Since the piston performs 1,000 revolutions per minute, RPM = 1,000 per minute. To know more about RPM, check out our RPM Calculator.

  1. Calculate the piston speed

Using the equation above, Piston speed = 2 * Stroke * RPM = 2 * 5 * 1,000 = 10,000 inches per minute = 833.3 feet per minute.

This means, on average, the piston is moving at a speed of 833.3 feet per minute within the cylinder.

The purpose of calculating piston speed

So, here come the big questions, "Why do we need to calculate the piston speed?"

  • Evaluate engine performance

Generally, a higher piston speed implies a higher productivity of the engine, leading to higher engine performance. This, however, is conditioned upon the proper intake/exhaust tract and valve timing.

  • Estimate expected durability

The piston applies forces to air and translates it to energy when it is at work. If the energy is too high, the cylinder will be overfilled, reducing the piston's durability.

  • Determine the required strength of the components

The component strengths of all the rods, bolts, and crankshafts are essential as it defines how fast the piston can work. Hence, engineers would normally make sure the piston speed is below the required strength of those components.

Other considerations

Even though piston speed is one of the most important metrics to calculate when evaluating a piston's performance, there are still some factors that you need to consider to make the right decisions:

  • Maximum piston speed

The mean piston speed is much lower than the maximum piston speed, so it is important to understand the maximum speed that the piston's cylinder needs to withstand.

  • Instantaneous speed

It is important to realize that the mean piston speed is not the piston's speed at a given time. Hence, it is important to determine the actual speed of the piston at a given time if you are looking at the piston performance at a particular point of the cycle.

  • Piston force

Piston speed can only provide so much information. If you need to carry out other further calculations, you might want to consider calculating the piston force or the hydraulic pressure.

Wei Bin Loo
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