Piston Engine Volume Size and Compression Ratios (Explained and Calculated)

In this videos we will be going over how to calculate an engine driven by pistons displacement (this is the volume you will commonly see engines advertised as), how to calculate the compression ratio of an engine and some of the advantages and disadvantages of high and low compression ratios. We will start out by finding the engine displacement volume. In order to find this we will need to know the stroke length, which is the distance the piston moves back and forth. In this case it is 10.6 cm and is marked in red. It is important to know the terminology for where a piston is at in an engine. If the piston is in the most compressed condition of the stroke it is top dead center (marked in blue). If the piston is at the least compressed part of the stroke it is at bottom dead center (marked in green) We will also need to know the engine cylinders bore diameter. In this case it is 9 cm and marked in purple. And finally the numbers of cylinders in this engine it has 2 cylinders. So to find the piston displacement we need to find the volume of the stroke of the piston. To complete this we can take the area of the cylinder bore in this case 9 cm squared times pi over 4 and multiply it by the stroke length of 10.6 cm. we get that each piston has a total displacement of 674.34 cm cubed. Now to get the displacement volume of the engine we need to take the numbers of cylinders in this case 2 and multiply it by the volume displaced by one piston. So after plugging in our numbers we get a total displacement of the engine of 1348 cubic centimeters. This is 1.348 liters. Like previously stated this is the common way engines volumes are marketed. Now let’s move onto compression ratios. To calculate the compression ratio you can take the volume of the combustion chamber at bottom dead center and divide by the volume at top dead center. In this example I am assuming a perfectly cylindrical combustion chamber in the real world this will more than likely not be the case. So if we take the volume of the chamber at BDC and divide by the volume at TDC we get a compression ratio of 8.07. This means that the volume at bottom dead center is 8.07 times larger than the volume at top dead center. Now you are probably wondering what is the importance of compression ratios. I have put together a simple matrix of advantages and disadvantages for high and low compression ratios. High compression ratio advantages are better fuel economy, more power due to being able to extract more mechanical energy from the air fuel mixture, and better emmisions (more efficient burn of fuel). High compression ratio disadvantages are higher heat (the more you compress a gas the higher the heat), expensive fuel (higher octane /grade needed to avoid engine knocking or detonation of fuel mixture before spark (non diesel engines) and generally more expensive and complicated to build and maintain. Low compression ratio advantages generally are cheaper to build and maintain, run on lower cheaper grade fuel and produce less heat Low compression ratio disadvantages generally are that they are not as efficient (don’t extract as much mechanical energy from air fuel mixture) and have more unburnt fuel in the emissions. That concludes this video hopefully you learned something and hopefully I earned a like share or subscription. Disclaimer These videos are intended for educational purposes only (students trying to pass a class) If you design or build something based off of these videos you do so at your own risk. I am not a professional engineer and this should not be considered engineering advice. Consult an engineer if you feel you may put someone at risk.