In the basics section I plan to go over various functions and components of the car and describe in plain language what they do and what it all means. Most of these Basics posts will be purely for educational and interests sake. I have no plans to go through the car in any comprehensible way so enjoy the schizophrenic ride. To kick things off I’ve chosen the Compression Ratio.
A typical car engine goes through 4 major strokes or stages hence the classification ’4 stroke’. The stages in order are; Intake Stroke, Compression Stroke, Power Stroke & Exhaust Stroke. These 4 stages happen in the cylinders of your engine. Think of it as the heart or centre of your power plant. Our subject today concerns the Compression Stroke.
You might have heard your friends talk about the compression ratio (CR) and one of them might have proudly thumped his chest as he announced his Honda S2000 had an increased CR of 13:1. You might have also heard that his engine blew up the following week. As with all things if you increase or decrease it to gain more power you are running the risk of damaging your pride and joy. If you are curious as to what the compression ratio is and what it does please read on.
During the compression stroke your piston squashes a mixture of air and fuel in a small chamber called the cylinder. If your cylinder contained 1,000 units of volume in its largest stage (start of the compression stroke) and only 100 units of volume at the smallest stage (end of the compression stroke) then your ratio will be 10:1. This means the piston has squashed the volume of air and fuel mixture from 10 parts into 1.
The reason your engine squashes this air/fuel mixture is two fold. By compressing the mixture it breaks up the fuel into smaller particles making it more even and easier to detonate. As the mixture condenses into a smaller area the temperature rises making it easier to ignite and gives us a powerful explosion to assist with out next stage called the Power Stroke.
Most modern engines have a CR of 10:1 and as you can imagine performance cars run higher ratios. As you increase the ratio you run into pinging (or knocking) issues which can be hazardous to the health of your engine. Pinging is the act of detonation of small pockets of air/fuel at the incorrect time in the cycle. In order to sustain the higher ratios without pinging octane rich fuels are required. This is why you see your friends with increased CR always filling up with the most expensive fuel available. As the CR is increased some of the internal parts of the engine also require replacement with stronger components to cope with the increased forces of the explosion.
In conclusion, compression ratio is the amount we squash our air/fuel mixture to create a bang. The higher the ratio the bigger the bang but also the more stress we put on the engine and more octane rich fuel we need to use.