Compression ratios?

Question

In car magazines when they quote the compression ratio, what does this mean? I know it must have something to do with the fuel/air mixture or something but what does it actually tell me about the car? Is a lower ratio better? I mean torque and BHP are obvious measures but this has always baffled me.

And while i'm on it, what about bore and stroke? Is bore the diameter of the piston and stroke the length of the rod from the piston to crank? And again, what do altering these things do?

Answer 1

Compression ratio is the differential between the space between piston at top of the cylinder at TDC and BDC.

So if the space left when the piston is at the top is 1/9th of the space when it's at the bottom then the engine has a 9:1 compression ratio.

The higher the compression ratio, generally the more power is made. Higher compression also means a propensity to pre-detonate so higher compression engines usually require higher octane fuel.

You are correct about bore and stroke.

Generally speaking, increasing bore makes more horsepower than torque and increasing stroke makes more torque than horsepower.

Engines designed to rev to high RPM's and make high horsepower usually have slightly higher bore than stroke. This is called an "oversquare" design. It's generally long lasting and can rev higher, but generates a bit more heat. This is what you'll find in motorcycles and a lot of sports cars. An extreme example would be the engines in Forumula one cars that have more than 2 times the bore than stroke and can rev to 19,000 RPM and make crazy horsepower.

A long stroke makes it harder for an engine to rev high but it increases the amount of torque produced. An engine with longer stroke than bore is called "undersquare". This is a practical design for producing some low-end torque, like what you'd want in a small engined commuter car that you'd drive in traffic and also in a truck that's hauling stuff. Not designed for high revs and high horsepower, but good at making power at low RPMs and it's easier to have a higher compression ratio with a long stroke motor which makes up for some of the power loss in the high end.

A "square" motor is an engine where the bore and stroke are equal.

Answer 2

Bore is the diameter of the cylinder
Stroke is the distance the piston moves from dead top to dead bottom.
Compression ratio is measure as, how much volume of air fits in the combustion chamber with the piston at dead bottom and how much fits with the piston at top dead center. ie 1 liter of air fits but at TDC only 100ml.
Most places restrict street legal compression to around 10 to 1. The more compression the higher the octane fuel you need.
Lower compression means less power, but also less wear and tear. It is a 2 sided sword.
People used to, and sometimes still have the motor decked and the head shaved making the combustion chamber smaller and that raises the compression ratio since the bore and stroke remained the same. Problem is this changes all the geometry of the internals like push rods and other things.

Still got questions? check out a local machine shop and ask them some questions.. most will be able to point out a few things if they are not too busy.

Answer 3

The compression ratio is the ratio of the space in the cylinder when the piston is at the bottom of the stroke to the space in the cylinder when the piston reaches the top of the stroke. The greater the compression ratio the greater the thrust on the piston as the spark plug ignites the petrol/air mixture as it enters the cylinder head. The higher the compression ratio at one time meant that engine needed a higher octane fuel. Low comp ratio engines ran on the economy fuel which was cheaper. Regarding the stroke and bore.A ford Mondeo for example with a 1.8litre engine has the same engine as the 2.0 litre. The different capacity is achieved by changing the length of the stroke by changing the length of the connecting rod from the crank shaft. I have tried to keep it simple but I do not doubt that you will have more questions now about crank shafts etc.
Happy New Year.

Answer 4

The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). It is a ratio between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Literally, high ratios place increased oxygen and fuel molecules into a reduced space, thus allowing for increased power at the moment of ignition. Higher compression ratios, however, also make detonation more likely.


Bore is the diameter of the cylinder
Stroke refers to the distance the piston travels

By changing the crankshaft the the stock can be varied and hence he compression in increased and decreased. The bore can be made large and hence more air/fuel can be compressed and the more explosive the mixture becomes. Its a combination of the two that gives the best performance.

Answer 5

Compression ratio is the ratio of volume in the cylinder at the bottom of the piston travel IE stroke to the volume above the piston when at the top of its stroke..The formula for calculating compression ratio is volume above the piston at bdc over volume above the piston at tdc.Example if the cyl held 10cc at bdc the piston goes up compressing the mixture so that at tdc there was 1cc above the piston the compression ratio wold be 10 to one.If this is not clear go to how stuff works auto section it will be explained by a small video clip.Mick UK

Answer 6

your compression ratio is the ratio of the volume of your cylinder at bottom dead center to the volume of your cylinder at top dead center.you are right about bore. stroke is the distance that the piston travels from top to bottom.

Answer 7

the hole mess you described is simple you want as much compression as possible the reason is the More you compress the air and gas the harder it explodes thus more power

bore is diameter of piston
stroke is how far up and down it moves
the bigger these are the more they can compress