Forum » General Engine Building Discussion » Compression ratio

Compression ratio

General Engine Building Discussion

Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.


Page 1
Author
278 Views

Hi Andrea. Great course so far have learnt heaps and really excited to learn more. But.. This compression ratio topic has got me really confused with all the squared and cubed stuff. I know the stuff we need to measure the comp ratio like the head gasket thickness by its radius bore radius an area the deck clearance and chamber size and piston dome.. But just can't piece them together. Can you please assist with helping sort this out in a more simple way? As in let's say we get all these things together.. And get the formula and the right way to measure them up for swept volume and then once each one has the formula to measure it then move onto the final equation of matching them all up and getting a result. Thanks mate! #moparornocar ;)

You can find the answer in google as your ask.

Forget about the formulas for calculating the compression ratio. It's not important to understand the basic concept. And don't confuse compression ratio with bore and stroke. For a gasoline engine remember these basic rules of thumb (simplifying the concept):

Higher compression ratio-->Better fuel efficiency, better torque & power, more tendency to knock.

Lower compression ratio-->Worse fuel efficiency, worse torque & power, less tendency to knock.

The torque and power thing is complicated because it interacts with knock and depends on the fuel you are using. For an engine with a fixed displacement, bore and stroke rules of thumb:

Bore bigger than stroke (Oversquare): Better breathing in the cylinder head (good for power and torque especially high revving non turbo engine), worse for knock and fuel efficiency.

Bore smaller than stroke (Undersquare): Worse breathing in the cylinder head (not as good for power and torque), better for fuel effiency and knock, especially for smaller(ish) displacement turbo engine.

All you really need to understand is that when it comes to calculating the compression ratio is that we're comparing the volume in the cylinder with the piston at the bottom of the stroke with the volume in the cylinder with the piston at the top of the stroke. This requires us to calculate the volume of the various components as discussed in the course. This requires us to start by calculating an area (which is 'squared') and then from the area we can calculate the volume (which is 'cubed'). Go through the module again as I believe it's explained as simply as we can break it down.