I am not real clear on the beniefits/differences in compression ratios. What do they measure? How would, say, a truck be different than a sports car? What can one do to modify these ratios?

 

diesels don't have spark plugs. they basically rely on the extremely high compression, and volitility of the fuel to ignite. its also a direct injection into the cylinder, rather than into the intake. don't quote me, but i think thats all correct.
the C/R of a jetta tdi is like 19-20. damn.

-PJ

 

Actually, diesels have glow plugs. They don't have conventional "spark plugs", but they have a heated tip at the top of every cylinder, and when the charge is heavily compressed towards the top, it self ignites. That's why, sometimes, when turn off your motor via the ignition, if the car continues to run on it's own before it finally shuts down, it's called dieseling. Just some useless facts.

As for compression ratios, as previously stated, they are one of the key determiners of power production. As a general rule, the higher the compression, the more power a motor will generate. There are actually two types of compression. There's static compression, which is the actual difference in chamber volume from bottom dead center to top dead center. For instance, a 9:1 compression ratio would mean that, at top dead center, the chamber volume is reduced to one ninth the total volume that the chamber has at bottom dead center. Then, we have dynamic compression. That's a combination of your static compression combined with volumetric efficiency. Volumetric efficiency (VE) is the motor's ability to breathe. The reason why people install cams, intakes, headers, exhaust systems, is to increase the flow of the motor. If your intake/exhaust sytem is efficient enough to fill a 2 litre motor with only 1.5 litres of air, that would give your motor a 75% VE. Without forced induction, most motors will never see 100% VE. That means that, even if you have a 2.0 litre motor, you will seldom, if ever, move 2.0 litres of air for a full cycle of the motor. Of course, resonance tuning and boost are two ways to get around this, but on the most part, without forced induction or some serious tuning, you won't see 100% VE. So how do you figure out your dynamic compression ratio?

Static compression ratio x VE = dynamic compression ratio

If your motor has a 9:1 static compression ratio and an 85% VE...

9.0 x .85 = 7.65 dynamic compression ratio.

So, either an increase in static compression ratio or VE would result in a higher dynamic compression ratio, or power output.

 

Excellent explanation!

To further the thought: Say you have a 10:1 static compression car (close to a 2.5 RS) and add a turbo. You now increase the VE and thus the dynamic compression ratio. This is where trouble can begin, because the dynamic compression ratio gets so high that problems occur, like detonation. Solutions to this include running higher octane fuel, retarding timing through engine management, limiting boost, lowering compression ratio, replacing rods, crank etc w/stronger parts etc.

jack