Please explain high compression pistons

[GeoffM]

Please excuse my drawing skills.
I know what compression is, and how it's made, but I don't understand this:
What goes in will always compress to be the same. Look at my drawing below. Normal compression goes up and down like normal. To me, a high compression piston would have to change it's head to get compression. From my understanding, this can be done with different head designs, but I can't see that making that much of a difference.


See how on the High side I've drawn the piston differently for each stroke? That's how I'd imagine it'd have to be, but I know that's impossible.

[deevergote]

I don't even understand what you are asking.

High compression pistons have a dome. They take up more area in the combustion zone, which results in the air/fuel mixture being compressed more than with a piston with less of a dome (or a concave piston, for that matter.)

[92exa6]

I think what he is saying is that the high-comp piston, with it's "dome" would allow a smaller amount of a/f mixture in the cylinder than a standard piston in the same cylinder. This holds true if you are measuring the total area above the piston in cc's while the piston is at BDC. Now when you actually run the piston through its stroke, which is unchanged, it draws roughly the same amount of a/f whether the piston is HC or STD. Bore and stroke stay the same, intake charge volume remains about the same, but now at TDC the HC piston fills more area of the combustion chamber, compressing the charge more than the STD piston, shown as a ratio, such as 8:1 vs 11:1. Put different, using the previous comp ratios, both start with 100% charge, the STD compression piston will compress that to 12.5% of its original volume, the HC piston will compress its charge to just 9% of its original volume, making more potential power. Volumetric efficiency, intake volocity,etc changes with pistons as well, but for simplicity sake it's best left out for now.

[GeoffM]

I don't even understand what you are asking.

High compression pistons have a dome. They take up more area in the combustion zone, which results in the air/fuel mixture being compressed more than with a piston with less of a dome (or a concave piston, for that matter.)

92exa6 pretty much got it. I was trying to ask how you can get higher compression when the pistons will always take up the same amount of space. The dome filling up more of the combustion zone makes sense.

[deevergote]

That's where the COMPRESSION comes into play. The dome takes up more space, and compresses the mixture further. The air/fuel mixture can be compressed.

That's why when you get water in the engine, you destroy it... water cannot be compressed.

[GeoffM]

That's where the COMPRESSION comes into play. The dome takes up more space, and compresses the mixture further. The air/fuel mixture can be compressed.

That's why when you get water in the engine, you destroy it... water cannot be compressed.

I know, I remember science class.
Pretty much I was asking that since the dome takes up more space, it sucks in less AF, so it has less to compress. I guess it makes sense.

[deevergote]

Ah, I see what you mean. You're referring to the overall volume of the cylinder when the piston is down as far as it can go... in that case, I suppose the dome is indeed taking up more space, leaving less room in the cylinder.