Compression ratio for IC engine

[xander janssen]

Hi all,

I'm planning to design and build my first IC engine where I will base my design on several plans I can find online e.g. Webster, Jan Ridders. Here I find various compression ratios ranging from ~3 to almost 10.

I would love to make a "slow running farm engine" like engine.

Does anyone know the effect of the compression ratio on engine speed?

Xander

 

[tornitore45]

Low speed can not stand high compression unless a large flywheel store enough energy to cycle through. But a heavy flywheel is hard to accelerate in the first place and tend to increase friction. So the sweet spot for a slow running engine is toward the low compression ratio range. Around 4 is a good compromise.

 

[Brian Rupnow]

I agree with Tornitore. 4:1 compression is about right for slow revving engines.---Brian

 

[Hopper]

On a side valve engine, if you go above about 6:1 compression the reduced area connecting the valve area to the cylinder bore area becomes so small it actually reduces performance. Same applies to a pocket valve or F-head design like the Webster. 4:1 is what those old pocket valve motorcycle engines ran back in vintage days, 5:1 for the side valve bikes.

 

[picks27t]

I am building a .500 piston on a 3/4 inch stroke what piston to head distance would it take to get to 4:1 any help Thanks.

 

[10K Pete]

I am building a .500 piston on a 3/4 inch stroke what piston to head distance would it take to get to 4:1 any help Thanks.

1/4 of 3/4 is how much?

Pete

 

[TonyM]

About 4.7mm:hDe:

 

[tornitore45]

Stroke = 3/4
Compression ratio = 4
Head to piston dead top center = (3/4)/(4-1) = 3/12 inch =6.35 mm

Piston Down volume = A x (Stroke + Top)
Piston Up Volume = A x Top
Compression = (Stoke + Top)/Top => Top = Stroke / (Compression -1)

 

[TonyM]

To avoid confusing the OP I was responding to 10K Petes question.

Calculator at http://www.gtsparkplugs.com/CompRatioCalc.html confirms tornitore45's answer.