Compression and charged intakes...

[bjcsc]

Recently in this thread I learned I how changing the head (among other things) can increase your compression ratio. Largely what I was missing in my previous understanding was the intake volume being "fixed" by the swept volume of the cylinder. The question I have now is: How does this apply to a charged (super or turbo) system where the intake is under pressure? Same effect but less?

 

[xtraham]

Re: Compression and charged intakes...

bj, compression wil be the same ratio on forced induction engines,
the forced air is required to have a richer fuel mixture ratio, the turbo/blower will inturn force more air/fuel into the combustion chamber, the more air forced into the combustion chamber along with the proper mixture of fuel creates a larger/more powerful explostion when compressed this makes the added power...

hope that helps

Tom

 

[bjcsc]

Re: Compression and charged intakes...

I already understand the forced induction and how the power is made, but what if you made a head change as in the other post. Would it limit the increase in compression you would gain because the system is under pressure?, i.e. would the pressure somewhat override the limit the swept volume of the piston is imposing? Granted few boats have room for superchargers but I'm still curious...

 

[xtraham]

Re: Compression and charged intakes...

no it wouldn't limit it, but raising the compression in a forced induction engine can destroy it, lets say we have a compression ratio of 9:1 wot @ cranking speed if in theroy we could turn the engine faster you would see a slight increase in compression, so if you were to induce the air of a forced air induction engine at a higher psi yes you would see an increase in compressed air ( compression ) @ the same rpm

 

[bomar76]

Re: Compression and charged intakes...

Granted few boats have room for superchargers

Not true.
Most boats have room for a supercharger.....it's just your wallets aren't thick enough.

Google "Procharger", one of several non roots style superchargers...they are a very compact installation.

 

[Buttanic]

Re: Compression and charged intakes...

The mechanical compression ratio will remain the same. You are still compressing the cylinder volumn into the combustion chamber. You will change the effective compression ratio. A 9 to1 normally asperiated engine at WOT will most likely have an inch or two intake vacumn which means the cylinder will be at less than atmospheric pressure so the effective compression ratio is less than 9 to 1. With a forced induction you may have up to 10 or 15 psi intake pressure so the cylinder will be pressurized above atmospheric. So although the mechanical compression ratio is the same there is more air and fuel in the cylinder of the supercharged engine so the effective compression ratio is higher than 9 to 1. Most forced induction engines have lower mechanical ratios than normally asperated engines. At some point the compression ratio can get too high which will result in detonation and engine damage.