Compression ratio

[Dvldog 8793]

Howdy
Question for those that know more than I(almost everyone).....
Seems that there are 3 different popular compression ratios out there.
Standard 6.5 - 22.2:1
New AMD GEP - 20.2:1
Hot Rod aftermarket - 18.0:1

My new engine is a 20.2:1. This engine is MUCH better performing than my old one was but I did allot of STUFF when I replaced the motor.
Can anyone out there explain why it is better to lower compression ratio and up the boost. Wouldn't be achaiving the same things just by different means? My old engine was running at about 10-11psi, this one I run at 15+(intercooled also)
Thanks for any info......
L8r
Conley Janssen
USMC 87-93

[rjschoolcraft]

See my current article Electronic 6.5L TD Power Project Part II: Building a more durable and more powerful engine. (http://www.thedieselpage.com/members/features/schoolcraft2.htm) for the technical reasons why to lower compression ratio. Then if you have more questions, I'll try to help clear them up.

[Cowracer]

{on edit} Damn, RonnieJoe Beat me to the punch while I was posting this. His is a much more technically accurate explanation.


The goal is to get as many little oxygen molecules into your cylinder as possible before the valve closes. Once the valve closes, you are stuck with that much oxygen, no matter what your compression ratio is.

Most engines are oxygen limited. Fuel will only burn properly in a strict ratio with oxygen. Too much fuel in the ratio, you will not have enough O2 to combust cleanly. As all piston engines (gas or diesel) are basically air pumps, they are air limited in how much power they make. In simple terms, getting air, and thus into (and out of ) the engine is the key to power. Turbos, tuned intake runners, port and polish, high lift cams, its all about getting as much O2 into the cyl as possible.

Lets look at a thoretical 1 cyl 1 liter normally aspirated diesel engine. This model assumes 100% efficiency at sea level.

When the piston completes its intake stroke, the cyl is filled with 1 liter of air at 14.7 psia. Oxygen content of air is ~20% so you have 200 cc of oxygen available for combustion. If the cyl compression ratio is 22:1 then that cylinders pressure would be 323 psi BEFORE factoring in pressure gains due to heating of the air. In the real world, the actual cyl pressure would be closer to 450psi. But for this instance, we will ignore heating pressure for the sake of simplicity.

So with only 200 cc of oxygen, we are limited to how much fuel we can inject, and thus, how much power we can make. We can inject WAY more fuel than this amount, but injecting too much will just produce smoke, and increase EGT's without making any additional power. There just isn't enough oxygen to combust this amount of fuel.

So, its obivous that we need more oxygen in the cylinder to allow more fuel to be burned. So we bolt on a turbocharger, and set it up to run at 14.7 psi of boost.

Now when the cylinder completes it intake stroke, It contians 1 liter of air at twice atomospheric pressure (29.4 psi) so you actually have DOUBLE the volume of air in the cylinder. So now we have 400 cc of oxygen available for combustion. we can inject TWICE as much fuel and make twice as much power. Life is good!

Except... Lets say our engine cannot take more than 550psi before bad things happen.

So with 14.7 lbs boost and a 22:1 com compression pressure at TDC is 646 PSI. and this would blow the head gaskets out of engine. So lets reduce the compression ratio to 18:1, this lowers the compression pressure to a more manageable 529 psi. But we still have the same 400cc of O2 available for comustion.

True, if you could build the engine to survive at 14.7 boost with 22:1 compression, it would out perform a comperable 18:1 engine, but a high boost 18:1 will make much more power than a low boost 22:1

So in summary, when we are limited to how much compression pressure the engine will tolerate, its better to pack extra air into the cylinder with higher boost, and compress it less, than to pack less air into the cylinder and compress it more.

That, in a nutshell is the benefits of an 18:1 compression engine. It allows more boost safely. The pistons in and of themselves will do nothing to enhance power. As a matter of fact, installing 18:1 pistons without increasing boost will reduce performance. They are the FOUNDATION to building power.

Tim

PS Don't flame me about intake air temps and exhaust backpressures and volumetric efficiencies and all that rot. They all factor into the "real world" but I kept them out of this model just to show the basic thought behind why 18:1 pistons.

[Dvldog 8793]

Howdy
Outstanding info. Thanks guys...
L8r

[Chuck1]

Cowracer good explination its all so clear now.What finally happened with the green power plant.

[Cowracer]

Getting a new one installed as we speak. See this thread

click here (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=009315)

Tim

[Hansh]

OK, that said, I was thinking earlier today.....are two small turbos better than one big turbo, each setup displacing the same amount of air.

[Herman_Bolger]

That was an excelent explanation, especailly for a confused former gasser like myself. I have a few freinds that are quite adept with diesels and they explain things to me and when their done things are clear as mud.But i actually understood the principles involved now. I am glad some asked. Now I know to.
I want to diesel everything now. Actually I would like to convert an S10. Anybody done that yet?

[HANK1948]

I want to diesel everything now. Actually I would like to convert an S10. Anybody done that yet?
I was consedering doing this because Iam sure a 6.2 would get around 30 MPG in a S10 , but the weight(engine) was a consern because I live in the country and drive alot of dirt roads and dont think the front suspension would hold up to the consant pounding of the dirt roads also the truck you pick couldnt have A/C because the evapoator housing would get in the way of the width of the engine. As long as you do city driving I think this would be a sweet ride

[Peter J. Bierman]

HH,

No, two turbo's are not better then one big one, just different.
I build a twin turbo 6.2 for two reasons:
1st, No one had it at the time so it's special
2nd, I had the turbos available

The twin setup is quick to spool up but the end result is the same.

The down side is that it's a more complecated set up and it's not for sale so you have to make your own parts.

Biggest plus: it looks and sounds very COOL :cool:

In the end both systems make boost and that's what it's all about!

Peter

[Diesel Dan]

I want to diesel everything now. Actually I would like to convert an S10. Anybody done that yet? [/QB]I started on this project once. Used a '85 S-10 2wd that had a 2.8L V-6 in it. Was equipped with a/c, p/s and P/B. A/C box was an issue and so was the power brakes and steering shaft. Dropped a 350 in it and shipped it. Now a 4.3L equipped truck might have more room but not sure it would be enough.