18:1 Pistons
 

I am trying to getmy head wrapped around low compression pistons in a IDI engine, i.e. the 6.5.

1) My understanding is that IDI diesel engines require a higher CR to run properly.
2) A lower CR (like 18:1) makes cold weather starting harder (longer glow plug times).
3) the only time you would need a lower CR is run higher boost for those times when you need the power which is "X" percent of the time and that in normal driving you might actually see less efficiency.

This from Chevron:

"The more rapid mixing of fuel and air achieved in IDI engines comes at a price, however. The high velocity flow of air through the narrow passage connecting the main cylinder to the prechamber, as well as the vigorous swirling motion in the prechamber itself, causes the air to lose significantly more heat during compression than it does in a DI engine. Coupled with a pressure drop from the main chamber to the prechamber, this results in an air temperature in the prechamber after compression that is lower than that in a similar DI engine.

Since rapid fuel autoignition requires a certain air temperature, an IDI engine needs a higher compression ratio to achieve the desired air temperature in the prechamber. IDI engines operate at compression ratios of about 20:1 to 24:1; while DI engines operate at ratios of about 15:1 to 18:1. The heat losses that necessitate these higher compression ratios have another, more important effect: they decrease the efficiency of the engine. IDI engines typically achieve fuel efficiencies that are 10% to 20% lower, on a relative basis, than comparable DI engines.

Even with the higher compression ratios, IDI engines may still be hard to start. Most IDI engines use glow plugs to heat the air in the prechamber in order to make starting easier. Glow plugs, which are small resistive heaters, are usually powered for only the first few minutes of engine operation."

So, is there a question there or was it a statement?

:humm:

Sorry,

I guess I was hoping for anyone with more experience with this than I have. I understand the reason for lowering the CR. It just seems with an IDI engine that it could be counter-productive.

Well, it all depends on what you're after...


Sent from my iPhone using Tapatalk

What do you want to do with this motor???

If you are looking for more towing power, or working the motor hard with a different turbo than a GMx, then lower CR will be better for the life of the motor, if you are looking for MPG only, stay with stock CR and still a better turbo.

Lower CR = motor life & stronger HP comes from lower CR and other mods combined.

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.

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.


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.

-Cowracer

The reason for lowering compression is due to a weak engine design. The Finns have no problem running German diesels to 30+psi and 100hp/L on stock 21:1 compression and rods.