Static CR Load?

[TeeCherry]

I'm considering building a SOHC 1.6L motor with a static CR that's pretty much as high as pump gas can tolerate; in the neighborhood of 12:1. I've heard before that too high of a static CR can actually become a bit of a load on a motor. Being that the motor works physically to overcome the high Static CR. Is this true? Thanks!
Regards,
Tee

 

[patprimmer]

Over simplified but yes extra pressure on the compression stroke increases load on the engine, but not nearly so much as detonation, or even controlled but higher combustion pressure build up.

Extra compression actually reduces load on the rod bolts at just before firing point.




Regards

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[TeeCherry]

Hello,
I'm wondering if there is a limit where raising the static CR hits a point of dimishing returns. Basically is there a tipping point where an increase in CR won't produce more power due to the physical load the motor will see? Thanks!
Regards,
Tee

 

[crysta1c1ear]

The more you compress air, the hotter it gets, as a child who has played with his finger on the end of a bicycle pump will know. If the fuel in an engine gets too hot, it can autoignite. So to prevent autoignition the intake valve should be closed later if the static compression ratio is higher, ensuring the air is compressed by a broadly similar amount and reaching a similar temperature.

Let's say one engine compresses 60 cc down to 6 cc with a 10/1 compression ratio. If you increase the compression ratio to 12/1 then you will be compressing into 5 cc space. To have that 5 cc at similar temperatures and pressure rather than autoigniting, you would have to ensure around 50 cc of air went in instead of 60 cc. The way to do that is normally to close the intake valve later.

With just 5/6 of the air going in, one would have lost some power, but possibly gained on other things, eg fuel efficiency, the power stroke being comparatively longer than the compression stoke, etc.

My figures are just meant to be illustrative, so you can see my point. I am not suggesting that a 10/1 compression ratio actually compresses by a factor of precisely 10 with cylinders being completely filled.

 

[TeeCherry]

Hello,
Thanks a lot for clarifying this for me. =)
Regards,
Tee

 

[patprimmer]

If you are chasing maximum power through increased compression, you can get increases up to the onset of detonation, at which point it drops off suddenly as you need to severly retard ignition to avoid destructive detonation. This retarding of ignition severly reduces power.

You can run higer compression if you have higher octane fuel.

A large number of factors effect the point at which detonation starts. These are:-

Compression ratio.
Fuel octane.
Air to fuel ratio.
Intake valve closing point (as mentioned by crystalclear).
Ignition timing.
Engine volumetric efficiency.
Spark plug heat range and design.
Combustion chamber surface temperature and heat rejection capacity.
Combustion chamber geometry.
Rod to stroke ratio.

I have probably forgotten a few.
Many points above can be further subdivided.


Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Greenlight]

Consider the dynamic compression ratio instead of the static compression ratio. Dynamic CR takes into account the camshaft timing, rod length, etc.

The dynamic CR will be a better indicator of detonation, idle quality, etc.

 

[stout1]

When building an engine go with the DCR not the SCR. It's a pretty easy calc, I set mine up in excel so I just need to plug in a couple simple numbers. A shop I was working at was building Harley drag engines that were running an SCR of 19.5:1 on race gas; it's all about having the components match. good luck

Michael