What´s maximum static compression ratio appliable to LPG? 1

[sraposo]

Sirs,

I need to know what is the typical maximum static compression ratio appliable to LPG.
An Diesel-Cycle engine was converted to Cycle Otto to run on LPG and its original static compression ratio (21:1) was maintained. I believe such compression ratio is highly improper and should cause pre-ignition. I´ve unsuccessfully searched for such information.

Regards

 

[ivymike]

Like the others, I don't have a hard figure for max CR. I can say that LPG has a pretty low Cat methane number of about 33 (Methane Number is an anti-knock index for gaseous-fuel engines), so it is much more likely to have knock issues than natural gas with about 87 MN(pure methane = 100 MN). Most engines that run these low-MN fuels have CR~8:!. I'm aware of one lean-burn engine that can run this fuel which has a static compression ratio of around 10:1 but uses valve timing to deliver an effective compression ratio more like 8:1. There are gaseous-fueled engines with higher compression ratios (a few up to almost 13:1) but they are all restricted to higher-MN fuels as far as I can tell and the higher the CR the narrower the operating range for transient loads, temperatures, etc. In all instances above I'm referring to turbocharged engines with bore > 135 mm.

 

[NickJ67]

My feeling on CR is that it might just live (for a while at least) at 11:1 without boost, but adding boost will kill it very quickly. To run even moderate boost Ivymikes' numbers look much more realistic. Then there are the very valid points raised by Gruntguru.

I wouldn't sell this engine life insurance.......

Nick

 

[sraposo]

Hello!

Here I come back to update you all about the converted engine.

Nowaday, it is operating pretty well, according costumer´s words. It was achieve only after some important corrections that have took a lot of day until they were finally done.
In order to save your precious time, specially after spending so much with guessing on a poorly defined scenario, below I list the main reasons (not all reasons, indeed) the engine was malfunctioning:
1) 21:1 compression ratio. It was reduced to about 12:1;
2) usage of 85mm pipes between spark plugs and cylinder head. Plugs were mounted directly to the head;
3) diesel-like GLP feeding system developed by costumer himself. Costumer intended to control engine speed by modulating GLP mass, keeping air flow unrestricted, as if it was a Diesel Cycle engine. Costumer modified GLP feeding device to, I believe, something that keeps A/F ratio constant.

Although we all know, so it´s dispensable to say, I will do it again: good engineering is for (preferably) good engineers with proper resources (money, time and knowledge of target application).

When I started this thread, I did not suppose that so many and important incompliances were present, so we´ve wasted some time trying to solve a problem the should no exist. So, for this extra reason, I thank you all for your kind help and interest.

Best regards,

Sérgio

 

[gruntguru]

Great to hear back from you on the final outcome - thanks. I assume they had to incorporate intake throttling for load control?

je suis charlie

 

[sraposo]

Hi, gruntguru.

Yes, engine speed control under variable load is supposedly done by a throttle. Something like that was installed, but I´m not sure if it was properly built.