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ktbundy (Automotive) (OP)
8 Apr 03 0:50
Is propane capable of supporting an engine with 12.5 to 1? The piston quench is near perfect at .045. The camshaft has a fairly large overlap also. I have seen 11.5 to 1 used, but I am wondering if this is pushing it a bit too far. Thanks in advance.
franzh (Automotive)
8 Apr 03 8:21
All fuels have a critical compression ratio, or the ratio at which the fuel and air mixture will spontaneously ignite due to compression generated heat.  Many other factors come into play:
Is the fuel liquid or vapor?  Liquid fuels may have different CCR due to heat absorption and latent vaporization.
What is the rate of compression (usually based on piston speed/engine rpm)?

You state that the piston quench is near perfect, for which fuel?  Engine design engineers have been trying for 100 years to find the perfect combustion chambers, and are getting very close, finally, mainly due to computer modeling.  The quench (squish) areas are generally used to initiate high speed air-fuel movement to accelerate the combustion process.  The need for squish areas are different for gaseous fuels than liquid fuels.

As for propane, some engines can operate at 14:1 which is generally considered above propanes CCR of around 13.5:1, but other factors come into play, as camshaft events, ignition timing, cylinder head design and material and so on.  As for what is the ideal compression ratio of propane?  That depends.  How long do you want it to last?  I have seen engine with 375,000 miles at 8.5:1, and industrial engines at 12:1 at 30,000 hours.  Of course, the usage and load cycling are different, but here is a case of an engine being optimized versus one being converted from a gasoline mode.

One last thing, propane is unique in that at different phases, it can change physical properties.  Extremely high compression ratios can approach the super critical liquid point.

Franz
ktbundy (Automotive) (OP)
8 Apr 03 9:42
I think I understand your point about different quench heights for different fuels. Mine was based more on gasoline. As far as the motor goes, it is built with hardened seat inserts, stainless valves and forged pistons.
This would be a vapor set-up utilizing 2 impco mixers on top of a tunnel ram intake. The motor is a 360 Mopar. As for timing, I have yet to fire the motor, but have done quite a bit of reading on the subject(including your info on the web Franz).
gasresearch (Automotive)
10 Apr 03 6:24
One of the best learning engines that we produced years ago was a 5.0ltr Holden V8 engine running 10.5:1, small cam (204deg at .050") and a squish area of .025".  We acheived this via a head gasket shim that was supplied incorrectly.  I would have to say that this engine was one of the most memorable engines that we have produced over the years.  Instant throttle response, perfect tune (around 28deg total timing and unusual with a high comp/small cam combo) and a great spread of power across the rpm range.  We have run similar packages with less squish area and not seen the type of operation that we acheived from this engine.  A high rpm engine would not suit this type of squish but only a little more would be ideal.  Say around .030"-.035".  I have always been a strong believer in this critical engine design area and have seen engines perform with higher comp ratios when the squish is at this size.  12.5:1 is fine with a cam of around 250deg at .050" plus. (with a 360cube eng) However, acheiving this comp generally means dome pistons(poor flame travel), or small closed chamber heads, (schrouded valves).  Both will cost power so generally I would set a high rpm propane engine between 10-11.5:1, running a closed chamber head with sufficent valve/chamber flow and a dish top pistons, this will centralise the air/charge. And of course a nice tight squish area.

Regards

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