Max compression ratio for propane 1

A question was asked of me regarding the maximum compression ration for a propane fueled engine.
I believe it to be 13 or 14:1. Anyway just hoping some one can help.
thanks
Gord

 

[dcasto]

propane is still under 100 octane, so maximum compression ratios will be about 10: 1.

 

[kenre]

Propane is rated anywhere from 104 to 110 octane.
Max compression ratio will depend on many things including camshaft timing.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[patprimmer]

There are variables as listed above, but as far as I now, 11:1 is generally considered the safe everyday use limit, maybe a little more if long duration racing cams are used or the fuel is known to reliable be pretty much pure propane and not to much butane.

Regards

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

 

[TDIMeister]

What anti-knock benefits propane gains from a higher octane rating, it loses from not having the charge cooling effect of evaporating liquid fuel. Furthermore, at high load operation, gasoline engines are enrichened beyond the stoichiometric AFR -- in many cases down to around 12:1 -- so that the extra fuel provides even greater charge cooling for knock resistance.

Introduced as a carburated-gas, propane provides no evaporative charge cooling, and therefore running richer than stoichiometric further does not provide any further cooling benefit.

There are systems out that inject propane as pressurized-liquid directly into the inlet tract, and these do provide significant charge cooling like gasoline. However, these systems are the exception rather than the rule of most aftermarket propane conversions, which pre-expand the propane before admitting it into the intake as a gas.

In summary, the tolerance for increased compression ratio in a converted propane engine is relatively benign compared to the baseline gasoline engine. The exception is a system that injects propane as liquid in the intake; there you can easily increase the CR by 1 or 2 full points if everything else is equal.

 

[dcasto]

According to ASTM D357 motor method, Propane is 97.1 octane. And according to the book by Edward F Oberty, "Internal combustion engines and air pollution", it would take 1.6 mg/liter of tetraethyl lead to raise the octane to ROM 100 per ASTM D909.

 

[BrianPetersen]

Kinda hard to add tetraethyl lead to a gaseous fuel, though ... and kinda bad for modern O2 sensors and catalysts!

 

Thanks for the comments. Not sure where the 13 or 14 to 1 came from. The application is I believe a pulling tractor using a 60 year old engine design so chances of a long duration cam with overlap wouldn’t be likely.
Anbody know where Glen Sali of propane drag car era is these days?,
Thanks again
Gord

 

[patprimmer]

Here is some data I found.

methane propane iso-octane
RON 120 112 100
MON 120 97 100
Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712
Net Heating Value (MJ/kg) 50.0 46.2 44.2
Vapour Pressure @ 38C ( kPa ) - - 11.8
Flame Temperature ( C ) 1950 1925 1980
Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31
Minimum Ignition Energy ( mJ ) 0.30 0.26 -
Lower Flammable Limit ( vol% ) 5.0 2.1 0.95
Upper Flammable Limit ( vol% ) 15.0 9.5 6.0
Autoignition Temperature ( C ) 540 - 630 450 415

Regards

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

 

[franzh]

One cannot rely on one fixed static compression ratio for any fuel, there are too many variables.

Common logic would dictate that a fuel with an octane rating of 100 would work best in an engine with about 11:1, but here we go with race engines running 14:1, and they are making lots of power. A friend of mine is a mud racer (big engine, little truck, 4 wheel drive) running a 540 CID with 14:1, on alcohol. Another friend is a local drag racer and on 104 octane racing fuel, he chooses 13:1.

Most engines use a lower compression ratio for emission controls and engine longevity. My lawn mower engine uses 7:1 as an example, and it runs, and runs, and runs, longer than I would care to admit, with minimal maintenance.

When we work on the engine in a dyno cell and are setting the initial timing curve, we work on a best torque limited curve, before detonation. Increased compression will usually require a lower total advance and less agressive curve before detonation, at best torque.

Propane behaves like the other fuels, but as mentioned, since it is a dry fuel with practically no heat absorbtion or charge cooling. When we have detonation from compression, reducing ignition timing will have little effect.

On a valve-in-block engine, one can tolerate higher compression ratio than a modern valve in head engine, due to a less efficient combustion process. In this case, its the engine, not the fuel that is dicatated by compression.

Personally, on propane, in a modern engine since the early 1990's, 10:1 is a good starting place, balancing engine longevity and power. 11:1 would give a little more power but the fuel mixtures and timing will have to be monitored more closely, and 12:1 needs constant vigilance on the above.

Franz

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[BrianPetersen]

So are those folks using propane as a power-adder for a diesel engine, pretty much running their engine on detonation??

 

[franzh]

They are adding fuel to fuel, on an engine already running on excess air. The combustion pressure curve on diesel is different than on propane, as is the initial peak and drop.

Question to ponder:

Does a diesel engine run on a constant detonation mode?

Franz

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[BrianPetersen]

There is an element of "detonation" involved in diesel combustion, but it only involves the first bit of fuel injected during the ignition-delay period. Following that, it is a more or less smooth combustion. But if you mix in a portion of fuel into the intake air, isn't that fuel going to self-ignite and "detonate" before the end of the compression stroke, if the temperature at the end of compression is above the ignition temperature?

I'm thinking those propane-injection power adders are sending peak cylinder pressure sky high.

For what it's worth, diesel engines seem to be moving towards a combustion system with more pre-mixed combustion (possibly entirely pre-mixed in the low-load regime), but with the rate of heat release (and peak cylinder temperature, hence NOx emissions) controlled by using extremely high EGR in these part-load conditions.

I somehow doubt that the propane power-adder folks are putting that much research into how the combustion is progressing.

 

Good comments! Learned more than I had hoped.
Gord

 

[stanlsimon]

More about LP gas fired IC engines:
If liquid state propane is introduced to the engine, there is change of state heat of evaporation required to vaporize the liquid, and then also roughly isenthalpic expansion which gives further charge densification. Per the Classius-Clapeyron relationship propane will have less cooling than larger molecular weight fuel.

I am guessing that the LP is vaporized prior to carburator so much of the cooling advantage is lost.

 

[dcasto]

All the propane fuel trucks I've run were started and run on gasoline until the engine water was warm enough to vaporize the propane. I gues you could start on vapors off the top of the tank and switch to liquid when the engine is warm.

The rule of thumb for home racing was 12.5 : 1 compression on Aviation gas at 105 Octane. My present car has 11.5 to 1 and requires 91 to 93 (depending on altitute). But it has more bells and whistle than you can believe. Thats how I got to 11 to 1 with you every day vehicle.

 

[cleanburner]

i wasn't aware of any low level compression issues for propane motors-engines when we built my 496 chev
so it's 13 to one in a street and strip trailer towing package..
works just fine, no detonation

cleanburner