CR vs Octane.

This is something I used to remember, but don't anymore.


Octane is 115 rated. Given an N/A engine with cast iron head and pistons the ideal CR is...?

Given aluminum pistons? (I don't even know if there's any that would work anyways)

That has lead in it right? Its also very expensive as well. There are going to be more factors than just CR including combustion chamber size, condition, timing, etc.

I'm aware that there is more factors. At least timing is adjustable after you build an engine. Besides, I could run 115 on a 8:1 CR by advancing the timing a long ways, but would that be sensible? Not really. I just need a ball park CR so I can start crunching numbers. It's rather hard to try figuring the correct CC size given I'm not sure what CR to shoot for.

I was thinking 12:1ish would be acceptable.


And no, it doesn't have lead in it.

DirtyMJ said:

I'm aware that there is more factors. At least timing is adjustable after you build an engine. Besides, I could run 115 on a 8:1 CR by advancing the timing a long ways, but would that be sensible? Not really. I just need a ball park CR so I can start crunching numbers. It's rather hard to try figuring the correct CC size given I'm not sure what CR to shoot for.

I was thinking 12:1ish would be acceptable.


And no, it doesn't have lead in it.

Click to expand...

115 racing fuel you could hold close to 12:1 if your Quench was around .045-.055.

Are you sure you want to go that hight on compression? Evan if your making it and "only race truck" ..............I would consider 11:1 and nothing over 10:1 for the street.

If your going to run and Hesco alu head then 12:1 And no hire then 11:1.........on the street w/ a mix of racing fuel!

Like what was said above, there is a lot of variable that would change theses recamendation..........like the cam you choose, and the "Dynamic Compression Ratio" that it create.

Flash

That's actually a line of research I'm working on at the moment - part of writing my chapter on "Exotic and Alternative Fuels."

There are an awful lot of factors involved in selecting octane against CR - cylinder head and engine block material, piston alloy, piston cooling, cooling system efficiency, quench, ... - so anything I dig up is still going to be just a starting point, but I'm trying to make it rather less guesswork than it currently is...

Also note that there are actually two different compression ratios for any given engine combination. There is the "static compression ratio," which is determined by the selection of cylinder head, cylinder bore, crankshaft stroke, piston, and the like; and the "dynamic" compression ratio - all of the above are still in play, plus camshaft selection (mainly intake valve closing timing.) It is this latter that will have more effect upon fuel octane requirements than the former - and that's what I'm trying to sort out...

Actually, I made this quite a pickle by not mentioning a few things.


The 115 octane fuel isn't anything amazing. Most people use it in their BBQs. It's propane...


All I want to do is build a stroker to replace my rather tired 4oh. And that would really be the time to get the CR somewhere closer to where it should be. The parts going in won't be anything amazing. A HO head if it works, maybe port matched. Probably not a stock cam, but maybe since I'm so cheap, although I'd much rather run one with a softer seating profile. And if the HO head works, a stock HO header. Things like aluminum pistons are something I'd buy if I either get stuck or feel like blowing some cash.

Anyways. I did a bit more reading and looked at some of my old notes. Apparently 12:1 is what I should shoot for.

I've sadly forgotten a lot about reciprocating engines. I only started thinking about this because hearing the O-320 that I rebuilt over the last two weeks running was kinda fun.

5-90, I'm thinking I'm wrong here... But I think we referred to effective CR (volumetric efficentcy corrected) as mean effective pressure. But that might have been the thoretical cylinder peak pressure? Durr...

Yah - propane is up around 112-115 (R+M)/2 octane, or thereabouts. It's also a nearly pure hydrocarbon - the only additive is beeswax, and that's why it burns so cleanly.

"Static Compression Ratio" - the ratio between the volume above the piston when the piston is at TDC, vice the piston at BDC. Determined by engine component selection, as above - and without respect for camshaft selection.

"Dynamic Compression Ratio" - the ratio between the volume above the piston when the piston is at TDC, vice the piston at the moment the intake valve is closed (which is usually sometime after the piston is travelling upwards.) This is always lower than the SCR, and can be changed by installing a different camshaft (even when all other engine components are the same.) I don't have the formula to hand, but it's readily available, and there are a couple of Javascript calculators online as well for the maths-impaired (there are a few of us out there.)

"Quench" - the distance between the deckline of the piston (flat surface) and the deckline of the cylinder head (flat surface.) "Ideal quench" is usually taken to be .040-.060", and this "squish band" forces the fuel/air mixture to be turned inside out in the combustion chamber as the piston reaches TDC. This inversion reduces the overall combustion chamber temperature, which reduces the potential for detonation due to compression ("spark knock" or "preignition.")

"Octane" - this can be taken as several numbers - "Motor Octane Number" (MON,) "Research Octane Number" (RON,) or "Pump Octane" (typically what is referred to, calculated as (R+M)/2.) A higher octane indicates a higher resistance to ignition, which means that the fuel/air mixture can be compressed further (gasses will heat up as they are compressed) before it will spontaneously ignite. This is what I'm trying to sort out - various minima for octane numbers (irrespective of other factors) with various DCRs. Factors that can affect octane requirements:

• Cylinder Head Material
• Engine Block Material
• Engine Cooling System Efficiency
• Engine Coolant Composition
• Heat Rejection of Radiator
• Direct Cooling of Combustion Chamber Components
• Combustion Chamber Quench Distance

For instance, oil jets can be directed at the bottom of the pistons to cool them (a common turbo-builder's trick,) which can allow the use of lower octane fuels.

There are a huge number of factors involved, but what I'm trying to do is generate a "jumping-off point" for stroker builders. An educated guess is better than a wild-assed guess anyday...

DirtyMJ said:

I'm aware that there is more factors. At least timing is adjustable after you build an engine. Besides, I could run 115 on a 8:1 CR by advancing the timing a long ways, but would that be sensible? Not really. I just need a ball park CR so I can start crunching numbers. It's rather hard to try figuring the correct CC size given I'm not sure what CR to shoot for.

I was thinking 12:1ish would be acceptable.


And no, it doesn't have lead in it.

Click to expand...

Well you'll want to find the best balance between advancing the timing (obviously not as far as you were saying) and CR. As far as the lead, i remember seeing it at the drag strip for C16 i believe that said lead somewhere on there. The octane number is just a rating for a given fuel, apparently propane and C16 have the same rating. Try this site out.

http://www.vpracingfuels.com/vp_01_fuels.html
It is under leaded btw.


C16
Used in turbocharged engines, blown engines and nitrous applications with CRs up to 17:1. Recommended by the top nitrous oxide companies. Spec Fuel for NHRA Comp Eliminator. • Color: Blue
• Motor Octane: 117
• Specific gravity: .735 at 60° F

What kind of pistons were you looking to use too? Is this a trailonly kinda rig?

It's blue and it smells good when it burns :sunshine: and it makes my bike go low 8's :greensmok

DirtyMJ said:

I've sadly forgotten a lot about reciprocating engines. I only started thinking about this because hearing the O-320 that I rebuilt over the last two weeks running was kinda fun.

Click to expand...

Does that Lycoming have iron pistons? All modern auto engines use aluminum pistons. Cast, forged, or hypereutectic are the choices.

They're cast aluminum. I think they stopped using iron pistons in the 30s.

It wouldn't surprise me if ASEL pistons were still made from iron- they are definitely stone age technology. Pretty reliable, though.

Gonna have to keep an eye on this one, looks interesting.

What a curious thread! I am wondering if the propane will be used as a liquid or a gas and if a liquid how do you deal with freezing? Sounds like a neat idea.

The propane is stored as a liquid, and remains liquid until it hits the regulator.

Impco E regulator, one I have lying around... Kinda dirty.



The regulator has an element in it to pump coolant through it and to help vapourize the propane. The output is a constant pressure (something like 1psi) and is fed through a hose to the mixer. If the output is liquid, it won't work at all. That can happen if it's cold enough (-45ish).

Impco 425 mixer.



The mixer is just like a carb, but less complex. The vapour is mixed with the air there and hopefully it makes its way into the cylinders and goes bang when it should and all those other things.

Ayuh - the propane ends up being vapourised by the converter (I think that's what it's called) and the beeswax carried by the stuff lubricates the converter parts. The converter wants occasional service - much like a carburettor - but it's a good deal simpler.

If you do this conversion, I'd appreciate a detailed report. I'm approaching it from a theoretical POV, and I'd like some "real-world" data to temper the theory. I've dealt with converted gasoline engines in forklifts and generators before, so I do know it can be done (it's just a matter of getting everything right.) I've even seen "dual-fuel" conversions - where it starts on gasoline, and switches to propane to run on.

I have a dual fuel mixer actually. Well, I think two of them. They're imcpo 300As. I was actually planning to modify one of them to piggyback on the stock TB but opted against it in the end (way too much time would be invested, and it might not work correctly).


It's not like the states here. In the 80s there was a big tax rebate given to vehicles that were converted. So there's a lot of older vehicles running around on propane, or dead in the junkyard. I've delt with a couple SBCs on propane, but never actually owned one. Hence I have a reasonable idea what to expect with everything.