Lil Mule

DRD. I looked at this thread, read and reread it numerous times and couldn't put my finger on what was missing.

Let me start off by saying that gasoline and propane have very similar properties on how they behave. This is why a gasoline engine can easily be converted to propane.

You mentioned auto ignition temps, but unfortunately thats number is at 1atm or atmospheric pressure at sea level (14.7psi)

Auto ignition temps at 1atm (14.7psi)
Propane 470*C / 878*F
Gasoline 370*C / 698*F
Diesel 254*C / 490*F

Gasoline/propane engines operate at about 10atm or 147psi
Diesel engines operate at twice that if not more. Or 20+atm

At these combution pressures, you drop auto ignition temps to levels that will ignite any of the 3 fuels readily.

Auto ignition temps are inversely porportional to pressure. As pressure rises, auto ignition temps drop.


Unfortunately my brain is now smoking and I'm putting away my chemistry book for the night. Been a while since I've opened it.

Theres more to this, hopefully someone else will finish this puzzle

DRD

In a diesel engine, combustion is initiated through autoignition of the fuel due to the high
end-temperature and pressure of compression. The autoignition temperature of propane
and especially methane are so high that these fuels are not suited for the diesel process.
For autoignition with methane, a compression ratio of 38:1 and for propane 29:1 is needed
/34/. Therefore, in a gas engine the gaseous fuel has to be ignited either by an electricalspark (Otto engine) or by diesel fuel (pilot injection).

Some interesting reading here:

http://www.iangv.org/html/sources/so...part2final.pdf

At boost levels over 40 PSI it might be possible to have auto ignition of propane, at least I think it's possible. Where's all the engineers when you need 'em?

Lil Dog

Interesting.. Well thats some education for me.. I was under the impression that the Propane would ignite below the 400 psi compression pressure.

The problem here is that the compression ratio they use is not clear.

In automotive terms, the compression ratio is based on volume ratio. In pure compression terms of a gas using theoretical models the compression ratio is the pressure ratio.

So in the Cummins the Volume ratio is 17:1 and the pressure ratio is 28.2:1 using 400 psi compression values. This is based using absolute pressures so its 400+14.7(atm) / 0+14.7(atm).

If you use the volume ratio, propane won't autoignite, if you use the pressure ratio, it will.

Now we need the engineer that wrote that paper..

ridofpwrstroke

After reading the thread i am confused. But i think that most cummins guys dont use propane is because injectors are very cheap and you can get almost any box cheaper than propane. I used to have a 97 f350 and if you look at my screen name i got rid of it. When I was looking to bump up the truck the chip was the only thing I could afford. The exhaust helped but it still has nowhere near the power of my 02. I remember when I was looking at aftermarket injectors for the ford, it was like 4000 bucks for a set of 8 compared to 450 bucks for a set of six for my cummins. Not many people can afford that much money so propane is a cheaper alternative for them. The cummins doesnt need propane, but the ford and chevy guys put on their boxes wanting more usually go propane because of the expense. JMO

wannadiesel

So if propane won't autoignite in a diesel engine, why can they run away on propane? And why do failures related to propane resemble over-advance failures?

DRD

I think diesels run away because they are using the diesel to light the fire and since they are not air starved continue to speed up as long as the gas being burnt. I think once the burn is started the sharp rise in cylinder pressure and temp. causes the propane to detonate. I think......

wannadiesel

Even if you turn off the key/shut off the fuel they keep running. See the post about the backhoe on the first page.

When you do the math on autoignition, keep in mind that you're starting the compression stroke with 150-200 degree air, not ambient temp air when you have warm engine under load.

If I thought propane was a safe way to add power, I'd have done it by now. I can get propane for free at work.

Tuffram

Shut off the key and it stops the Diesel fuel, however, There is another source of fuel you are forgetting about. It's in the crankcase!! And sucking it past the rings is not a problem for an engine in run away.... an awful lot of BTUs in the oil pan once it gets burning....