Compression vs spark ignition efficiency?

[BioHazard]

Well, today diesel hit $4.00/gallon locally for the first time ever. Propane is still at $2.79 at the same gas station, even cheaper if you have a propane company deliver to your house. If my math is correct that calculates out to $3.20 for 100,000 BTUs of diesel and $3.07 for 100,000 BTUs of propane. (and $3.07/100KBTU for gas) Propane hasn't been cheaper than gasoline or diesel since at least the mid 90s as far as I know, when propane vehicle conversions were more common.

If this trend continues, everything I own is going to end up being powered by propane. (or natural gas) What I want to know more about is how the efficiency changes when converting a diesel engine running on propane to a spark ignition engine running on propane? Is it possible for a diesel, compression ignition engine to run on 100% propane, or must there be a 10% or so diesel pilot?

Now take that same diesel engine, lower the compression ratio to something manageable (but still high) and add spark ignition like Arrow did with the Petteroid, or the Kubota 3 cyl gas engine. Is this engine going to be less efficient than the same engine with compression only ignition? If so by how much?

[mobile_bob]

engineering text states that the spark ignition engine will be less efficient than the compression ignition engine

how much is anyones guess, however if i had to bet

i would bet that there would be around a 5 point drop in efficiency, that is only if the engine is optimized to run on propane
if not i would expect an even wider disparity.

no a diesel cannot run straight propane without pilot injection, how much offset is something that will have to be arrived at by trial and error
depending on engine type, loading, and a few other parameters.  90/10 might be possible, but i would expect maybe 70/30 as more realistic
without having to do some optimization and operating under a fixed rpm and more or less fixed loading.

ymmv

bob g

[Carlb]

I setup my listeroid with natural gas and diesel. I use a needle valve to control the amount of natural gas that enters the engine.  I do not adjust the natural gas needle valve based on load.  When i set it up i was using about 15% diesel and 85% natural gas at 1500 watts.  Since this is for backup only and I didn't plan to run any thing other than small stuff in a power outage (no washer or dryer and no air conditioning ) 1500 watts is about what the lights, frig, computer, tv and some other small stuff takes. 

If we use the microwave or coffee maker the rack just opens to maintain the proper rpm. Under a 2700 watt load its about a 50/50 mix of diesel and natural gas.  I did not do lengthy scientific testing, but at 1500 watts the rack is closed to the same point as the engine would be idling with no load 

I can tell you that here Natural gas delivered is about 1.50 Per therm or 100,000 BTu's
So even if the efficiency with this combo is a little off because it is not measured precisely, it is still a lot cheaper than running propane or diesel alone.

here is a video of my setup

http://www.youtube.com/watch?v=AwPPui8Fyi0


Carl