Natural gas vs Propane hp derate 1

[Fmangas]

I'm being told if you make a switch on a spark ignition engine from Ng to Lpg you need to hp derate. Same engine at stoich on both LPG and NG. This just seems counterintuitive to me but I've been wrong before.
Thanks
Frank

 

[Compositepro]

It should be the opposite. Propane is denser, so you should be able to get more fuel and air into the cylinder. Propane will also produce slightly more moles of combustion products per mole of fuel and air.

 

[Fmangas]

That was my thought too, but I've been having people that I have a lot of respect for tell me I have to derate

 

[catserveng]

From the industrial engine side of it, it had more to do with detonation limits than fuel energy content. We typically run propane in lower compression engines, around 9:1 where the NG engines up to about 12:1. Actual power ratings and performance is based off methane number of the fuel.

In some older Caterpillar (and some Waukesha) models there were dual fuel configurations with different fuel trains and switchable ignition timing to support the fuel change, and in some cases a power derate was applied when operating conditions and fuel quality put us in detonation on propane. CAT had an older nice EDS sheet (Engineering Data Sheet) on the topic but can't find it and can't get online with the system now.

Hope that helps, Mike L.

 

[Lou Scannon]

It depends. If the engine is optimized for one fuel, it may be difficult or impossible to reliably get the same output from a different fuel, for the reasons given above. Assuming the engine is optimized for natural gas (especially compression ratio), it may well be necessary to derate for propane due to detonation. If you have the ability to adjust spark timing and boost pressure (if turbocharged), that may minimize the amount of derate necesssary.
BTW, everything said above by catserveng is on the money, in my book.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[turbocohen]

Yeah, what Hemi said..

 

[Fmangas]

Thanks guys, I never thought about it from that direction

 

[franzh]

Gaseous fuels displace air at stoich, about 10% for NG and about 5% for LPG. Displacing any air has the effect of derating an engine by that relative amount. Taking an engine designed for gasoline and simply substituting the fuel will result in lower output unless the engine is optimized for the fuels properties. Since both of these fuels have different octane ratings and resistance to knock, they benefit from enhanced timing. Still, advancing the timing to knock limits wont restore the power difference.

Franz



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[Tmoose]

Charge cooling due to evaporation can significantly affect inhaled charge temperature and thus density and volumetric efficiency. Alcohol requires significantly greater volume of fuel ( ~2X gasoline for methanol) yet can be counted on generally for +10% power over gasoline in some part due to cooling.

 

[turbocohen]

Same for liquid phase propane injected at the port. However you lose much of the gains with DI LPG..