Some propane conversion questions

[Ziggle]

I just purchased a pickup with a 454 Chevy engine, and I also have a boat with the same engine. As you can imagine, they suck gas like it's free. The boat gets about 1.5mpg on premium gasoline. I am considering converting both to propane, but I have some questions.

First, I know in a stock engine propane will have less power and mileage than gasoline. What if I increase the compression to around 11:1? I know that will increase the power and mileage from propane, but is it still less than gasoline?

Secondly, if I do this I'll probably get a bulk tank for my house so that I can fill the boat and truck from there. How much cheaper is delivered bulk propane as compared to gas station prices? Is tax already added to the propane at gas stations, and not added to bulk propane? I don't have to pay road taxes on the fuel the boat uses, so it would be nice if I could buy it tax free rather than getting a refund every year.

Thanks for the help!

 

[franzh]

Like in real estate, it depends on Location, Location, and Location.
Delivered LPG here in Texas is running from $1.95 to $2.45 per gallon. Motorfuel is running about $1.65. This is one reason LPG has not really caught on as a motorfuel alternative although there are certain advantages.
Next, the modern engines do not handle gaseous fuels well, primarily where the air-fuel mix is fumigated into the air inlet. Port vapor injection is MUCH better but there are only a few such systems available. Blame the intake manifold here.
On standard compression engines, expect about a 15% loss in power with the same increase in fuel usage.
Interestingly, in Australia and other parts of the world, LPG is widely accepted as a performance fuel because of its compression tolerance and significantly lower cost of operation.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

I would expect the fuel consumption to go up by more than 15%, as the energy content of liquid propane by volume is about 72% that of gasoline.

 

[franzh]

Depending on how efficient the engine is, I have seen as much as a 50% difference, and as close as 5% difference. More modern systems with full management are very good at replicating the gasoline performance profile to the point of being almost totally transparent to the driver. LPG does have about 75% of the calorific value as gasoline, but the power and economy drop is quite dependent on engines, design, efficiency, mechanical condition, type of fuel management and carburetion, etc.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

Agreed. But on "standard compression engines", I think 15% is optimistic.

 

[crysta1c1ear]

I'd agree with hemi that 15% worse HP is optimistic on a simple gasoline to propane engine conversion. My calculations are like this, so if you want to blow a hole in my argument you can see where to do it.

Energy density of Octane, from enthalpies of reaction with air.
44,877,982 Joules per kilogram
Energy density of propane derived the same way
39,870,909 Joules per kilogram

Air fuel ratios calculated using molecular weight of propane 44 and octane 114,
AFR octane 15.06,
AFR propane 15.61

energy per kilo of air into the engine ...
Octane 2,980,247
Propane 2,554,835

So as a percentage, propane provides 86% (ie 14% less) energy for a given amount of air into the engine.

Now a fixed amount of energy will be used to run oil and water pumps, reverse the direction of motion of the pistons, run the valves, compressing the air, and maybe even running alternators, air conditioning, power steering, etc, depending on the engine application.

So like a 14% pay cut damages disposable income by far more than 14%, a straight forward gasoline to propane engine conversion ought to cut power by well more than 14%, by my estimations.

=

If propane has other properties like higher octane ratings and an engine is modified to run higher compression ratios then it might be another story. I've not considered that. I'm not thinking miles per gallon here. I'm not thinking of running costs. I'm just thinking of the power you might get running a converted gasoline engine at the same RPM and airflow through the engine, with a change of fuel and adjusting the air fuel ratio (AFR) to use up the oxygen without wasting fuel.

I've not assumed the propane's volume displaces any air either. With that assumption, and engine converted to propane would suffer too.

=

I think I once calculated on here that $1.05 gasoline was financially breakeven with $0.77 LPG. That was based on less kilograms per litre with LPG - lower physical density. Combined with lower energy density (ie less Joules per kilogram) and the disposable income effect (greater percentage being used just for running costs) I don't think I'll be doing any LPG conversion to my car in a hurry.

Am I on track here, or have I lost the plot?

 

[Lou Scannon]

Well actually, the 15% figure was originally offered as a fuel consumption penalty estimate, not a power penalty estimate. My contention was that, with only 72% of the energy by volume and the same efficiency (same compression ratio), the penalty will be more than 15%.

 

[rdd48856]

Curiously I found opposite results.
Combustion Enthalpy (source – “Chemistry - An Experimental Science”; Pimentel, George C.; University of California, Berkeley and “Organic Chemistry”, Pine, Stanley H., McGraw-Hill):
Propane - 531 kcal/mol (or 12068 kcal/kg) density = 0,501 kg/l M(C3H8) = 44 g/mol
n-Butane - 688 kcal/mol (or 11862 kcal/kg) density= 0,579 kg/l M(C4H10) = 58g/mol
n-Octane – 1318 kcal/mol (or 11561 kcal/kg) density= 0,703 kg/l M(C8H18) = 114g/mol
C3H8 + 5O2 --> 3CO2 + 4H2O + 531kcal that is 106kcal/mol O2
2C4H10 + 13O2-->8CO2 + 10H2O + 1376kcal that is 106kcal/mol O2
2C8H18 + 25O2-->16CO2 + 18H2O + 2636kcal that is 105kcal/mol O2

So the energy released per mol O2 (and per “mol of air”) is about the same.
On the other hand we have an AFR(molar) = 25. But propane is on gaseous phase so it displaces 1/25 (molar) of O2 thus resulting in a theoretical loss of power of 4%( other things being equal). However LPG delivering systems hardly resembles gasoline ones (venturi mixers, for instance, introduce intake restriction losses). When multipoint sequential gaseous injection is used typical power losses are about 5 %. When multipoint sequential liquid injection (OEM only) is used actually gain is obtained (due to temperature intake reduction which increments air density). Results obtained on Dyno tests.

Mileage is a completely different story. Propane has a density of aprox. 0,50kg/l, butane 0,54kg/l and n –octane 0,70kg/l. We have a theoretical volumetric fuel consumption increase of about 34% for pure propane(which I believe is the LPG in United States) and of 18% for pure n-butane. However alkane gaseous fuels combustion tends to be more efficient than liquid ones (better mix, softer combustion, etc.). Typical increase in fuel consumption for multipoint sequential gaseous injection (volumetric) is of about 20% for an LPG made of 80% butane(that’s what we have here on the south of Europe) and should be of 30% for 100% propane. On older cars (non catalysed) it was possible to improve mileage running slightly lean but this is no longer the case.

 

[franzh]

I have seen power gains on LPPFI systems but the fuel economy does not show the advantage.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[rdd48856]

That's right! There's a long way to go on LPPFI...

 

[pbloore]

I would agree with franzh based on personal experience. My series I Discovery which had a relatively uncomplicated gasoline PFi (OBD 1 capable) got about 14 (11.7 US) MPG. On a closed loop venturi system it returned 13 (10.8 US) MPG, but when fitted with a relatively simple closed loop continuous flow LPG PFI system and with some ignition timing adjustments it returned 14.5 (12.1 US) MPG.

Admittedly the powertrain control system and engine on this vehicle were pretty old, but it still shows that with some optimisation good results are achieveable.

My Range Rover fitted with a much more modern Powertrain Control System, a larger version of the same engine, and with 16% more weight returns 16 (13.2 US) MPG on gasoline and just over 15 (12.5 US) MPG and that's running the standard ignition timing. It is fitted with a sequential PFi LPG system running as a slave to the gasoline system. I haven't come up with a reliable method of altering the ignition timing on this system yet, but am hoping to give this a go.

Paul