Should I convert my 4-stroke engines from regular gas to propane

Question

I am planning on converting my portable-generator/lawn-mower/snow-blower from gas to propane. This was my analysis; could you please comment on the facts, analysis & conclusion? Am I on the right track?

Question: Are my facts correct?

• Gas: BTU/gallon: 114,000 $/gallon: $3.25 Weight: 6 pounds/gallon

• Propane: BTU/gallon: 91,502 $/gallon: $2.93 Weight: 4.2 pounds/gallon

Question: Is my analysis correct?

• Gas is more cost effective (35K BTUs/$ for gas vs.31K BTUs/$ for Propane)
• Propane is eco-friendly (societal factor)
• Propane does not destroy the engine (maintenance cost)
• Propane can be stored for a longer time without deterioration (cost of waste)
• Propane is lighter to transport

Is my conclusion correct?

Gas is 12% cost effective; however, propane compensates for lower energy/$ due to other practical factors; hence gas-to-propane conversion is worthwhile.

Answer 1

cost

• at ~$3/gal for propane that might be low, it will depend on where and how you buy. My 120 gal tank for home I pay around $4.50/gal but that's one fill every 2-3 years paid cash no service plan no tank rental. With larger propane deliveries and plans the cost [per gallon of propane] can get heavily discounted to below $3/gal.
• typical store bought 20 lb gas grill propane... are never 20 lbs they are 15 lbs filled; 15/4.2 = 3.5 gallons for $14-20 out the door tank exchange from walmart/depot/lowes = $4.00 to $5.71 per gallon propane.

power

• a gallon of propane has 27% less energy than a gallon of gasoline... or propane = gas * 0.73
• octane of propane is 110, versus gasoline of 87-93.
• it's often comparable to mileage economy in a car gas vs propane: car gets ~20mpg for gas vs 14 mpg for propane: take this link for what its worth. https://dr1.com/forums/showthread.php/119032-Propane-vs-Gasoline-did-research-here-are-the-numbers
• so gasoline is more cost effective than propane (or natural gas)

destroy the engine

• https://www.machinerylubrication.com/Read/524/natural-gas-engine-oil-analysis
• for outdoor power equipment dust will likely contaminate oil in same amount of time vs cleanly maintained gasoline necessitating oil change; so you can't just assume propane or nat. gas does not degrade the oil (but non-sour propane would probably keep engine oil cleaner vs gasoline strictly speaking

• https://www.enginebuildermag.com/2015/05/cng-and-propane-engine-builds/

  • Because CNG/LNG/JP are gases, they provide no lubrication or cleaning for the intake valves. Consequently, engines that run on dry fuels usually require hardened valves and valve seats to minimize valve wear and valve recession. On the other hand, dry fuels mean less wash down of the cylinder walls so there’s better ring lubrication than with wet fuels.
  • that said, running a mower engine on propane which was built [cheaply] to run on gasoline will likely not last as long as on clean gasoline with proper maintenance... but hard to say for a 1 cylinder < 10hp engine.

conclusion

you say conversion to propane worthwhile for

• generator
  • purpose built natural gas or propane engines in electrical generators are great because the liquified gas stores indefinitely unlike gasoline or diesel; but your question is conversion so this doesn't apply to you it
  • thinking of a ~5000 watt generator I can pick up at depot right now (if that does apply to you) that's a single cylinder engine no different than mower or snowblower so no, not worthwhile because the engine will operate at reduced power because first it doesn't have the higher compression (because it was built for gasoline) to take advantage of the higher octane of propane to extract 100% of the power out of a gallon of it, at which point is then 27% less so double wammy your 5000 watt gasoline generator will likely only be able to produce 3000 watts; i wouldn't call that worthwhile unless there's a gasoline shortage.
• mower, snowblower, other small outdoor power equipment
  • still not worthwhile (unless you like tinkering) for same reasons as generator reduced power power; only good reason if there's no gasoline available.
  • the only attractive point of LP is it's indefinite storage but carb gaskets and hoses deteriorate and will leak just the same; the emissions calculations are iffy because of 27% less power per gallon so you will be burning more of it regardless and then doing so in a low compression gasoline engine (double wammy) may likely end up with total amount of emissions produced mowing your lawn : Compared to gasoline, propane yields 12 percent less carbon dioxide, about 20 percent less nitrogen oxide, and as much as 60 percent less carbon monoxide (World Liquid Propane Gas Association, January 2003; California Energy Commission, January 2003). And these emissions numbers would be for a purpose built engine to run as best as possible on LP not a one cyl honda or briggs at 8.5:1 retrofitted for LP it comes down to BSFC.
• propane @ 4.2 lb/gal versus 7 lb/gal gasoline is ligher per gallon but propane must be in a pressure vessel typically at 177 psi to keep it liquid so factor in weight of steel tank: tare weight of full 20 lb propane tank with 4.5 gallons is 37 lbs. In a 1 lb plastic gas can with 4.5 gallons of gasoline totals 28 lbs so handling gasoline is lighter practical sense at the ~5 gallon quantity.

Answer 2

As you already know, there are pros & cons - it isn't a simple decision. So take each device separately as they are actually quite different:

Portable Generator

How often do you use it? Job site or fixed location? Power outage or "some tasks always need a generator"?

The more you use it, the more replacement with propane may make sense. But if it is only occasional usage then the environmentally & financially speaking the fixed costs (i.e., energy used in manufacturing and cost to buy it) will have a very long payback time.

Lawn Mower

Depending on:

• Size of your property
• Usage elsewhere (e.g., if you use the mower to mow other people's lawns)

you may be far better off with a battery electric mower. There will still be emissions, but at the utility plant which are (a) typically (unless coal) cleaner and (b) don't have the same direct impact on your (and your neighbors') air quality.

Snow Blower

Battery electric may make sense here too, but depending on how frequently you get snow-blower-level snow (the more frequent, the faster the environmental and financial payback of switching) and how much power you need, sticking with gas may make a lot of sense.

Answer 3

Thanks to everyone that provided thoughtful and well reasoned responses.

I put some further thought into your responses and cautiously and successfully experimented a propane retro-fit to my (normal) gas-generator.

My analysis:

• A large capacity gas cylinder is fine for a generator but impractical for a snow-blower and/or lawnmower
• A light-weight 1 lb. small capacity propane camping-type cannister (portable for lawn-mower, snow-blower) costs about the same as a gallon of gas. Considering lower BTU/gallon for propane, gas is more cost-effective by about 25% for lawn-mower and snow-blower
• You need at least 2 adaptors to connect a camping-style 1 lb. propane cannister to connect to engine-inlet (cannister adaptor + control-valve). These are 2 potential points of leaks. I realized this fact after smelling low levels of propane after hooking it up for about an hour. It was a very slow leak despite the fact that I did as best as I could to secure the connectors and prevent gas-leak
• If the retrofit blows-up and the house/shed catches fire, I would NOT have any recourse to insurance claims as the product was tampered with and not used as designed.

Conclusion:

Propane RETRO-FIT to a gas lawn-mower, snow-blower, generator is NOT cost-effective and could be a potential FIRE-HAZZARD and a LIABILITY NOT COVERED BY NORMAL HOME INSURANCE POLICIES.