Does LPG's higher octane rating compensate for its lower calourific value?

[NICAT]

Hi.
Gasoline price was increased last year in my country.
It is 2x more expensive than LPG. There is generally heavy traffic in the city. So the consumption is as much as 17L-18L/100km

According to what i found after google searches,
LPG has less calourific value, which means reduced power and increased fuel consumption.
But it also has higher octane rating, which let engine use it as a premium fuel, so more advanced ignition timing, more thermal efficiency.
So in theory, it may compensate for its lower calourific value at some level.
But, since gasoline is liquid, when it evaporates, it cools down the cylinders. But "standart" LPG conversions inject LPG as a gas to cylinders, means its higher octane rating wouldn't be as effective as expected.
Car's engine is M54, with 10.8:1 compression ratio.

If any of you has used it, would you recommend it ? How much difference is there in fuel consumption ?

 

[DrDanger]

You have your facts right.
Running LPG on a 2004 Peugeot 407 2.0l for 50000km.
I use 20% more LPG than petrol, have it tuned very rich.
I had one sensor go bad on it.
I changed the filters and it has run reliably.
Very happy with fuel consumption and engine performance. I get 13,5l/100 city on LPG to 11,5 to 12l per 100 on petrol. Highway mileage depends - mostly around 8l/100 on both.
Get it tuned a bit rich to ensure you dont burn an exhaust valve.
It is a long thermodynamic story to explain why. I'm no expert but - due to lower efficiency of the burning, more heat is released. A richer mixture ensures a small bit of
extra fuel is unburned, which keeps temperatures lower. Also more fuel injected cools better in the intake and compression strokes.
Also this way you car will perform the same or maybe even better than on premium fuel.
Hope it helps.

 

[mk378]

LPG still has 80% of the energy per liter but 50% of the cost, seems like a no-brainer.

 

[NICAT]

Originally Posted by DrDanger
You have your facts right.
Running LPG on a 2004 Peugeot 407 2.0l for 50000km.
I use 20% more LPG than petrol, have it tuned very rich.
I had one sensor go bad on it.
I changed the filters and it has run reliably.
Very happy with fuel consumption and engine performance. I get 13,5l/100 city on LPG to 11,5 to 12l per 100 on petrol. Highway mileage depends - mostly around 8l/100 on both.
Get it tuned a bit rich to ensure you dont burn an exhaust valve.
It is a long thermodynamic story to explain why. I'm no expert but - due to lower efficiency of the burning, more heat is released. A richer mixture ensures a small bit of
extra fuel is unburned, which keeps temperatures lower. Also more fuel injected cools better in the intake and compression strokes.
Also this way you car will perform the same or maybe even better than on premium fuel.
Hope it helps.


Very informative, thank you

 

[FlyNavyP3]

Unless the car is already dual fuel this may take a long time to recoup the conversion cost. If the conversion is required how much time will be required to just break even.

If you have to spend mo ey to save money in the long run work out that math first to see if you can break even and start seeing a savings in an acceptable timeframe.

If the break even point is 10 years from now there's really no point in converting.

If the car is already propane capable why not just try it out?

 

[Linctex]

The reduced cost is definitely a deciding factor.

However, if you are looking to regain lost power and fuel economy,
You would need to raise the compression ratio of the engine.

 

[NICAT]

Originally Posted by FlyNavyP3
Unless the car is already dual fuel this may take a long time to recoup the conversion cost. If the conversion is required how much time will be required to just break even.

If you have to spend mo ey to save money in the long run work out that math first to see if you can break even and start seeing a savings in an acceptable timeframe.

If the break even point is 10 years from now there's really no point in converting.

If the car is already propane capable why not just try it out?


The car is not dual fuel. LPG sysem must be installed.
The most well known company offers it for 1000$, Roughly after a year, i would recoup the conversion cost.

 

[BrianF]

My father ran dual fuel Dodge 150/1500's since the 70's. Had a LPG factory ordered D250 van as well. I recall in the 80's and into the early 90's it made sense. The price was a fraction of what gas was. If you converted the engine for the LPG and raised the compression, the power was not that bad.

What happened is that companies priced themselves out of conversions and LPG price just kept going up. I have not looked at the price lately as a lot of places no longer have auto but it was priced higher than gas.

 

[hpb]

If you can recover the cost of the conversion after just one year, I say go for it.

 

[Linctex]

Originally Posted by BrianF

What happened is that companies priced themselves out of conversions and LPG price just kept going up. I have not looked at the price lately as a lot of places no longer have auto but it was priced higher than gas.


I remember when it was $0.50 a gallon

A LOT (and I mean a LOT) of older tractors in Texas are propane powered.

The oil refineries would GIVE IT AWAY to whoever would haul it.

I talk to old farmers who paid $0.08 a gallon for propane back in the day (the cost to haul it)

 

[pkunk]

I had several Pickups and a drill rig on propane back when it was $.25 a gal and gas was a buck. After 20k mi. the oil still looked like it was fresh out of the bottle. Fuel tax ultimately raised the price to equal gas but I put a wet line on one of my home tanks and filled up at home. Probably illegal but the propane company sold the parts to me. Still use the wet line to fill the little tank on my motorhome and a few bottles, saves some and a trip to the dealer.
It was a real saving in the 80s and early 90s and engine longevity was amazing. Had a Ford 300-6 with well over 300k and the oil was never dark, ran well, and started on the coldest mornings.

 

[Dinoburner]

When comparing any fuels its useful to use BTUs per unit. Propane is 90,000 btus per gallon and gasoline about 130,000 btu, the approximate octane equivilent for LP is 110 which permits a higher compression ratio which can help obtain peak efficancy. The btu content still rules. One other benefit of LP though is it burns very clean allowing longer OCIs.One downside is in cold climates below 0 temps starting ican be difficult. It`s been 40 years since I drove a propane bobtail but it seemed to be a trouble free system.

 

[hpb]

Originally Posted by Dinoburner
One downside is in cold climates below 0 temps starting ican be difficult.


Dual fuel systems like the OP would install start the engine on petrol, and then automatically switch over to LPG after a few seconds, eliminating the cold start issues.

 

[Garak]

I had an LPG only vehicle here, and simply plugged it in below -15 C. I started it at -40 and below.

 

[Shannow]

Can't just look at the energy per gallon, you need to look at the stoichiometry as the next step.

That's how much energy is contained in a full cylinder of air fuel mix at the right air/fuel ratio to get the job done.

Per Heywood (book cost me $124 with student discount in 1990), the energy content of a stoichiometric mix is
* Gasoline - 2.83MJ/Kg
* Propane - 2.77MJ/Kg

The energy difference is only 3%...but that's burned in a calorimeter.

Propane DOES however have 112 RONs, and 97 MONs.

It's also mixed, in a mixer, so there's no rich or lean cylinders, like on a centrally mixed (carbed) gasoline engine.

Tuned properly...gas with a mixer CAN give you better power than an Gasoline...I had a dual fuel Holden V-8, and can vouch for that. As gas was so cheap, I used to boost the octane of the gasoline so that it ran OK if needed, and ran 1 tank of modified gasoline (78 litres) to 4 tanks of LPG (130L each), so it was worth it.

Guy at work has converted a V-8 Audi to duel fuel, with port liquid propane injection, and the injectors fired off the OEM computer, but pulse width modified by a piggyback set-up...way too complicated for me, but with the phase change cooling the charge air, it goes pretty hard.

 

[NICAT]

Originally Posted by Shannow

It's also mixed, in a mixer, so there's no rich or lean cylinders, like on a centrally mixed (carbed) gasoline engine.
.

Can you please elaborate on this ?
Does this mean, compared to gasoline engine, LPG engine has more stable Air fuel ratio ?
Regards

 

[DrDanger]

LPG here is a mix of propane and butane(10s of others at low quantities).
Content varies batch for batch. Summer and winter lp varies greatly.No EU regulations, no nothing. They can sell be selling you whatever.
Makes perfect tuning nigh on impossible.
As per my gas garage propane needs 20l/air and butane 26l/air. LPG is anywhere in the 20-26l range. Another garage claim stoicomety is at 15,7:1. Have not bothered with this as long as my OBD shows a rich mixture.
Tuning is always a compromise even on a brand new OBD II feedback system.

 

[WyrTwister]

Originally Posted by Linctex
Originally Posted by BrianF

What happened is that companies priced themselves out of conversions and LPG price just kept going up. I have not looked at the price lately as a lot of places no longer have auto but it was priced higher than gas.


I remember when it was $0.50 a gallon

A LOT (and I mean a LOT) of older tractors in Texas are propane powered.

The oil refineries would GIVE IT AWAY to whoever would haul it.

I talk to old farmers who paid $0.08 a gallon for propane back in the day (the cost to haul it)


I was raised on a farm until about the 5th grade . Propane / butane for house heating & Dad's 720 John Deer tractor . I remember Dad telling me he paid $ .06 - $ .08 a gallon for it ( no tax ) .

A lot of farmers had conversions on their pick up trucks . They were not suppose to run non-taxed propane / butane in motor vehicles , but a lot of them did . A few got into trouble & were fined .

I also remember being told it burned " cleaner " than gasoline , potentially making the engine last longer .

I also personally witnessed an overhaul when I was a High School Senior of a farmer's truck with horribly burned valves . Asked the mechanic what caused that . I was told , penny pinching farmer running the fuel mix too lean .

If your engine has a knock sensor , it may advance the timing because of the higher octane rating of propane / butane . This may help compensate a little for less energy per pound of fuel ?

 

[Donald]

LP gas engines will burn valves and unsure if they use special metals for engines built specifically for LP gas. Or they expect to do a valve job over the life of the engine.

 

[Garak]

We did fine with LPG in the taxis. On occasion, a valve was burned, but usually an engine that was designed for unleaded gas (yes, this was long enough back that some of the engines were still using leaded gas) was durable enough on LPG for hundreds of thousands of kilometres.