Propane Butane Fuel, LPG Composition and Properties

Motor Fuel LPG - Liquefied Petroleum Gas is also known as Autogas and GPL, Propane Butane Fuel. Despite of multi-refering, it is a mixture of different lower hydrocarbons (C1 – C8), mainly composed of propane and butane. At standard temperature and pressure it is a gas or liquid which will rapidly evaporate. Compressing at slightly higher pressure it becomes a liquid. Though its combustion produces CO2, it is a more environmental friendly then gasoline and diesel. LPG is being extracted from wet (crude) natural gas and crude oil through treatment process on the oilfields and as the lightest product when crude oil is being refined.
LPG composition is varying on producer and the seasons. It can be composed of pure propane or pure butane, most often 49% - 49% including about 2% other unsaturated hydrocarbons, sulfur and nitrogen oxides. While butane and propane are different chemical compounds, their properties are similar enough to be useful in mixtures and improve fuel characteristics. They do not react with other.
Propane Butane mixture has energy density about 26 MJ/lit (0.112Btu/gal). It is lower than either gasoline or diesel. Compared to gasoline based on energy density and my personal experience consumption increase about 10%. It does not decrease mileage. Almost everywhere LPG is about double cheaper then gasoline.

Propane Butane Mixture Properties

• Combustion:

Propane: C3H8 + 5 O2 → 3CO2 + 4 H2O + energy
Butane: 2C4H10 +13 O2 → 8CO2 + 10H2O + energy
Combustion is almost complete with minimal carbon monoxide emitting. It is clear those propane and butane combustions emit CO2, but it is 20% - 30% less amount then same volume of gasoline emits. Combustion is almost complete, so amount of carbon monoxide is minimal. There are no any other pollutants such as sulfur oxides and unburned aromatic compounds, harmful gases for human health. Generally, with Propane - Butane fuel being up to 75% cleaner on petrol and up to 90% cleaner on diesel.

• Density:

Propane: 504kg/m3; Butane: 582kg/m3; Mixture: about 540 (depends of ratio);
Density of propane butane mixture is lower then density of gasoline. Therefore a mass unit has similar energy density as a gasoline, thought a volume unit has 20% less.

• Octane number:

Octane number is a measure of how resistant gasoline is to premature detonation (knocking).
The octane value (RON) of propane butane mixture is usually regarded to be 100 to 110. It is means that it is a very engine very friendly fuel with better knock-resistant characteristics then octane number testing standard 2,2,4-trimethylpentane (octane).

• Boiling Point and Volatility:

Propane: -42C; Butane: +9C; Mixture: depends of ratio;
Boiling point is a reason for varying of the composition. LPG Fuel Mixture consists of more propane in winter season as well as more butane in summer season.

• Impurities and Free Water:

It is easy to remove all impurities from each propane or butane during the processing. Therefore usually no impurities, free water or carbon residue. The combustion is clean.

LPG is used to run internal combustion engines meant for gasoline use or as a fuel in heating appliances. It can replace gasoline. However LPG conversion requires small modification of fuel injection system and additional tank. The modification is very simple, safe and cheap. My mechanist installed it for less then two hours, ask 270€ (380$) and said thanks, bye. Installation included electronic switcher on control desk which enable me to change the fuel during the driving. Installation expense was recouped after 8000km (5000miles) based on price difference between gasoline and Propane Butane gas in my country. Installation set for newer vehicle models is higher cost, but anyway this money can be compensated in one-two years with the money you save at the pump.
Propane Butane Mixture is perfectly safe if handled correctly and monitored very closely, although it is dangerous substance, colorless and odorless. It is being stored into strengthen tank then gasoline as well as leakage can be detected by distinctive smell adding a special stenching agent. It is non-corrosive, nontoxic but very flammable due to high evaporation ratio. As a liquid, LPG is not explosive, although as a gas can explode in air. LPG in any ratio of propane and butane has a higher energy density and than natural gas (LNG), which means that LPG can not simply be substituted for natural gas.

With increasing demand for LPG, many vehicle manufacturers are creating cars with bi-fuel tanks - running on both Propane Butane Mixture and gasoline. LPG (or propane and butane separately) is so much better for the environment and at the same time save the money. Worldwide millions of people have been enjoying from converting their car to run on propane - butane. It is cleaner and cheaper.