Key words: LPG, LPG, fuel gas, composition, characteristics.
LPG is a mixture of hydrocarbons having a low molecular weight with three or four carbon atoms, that is to say: propane, propylene, n-butane, isobutane, butenes, in varying proportions. The manufacture of this fuel is derived from the processing of crude in refineries and the separation (degassing) of natural gas (methane-ethane).
Liquefied petroleum gases can also contain small amounts of methane, ethylene, pentane and pantenes and, exceptionally, hydrocarbons such as butadienes, acetylene and methylacetylene.
These latter hydrocarbons are present only as by-products of the production of olefins for petrochemical use. In addition to hydrocarbons, sulfur compounds (mercaptans and alkylsulphides) will sometimes be found in extremely small quantities, but which are of some importance with regard to the corrosivity of the product.
The main features
LPGs are easily liquefied gases at ambient temperature under low pressure (4-18 atmospheres): this feature allows easier storage and transport than for non-condensable gases such as methane, ethane, ethylene , which require very high pressures in order to be liquefied at room temperature.
· Refined LPGs are generally almost odorless and extremely flammable due to their high volatility. They can give explosive mixtures on contact with air. To better recognize them or detect possible leaks, they are given a particular odor by means of suitable substances (mercaptans).
- LPGs are not really toxic: they have at most a slight anesthetic power, if they are inhaled for a long time and can cause migraines and stomach aches.
- LPG, when it spreads in its liquid form, out of a pressurized container, evaporates producing cold: on contact with the skin, it causes characteristic burns called “cold burns”.
The physico-chemical characteristics of LPG (distillation curve, vapor pressure, specific gravity, calorific value, efficiency in engines, etc ...) depend on their content of various hydrocarbons.
Commercial products are very different from each other. In addition, their vapor pressure, specific weight and anti-knock properties are very sensitive to variations in ambient temperature. The methods of calculating the number of octane are recent (ASTM-CFR engine under operating conditions Motor Method Standard ASTM D 2623).
Tests have shown that an index of 92 should be considered the minimum value for fueling cars that use this type of fuel. LPGs containing olefinic hydrocarbons (more particularly propylene) can give rise to detonation and pre-ignition phenomena which are all the more sensitive as their olefinic hydrocarbon content is greater and the engine compression ratio is higher. .
The same can be said for LPGs with a high n-butane content. In this regard, the NGPA, the body responsible in America for the unification of standards, provides that LPGs (specification HD-5) must contain a maximum of 5% by volume of propylene.
Comparison with gasoline
The calorific value of LPG is practically equal to that of gasoline if expressed in kilocalories per kilo of fuel, but these values will be very different if they are expressed in kilocalories per liter of liquid fuel at 15 ° C.
This diversity comes from the difference in density between LPG and gasoline: on average, the density at 15 ° C of an LPG is 0.555 kg / liter and that of gasoline is 0.730 kg / liter. An engine powered by gasoline develops 10 to 12% more power but also exhibits higher specific consumption and lower overall efficiency than an engine powered by LPG.
Since the heating values of the two fuels are practically equal, the decrease in power observed with LPG is due to a lower filling of the cylinders, the causes of which are:
- The presence of a mixer between the air filter and the carburetor (the pressure drop in the intake duct causes a decrease in power of 5 to 6%). An adequate arrangement of the gas inlet, obtained by perforating the carburetor and applying a nozzle which sends directly into the narrowest section of the Venturi, would allow this loss of power to be considerably attenuated.
- A warmer mixture, and therefore less dense, because the vaporization of LPG occurs in a reducer-vaporizer. The fuel enters the carburetor already hot while the air / gasoline mixture is cooled by the latent heat of vaporization of the gasoline. The recorded power loss is of the order of 5-6%, on the other hand, it is inevitable since, in order to guarantee a constant air / fuel ratio, the supply device must send the LPG already present at the gaseous state in the narrowest section of the carburetor.
Better performance for LPG
The increase in the overall efficiency of LPG compared to gasoline can be explained by better combustion due to greater homogeneity of the gas / air mixture and by the fact that the adjustment of the mixer, carried out so as to obtain a maximum of power with minimum consumption, provides slightly leaner mixture. But, since the LPGs of different compositions also have a different specific gravity, this results in a different consumption by weight for the same mixer setting.
Since it can be considered that at a constant speed the amount of air required by the engine is also constant, a different air / fuel ratio will correspond to each gas flow. As a result, for LPGs of different compositions, different consumptions and yields will be obtained, which does not detract from the fact that with an adjustment of the mixer adapted to each type of gas, a maximum power will always be recorded with a minimum of consumption.
Assuming therefore that the use of LPG causes a loss of power of around 12%, liquid gas installations do not allow less to obtain, if they are properly regulated, a lower specific fuel consumption, that is, that is to say a greater number of horses per kilo of LPG.
Mechanical advantages of LPG
Apart from the exclusively economic factor, another reason should make the use of LPG preferable to that of gasoline: it ensures a longer engine life of about 50%
- Its combustion being more complete than that of liquid fuels, it results in a reduction of deposits in the combustion chamber and on the pistons: the flexible operation, without detonation, results in better working conditions of the connecting rods, the bearings and the ancillary organs.
- The gaseous nature of the fuel as it enters the engine eliminates the action of washing the walls of the cylinders during high acceleration phases, with an appreciable reduction in the wear of the liners of the cylinders, pistons and rings.
- Valves and candles, despite the higher operating temperatures, also have a longer life.
All these factors make it possible to space out periodic engine overhauls, which can increase normal operation by 50 to 200%. The fact that there is no washing of the cylinders by the fuel prevents the dilution of the lubricant, and it is thus possible to space the oil changes much longer.
Precautions with LPG
If the supply of LPG causes an increase in the viscosity of the engine oil, it causes, on the other hand, a greater oxidation of the lubricant because of the heat released, higher than with gasoline and favored by the absence insulation on parts (deposits on the piston head)
To avoid a decrease in efficiency, it will therefore be necessary that the LPG engine be lubricated with an oil less viscous than that used for gasoline engines - for example an SAE 30 instead of an SAE 40 - and that the restoration of the level or carried out with SAE oils with a viscosity of about one unit less than that of the oil used after the change.
In return for the advantages presented by the LPG, corresponds a greater wear of the seats of the valves, which results in a lack of play of the pushers and the toasting of the valves, which remain partially open.
This phenomenon is more noticeable when the engine is lubricated with oil that does not contain ash and organometallic additives. When switching from gasoline to LPG supply, the use of spark plugs with a cooler thermal value is necessary because, if with the gasoline supply the internal walls of the cylinder and the gas chamber explosion are sprayed with very fine droplets and, therefore, cooled, this phenomenon is less noticeable with LPG supply which causes greater heating of the explosion chambers and spark plugs: it follows the formation of a less efficient spark . Optimal operation can be restored by precisely using cooler candles.
The supply circuit of an engine running on liquid gas consists of a tank, a filter, a pressure regulator, a vaporizer, a carburetor and the corresponding pipes.
The sample is taken by means of a tubing plunging into the bottom of the tank, where the gas is always in the liquid state. The upper part contains only vapors that would not allow the engine to run at high speeds.
Finally, if the LPG was taken from the upper part of the tank, the composition of the remaining liquid gas would gradually be enriched with butane, due to the more rapid evaporation of the propane. This would result in a decrease in pressure in the tank and a decrease in the octane number of the fuel. By drawing liquid LPG from the bottom of the tank, the mixture therefore remains practically constant. The LPG passes through a first filter then passes, still in the liquid state, into the high pressure part of the regulator (primary regulator) where the pressure is reduced to values varying between 0,3 and 0,7 kg / cm2, against 10 to 14 kg / cm2 in the tank.
It then passes into the “vaporizer” (generally incorporated into the pressure regulator): this is a coil submerged in hot water coming from the engine, in which the LPG will turn into gas.
This gas then enters the low pressure part of the regulator (secondary regulator), which brings the pressure to a value slightly lower than atmospheric pressure (about 5 mm of water) The regulation of this depression is fundamental to achieve a correct dosage fuel in the carburetor. The regulator will be sensitive to variations in atmospheric pressure and temperature acting in such a way that the final pressure is always slightly lower than atmospheric pressure to prevent gas from escaping freely into the atmosphere during engine operation.
From the secondary regulator, the fuel will pass into the carburetor where it will be mixed with the air which will be drawn into the intake duct.