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Keywords: 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 production of this fuel is derived from crude processing in refineries and separation (outgassing) of natural gas (methane ethane).

liquefied petroleum gas may also contain small amounts of methane, ethylene, pentane and pantènes and exceptionally hydrocarbons such as butadiene, acetylene and methyl acetylene.

These hydrocarbons are present only as byproducts of the production of olefins petrochemical use. In addition to hydrocarbons, also sometimes find sulfur compounds (mercaptans and alkyl sulfides) in extremely small amounts, but have a certain importance with regard to the corrosiveness of the product.

Main characteristics

LPG is easily liquefy gas at room temperature under low pressure (4-18 atmospheres): this allows storage and transportation easier for non-condensable gases such as methane, ethane, ethylene which require very high pressures to be liquefied at ambient temperature.

· Refined LPG are normally almost odorless and highly flammable, given their volatility. They can give, upon contact with air, explosive mixtures. To better recognize or detect any leaks, given a particular odor by appropriate substances (mercaptans).

  • LPG are not really toxic: they have at most a slight numbing power, if inhaled long and can cause headaches and stomach aches.

  • LPG, when it spreads in its liquid form, out of a container under pressure, evaporates while producing cold: in contact with the skin, it causes characteristic burns called "cold burns".

The physicochemical characteristics of LPG (distillation curve, vapor pressure, density, calorific efficiency in engines, etc ...) depend on their content of various hydrocarbons.

Commercial products are very different from each other. In addition, their vapor pressure, their density and their antiknock properties are very sensitive to changes in ambient temperature. The calculation methods of octane are recent (ASTM-CFR engine in operating conditions Motor Method ASTM D 2623).

Tests have shown that 92 index should be considered the minimum value to power the cars that use this type of fuel. LPG containing olefinic hydrocarbons (particularly propylene) may give rise to phenomena of detonation and pre-ignition even more sensitive than their content of olefinic hydrocarbons is greater and the engine compression ratio is higher .

One can say the same for LPG with a high content of n-butane. In this regard, the NGPA, body in America unification of standards, provides that the GPL (specification HD-5) must contain a maximum of 5% by volume of propylene.

Comparative face petrol

The calorific value of LPG is almost equal to that of gasoline when expressed in kilocalories per kilogram of fuel but these values ​​will be very different if expressed in kilocalories per liter of liquid fuel 15 ° C.

This diversity comes from the density difference between LPG and petrol: on average, the density 15 ° C LPG is 0.555 kg / liter and of gasoline 0.730 kg / liter. An engine fueled with petrol develops a higher power to 10 12% but also charges a higher specific consumption and overall efficiency below an engine powered by LPG.

Calorific values ​​of the two fuels is practically equal, the power decrease seen with the GPL is due to a lower cylinder filling, the causes are:

  • The presence of a mixing between the air filter and the carburetor (the pressure drop in the inlet duct causes a power decrease to 5 6%). Adequate development of the gas inlet, obtained by perforating the carburetor and applying a spray that sends directly in the narrowest section of the venturi, would significantly reduce the power loss.

  • A warmer mixture, and therefore less dense, because vaporization of the LPG takes place in a reducing-spray. The fuel arrives already hot in the carburetor while the air / fuel mixture undergoes cooling due to the latent heat of vaporization of gasoline. The power loss is recorded in the order of 5-6%, on the other hand, it is inevitable given that to ensure a constant air / fuel ratio, the feeder should send the GPL when they are at the gaseous state in the narrowest section of the carburetor.

Better performance for LPG

The increase in the overall yield of LPG compared to gasoline can be explained by better combustion due to greater homogeneity of the gas / air and that the blender setting, performed in order to achieve maximum power with minimum consumption, provides slightly leaner mixture. But, given that LPG different compositions also have a different specific weight, it follows a different weight for the same consumption mixer setting.

Since we can consider that at a constant speed the amount of air required by the engine is also constant, an air / fuel ratio different match each gas flow. The result is that for LPG different compositions will be obtained different consumptions and yields which does not alter the fact that with an adjustment of the mixer adapted to each type of gas, maximum power is always register with a minimum of consumption.

Admitting that the use of LPG causes a power loss of about 12%, liquid gas facilities do not allow to get less, if properly regulated, a lower specific fuel consumption is -to say a greater number of horses per kilo of LPG.

LPG mechanical advantages

Regardless of the exclusive economic factor, another reason should prefer the use of LPG than petrol: it provides superior engine life of about 50%

  • Its combustion is more complete than that of liquid fuels, resulting in a reduction of deposits in the combustion chamber and the pistons: flexible operation, without detonation, resulting in better working conditions, connecting rods, bearings and ancillaries.

  • The gaseous nature of the fuel as it enters the engine cylinder walls eliminates the washing action during the thrust acceleration phases with an appreciable reduction in wear of cylinder liners, pistons and piston rings.

  • Valves and spark plugs, in spite of the higher operating temperatures, also have a longer duration.

All these factors help to space out periodic reviews of the motor, the normal operation can be increased so has 50 200%. The fact that there is no washing of the cylinders by the fuel prevents the dilution of the lubricant, and it is thus much more space drain.

Precautions with LPG

If LPG fueling causes an increase in viscosity of the engine oil, it causes, by cons, greater oxidation of the lubricant due to the heat, higher than with the gasoline and favored by the absence insulation on parts (deposits on the piston head)

To prevent a reduction in yield will therefore require the engine to LPG lubricated evening with a less viscous oil as used for gasoline engines - eg SAE 30 40 instead of SAE - and that recovery level is carried out with SAE oils of a viscosity of about one unit relative to that of the used oil after draining.

In consideration of the advantages of LPG is greater wear on the valve seats, which results in a lack of playing push and roasting valves, which remain partially open.

This phenomenon is more noticeable when the engine is lubricated with oil containing no ash and organometallic additives. When switched from gasoline to food to that of LPG, the use of candles to a colder heat range is necessary because, if with food gasoline internal walls of the cylinder and the chamber explosion sprayed with very fine droplets and thus cooled, this phenomenon is less noticeable with LPG fueling what causes more warming of the combustion chambers and candles: it follows the formation of a less efficient spark . Optimal operation can be restored using precisely colder plugs.

The LPG system

The power supply circuit of an engine running on liquid gas consists of a tank, a filter, a pressure regulator, a spray, a carburetor and corresponding pipes.

Sampling is effected by means of a tube dipping tank bottom, where the gas is still in the liquid state. The top portion contains only vapors that would not allow the engine to operate at high speeds.

Finally, if you were to scrape LPG in the upper part of the tank, the composition of the liquid remaining gas would be enriched gradually butane, due to more rapid evaporation of propane. Thereby yielding a decrease in pressure in the reservoir and a reduction in the octane number of the fuel. Drawing the liquid LPG to the tank bottom, the mixture remains virtually constant. LPG through a first filter and then passes, always in the liquid state, in the high pressure part of the regulator (primary controller) where the pressure is reduced to values ​​ranging between 0,3 and 0,7 kg / cm2, against 10 14 kg to / cm2 in the tank.

He then switches to the "spray" (generally incorporated in the pressure regulator): it is a coil immersed in hot water from the engine, wherein the LPG will turn into gas.

This gas then enters the low pressure part of the controller (secondary regulator), which lowers the pressure to a value slightly lower than the atmospheric pressure (5 mm of water) The regulation of this depression is fundamental to achieve a correct dosage fuel into the carburetor. The controller will be sensitive to changes in atmospheric pressure and temperature acting so that the final pressure is always slightly less than atmospheric pressure to prevent the gas from escaping freely into the atmosphere during the engine operation.

The secondary regulator, the fuel pass into the carburetor where it would be mixed with air to be sucked into the intake duct.


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