CNG - Compressed Natural Gas is another alternative fuel substitute for gasoline (petrol), diesel, or propane fuel. Its combustion does produce greenhouse gases so it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels. The natural gas used for NGVs (Natural Gas Vehicles) is the same natural gas that is piped to millions of homes for cooking and heating.

CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of its volume at standard atmospheric pressure. It is stored and distributed in hard containers, at a normal pressure of 200–220 bar usually in cylindrical or spherical shapes.

Natural Gas is quite simple fuel. Around 90% of natural gas is methane (CH4),which is just one carbon atom with four hydrogen atoms attached. The only simpler fuel is Hydrogen, however till this moment there is no economic methods of creating and distributing large quantities of hydrogen. As a result Natural Gas is going to remain important alternative source of clean energy for some incoming years.

Compressed Natural Gas is the most common application for NGVs though Liquefied Natural Gas (LNG) use is becoming increasingly common. Natural gas occupies more volume than traditional liquid fuels thus it must be compressed or liquefied to make it practical for transport applications.

When comparing fuel prices it is important to consider energy content of fuels. CNG is sold either by the kilogram or the cubic meter (m3) and LNG is measured in liters.

LNG - Liquefied Natural Gas is natural gas (primarily methane, CH4) that has been converted to liquid form for ease of storage or transport. This process involves removal of certain components, such as water dust, helium, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid by cooling it to approximately −163 °C (−260 °F).

The reduction in volume makes it much more cost-efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by specially designed cryogenic sea vessels (LNG carriers) or cryogenic road tankers. Octane value of LNG amounts to around 130.  

In comparison with diesel and unleaded petrol all alternative fuels described above are safer to our environment. Harmful substances are almost not present in liquid gases. Besides, in comparison with other kinds of fuel, the harmful exhaust fumes are significantly limited.

A correct assembling of gas installation is proven to be an absolutely safe way of a car operating. Moreover, all the components used in gas installations undergo a series of obligatory tests, which guarantee safety of their use. These results are approved by certificates. In the Europe it is full compliance with ECE67R01 (LPG) and ECE110R (CNG). Obviously, choice of an appropriate installation depending on the car type is a separate issue. 

Basically CNG is completely different fuel. Burning process is longer than in case of gasoline and LPG. Therefore there are special electronic device required to support CNG/air mixture burning. Authorized and qualified personnel surely know how to select adequate equipment. Another big difference is CNG container pressure that is much more higher than the pressure inside LPG Tank. Obviously in CNG installations due to this higher pressure only CNG dedicated accessories should be installed. Both types of installations are absolutely safe, provided by the equipment is installed in a car is certified and installed by qualified personnel. Please make sure about certifications that is obligatory in your country.

The main and quite visible difference is lower cost of GAS car operating usually in the region of 50%. Besides, the easily seen difference is an outer gas cap and a petrol/ gas switch. In case of injection cars it is enough to press the accelerator pedal, and the vehicle will automatically switch to gas. The most modern IV generation systems such as SGI VERSUS system are almost completely automatic. Everything was automated to make the drive more comfortable. Here, the only difference is a regular servicing (average once on 10,000 km) 

a) Proper GAS powered vehicle operating guarantees benefits of the gas installation. First of all the most important thing is to care for a technical condition of a vehicle and especially these elements that have a direct influence on proper working of a gas installation, these are: ignition system (spark plugs, coils, high- voltage wires), intake manifold - especially air filter. In case of fuel the ignition system is subsidiary, whereas when it comes to gas this is the main cause of GAS installation’s problems. Because of its properties GAS burns slower than petrol so any spark jump at an open valve leads to a mixture detonation in the intake manifold (in case of I and II generation installations). It is worth noticing that the problems described above do not concern IV generation installation, where the risk of fires or any engine damages resulting from failure in ignition system, simply do not exist

b) Lubricant. Engine running on GAS does not require using any specific oil and most available oils are appropriate to be used in LPG powered engines (there are also purpose - built oils). Unlike petrol, the propane-butane mixture does not cause washing away of the lubricating layer from the cylinder walls. Because of gas combustion there are no carbonation problems. Thanks to this phenomenon color of the exchanged oil, when the engine runs on LPG, remains basically the same. 

c) Also a very important matter is a systematic gas installation servicing - gas filters replacement - of liquid and gaseous state filters, reducer cleaning, etc. Inspections should be made in specialized workshops only by qualified staff.

d) Another and not less important issue is starting the engine on petrol and switching to gas at a proper engine temperature (recommended 30-40°C). It is important especially in case of LPG. If the switching to gas is immediate and the reducer body has not reached a proper temperature, LPG is not able to evaporate; furthermore, collecting the heat from the environment resulting from gas evaporation may cause freezing of the reducer. It usually does not concern the Sequential Gas Injection where the engine automatically switches to GAS at a proper (earlier programmed) temperature reached by the reducer.

 Studies over GAS usage and its influence on the engine life have been conducted for many years. They have shown that GAS does not shorten the engine life; on the contrary, it is established that engines running on GAS last longer. It is caused by chemical components of this fuel, which is seen during combustion (less pollution). GAS does not lower oil density; on the contrary, it increases its viscosity, which reduces wear of the engine (pistons, rings, cylinder head’s walls, etc.) Generally, GAS burns more completely and does not cause carbon deposit. However, there is a small disadvantage in case of 12- 20 year old cars. Namely, the carbon deposit makes kind of a ‘sealing crust’ from the inner side of the older engine and when it runs on GAS this ‘crust’ melts and may make a small oil leak bigger. Obviously, a proper installation and consequences of the ‘acetylene blowpipe’ phenomenon is a separate issue. So the installation must be appropriately selected depending on the car type. Its appropriate selection and later operating does not have any negative effects on the engine life.

 In case of LPG tanks it holds 80% of the amount. So when its capacity is 40 liters it will hold around 32 liters. It results from physical and chemical properties of an LPG gas (a mixture of propane- butane and isobutane) - LPG when being heated increases its volume. This guarantee safe use of the LPG installation almost in 100%. Tests on LPG tanks show that the temperature of the LPG over 80% causes that the tank is filled completely because of the volume increase; however, this temperature is almost impossible to reach in a standard use of the LPG installation. As for an automatic switch to petrol - IV generation system - Sequential Gas Injection is equipped with appropriate pressure sensors, also on the supply GAS Injectors. Driving when the tank is almost empty is impossible both with LPG and CNG - when the pressure on the gas panels falls, the engine automatically switches to petrol to prevent poor mixture phenomenon. It prevents additionally from an excessive heating of valves, valve seats, and a cylinder head.