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Saturday, May 27, 2017

LNG – Liquefied Natural Gas

Posted by Krzysztof Lis on January 30, 2014

LNG, or liquified natural gas, is natural gas in a liquid state. It has been cooled to cryogenic temperatures, and because of that in atmospheric pressur it is in liquid state.

LNG can be used as car fuel, as well as CNG (compressed natural gas) can. Both these fuels have significant advantages over gasoline and diesel fuel.

Because of the liquid state, LNG is more than twice as dense as CNG. It needs to be stored in insulated tanks because of the low temperature, but under pretty low pressures, as compared to compressed natural gas. The latter requires over 200 times atmospheric pressure (3,000-3,600 psi, or 21 to 25 MPa), while LNG requires only 70-150 psi (0,5-1,0 MPa, 5-10 times the atmospheric pressure). LNG is supplied to the engine in liquid state and then vaporised, similarly to LPG (liquefied petroleum gas, propane-butane mix, also known as autogas). 

Liquefied Natural Gas in car engines

LNG can be used in spark ignition (gasoline-powered) engines, that can be modified to run as dedicated for natural gas, or bi-fuel. In some cases it is possible to convert the engine to tri-fuel capacity (for CNG/LNG, LPG and gasoline), but this is mostly used in generator engines, and not in cars.

Compression ignition (diesel) engines can run on LNG as well, but only to a certain point. LNG cannot be ignited by compressing it in the engine, so a small dose of diesel fuel (pilot dose) must be maintained in order for the engine to work. On the other hand, the engine can be retrofitted with spark ignition system.

LNG vs gasoline, diesel and LPG

LNG seems to wear out the engine parts slower than gasoline does.

Emissions from natural gas are cleaner, as this fuel consists mainly of methane (CH4), very simple hydrocarbon, easy tu burn clean and efficiently. Natural gas engines produce much less particulate emissions, unburnt carbon (soot) and carbon monoxide.

LPG has different chemical composition (mainly propane C3H8 and butane C4H10), is much denser as a liquid and can be a liquid in room temperatures under pretty low pressures. Because of that it’s much easier to liquify autogas than natural gas (it requires much less energy). And this means it’s significantly cheaper. For example for the CNG the cost of compression is 20% of total fuel cost!

On the other hand, LPG is heavier than air (methane is lighter), and this causes it to collect in low spots, making it more hazardous to use.

Liquefied natural gas distribution

Natural gas is distributed under pressure via natural gas lines. In theory, filling stations for LNG cars could be located anywhere, as long as there is the natural gas supply. In practice, liquification plants require enormous investments and because of that are built on large scale.

Because of that it is assumed that LNG will be distributed using tankers, and not produced locally.

LNG conversion kits

It’s possible to get conversion kit to LNG for gasoline or diesel engines in many countries, and to get a vehicle converted by a qualified professional.

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