CNG

Article

January 27, 2022

CNG (Compressed Natural Gas) or compressed natural gas is a mixture of high pressure stored hydrocarbon gases used as fuel for motor vehicles as well as for heating. (Due to the different compressibility of the natural gas components, gaseous CNG is already pure methane, as the other components - water vapor, ethane, propane, butane, etc. - are already liquefied at a pressure of 200 bar and accumulate at the bottom of the high-pressure tank.) Natural gas is delivered via pipelines to household and industrial consumers. The gas used as a motor fuel is essentially of the same quality as that used in the household, but the gas coming from the mains is usually still dehumidified at the filling stations, so less water enters the fuel system of the vehicles. Prior to demisting, the gas is compressed to 220 bar with a high-pressure compressor (which may vary slightly from country to country) and refueled in vehicles. The pressure of the natural gas in the CNG cylinder changes due to temperature fluctuations, but this can be ignored for engine operation. In vehicles, gas tanks are usually cylindrical or very rarely spherical in shape to apply uniform pressure to their walls. The nominal pressure is usually 220 bar. The operation of the engines is not affected by the pressure, but by the calorific value of natural gas, this may vary from settlement to settlement, but within a settlement the difference may not exceed 5%.

Composition

Natural gas consists of various hydrocarbon compounds. It usually contains 90% methane (CH4). It contains ethane (C2H6), propane (C3H8), butane (C4H10), carbon dioxide (CO2) and nitrogen (N2).

Properties of CNG

Some properties of gas depending on composition at atmospheric pressure relative density: 0.55 to 0.71 (according to ASTM D-3588 relative to air) density: 0.7-0.8 kg / m³ (at storage pressure 128 kg / m³) molar mass 0.016043 kg / mol Auto-ignition temperature: 480-640 ° C lower and upper inflammatory concentration limit (in terms of volume fraction): 5-15% flame propagation speed: 0.33 m / s lower calorific value: 0.028-0.041 MJ / L lower calorific value: 47,000 kJ / kg upper calorific value: 52,000 kJ / kg The amount of air required to burn 1 liter of natural gas is theoretically: approx. 9.5 liters octane number: approx. 120 energy density in the tank: 46.5 to 49 MJ / kg (6.7 to 7.9 MJ / L) Proportional weight ratio of propellant to air: 17: 1 (theoretical air demand for combustion) boiling point: −163 ° C freezing point: -183 ° C critical temperature -89 ° C specific heat capacity: 2.18 kJ / kg × K

Technology

CNG can also be used in Otto engines (petrol) as well as Diesel engines. Lean gas Otto engines can achieve higher efficiencies compared to stoichiometric Otto engines at the cost of higher NOx and hydrocarbon emissions. Electronically controlled stoichiometric engines deliver lower emissions and the highest possible power output, especially in combination with EGR (when part of the exhaust is returned to the intake manifold), turbocharging, intercooler and three-way catalytic converter, but with higher fuel consumption , and their heat recovery is lower. A properly designed compressed natural gas engine offers the highest shaft power compared to a gasoline engine because the octane number of compressed natural gas is higher than that of gasoline. CNG tanks can be filled at low pressure (so-called "slow filling") or high pressure (so-called fast filling). The difference is the price at the charging station and the charging time. Nowadays, the technical possibility is already available for overnight charging via the normal gas network,

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