The ecological alternative
The natural gas known as CNG (Compressed Natural Gas) is not to be confused with LPG (Liquefied Petroleum Gas). Natural gas comes from fossil deposits located between a thousand metres and several kilometres beneath the Earth’s surface and consists primarily of methane.
The distinction is made between single-fuel and duel-fuel compressed natural gas (CNG) vehicles, that is to say those running solely on CNG and those running on both CNG and petrol.
When you fill your tank, natural gas is stored in the form of a gas in a high-pressure tank at 200-240 bar, with the pipe connecting to the petrol tank located next to the gas connection. In the event that there is no natural gas station around you can still continue on your journey using your petrol reserve. For your safety, the gas and petrol tanks are well protected beneath the floor, with the gas tank having to be able to withstand pressures of up to 600 bar. In order to improve capacity with regards to the size of the tank, the gas is compressed at over 200 bar, hence the name ‘compressed natural gas’. Depending on the vehicle and the size of its tank, you will find you can travel 400-600km on natural gas, not to mention the fact that you can continue your journey using the petrol engine.
CNG is not measured in litres but in kilograms, with one kilogram of natural gas the equivalent of 1.3 litres of diesel or 1.5 litres of petrol.
A CNG engine works on a similar principle to a petrol engine, the difference being that high-density gas is reduced to around 5-7 bar by means of a pressure reducing valve so that it can be fed into the engine via a low-pressure pipe. The air is sent to the gas distributor and a combustible mixture of air and gas reaches the cylinders. A turbo can also be added in order to achieve sufficient engine power.
The benefits of gas-powered vehicles are obvious; indeed, CNG takes longer to burn, reducing engine noise by up to 50% in comparison with a diesel engine.
Natural gas emissions also contain almost none of the fine particles that are harmful to our health and up to 90% less nitrogen oxide than diesel vehicles, which is, of course, the main advantage of this form of energy. Furthermore, the CO2
emissions produced are around 20% lower than with a petrol engine, not to mention the fact that CNG mixes better with the air since it is already in the form of a gas. The consistent mixture creates less combustion residue and results in a reduction in deposits in the combustion chambers, which consequently improves the reliability of the engine. The already unquestionably beneficial characteristics of CNG emissions can be even further improved using biogas. Biogas is produced from organic waste such as green waste and sewage sludge and is almost CO2 neutral. Mixing it with traditional natural gas can further improve the CO2 footprint. With regards to cost, it is possible to save up to 40% for every 100km travelled in comparison with a petrol engine and up to 30% in comparison with a diesel engine. This results in significantly lower prices at the pump, since natural gas is taxed at a lower rate owing to its lower CO2
footprint. Natural gas is transported via underground gas pipes, meaning that it does not contribute to road traffic.
One of the drawbacks of CNG is the higher acquisition costs, with an identical vehicle costing up to three thousand euros more than it would with a petrol or diesel engine. Luxembourg currently has six service stations selling CNG. The main argument put forward by critics is that in France, for example, the network of service stations supplying natural gas is not very extensive, and the various accidents that have occurred whilst refuelling, during which the gas tanks of a number of vehicles have exploded, have certainly done nothing to encourage the use of the technology. These accidents were caused by rusted tanks that had not been changed in time, despite the reminders issued by the manufacturer of the vehicles. We can therefore deduce from this example that the accident was not caused by the natural gas technology.
Natural gas vehicles are clearly as safe as any other, since CNG does not burn easily; its ignition temperature is in fact 650°C, which is much higher than that of petrol or diesel (from 220°C). Furthermore, a series of safety valves prevent any build up of excess pressure within the gas tanks.
It is also important to bear in mind that natural gas is less dense than air (0.6), meaning that it can very quickly disperse into the atmosphere, which is why it is permitted to park natural gas vehicles in indoor car parks.
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