What is CNG? How does it work? Everything you need to know about the fashionable fuel

Of all the fuels used to supply the energy that drives a car, compressed natural gas for vehicles is perhaps the least well known, even though it is the same gas that many Spanish households use to heat their homes and light their stoves. A very clean gas which, of all fossil energy sources, is by far the one that poses the fewest environmental problems and, especially, to the air we breathe.

Natural gas for vehicles, both in its gaseous form (compressed natural gas, CNG) and in liquid form (liquefied natural gas, LNG), is a fossil fuel extracted from deposits that need not be linked to petroleum. It is the fossil energy source with the least negative impact on the environment and, according to the latest data, the world has natural gas reserves sufficient to cover global consumption for 537 years. Reserves in Europe alone could cover consumption for 161 years. Best of all, many reserves remain to be discovered and many more to be brought into production.

According to various forecasts, it is estimated that, in the medium term, between 25 and 50 percent of diesel engines will be replaced by engines that run on natural gas as fuel. For now, of the more than 70,000 urban buses serving the main European cities, almost 13 percent (more than 9,000) already run on CNG, and of the 20,000 refuse trucks, more than 15 percent (around 3,000) also use it. All of this brings not only advantages in terms of emission reduction, but also in terms of noise pollution, since vehicles that use natural gas for propulsion are much quieter than the conventional diesel engines they replace. Natural gas for vehicles is currently used by more than 20 million vehicles worldwide: passenger cars, vans, trucks, buses, forklifts, trains and ships all move using natural gas as their energy source.

EU directives are clear on this. In all those relating to transport, member states of the Union are told that, by 2020, the main routes in each country should have CNG filling stations installed every 150 kilometres and LNG stations every 400 km. The former should serve passenger cars and light vehicles, while the latter are more oriented toward serving heavier transport. In Spain the forecast is to have 300 gas stations or service stations supplying CNG or LNG by 2020. According to the latest data, our country already has 57 natural gas vehicle stations open to the public — which should number almost 100 before the end of this year — and 6,100 vehicles equipped with this technology are on the road.

With the promotion of alternative energy sources in general and of natural gas for vehicles in particular, the European Union seeks not only to reduce Europe’s traditionally high dependence on oil, but also to improve the air we breathe every day in our cities and with it the health of citizens, because natural gas for vehicles is, after electricity, the cleanest energy source currently operational.

Differences between CNG and LPG

Many people confuse natural gas for vehicles with LPG (liquefied petroleum gas) when the two have nothing to do with each other. Natural gas is methane, whereas LPG, or Autogas as it is commercially known, is a mixture of gases, mainly propane and butane. Natural gas for vehicles can be found both in gaseous state (CNG) and in liquid state (LNG), whereas LPG is always offered in liquid state. This is an important difference, both in terms of emissions and consumption and in terms of safety.

In general, vehicles that use compressed natural gas (CNG) run on internal combustion engines with spark ignition, although there are also models using dual-fuel engines that run on diesel, the only difference being the fuel storage and supply mechanisms.

Due to the characteristics of natural gas, the engine delivers slightly less power (12.7% less than a petrol engine), but it allows adjustments to optimise its operation. Thus, to avoid power losses the compression ratio can be increased thanks to the excellent anti-knock behaviour of natural gas (130 octane), so that effective pressure rises and performance improves. It also allows the ignition to be advanced, improving cyclic dispersion. Moreover, the formation of a more homogeneous mixture leads to engine behaviour with less vibration and, consequently, less noise and greater durability. The fuel consumption is therefore lower than that of petrol, with optimal engine operation at lower revs and higher load levels. This reduction in engine speed means a gearbox with a smaller reduction ratio is needed.

Efficient, safe and low emission

Unlike LPG, CNG is always in gaseous state, whereas, as its name indicates, LPG is in liquid state — as is LNG — until, on contact with the atmosphere, it turns back into gas. This is very important. CNG is a very volatile gas, that is, it disperses very easily in air, so its flammability rate is much lower than that of other fuels; in fact it is the lowest of all those currently marketed for car use. This means that in the event of an accident, a car running on diesel, petrol, kerosene or LPG will catch fire before one running on CNG.

Very often, we hear CNG’s detractors talk about the risk of explosion that carrying a gas tank on board supposedly entails. It is a myth. As we have already mentioned, a natural gas vehicle has the same or even lower risk of ignition or explosion than a conventional vehicle. If the information were true, how many taxis or buses would have exploded or burned by now? These types of vehicles have been running for many years with dual CNG-petrol engines without incidents to date. And some of them have done so since the last century.

CNG is in gaseous state and to store it in the car it is kept under high pressure, because otherwise the energy it would provide in the car’s tank would be very little. To put it in perspective, one cubic metre of CNG is energetically equivalent to 1.1 litres of petrol, while one kilogram of CNG is equivalent to 1.5 litres of petrol. The CNG tank of a car can hold up to 15 kg of this gas, although for safety reasons only 11 are normally stored.

CNG also has important advantages in terms of emissions, as it is the fuel that produces the fewest waste emissions, as can be seen in the attached chart.

Fuel cost

Price is one of the main differentiating factors between natural gas for vehicles and petrol. One of the factors with the greatest influence on the price of both fuels is the hydrocarbon tax levied on fuel use, which is far more lenient on natural gas than on traditional fuels. For natural gas this tax amounts to approximately €0.05 per kilogram of CNG, whereas in the case of petrol the hydrocarbon tax ranges from €0.40 applied to each litre of 95-octane unleaded petrol to €0.43/litre on 98-octane unleaded. Natural gas for vehicles costs around €0.93/litre, while the price of a litre of petrol ranges between €1.15 and €1.58/litre. A mid-range car consumes around 3.5 kilos of CNG per 100 kilometres of use. As each kilo costs €0.93, this amounts to an approximate fuel cost of €3.25.

For a car with a useful life of 200,000 kilometres, the fuel saving with CNG would be €1,800 per year compared with diesel, €5,560 per year compared with a petrol engine and €1,160 per year compared with LPG. The difference is considerable.

Purchase grants

Being a clean energy helps. The purchase of vehicles using natural gas for vehicles as their propulsion source is subsidised through various types of aid from the different public administrations. Thus the Institute for Energy Diversification and Saving, IDAE, provides different types of aid for the transport sector when renewing vehicles, including financial aid for users as well as for companies interested in creating charging stations. These aids are specified in the MOVEA plans (Plan to Boost Mobility with Alternative Energy Vehicles), whose main objective is to encourage the purchase of vehicles with alternative fuels, allocating for this purpose a budget of €16.6 million.

In this way, passenger cars running on natural gas had — under MOVEA 2016 — a €3,000 grant from the Government, plus €700 extra if you scrapped an old vehicle and a further €1,000 from the dealer. For vans and light commercial vehicles, the grants ranged between €3,500 and €6,500, and for buses and trucks between €11,000 and €21,000.

These figures were those set by the last MOVEA Plan, which remained in force until the funds allocated to it ran out. MOVEA was followed by the MOVALT plan, specifically aimed at boosting sales of alternative-energy vehicles with similar or even higher aid than MOVEA, ranging from €500 to €19,000. This plan initially had a budget allocation of €35 million. We are currently awaiting approval of the State General Budget to know the definitive allocation of the new plans for 2018, although everything suggests that the new MOVALT will have a total of €50 million.

In addition, these grants do not prevent beneficiaries from also receiving those offered in a complementary way by some regional and local administrations on top of those provided by the Government and car manufacturers. Depending on the region and the municipality, these range from additional price discounts to exemptions from parking fees on the street, or the possibility of driving in traffic-restricted zones during pollution episodes.

Another advantage of natural gas vehicles is that they can reduce by 20 percent the value of the car in Personal Income Tax for professional-use vehicles that are energy efficient or can use alternative fuels such as CNG and comply with Euro 6 emission limits.

Finally, it should be remembered that, as the vast majority of vehicles using CNG as fuel have very low emissions, those that do not exceed 120 g/km of CO§h267Lanciah268§ will be exempt from the registration tax.

Maintenance

The service intervals set by manufacturers for correct maintenance are very similar for cars with conventional combustion engines and for those equipped with CNG engines, if not exactly the same.

That said, the European regulation governing these models, ECE R110, requires an annual inspection of the fuel tank and, every four years, another inspection of the gas lines, in order to verify that there are no gas leaks and that the safety valves work correctly. The rest of the car is not subject to such a limitation, but the gas storage tanks on board, for which a maximum useful life of 20 years is established, must be replaced with new ones after that period.

Price

Vehicles equipped with engines that can use CNG as fuel are slightly more expensive than their petrol counterparts, although in no case are the differences as large as those for electric vehicles compared with equivalent petrol and diesel versions. These differences amount to around €2,000 to €2,500. The good news is that these differences can be offset, at least in part, thanks to financial aid from both central and regional government. Likewise, the commercial promotions that some manufacturers run for their models with this technology bring their price fully in line with the equivalent diesel or petrol variants.

In any case, to amortise the price difference between a CNG vehicle and a petrol one simply by taking into account the savings obtained from the lower cost of a kilo of CNG compared with petrol, the driver of a gas-powered vehicle would need to cover approximately 82,000 km, a figure that at normal usage rates would take about four years.

Range and refuelling

Among the greatest concerns of those interested in new energy sources for cars are two that stand out above all: the range of the model and its refuelling or recharging. In this respect, CNG cars come out especially well, since refuelling is practically the same as for a conventional car, and one could even say it is safer and takes the same time. Simply connect the “pump hose” to your car and in just 2-3 minutes the operation is complete. The hose is not activated manually; instead, a customer card must be swiped so that the system can check that it is properly fitted and dispense the requested amount of gas.

Supply is guaranteed. Spain is the European country with the greatest experience in CNG transport and logistics, despite not being the main consumer. The country currently has a fleet of 240 tankers in the distribution network, surpassed only by China — 4,000 — and Japan — 600. Moreover, despite the small size of the CNG vehicle fleet in Spain, the existing network of stations, although limited, is sufficient to cover the main destinations on our road network.

There are currently 34 service stations open to the general public (there are more but they are for private use by companies) for the 4,400 passenger cars and light commercial vehicles that use this fuel in Spain, and it is expected that by 2020, when the CNG vehicle fleet reaches 17,200 units, there will be a total of 85. Of these, 20 are already under construction and 31 more are at the planning stage. As can be seen, this time both companies and administrations have done their homework and are working together on the development and roll-out of this fuel in our country, which will undoubtedly see a far faster introduction than that of unleaded fuels, which caught many parts of the country off guard.

Currently, natural gas supply companies are working to obtain the permits and approvals that would allow the installation of supply points at the user’s own home through a device the size of a boiler connected to the natural gas network. In this way, both communal and private garages could have charging points where their users could easily refuel their cars.

Advantages of CNG

• Significant reduction in emissions, especially of the most harmful ones (CO2, NOx, CO, HC, SO2) and practically no emission of harmful particulates (PM10).
• Significant energy saving and, consequently, lower fuel consumption.
• By operating at fewer rpm than a petrol engine and generating less vibration and waste, CNG engines last longer.
• Thanks to lower polluting emissions, maintenance is also cheaper. Oil changes are spaced further apart, and the whole exhaust and gas outlet system suffers less wear.
• Natural gas has 50% less tax burden on the hydrocarbon tax. Its low price allows rapid amortisation of any price difference in purchasing the vehicle (especially for fleet cars with more intensive use).
• In addition to possible government grants for buying a new car (the PIVE Plan), CNG vehicles add other tax incentives and aid, such as those provided under the MOVEA plan (up to €3,700 for a passenger car), the 20% deduction on benefits in kind for income tax, etc.
• CNG vehicles are even safer than conventional ones, since in contact with the atmosphere, compressed natural gas presents lower chances of ignition and detonation than petrol.
• CNG engines emit less operating noise and vibration than petrol and, especially, diesel engines.
• The CNG refuelling system is more efficient than that of traditional fuels. Losses from fuel volatilisation that occur in hoses are eliminated, since these are completely sealed.
• Natural gas reserves are more abundant than oil reserves (guaranteed supply for around 160 years in Europe). Engines can run on both biogas and biomethane, and thanks to the bifuel system CNG can also be used in petrol engines. This last option is currently the most widely accepted in Europe.

Disadvantages of CNG

• Storage in gaseous state requires larger-volume containers, which means that, especially in converted cars, part of the boot space must be given over to housing them. If liquid storage is chosen, the vehicle’s range will suffer.
• Due to the high pressure of the storage tanks, the tanks and their lines require specific tests carried out annually to verify their correct condition. These tests are compulsory for vehicles adapted to run on natural gas. If the car is originally a CNG vehicle, i.e. it leaves the factory as such, inspections follow the plan recommended by the manufacturer and the scheduled ITV calendar, the same as for a conventional car.
• Using CNG can cause problems with the three-way catalytic converter. The lower exhaust gas temperature produced by natural gas compared with petrol may mean that the three-way catalytic converter does not reach its optimum operating temperature, accelerating its ageing.
• The supply infrastructure for this type of fuel in Spain is still insufficient. There are barely 42 CNG supply points in the whole country.
• Slightly higher vehicle price (+/- €2,500) than its petrol or diesel equivalent

CNG cars already on sale