Editor's choice
17 Jan 20

Should you replace diesel by CNG in your fleet?

Compressed natural gas has a lot going for it. It can cut fuel bills by up to 40% and it creates less emissions than diesel. So what ‘s not to like? We compared both fuels on different aspects and came up with these pros and cons to help you decide whether you should consider CNG as an alternative for diesel in your fleet. 

1.    CNG is cheaper than diesel. The average price is about 1 euro per kg. The fuel consumption is comparable to that of a diesel, so you basically look at the fuel price to calculate the costs you can save.
2.    It helps reduce CO2 emissions. The difference with diesel in terms of tailpipe emissions may be limited, but if you take into account the production and the transport of the fuel, natural gas wins hands down. Gas pipes are everywhere and supply the filling station directly, whereas diesel needs to be refined and then transported by truck.
3.    CNG creates less particulate matter. Diesel cars come equipped with particulate filters, but these still create temporary pollution spikes when they regenerate. The regeneration takes place every 500km, more or less.

1.    The number of available models is quite limited, even though VW Group has been expanding its offer. You can get a natural gas engine in the A, B and C segment models of Seat, Skoda and Volkswagen, and even in the C-segment Audi A3 and the D-segment Audi A4 Avant and A5 Sportback.
2.    Limited fuelling infrastructure. Here too, things are changing for the better. Today, outside Austria, Belgium, Czech Republic, Germany, Italy, Netherlands and Sweden, the European distribution network is pretty meagre, but the EU is investing €160 million in the development of CNG stations, so that they will be located at a maximum distance of 150 km all over Europe by 31 December 2020. Especially France and Spain are catching up fast.
3.    Less range than diesel. The bulky CNG tanks can only hold so much compressed gas. In the case of the Skoda Octavia, for instance, you can drive some 400km on 17.7 kg of CNG realistically speaking. When the engine switches to petrol, you have about 150km to find a fuel station; the petrol tank only holds 11,8 litres.

CNG: how clean is it to produce?

Natural gas comes in two shapes: fossil natural gas, i.e. the gas that can be found in nature, and ‘synthetic’ natural gas (methane) that is produced artificially.

The global reserves of fossil natural gas exceed those of oil (>200 years). New discoveries are made regularly. Natural gas is extracted from underground gas fields, located at various depths. While extracting natural gas, some of it escapes in the atmosphere. Methane, which makes up between 80 and 90% of natural gas, is a far more aggressive greenhouse gas than CO2, so leakage is a big issue and compromises the green credentials of the fuel.

“In America, most gas is extracted through fracking of shale gas (i.e. hydraulic fracturing of rock to get to the trapped gas; editor’s note). Taking into account extraction, transport and storage, the leakage rate is estimated at 2.5%. In Europe, the average leakage rate is between 0.26 and 0.31%, depending on the actual gas mix, i.e. the combination of gases coming from Norway, the Netherlands, Russia, Qatar, and so on ”, explains Maarten Van Houdenhove, PR manager at Belgian fuel network DATS24 (Colruyt Group). “Even with part of the gas escaping into the atmosphere, CNG is a cleaner fuel than diesel or petrol if you look at the whole picture. For consumers willing to do something about the environment and at the same time watch their budget, it offers the best possible balance between ecology and economy.”  

As to non-fossil, i.e. synthetic natural gas: it makes the picture even better. It can be made in two different ways. The first method is to refine biogas produced by the natural breakdown of organic material in waste from agriculture, sewage or food. This makes it sustainable and carbon neutral. The same goes for the second method, which consists of combining CO2 with hydrogen. The hydrogen can be made with surplus energy from wind farms or solar panels, and this energy can also be used to create the necessary heat and pressure to methanize the hydrogen, but the result is a less favourable ‘green’ balance than with biogas.

Synthetic natural gas can be injected into the distribution grid or used directly by CNG vehicles. In the latter case, the CO2 balance becomes unbeatable, but unfortunately the number of installations that distribute pure green gas is very limited. Also, the amount of green gas injected in the grid is neglgible today.

What about NOx?

Nitrogen oxides (NOx), or the difficulty to control them in the case of diesel combustion, is what caused manufacturers like VW to install cheating devices. In actual fact, Euro 5 diesels emit tens, sometimes hundreds of times more NOx in real life than what the emission standard allowed.

A lot has changed since Dieselgate and with the introduction of WLTP and RDE (real driving emissions), the Euro 6d-temp emission standard now keeps NOx emissions effectively below 80mg/km. Recent research shows that the latest diesels from BMW and Mercedes emit less than 20 mg/km. Today, diesel is the culprit of NOx pollution, but tomorrow, modern diesel engines can even clean the air in highly-polluted areas, emitting less NOx than they absorb from the ambient air.

Euro 6d-temp and Euro 6d diesel vehicles seem to have cleaned up their act in terms of nitrogen oxides, but cold starts and high loads are still a challenge. Not so with CNG, which gets an “all clear” for NOx. It does have other issues, though: natural gas powered cars emit more carbon monoxide and unburnt hydrocarbons during the first minutes of operation, when the catalyst hasn’t reached its operational temperature.


Authored by: Dieter Quartier