CNG (Compressed Natural Gas)

CNG stands for compressed natural gas, which is used as a fuel for motor vehicles. The advantage of running on CNG is its cost and lower environmental burden.

CNG gas station logoThe abbreviation CNG comes from the English Compressed Natural Gas, which translates to compressed natural gas. Natural gas is considered a cleaner and more ecological alternative to gasoline or diesel, as it contains about a quarter less carbon. For example, converting a diesel engine to CNG reduces emissions to the level of Euro 5. CNG is a fossil fuel, a mixture of hydrocarbons with methane as the main component (96 – 98% by volume). Natural gas is obtained from natural sources or during oil extraction. Compared to oil, natural gas reserves are about twice as much. Natural gas is divided into two categories based on quality: High (87-99% methane) and Low (80-87% methane). The higher the methane ratio, the higher the combustion efficiency. CNG is often incorrectly confused with LPG.

The technology of burning natural gas is fully developed and tested. In 2015, there were more than 23 million vehicles worldwide with a natural gas drive. Special gas tanks and a modified injection system are required to use natural gas as an energy source for a motor vehicle. Gas can be used in compressed form (CNG) or even in liquefied form (LNG). However, CNG is currently the preferred form. A vehicle with a CNG drive usually has two separate tank systems. One smaller tank is for the original fuel (usually gasoline), and the second tank (pressure vessels) is for compressed natural gas. The gas in these tanks is compressed to a pressure of 20 – 30 MPa. The vehicle automatically switches between the two fuels, and their ranges are added together.

Advantages of CNG:

  • It is cheap
  • It has a high octane number of 128
  • Reduces noise levels in operation by several decibels
  • Double reserves of natural gas compared to oil
  • The even distribution of deposits contributes to a more balanced economic market and simplifies gas distribution to users. Note that today natural gas is transported by established gas pipelines, eliminating the need for complex, expensive, and environmentally unfriendly tanker transport.
  • CNG has a higher ignition temperature compared to LPG, reducing the risk of unintended ignition
  • Natural gas is lighter than air, so in the event of an accident or malfunction, it rises upward
  • Natural gas burns the cleanest among all hydrocarbon fuels. Emissions of nitrogen oxides are up to 60% lower than gasoline!
  • CNG vehicles are generally subject to minimal consumption tax in most countries and are exempt from road tax, which is most noticeable for large multi-axle vehicles

Disadvantages of CNG:

  • unlike LPG, operating a vehicle on natural gas reduces engine performance, see the section below.
  • more expensive maintenance – Czech legislation requires annual leakage checks and system inspections for non-original conversions.
  • prescribed replacement of spark plugs every 15,000 km (for gasoline engines, it’s 60,000 km)
  • reduction of luggage space due to the placement of the CNG tank. However, this disadvantage is more relevant to conversions. In original cars, designers try to place pressure tanks, for example, in the space of the spare wheel. The placement of heavy tanks also negatively affects the vehicle’s driving characteristics.
  • CNG pressure tanks must withstand higher pressure than LPG pressure tanks.
  • another frequently discussed disadvantage is the relatively small network of public refueling stations. This disadvantage is gradually disappearing with the growing network of these stations.
 CNG – reduction of engine performance

Operating an engine on CNG reduces its performance. For optimal combustion, the engine needs more air in the case of natural gas. This necessary amount of air can be obtained ideally through supercharging, so it is possible to achieve the same performance using natural gas as with gasoline in supercharged units. However, the performance difference between gasoline and CNG is noticeable, as seen, for example, in Octavia 1.4TSI vs 1.4TSI G-TEC.

In atmospheric engines, the performance drops by at least 10%, in practice, even more! To prevent such a drop, for example, in the Škoda Citigo G-TEC, the engine received modified software (ignition advance, fuel map) and a larger compression ratio.

The branded conversion of the atmospheric engine Škoda Fabia 1.0 MPI at Havex auto dealership for CNG combustion showed the following measured performance drop:

Škoda Fabia 1.0 MPI performance torque
gasoline 55.6 kW at 6278 rpm 97.2 N.m at 3639 rpm
CNG 41.1 kW at 5767 rpm 82.7 N.m at 3564 rpm

source: SvětMotoru

 vehicle with CNG drive  filling neck of gasoline and CNG tank  vehicle with CNG drive

According to the method of filling, refueling stations are divided into:

  • fast refueling stations – refueling time is comparable to pumping liquid fuels (3 – 5 minutes).
  • slow refueling stations – refueling with natural gas is done directly using a compressor, refueling usually takes several hours.

Did you know:

What is the consumption of a CNG car and how is it specified?

Since CNG is a gas, consumption is most often specified in kilograms per 100 kilometers driven. Like with traditional fuels, consumption varies depending on many factors, including driving style. As an example, the Škoda Octavia G-Tec can achieve a consumption of 3.4 kg (5.3 m3) of CNG per 100 km. In gasoline mode, the same car consumes about 5 liters per 100 km.

How much energy is in CNG?

In 1 kg of natural gas, there is as much energy as in, for example, 1.5 liters of gasoline or about 1.3 liters of diesel.

CNG as an eco-friendly fuel

During the processing of natural gas, sulfur and nitrogen are removed. Compared to gasoline, natural gas has a quarter less carbon. For these reasons, CNG can be considered an eco-friendly fuel compared to diesel or gasoline, reducing the levels of exhaust pollutants CO2 and NOx by 25% and CO levels by up to 50%.

Parking with CNG in underground garages

In most European Union countries, vehicles with natural gas drive can park without restrictions, even in ventilated underground garages. This is a significant advantage over LPG vehicles, which cannot park in underground garages (LPG is heavier than air and accumulates on the ground in case of a leak).