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Natural gas operation

1Natural gas controller
2Selector switch
3Petrol controller

1 kg of CNG (H) -> 1,33 L of Diesel or 1,5 L of petrol

Compression: Petrol 10,0 : 1 -> Natural gas 13,5 : 1


If the car is used a lot, switching from petrol to natural gas can lower the fuel costs by more than 50%. Currently (2005) the price of approx. 75 cents for a kg of gas, faces the approx. €1.20 for a liter of petrol. The reduced mineral-oil tax is guaranteed until 2018. At a natural gas filling station the customer still has the feeling of being the most important person and is lured with vouchers, among other things.
Above all things, natural gas-operated vehicles are environmentally friendly. This is obvious from the fact that the oil remains much cleaner if the engine is operated with natural gas and the spark plugs have to be changed less frequently. Because natural gas, in contrast to Liquified Petroleum Gas, is lighter than air, there are no general legal restrictions for underground parking. In addition, natural gas reduces the CO2 emission by nearly 20 percent.


There is really no need to fear an explosion. The system features more safety enhancing components than almost any other system used in the motor car. In addition, natural gas has an odour-producing additive mixed in to it. Escaping natural gas would thus, be easy to detect and suitable measures could be taken long before an ignitable mixture is formed. The tanks are made of both steel (see figure 2) and from fibre composite materials. To prevent the tanks from accident-damage, they have to submit to an impact-test. Only certain upgrades, built in afterwards, might be worrisome, because in the case of a rear-end collision, the rear-seat passengers may be endangered if the tank is mounted in the boot.
Perhaps the biggest disadvantages are the higher vehicle weight and the reduced luggage-space if the system is later fitted, also the cost of converting the car is much higher than it would be if a Liquified Petroleum Gas system were fitted.Additional disadvantage: At the moment, a max. range of 300 kms. during natural gas operation. With natural gas, the performance, the torque and the efficiency are different. There are engine control devices that make use of the changed ignition behaviour of natural gas (120 octane) so well, that no difference to petrol- operation is noticeable. In monovalent operation (gas only), the engine can be even better adjusted to the different type of fuel. Perhaps with the introduction of variable compression, the higher octane rating of natural gas will be even better usable, also for bivalent operation. In addition the fuel tanks must not always take up precious space, if, as a compensation, the size of the petrol tank is reduced. Thus, due to the high costs (€2500 - €3500), the later installation does not always make sense, so the cost of upgrading, must be compared with the advantages of a new bivalent vehicle.


Natural gas is stored in one or more thick walled containers (see figure 3) at a pressure of approx. 200 bar. If they are mounted directly underneath the vehicle floor, they have to be checked every 5 years, which is relatively expensive. The piping is fitted with a pressure gauge leading to the fuel gauge and the connection for the filler neck. The equipment is not compatible with that used in LPG-systems which operate under considerably lower pressure (max. 20 bar). There are also the relief valve and a further valve which reduce the high pressure, depending on the coolant temperature, to approx. 8 - 10 bar.
There are vehicles which are designed solely for natural gas operation and vehicles in which one can alternate between natural gas- and petrol operation (bivalent operation). The latter may have a common control device or, like in the above figure, two separate control devices. The natural gas control device provides, via solenoid valves, for the injection into the inlet manifolds of the individual cylinders depending on the inlet manifold pressure, the Lambda tension, the throttle flap position and the engine RPM.
The engine is generally started using petrol, although occasionally natural gas is also used directly. Should there be a fault in the natural gas system, or if the pressure drops below approx. 10 bar, the switch-over to petrol operation is automatic. Because of the temperature decrease caused by the decrease in pressure, the natural gas system must be connected to the cooling circulation.

Direct injection

The VW-company envisages an important future for natural gas driven vehicles. Although nowadays, the majority of their engines are already equipped with direct injection. What would be more logical, than to combine the savings-effect of direct fuel injection with that of the natural gas engine. This type of combination already exists, a newer version in fact, in the VW-Passat. Indeed, at the moment, apart from the higher cost, there appears to be an insoluble problem: Direct injection is (still) not possible for purely natural gas operation.
In addition, one should be aware, that petrol evaporates completely in the combustion chamber and only in the combustion chamber. The enormous evaporation-heat is very favourable for a higher compression ratio, which makes up a large part of the consumption advantage of direct injection. Indeed, because natural gas is already vaporous, the cooling is lacking, which apparently, makes the flawless use of gas-intake-valves impossible. Precisely for this reason, depending on the operating conditions, approx. 15 to 20 percent petrol is added, which of course, considerably reduces the additional savings-effect. By the way, a similar situation exists for LPG as well.

Suspicion of fraud

Should you, with a particular amount of fuel, only be able to travel slightly more than half the usual distance, then you have probably been filled-up with L-gas (8,5 kWh/kg) instead of H-gas (up to 14 kWh/kg). The engine will also run on this gas, however, because of it's much lower heating value, the Lambda control requires much more of it. 04/13

Read about Audi A3 g-tron.