On The Green Route

Print Edition: July 2011

Although alternative fuels, such as CNG and LPG, have been powering vehicles on Indian roads for some time now, the public awareness of these cleaner and cheaper options still remains inadequate.

How is natural gas formed?
Natural gas is a fossil fuel formed from organic matter stored underground, without the presence of air, for a long periods subject to high pressure. Natural gas deposits are usually between 15 and 600 million years old. Petroleum is also formed under similar conditions and is the reason why both fuels are usually found together. Of course, sometimes, gas may be found without the oil deposits.

Do CNG cars explode often?
No, there is no evidence to show that natural gas-powered cars are unsafe. Whether the car is equipped with a factory fitted CNG engine or whether the engine was re-tooled in the aftermarket to adapt it for CNG use makes no difference to the safety. As along as the fitment is done properly by authorised personnel, there is no extra danger since a cutoff valve prevents build-up of extra pressure.

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How long can gas vehicles go without refuelling?
This is dependent on volume of the fuel tank. CNG cars can be equipped with tank sizes of 12-30kg, allowing the vehicle to travel between 200-500km on just gas operation. With LPG vehicles, the tank sits in the spare wheel well, which is approximately 50 litres. Depending upon driving and consumption this is enough for distances between 350-600km. Larger tanks would have a greater range.

Can a carbureted car be fitted with LPG?
Carburettor engines can indeed be re-equipped to run on LPG as well. The engine functions on the Venturi Principle and the LPG metering valve uses the negative pressure in the siphon or inlet manifold to ensure gas flow. The equipment is fitted only if the examiner is convinced the reequipment will function properly. It should also be clarified before the installation whether the engine type is conducive for the use of CNG or LPG as fuel.

What are the problems in refitting an engine to accept LPG as fuel?
Protective additives are not present in LPG. This can cause problems for highly thermally-loaded exhaust valve seat rings. If these are too soft, the valve works itself into the valve seat, which shrinks from valve clearance, leading to damage to the engine. With some engines the original valve seat needs to be exchanged for a hardened ring for better functioning.

What does LPG consist of?
LPG is a fossil fuel that comprises propane and butane. The butane percentage it possesses dictates the octane number it gets, which can range from 105 to 115 RON (research octane number-denoting its resistance to detonation). The higher the RON value, the lower is the density, with the lowest density being 0.54 kg per litre. It is stored as a liquid.

Can LPG be used instead of CNG?
No, under no circumstances can the two different gases be replaced with each other. Although both are gases, CNG (compressed natural gas) and LPG (liquefied petroleum gas) are stored under different pressures and are only suitable for use in vehicles that have been specially equipped to accept one of the two fuels. While the operating pressure of natural gas vehicles is 200 bar, the operating pressure is only a scarce 10 bar for LPG. Any attempt to fill an LPG tank with CNG therefore would inevitably end with an explosion of the tank. In addition, LPG is stored in liquid form while natural gas is stored in gaseous form. Of course, the filler neck for CNG at any station will not fit the adapter of the LPG vehicle. Hence, a mistake is not physically possible.

What is the difference between the two?
Compressed natural gas, also known as CNG, consists mostly of methane and is lighter than air and is stored at a pressure of approximately 200 bar and is measured in kilograms. LPG (liquefied petroleum gas) on the other hand consists of a propane butane mixture. It is produced from natural gas or oil and is stored as a liquid at a pressures of around 10 bar. More importantly, LPG also gives greater mileage and results in lower loss of power. However, a greater number of CNG stations are found in the country while LPG can be bought in cylinders designed for cars.

Can one re-equip diesel engines for LPG?
The process is labour-intensive and complex. The self ignition would have to be provided with spark plugs instead of glow plugs and the appropriate ignition controls for the engine have to be adapted. Even so, we already have numerous examples of diesel engines converted to run on CNG in public transport buses.

How do I refuel gas vehicles?
Refuelling LPG and CNG is done differently using different nozzles. Apart from the CNG refuelling method shown below (left), the gas is sometimes also filled from under the bonnet. It depends on the car and the kit it is fitted with. Also, LPG is less available compared with CNG.

Which companies offer LPG or CNG ex-factory?
Chevrolet, Ford, Maruti Suzuki, Hyundai and Hindustan Motors offer different vehicles with both gas and petrol drives. All retain their petrol tanks besides the additional required gas tank. With LPG, consumption increases by approximately 20%.

How cost-effective are CNG and LPG?
Will I find out if there is a leak?
An odour is added to the normally odourless gas (CNG or LPG) so that a leak does not remain unnoticed. Even smallest of leaks are detectable. If there is an unpleasant odour inside the car, pull over, switch the engine off and call the fire brigade.

Will CNG and LPG remain cheap?
CNG and LPG will remain cheaper for many years as compared with conventional fuels since it has been highly subsidised by the government.

But in spite of the tax reduction, prices of both CNG and LPG may rise in the coming years as demand for them increases but not at the same pace as diesel and petrol.

What are the effects of using the gas operation on the environment?
In comparison with petrol or diesel fuels, gas burns more cleanly and emits fewer toxic gases on combustion. As a result the dangers of environmental pollution are reduced if a vehicle is made to run on gas instead of conventional fossil fuels like diesel or petrol.

Is it worth re-equipping new vehicles with LPG?
If your daily commute is over 50km you can easily recover the money in one year. In principle, all ex-factory offerings of LPG vehicles are adapted from petrol engines so that the volume for the latter remains unchanged. If a new vehicle is re-equipped, an auxiliary warranty and insurance should be also be taken.

Courtesy: Auto Build

CNG Demystified

The use of CNG reduces carbon monoxide (CO) emissions by over 70% and hydrocarbon emissions by over 40% as compared with cars using conventional fuel. Here, we explain the various components, their functions in a typical CNG kit and what to keep in mind to maintain the machinery to avoid taking unnecessary risks.


CNG is stored at a very high pressure (200 bar). The job of the reducer is to reduce the pressure before it enters the intake manifold. Get it checked if the car's performance dips even slightly.


It is the control unit (ECU) of the fuelling system. It controls the amount of natural gas that is to be supplied to the engine depending on the load. It requires no manual adjustments and can automatically adapt to the different working conditions.

Changeover Switch
Changeover switch

The changeover switch is used to switch between petrol and gas. The car starts on petrol and reverts to CNG automatically and can be switched back to petrol while driving if low on CNG.

Hi-pressure piping
Hi-pressure piping

A high pressure pipe travels from the cylinder in the boot to the engine bringing the gas. It is fitted underneath the car, so get the underbody checked regularly. Scraping the undercarriage could damage the piping.

Shut-off valve
Shut-off valve

The valve is used to shut down fuel supply to the engine. If the car is not going to be used for some time, start the engine and let it run on CNG. Then close the valve and let the engine die down.

Refuelling valve
Refuelling valve

The refuelling valve has bushes in it to hold the nozzle while filling the gas. With use over time there is a chance of the bushes wearing out. A replacement costs around Rs 1,000 depending on the manufacturer.

Timing Advancer
Timing advancer

A car running on gas runs at almost 15% less power output while running on CNG . As CNG has a slower combustion time in comparison with petrol, the spark needs to ignite in advance. The timing advance processor modifies the original ignition point.

CNG cylinder
CNG cylinder
CNG cylinder

A CNG cylinder is usually placed in the boot on a raised frame. They are built to withstand pressures of 300 bar though working pressure is only 200 bar. A 100-litre cylinder can hold up to 24.88 cubic metres of gas.

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