GETTING GASSED

IN a bid to encourage cleaner fuels the government has been promoting CNG and LPG as alternatives to conventional fuel (petrol and diesel). And with the public transport in the capital and other metros already running on CNG, the problem of limited refuelling outlets too seems to be disappearing. All this and the fact that green fuels mean lower running costs compared to conventional auto fuels, has a significant number of private vehicle owners interested in the CNG option.

To assess the cost factor, for a car like the Premier Padmini, a kilogram of CNG costs round about Rs 20 and offers a range of almost 20km per kg translating into running cost of rupee a kilometre. On the contrary, a litre of petrol, which costs about Rs 35, offers a running cost of almost Rs 3 per kilometre.

Even if we were to keep the better economy factor aside, CNG still offers a number of benefits. The fact that CNG enters the engine in the form of gas instead of spray or mist prevents the wash down of the lubricating oil from the top piston ring area keeping the engine’s rate of depreciation to a bare minimum. In addition, the octane number of CNG being 130 as compared to 87 of that of petrol helps thorough and rapid mixing of gas and air in the correct proportion, thereby improving the combustion efficiency.

Talking about good conversion systems, it’s imperative that the system operate as close to petrol as possible. In addition the risk of leakage should be marginal. So how much does a good conversion system cost? It can cost from around Rs 35,000 for a carburetted engine to Rs 45,000 for fuel injected ones (installation costs included). For taxis though it’s available on a discount at Rs 32,000 while for three-wheelers the price touches Rs 25,000. Most of these kits sold in India are brought in from Italy, while the remaining are manufactured in Gujarat.

A CNG kit includes a conversion kit and cylinder wherein the former is placed in the engine bay while the latter goes into the boot. The kit may be installed at any of the authorised CNG workshops, which are increasing by the day. But attention must be paid to the safety aspect while installation, which takes a simple soap test to ascertain if the safety aspect is adhered to. The test involves covering the lines carrying CNG with a soap solution, which indicate a leak by forming bubbles at suspect areas.

The basic components of a conversion kit include a pressure regulator to provide the system with fuel at a consistent operating pressure in addition to a mixer to ensure correct air/fuel mixing in both stationary and dynamic conditions improving the drive of the vehicle thanks to improved mixing. Besides a high pressure solenoid to allow the flow of gas and a petrol solenoid to cut off the flow of petrol to the engine during gas operation.

A filling valve installed in the engine compartment between the storage cylinder and the regulator aids easy refilling. The valve is connected to the refuelling system and a manual ball device to open/shutoff gas in case of an emergency or for maintenance.
In the case the car runs out of gas, a change over switch is provided which is an electronic control module allowing the user to switch to petrol. The gas from the cylinder goes to the engine compartment through a pressure pipe, which is also connected to the refuelling system. It is all part of the high pressure piping system that works overtime to avoid leakage. A pressure indicator is also provided to display the level of natural gas remaining in the storage cylinder.

Installation of the CNG kit in a three-wheeler (below left), and a taxi refuelling at a CNG station. Though refuelling takes just a few minutes the wait can be arduous (below). The three major parts employed for a CNG kit installation: mixer employed for correct air-fuel mixture, filler valve, cylinder in the boot (top to bottom, right)

Safety aspects
Pressure vessels, in this case the storage cylinder, are ideally made of high strength steel or aluminium over wrapped with a composite material like fibre-glass. There have been reports of tank rupture upon refuelling due to the damaged fibre over-wrap thanks to the use of spurious cylinders, especially by unauthorised agents who carry out conversions from petrol to CNG. But this problem can be addressed through adequate monitoring and implementation efforts. CNG calls for high-pressure on-board storage of the fuel around 200 atmosphere (3000 to 3500psi) or more. Providing them as after-market retrofits on autos and cars without proper inspection would lead into disastrous results from the safety point of view.

The diesel option
In diesel engines, CNG can be used either as a dual fuel or mono fuel. In dual fuel mode, diesel provides combustion initiation for CNG burning. Kits can be retrofitted to existing vehicles for them to operate on diesel/CNG mode. However, under city driving conditions, the substitution of diesel with CNG is quite low and hence the gains in emission reduction are also low.

Considering the limitations of emission reduction in a dual fuel engine, CNG can be used as a mono fuel by converting the diesel engine to a spark ignition engine. This conversion warrants major changes in the engine and the operation of the vehicle becomes CNG-dependent.