B.S. Murthy, Emeritus Professor - IIT Madras

In the interest of public health, environmentally friendly fuel, namely the compressed natural gas(CNG) for vehicles came into the scene at the capital city of Delhi as remedy to reduce the high level of atmoshperic pollution. After witnessing the recent chaotic situations owing to the failure to change over from the conventional liquid fuels to CNG, it is appropriate now to quickly review the technological and safety features, which are necessary for smooth transition from conventional fuels to gaseous petroleum fuels.

For societal good, there is no doubt that any transition from an established conventional system to a new system must tbe gradual and smooth. Changing to fuel options like CNG, LPG or alcohol fuels are no exceptions although they are proven environmentally cleanr alternate fuels in replacement of diesel oil or petrol. First requirement is to make a thorough techno-economic survey of availability and adequate of supply of these alternate fuels. Next in priority is to plan and establish ample storage and distribution infrastructure. If these basic issues are not fulfilled, the scheme of change over is bound to cause confusion and eventually chaos as is evidenced in the capital city of Delhi. Examples from countries where such changes are made would provide good guidelines.

First phase of change over should be directed to governmental and other organized fleet vehicles. Here there is a centralized control for fuelling and conforming to safety measures. In case of short supply, a flexible CNG-Fuel system (applicable to petrol driven vehicles only in India as it stands) would permit operation on petrol till the supply position of CNG is re-established. CNG refueling capabilities range from very small slow-fill to large fast-fill which can refuel CNG vehicles in a few minutes. The costly portion of the fast fill system is the expensive compressor and the cascade of storage tanks. Sometimes the actual driving range could be less than the advertised range when fuelling at fast fill stations that cannot fill the tanks to 100% capacity because heat of compression reduces the gaas density. For small vehicles like autos and passenger cars, large guel tanks would adversely affect the passenger and luggage occupancy and hence they have to have reasonably smaller tanks. This would mean more frequent refueling for the same distance covered over the trips. Having gained experience with fleet vehicles, the transition should be made to private carries, personalized passenger cars and lastly to the much needed public-vehicles like auto-rickshaws.

Most important features of gaseous petroleum fuels is safety. CNG calls for high-pressure on-board stroage of the fuel around 200 atmosphere (3000 to 3500 p.s.i) or more. Since pressure vessels (usually high strength steel or aluminium over wrapped with a composite material like fiber glass or all composite material) require critical standard requirements, it is a wise decision to insist on the vehicle manufacturer to conform to this. Providing them as after-market retrofits on autos and cars without proper inspection would lead into disastrous results from the safety point of view. Even with proper inspection and maintenance, tank rupture upon refuelling in he Los Angeles County Transit Agency was reported due to the damaged fiber over-wrap. Security lapses in non-fleet vehicles like autos are difficult to monitor is crowded and unregulated traffic situations as encountered in India.

On the brighter side of CNG as a clean fuel, people are aware that CNG has limited health concerns. It is non-toxic. CNG is typically 85 to 99% methane. Combustion of methane does not produce higher hydrocarbons, which are reported to be carcinogenic. No smoke or particulates since sulphur is practically absent. The unburned methane is not phote chemeicaly active and that is why the lmits in the standards psecify only non-methane hydrocarbon emission standards. But the other side of the story would be that the green house effects of the escaped methane, which would be substantial and methane is prone to produce formaidehyde. With extensive use of CNG, environmental safety would bring unburned methane also under regulation. CNG cannot be adultered as easily as petrol or diesel oil since it is not possible to tamper with the compressed storage system. But variation of the composition of methane as it occurs in nature would lead to frequent variations in vehicles performance.

The darker side of the engine technology must be noted with care. As it is used in a diesel engine, the original diesel engine has been modified by reducing the compression ratio to about 10 to suit spark ignition system with mixture control for regulating the load. The engine does not shed its heay weight designed for the original diesel engine. But it loses the important values of the high compression ratio responsible for the superior diesel efficienty and lean burn characteristics, which is the birthright of a diesel engine. Consequently, for the same power developed, the diesel engine rejects less heat to the exhaust, and this becomes smaller at part loads. Bu the CNG vehicles as it stands now do not enjoy this benefit of cooler exhaust. Driving in city limits, with stop and go driving, would compound to this problem and the drivers would be exposed to greater heat levels. True, you can provide a bigger cooling system but this adds more weight to the vehicle. Why produce moe heat and then cool the engine again!

Advanced technology has been developed to design lean burn engines for CNG and also methods to directly inject CNG into the cylinder retaining all the diesel features like high compression ratio and loan cntrol by altering the fuel injected. Our Indian Industries had no lead – time to develop these refinements. The only method available was to downgrade the diesel engine and use it as a homogeneous charge spark-ignition engine.

The petrol-driven vehicles like passenger cars and autos do not require major engine modifications. Just the conversion kits and storage cylinders are additional items. They can be adopted as flexible fuel vehicles just in case there is a failure of CNG supply. But it must be noted that if we do not need flexible fuel capability (when there is an uninterrupted supply of CNG), the engine can be designed for higher compression ratios of about 10 and obtain higher fuel-efficiency.

The common drawbacks of both CNG-converted diesel and petrol engines are the extra weight and volume needed for on-board storeage of fuel. For an equivalent range of distance to be covered, the CNG has an additonal fuel volume capacity of nearly 4.3 times that required by diesel fuel, while in terms of weight the factor would be about 3.7. There would be a corrsponding penality in passenger occupancy and payload. Approximately, a significantly larger number of CNG buses or trucks are required to replace the smaller number of diesel buses or trucks to carry the same number of passengers and pay load assuming the same vehicle performance in terms of efficiency. Another performance compromise is the slight power loss experienced by most light duty CNG vehicles. This primarliy because the gas displances some of the air in the intake charge, and the engine power is ultimately related by the rate of which the engine can process the air.

Plenty of technical studies are required before the country changes from liquid fuels from petroleum sources to gaseous fuels like CNG and LPG. Altenate fuels like alcohols have also proved to be excellent fuels for vehicular use. India has proven record on alcohol fuel technology. Along with CNG, alcohol fuels can also be tried as alternatives.

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