FAQs : Converting Diesel Buses to CNG

Why are countries shifting from diesel to CNG for their public transport bus fleets?
Substitution of diesel by CNG in public transport buses has several advantages. These vehicles have a high specific fuel consumption and high mileage. On the other hand, only a few centrally located filling stations are required in view of their limited range of operation. Thus investment in the CNG infrastructure can be controlled and the amount of fuel required is predictable. Also the environmental and economic benefits of using CNG can be obtained only if the conversion takes place on a large scale.
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How does converting public transport buses from diesel to CNG help improve the quality of life?

Large scale use of CNG operated buses brings about an improvement in the quality of life, in terms of reduction in air and noise pollution. There is a lower visible particulate emission (black soot) which helps improve the health of the population. Engine noise is reduced by up to 5 dBA which corresponds to a 50 per cent reduction in noise emission. Noise pollution in congested areas often leads to additional stress and therefore further ailments. According to the US Environmental Protection Agency, “Compared to gasoline or diesel vehicles, those powered by natural gas emit 80 percent fewer ozone precursors and over 95 percent fewer particulates.
What are the various kinds of conversion that are possible?
The following options are available:
(i) Conversion of a diesel engine to dedicated CNG (spark ignition) combustion
(ii) Conversion of a diesel engine to dual fuel (gas and diesel combined) combustion.
Which option is ideal for converting a public transport fleet?
Conversion of a diesel engine to dedicated CNG is the better option for a public transport fleet. This is because the bus routes are fixed and the operating routines known in advance. A UN ESCAP report based on studies conducted in Karachi, Pakistan states, “The primary target market for the conversion of diesel vehicles to CNG fuelling are buses and trucks in urban transport. Among them, preference should be given to city buses due to their limited and regular operating patterns and there, in many cases, large fleets which might justify the establishment of own CNG refuelling stations.”
What are the technical aspects of converting a bus from diesel to CNG?
New or modified pistons will be required. CNG works best with a compression ratio of 10 & 12 to 1 as compared to diesel engines which have a compression ratio between 16 and 18 to 1. Diesel buses work with diesel fuel injectors and not spark plugs. Therefore, a conversion to CNG would require replacing the injector with a spark plug. Since spark ignited engines become hotter, thermal management components will have to be upgraded. This means that larger radiators, oil coolers and heat shields will be required. A catalytic converter to adhere to emission norms and an engine management system as per requirements is also essential.
What factors must be taken into account before launching a conversion programme?
The conversion must be undertaken by suitably qualified mechanics on appropriate vehicles. Substandard conversions and conversions of inappropriate vehicles can result in inadequate vehicle performance.
Have large scale CNG conversion programmes been successful?
Yes, large scale CNG conversion programmes have been very successful. Pakistan, US, Canada, Argentina, India and South Korea are just a few countries which have used CNG to control pollution levels in urban areas. New Delhi’s CNG conversion programme has been credited with dramatically improving the air quality in the city.