Paper presented at the 33rd Annual Southern African Transport Conference 7-10 July 2014 "Leading Transport into the Future", CSIR International Convention Centre, Pretoria, South Africa.
To investigate the use of clean burning methane in the form of compressed natural gas (CNG) and compressed bio-gas (CBG) in public transport in South Africa, the Industrial Development Corporation of South Africa and Cape Advanced Engineering (Pty) Ltd implemented a real world vehicle fleet trial. The vehicles involved were mini-bus taxis and urban buses. Vehicles were fuelled at three commercial filling stations in Gauteng, of which two provides pipeline CNG and the third a blend of compressed bio-gas and pipeline CNG.
The 10-month programme investigated fuel consumption, vehicle operating cost, oil analysis – also to assess extending the oil drain interval to reduce related operating costs – and the financial viability of conversion to bi-fuel or dual fuel.
The fleet of mini-bus taxis obtained a 22% saving in fuel operating cost when using gas or petrol (bi-fuel) compared to standard (petrol) vehicle operation. The actual savings depend on the level of petrol displacement with CNG, the route operating conditions and the efficiency of the bi-fuel operation. Oil degradation and engine wear were found to be favourable with bi-fuel operation compared to standard petrol operation, thus it seems that equivalent or better engine life could be achieved with bi-fuel operation.
Financial feasibility was favourable, as converting commuter mini-bus taxis to bi-fuel showed a payback period ranging from less than a year (for 225km travelled per day, at a R20,000 conversion cost and a 25% lower than petrol CNG pump price), to six months (if the daily distance increased to 300km and the conversion cost reduced by 20%).
Evaluation of the diesel dual fuel commuter bus compared to a diesel (standard) commuter bus of similar configuration found a fuel operating cost saving of 76c/km (19.2%), based on a diesel substitution of 71%. There is clear indication from the measured oil analysis results that the oil drain interval could be extended by up to double depending on the substitution of diesel by CNG, although this would require more extensive monitoring and trials.
Measured fuel cost savings showed that the conversion and operation of diesel dual fuel vehicles are economically feasible if the vehicles operate over a distance of more than 220 km per day and/or if the conversion costs is limited to R150 000 and/or if the CNG fuel can be purchased at a discount of at least 15% relative to the existing retail price of diesel on an energy equivalent basis.
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