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Cleveland's LPG bus is qualified success
Page 30
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A QUALIFIED success for Cleveland Transit's pioneering lpg-powered doubledecker is reported in a comprehensive analysis of the vehicle's first 12 months in service published by the undertaking this week. Because the fuel is not subject to tax rebate like derv, the vehicle is costing about 2.5p per mile more to run in fuel cost. Even so Cleveland is to have a second LPG bus with a fully-engineered conversion and an Allison AT 540 automatic gearbox.
The report published this week outlines the background to the project which involved replacing the Gardner 6LW engine in a Daimler Fleetline doubledecker with a Rolls-Royce B81 gas engine. It then goes on to relate the problems encountered in operational service. One of the first of these was the difficulty encountered by drivers in changing gear at the correct speeds because the engine was so quiet. Following road tests change-up points were marked on the speedometer at the following speeds: 1st to 2nd gear, 6krn/ h (5 mph); 3rd gear, 22.5 km/h (14mph); 4th gear, 35km/ h (22 mph). Following this there was a marked improvement in fuel consumption.
In the conversion the semi-automatic Daimatic transmission was left virtually unaltered. A reduction box was added to cope with the higher engine speed and this gave trouble. Fluid flywheel slip was also a problem.
After 15,000 miles it was decided to strip down the engine for analysis by RollsRoyce engineers. It was discovered that an incorrect pull-off spring had limited maximum engine speed to 2,800rpm instead of the designed level of 3,200rpm. Under testbed work, number three cylinder began to fail and it was discovered that the exhaust valve head was burnt away. The other seven exhaust valves showed signs of similar burning. Piston crowns were eroded and a hard deposit was discovered in the combustion chambers. Subsequent analysis by a Dutch research institute showed that the engine oil used — Shell Rotella — had given rise to the deposits. Softer valve stems seals were also recommended. Because LPG has no "pinking property" it was discovered that the ignition was too far advanced, to 151/2 degrees instead of eight.
Following this, modifications were made to the engine before it was placed back in service. A further 8,500 miles were completed before March this year when another examination was made. This revealed that certain vibration had arisen due to fan-drive shaft difficulties but otherwise there were no major problems. An operational problem was the storage of the fuel in four separate tanks in the vehicle which needed the driver to switch from one to the other. It had proved impossible to fit an indicator showing the amount of fuel left in the fourth tank.
Over the 44 weeks of operation the prices of both dery and LPG have changed considerably and the following are average ones. The LPG bus has averaged 1.3 km/1 (3.62mpg) at a cost per km of 3.4p (5.5p per mile). An equivalent diesel vehicle which has "shadowed" the test bus has returned 2.5km/I (7.15mpg) at a cost of 1.85p per km (2.97p per mile). Because LPG is not subject to tax rebate this means that the LPG bus was about 1.6p per km (2.5p per mile) more expensive in fuel costs.
Over the test period lubrication oil consumption has averaged only about 965 km (600 miles) per gallon. The Shell oil has been replaced by a Castro! product. Noise tests conducted by the DoE show the LPG bus to be little different in external readings from diesel buses but that internal noise levels were markedly better. Measurements showed a maximum interior level of 79dBA for the RollsRoyce engine, 88d BA for the Gardner and 94dBA for a Leyland 680. Further attention is to be paid to fan design and exhaust note in an effort to reduce the exterior noise level (85d BA).
When the project began it was hoped to reduce vehicle weight but because of heavier fuel and storage tanks this saving was only about 50 kg (1 cwt).