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engine. The petrol/gas changeover valve has the normal carburettor mounted above it and is controlled from the cab. The engine is normally started on petrol and then the valve slowly moved over so that a petrol/gas/air mixture is used and then gas/air only.
Operating Routine
The daily fuelling, maintenance and starting procedure took two men about two hours of dusty and dirty work. Charcoal was bought in villages in stick form (rather larger than usually found in Hong Kong). It was then broken into pieces 1-1/4” size, sifted and put into bags. This was often done at the end of the day's run. If charcoal was good and cheap, 300 lbs or more would be bought. Consumption would be a 100 lbs, or so a day.
In the morning before starting the whole gas system was cleaned, the firebox door dropped, ash removed, cyclone emptied and bag filters removed, turned inside out and shaken (the dirtiest job!). All was then replaced, the hopper filled with the broken charcoal and all doors to the system made air tight. Any air leakage meant loss of power. The fire was started using a torch dipped in oil and brought up to heat using a hand wound centrifugal blower mounted on the tuyere. This could be 15-20 minutes work performed by the junior crew member while the driver took a quick breakfast. The engine was then started on petrol and changed to gas/air. Passengers were loaded aboard and the journey recommenced.
Efficiency of Performance
As has been mentioned previously, the efficiency of the charcoal conversions improved with time and experience. The contributing factors were:--
1. Raising the compression ratio of the engines. This was done by machining off the cylinder heads by (writing from memory) up to 0.030" on the Chevrolets and 0.080" on the Dodge trucks. The first truck on which this was tried was No. 38 and was christened "Anne Boleyn" in consequence. She was a well behaved lady after the operation.
2. Fitting a manual advance/retard control to the ignition. Gas/air, with a slower burning rate than petrol vapour, requires the spark earlier in the cycle.
3. General improvement in construction and air tightness of the gas systems.