MINI STEAM JEEP (STEMKA) 1




The steam-powered mini-jeep here described was not intended to be a contribution to the cause of motoring. It was the realisation of a desire to produce a steam-powered vehicle that gave a riding experience to its driver and passenger with freedom from the constraints of an earlier 7¼-inch gauge railway. The design was based on the American army jeep. No accessories requiring other installed power were included.

The actual construction time was 2050 hours spread over 5 years from 1987 to 1992. The total cost of materials and purchased trade components came to 2050 dollars, by coincidence a dollar spent for each hour of productive work. Double the workshop time was spent on scheming, planning and drawing. It was a labour of love, my idea of leisure and pleasure. Following the preliminary design work a wood and cardboard scale model was made and construction commenced with the details being determined as the project advanced. The objective was to create a fun machine relying on solid fuel firing, and not aimed at highway travel or commercial operations.

The flash type steam generator consisted of 65 feet of ¼” nominal bore steam pipe formed into a stack of ten zigzag grid elements located in the combustion gas flue of a wood burning furnace. The heating surface area was 9.2 square feet and the fluid content was 1.1 litres. The maximum working pressure was 600 pounds per square inch. The furnace volume was 1 cubic foot. The engine was a duplex open frame type with twin double acting cylinders of 1 5/8” bore X 2” piston stroke. The ¾” diameter X 7/8” travel piston valves were actuated by the Marshall variant of Hackworth valve gear that provided for expansive working and reverse. The 1 to 2½ ratio chain drive from crankshaft to differential gave an engine speed of 1100 RPM at 40 KPH on 16” diameter road wheels.

At start-up the wood chip fire was established and some water was pumped into the boiler by a few strokes of the foot pedal operated feed-water pump. Steam immediately generated and was blown through the engine to warm up. As pressure increased the car literally chuffed off. When the vehicle was running the boiler feed-water supply was taken over by the rear axle driven feed pump. 100 PSI was sufficient to motor on flat road.

It is important that boiler feed-water supply is matched as closely as possible to the steam demands of the engine. Ideally such control should be completely automatic but it was difficult to achieve a balance in this application because of the inexact practice of hand firing solid fuels plus the wide variations in demand on the road motoring. Various patterns of boiler pressure actuated feed-water control valves were made and fitted before being abandoned in favour of a manually controlled water metering and bypass valve that returned acceptable results providing that the driver remained alert to the cause and effect performance signals. The state of the steam generator was judged from the pressure gauge, the feel of the throttle and engine responses and the forces required on the priming pump pedal. With all of this in mind the furnace conditions had to be sustained by the frequent addition of wood blocks or pine-cones. Fuel consumption worked out at eight to ten pine-cones per kilometre. It was true seat-of-the-pants motoring.

The forty litres of feed water carried served about 2 hours of start/stop motoring around local streets, grassed recreation fields or on hard sand flats of the local tidal estuary. The car was driven on the hand or foot-operated throttle with high boiler pressure, or with minimal boiler pressure and the throttle full open with the speed controlled by the regulation of feed water delivered to the boiler. My wife and I once drove the car by this method in a town festival street procession. She was the fire attendant.