RCS Motor Club

Mechanics

Dennis Bros. Started production of the N-Type fire engine in 1908, the same basic design of engine was kept until 1931, when the last of the White and Poppe engine Braidwood body appliances were produced by Dennis.

Routine Maintenance

Alice Rowlands working in Jez’s engine bay

The Engine

Inlet side of engine

Jezebel is fitted with a 9120cc engine rated at 60hp. It is of a four cylinder design (cylinders cast in pairs), with a five inch bore and seven inch stroke. The cylinder blocks are designed with a valve on either side of the cylinder with a cap above each valve to allow its extraction, the combustion chamber is otherwise enclosed (no removable head). The valves are driven from a camshaft at either side of the engine, with a roller follower on each cam, and a packing piece on each tappet for “silent” operation. The little end and camshaft bearings are splash lubricated bronze bushes, but the main and big ends are white metal with an interesting lubrication scheme. The oil is scavenged from the sump and pumped to an external oil tank (this has water flowing through it during pumping at a fire). From here it is pumped to no.1 main bearing which has a hole drilled through the crankshaft, a slot in the bottom shell and four holes in the top shell, to distribute the oil to the other 4 mains. The oil is taken from here to the other main bearings via external oil pipes and to troughs above each of the bearings where it is allowed to run in. The overflow from the mains runs into troughs below each of the big ends that have scoops to collect this oil.

Fuel

The engine was originally fitted with a White and Poppe Carburetor, but these were often replaced with Zenith Carburetors in service (a much better design). This was the case when Jezebel came to college and she has kept this carburetor since. The fuel is fed from a 20-gallon tank under the driver’s seat; she delivers around 4mpg in traffic and around 8mpg on the open road. There is a water jacket around the inlet manifold to aid the vaporisation of the fuel. Jez was designed to run on whatever fuel was around at the time (less than one star and unleaded) so will run on most things with volatility.

The Body

The body is constructed from ash panels, and is designed to be able to carry over 1500 feet of hose. The crew is seated on the body in the front seat and on bench seats either side.

The Pump

Gwynnes of London built the original pump fitted to Jezebel. However when Jezebel was in service with Crosfield’s the priming pump gear failed and as Dennis Bros. were no longer able to supply this item the pump was removed and Jez towed a trailer pump for the rest of her working life. When Jez came to college the search began for a replacement pump, as a Gwynne pump could not be found, a Dennis no.2 pump was fitted. This is a slightly later design (1921), fitted to the late 20’s Braidwood body Dennis Machines. The pump is driven off the engine via a power take off box mounted just behind the clutch.

The transmission

The transmission consists of a number of universal joints and propshafts. Only one of the universal joints is of the rotating pin design, the rest are what are phrased as “Box Joints”. These consist of a box open on one side, a knuckle with radiused edges and a pair of bronze “slippers” to give the required rotational and longitudinal freedom. Drive is taken from the engine via a leather cone clutch through the PTO Box to the Gearbox. The Gearbox is of a standard straight cut crash box design. Drive is then transferred via another prop shaft to the Dennis Patent Overhead Wormdrive Differential. Drive is finally through plate Sankey wheels to solid tyres.

The Brakes

The brakes operate on the rear wheels only. This obviously make stopping distances slightly longer than on a modern car when the fully laden appliance weighs around 6 tons. There are expanding shoes in drums bolted to each of the rear wheels. This is the means by which all braking is done under normal circumstances. There is also a pair of contracting shoes on a brake wheel mounted on the rear of the gearbox, this is rather effective, but should be reserved for emergencies only.

The cooling system

The cooling system consists of a large radiator that can be seen on the front of Jez, and a pump which, circulates this water through the cylinder blocks. In addition when pumping at a fire there is an additional feed from the pump at the rear of the appliance to cool the engine. This water passes through a coil in the oil tank then to the radiator.

Contact Us!


Email: jez@imperial.ac.uk
Write: RCS Motor Club c/o Student Activities Centre, Imperial College Union, Beit Quad, Prince Consort Road, South Kensington, London, SW7 2BB
Tel: +44 (0)20 759 48085 Not always attended
Facebook: www.facebook.com/jezebel.dennis

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