Richard Trevithick constructed an experimental steam-driven vehicle (Puffing Devil) at Camborne, Cornwall. It was equipped with a firebox enclosed within the boiler, with one vertical cylinder, the motion of the single piston being transmitted directly to the driving wheels by means of connecting rods. It was reported as weighing 1520 kg fully loaded, with a speed of 14.5 km/h (9 mph) on the flat. Trevithick ran this for several hundred yards up a hill with several people hanging on to it. Unfortunately, while the driver and passengers were in a pub celebrating the event, it set fire to a shed in which it had been left unattended, and was destroyed.
Description
Not all the details of the carriage are known but the drawings which accompanied the original patent have survived, as have contemporary drawings made by a naval engineer who was sent to examine it. Further information has also been obtained from eyewitness accounts. The carriage had 8-ft-diameter driving wheels which were intended to smooth out the road surfaces of the time, to help the fire from being extinguished by shaking. A forked piston rod reduced the distance between the single cylinder and the crankshaft and was considered a singular innovation at the time. Spring-operated valve gear was used to minimise the weight of the flywheel, overcoming one of the drawbacks of industrial steam engines. The engine had a single horizontal cylinder which, along with the boiler and firebox, was placed behind the rear axle. The motion of the piston was transmitted to a separate crankshaft via the forked piston rod. The crankshaft drove the axle of the driving wheel (which was fitted with a flywheel) via a spur gear. The steam cocks (used to blow out water condensate from the steam chest), the force pump and the firebox bellows were also driven by the crankshaft.
The patent shows two features which may have been incorporated by Trevithick to discourage unlicensed copies: if the engine had been assembled as per the patent drawings it would have been able to run only backwards; and if the water pump had been arranged to be driven by the valve spring as shown, it would have run unevenly if at all.
