By the early part of the last century it was evident that private cars were here to stay, and numerous firms were attracted into motor manufacturing to exploit the growing market. One such firm was the Thornycroft Steam Wagon Company Ltd, which entered the fray in 1903. Thornycroft's first petrol models were a twin-cylinder 1,788cc car known as the 10hp, and the 3,576cc four-cylinder 20hp. Both cars had water-cooled in-line engines, with splash lubrication, and cylinders for the 10hp car were cast as a pair in a single block, in which the water jacket, cylinders and cylinder heads were cast in one piece. The engine for the 20hp "four" was simply a doubled-up 10hp unit, comprising two separate twin-cylinder blocks. There were two valves per cylinder of ioe layout, and a point of interest was the automatic overhead inlet valves used on both 10hp and 20hp cars, which were opened by atmospheric pressure during the induction stroke; although long since discarded, automatic inlet valves were not uncommon in the very early days of motoring. Both cars had camshaft-operated side exhaust valves.
Power was transmitted to a differential mounted on the back axle, through an expanding clutch with cylindrical pressure surfaces, a three-speed gear box with reverse and direct-drive top, and a propeller shaft supported on a universal joint at each end. This power train layout became almost universal in cars for about 50 years until transverse engines and front-wheel drive became widespread, but in 1903 chain drive was not uncommon. The differential was braced by a radius rod to the gearbox. The expanding clutch was replaced with a metal-to-metal plate type for 1904.
Thornycroft followed convention by not fitting brakes to the front wheels of its cars, a feature which should be seen in the light of lower traffic densities and speeds in the early days of motoring - e.g. 30 to 40mph (48 to 64kph). Braking was achieved with brakes on the rear wheels and a transmission brake, an arrangement which was used on all Thornycroft cars.
Wooden artillery-style wheels were fitted to beam axles fore and aft, suspended by semi-elliptic springs. Spark ignition was fitted, with one spark plug per cylinder. Ignition on both cars was by HT coils, supplied on the LT side by a battery through a distributor. On the 10hp, at least, the two HT coils were trembler devices. The battery for energising the HT coils was charged by a dynamo, belt-driven from a pulley mounted on a crankshaft extension behind the engine.
Thornycroft's first car sale was vehicle no. 199, a 10hp which went to J Dent on 29 June 1903. Although the 20hp was introduced in 1903, Thornycroft had to wait until 18 June 1904 to sell the first one, when vehicle no. 350, a side entrance car, was bought by A. Blackburn. This was nearly 18 months after the 10 and 20hp cars were first shown at the 1903 Crystal Palace Automobile Show. Both the 10hp and 20hp cars were continued for 1904, the only types produced that year.