Tractors with hot-bulb engines are still major attractions at every historic tractor meeting. These vehicles without starter motors and spark plugs are unique in the history of tractor manufacturing. The hot-bulb engine, also known as the "Akroyd engine", became known on account of its inventor Herbert Akroyd Stuart. From 1891 onwards, hot-bulb engines were already being produced in England at Richard Hornsby & Sons, later on in Sweden at Bolinder-Munktell, at Landini in Italy and at SF Vierzon in France, and were being used in the construction of tractors. In Germany the "Bulldogs" from the Mannheim-based company Heinrich Lanz AG, in particular, shaped agriculture after 1921 to such an extent that the term – especially in southern Germany – actually became the deonym for "tractor". In its function the hot-bulb engine combines the principles of a petrol and a diesel engine. Due to the low compression, however, the cast-iron hot bulb in front of the engine must be heated prior to starting with a heating lamp (or a gas burner). In order to actually start the engine of a Lanz Bulldog, the steering wheel and the steering column are then removed and placed aterally in the crankshaft. The engine is started by means of pendulum movements against the compression resistance. Due to their low compression ratio, hot-bulb engines only achieve modest efficiency with correspondingly high
fuel consumption. Nevertheless, they were very successful since they represented an inexpensive alternative to steam engines, especially in industry and agriculture. "Hot bulbs" also managed to hold their ground for many decades against the increasing competition of diesel engines since they could be produced more cheaply and placed low demands on fuel. They can be powered by normal diesel, but also by heavy oil, fish oil, paraffin or even "salad oil". The incredibly rustic technology, the "beefy" appearance and, naturally, the fascinating sound of the large single-cylinder engines have fascinated not only tractor enthusiasts – right up to the present day.