Part 1. By Julius de Waal

The Lanz Bulldog was born out of necessity after the First World War when German farmers were short of conventional fuels like petrol (gasolene) and diesel. In response Lanz designed a tractor using an engine based on a design by an Englishman, Herbert Ackroyd Stuart, which ran on almost any liquid fuel, including crude oil, tar oil, spirit, vegetable oil, and so on, by using a hot bulb.

This two-stroke engine differed from the diesel which compresses air and fuel is injected into the hot air at the top of the stroke. The diesel requires a high compression ratio.

In contrast the hot bulb engine injects fuel via the pre-heated bulb and ignition takes place when the rising piston forces air into the cylinder. That requires a compression ratio of only 5:1 rather than the 20:1 of the diesel.

The rest of the tractor was designed to be simple for the farmer to use and maintain. To start the engine he had to pre-heat the bulb for 20-minutes using a blow torch.

Last week Find Hansen described his model of the smallest two-stroke hot bulb engine. This design is too small for most model engineers to make a working hot bulb engine and so includes a vacuum engine to make an interesting alternative for a model in this scale which retains an external heat source. The drawings are based on those originally by C. P. Hendrikx.

Click on drawings to download - for personal use only.