HERBERT Stumm’s delightful Gold Medal winning model of the Lanz Bulldog tractor is one that visitors to exhibitions never tire of. Like all of Herbert’s model it is a working one. Like all of his models it is something a bit different.

The Bulldog was born out of necessity after the First World War when German farmers were short of convention fuels like petrol (gasolene) and diesel. In response Lanz designed a tractor using an engine based on a design by an English man, 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 differs 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.

Herbert had the idea to build the Lanz Bulldog on a visit to the inspirational Sinsheim Technical Museum. After being given a copy of a history of Lanz with drawings, Herbert was able to start the project. Later he was able to inspect a tractor thoroughly and start work on the 1:6 scale model.

Herbert rarely makes drawings, and only prepares sketches “if a part is too complicated to carry all the dimensions in my mind.” Although he does make use of readily available flywheel castings, generally he avoids castings, preferring to carve complex shapes out of solid sections. In this case the chassis and crankcase were cut from aluminium alloy blocks.

The model is of a road going tractor and fitted with rubber tyres - actually modified garden mower tyres bought at a car boot sale.

In total it occupied 18 months spare time.