Early Swedish Hot-Bulb Engines - Efficiency and Performance Compared to Contemporary Gasoline and Diesel Engines

“Hot Bulb engines” was the popular name of the early direct injected 2-stroke oil engine, invented and patented by Carl W. Weiss 1897. This paper covers engines of this design, built under license in Sweden by various manufacturers. The continuous development is demonstrated through examples of different combustion chamber designs.

The material is based on official engine performance evaluations on stationary engines and farm tractors from 1899 to 1995 made by the National Machinery Testing Institute in Sweden (SMP). Hot-bulb, diesel and spark ignited engines are compared regarding efficiency, brake mean effective pressure and specific power (power per displaced volume).

The evaluated hot-bulb engines had a fairly good efficiency, well matching the contemporary diesel engines. At low mean effective pressures, the efficiency of the hot-bulb engines was even better than that of subsequent diesel engines. This is quite surprising since the compression ratios were much lower than in the diesel engines. The development of the hot-bulb engines lasted approximately until the end of the 2'nd world war, then the spark ignited and the diesel engines became superior. The measurements and evaluations made at SMP show that the overall engine development during the 20^Th century resulted in a more than 3 times higher efficiency and a 10 times higher power per displaced volume in engines for rural applications like farm tractors and stationary engines.