At the 1900 Paris World's Fair, Rudolf Diesel's engine ran on what unusual fuel?

Answer

Diesel was interested in engines that could run on vegetable oils.

Context

At the 1900 Paris Exposition, a small diesel engine built by the Otto company ran on peanut oil, according to Diesel's own later accounts. He said the test was run at the request of the French government, which hoped that groundnut oil produced in its African colonies could power local engines. That detail explains why the fuel was chosen.

The engine itself was not specially built for peanut oil. Diesel said in 1912 lectures that it was designed for mineral oil and ran on vegetable oil without changes. A biography reporting the opposite is contradicted by his own account, according to the AOCS history of vegetable-oil fuels. Diesel's compression-ignition engine ignites fuel by heat from compressing air, so it does not need a spark and can burn a range of oils. The ignition-by-compression principle was patented by Diesel in the 1890s.

Diesel did see potential for small-scale users such as farmers and craftsmen, but the engine's commercial success came through petroleum-derived fuel, which was cheap and abundant. Vegetable oils are thicker and can leave deposits, so modern engines usually use biodiesel (chemically processed oil) or blends instead. Diesel died in 1913 in circumstances that remain unexplained, after disappearing from a steamer crossing the English Channel.