What type of fuel did the first diesel run on?

The first diesel engine did not run on diesel fuel specifically. Rudolf Diesel, the inventor of the diesel engine, originally designed it to run on vegetable oil, such as peanut oil. However, due to the high cost and limited availability of vegetable oils at the time, Diesel later successfully demonstrated his engine using petroleum-based fuels such as kerosene. It wasn't until later that the engine was optimized to run efficiently and reliably on diesel fuel, which is a heavier and more energy-dense distillate obtained from petroleum crude oil.