On September 30, 1913, Rudolf Diesel’s cabin aboard the steamship SS Dresden was found empty, his bed unslept in, his nightshirt laid out, and his pocket watch propped up beside it. The engineer whose name is on fuel pumps worldwide had vanished into the English Channel, leaving one of the most debated mysteries of the twentieth century.

This episode traces Diesel from his birth in Paris in 1858 and his family’s expulsion during the Franco-Prussian War to his studies under Carl von Linde, his obsession with the Carnot cycle, the explosions that nearly killed him, and the compression-ignition engine that transformed industry. We then weigh the evidence behind three competing theories about his disappearance.

• Why steam engines wasted up to 90 percent of the energy in their coal
• How compressing air until it is hot enough ignites fuel without a spark plug
• The engine that ran on peanut oil at the 1900 World Exhibition
• The bag of 20,000 marks, empty bank accounts, and a cross drawn in his diary
• Suicide, assassination, or a British ruse: the theories behind his disappearance