Compression replaced the spark plug
A diesel engine does not need a spark to set fuel burning. Its piston squeezes air until the air becomes extremely hot; fuel injected into that compressed air ignites. That difference gave Diesel’s engine flexibility in what it could burn: the key was whether fuel could be delivered into the hot air and ignite, not whether it matched gasoline’s behavior in a spark-ignition engine.
The fuel still has to move through the machine
Peanut oil can burn in a diesel engine, but it is thicker than the petroleum diesel used in modern engines. Thickness matters because the fuel system must push the liquid through narrow passages and break it into a fine spray. Poorer atomization can leave fuel burning incompletely, while unburned or partly burned material can build up in the engine.
So a successful demonstration did not mean that every vegetable oil was a ready-made, trouble-free replacement for diesel fuel. Processing can change plant oils into biodiesel, a fuel designed to behave more like conventional diesel; using raw oil raises different practical challenges, especially for regular operation.
A flexible engine, a constrained fuel choice
The striking point is not that peanut oil was identical to petroleum diesel, but that compression ignition made a wider range of fuels technically possible. Whether those fuels became useful in everyday transport depended on more than the engine: fuel handling, reliability, and the supply of a consistent fuel all mattered. The engine could tolerate an unconventional fuel; the fuel system and the market determined whether that possibility could last.
