Diesel engine
The diesel engine is a compression-ignition internal combustion engine that replaces spark-based ignition with heat from air compression. It solves the low efficiency of steam engines by achieving 26.2% effective efficiency by 1897 — 75% above steam’s 10% theoretical limit. Its core mechanism works: air compression alone ignites fuel. But its original isothermal-cycle theory failed — abandoned after criticism revealed it demanded physically impossible compression ratios. Diesel corrected to a constant-pressure cycle by June 1893, filed two patents, published a treatise, and proved the concept with petrol ignition on 10 August 1893. The first successful engine, Motor 250/400, was tested in 1897. It changes energy conversion by establishing compression ignition as a scalable, high-efficiency alternative — but only after discarding its founding premise. Worth your time if you work on thermodynamics, engine design, or historical technology development.