Spontaneity and Efficiency
Diesel engines convert the chemical energy in fuel to mechanical energy which moves pistons up and down inside cylinders. The pistons are connected to the engine’s crankshaft, which changes their linear motion into the rotary motion needed to propel the vehicle’s wheels. Energy is released in a series of small explosions (combustion) as fuel reacts chemically with oxygen from the air. The chemical equation of diesel fuel combustion is as follows, C13H28 + 20O2 → 13CO2 +14H2O. Combustion reactions are spontaneous yielding a -∆G. The reaction goes from 20 moles of O2 gas to 13 moles of CO2 yielding a -∆S. Combustion reactions break bonds between the molecules signaling an exothermic reaction or -∆H.
German engineer Rudolf Diesel theorized that fuel could be made to ignite spontaneously if the air inside an engine’s cylinders became hot enough through compression because air heats up when it’s compressed. Achieving high temperatures meant producing much greater air compression than occurs in gasoline engines, but Diesel calculated that high compression should lead to high engine efficiency. Part of the reason is that compressing air concentrates fuel-burning oxygen. A fuel that has high energy content per gallon, like diesel fuel, should be able to react with most of the concentrated oxygen to deliver more punch per explosion, if it was injected into an engine’s cylinders at exactly the right time. Diesel’s calculations were correct. As a result, although diesel engines have seen vast improvements, the basic concept of the four-stroke diesel engine has remained virtually unchanged for over 100 years.