Nikolaus August Otto, is the father of the infernal combustion engine; and I have had to be VERY harsh and not indulge in writing a biography of the fellow and his associates, who included Gottlieb Daimler and Wilhelm Maybach, who are also eminent pioneers of the Automotive industry. So I have made do with simply linking their names to their entries in Wilkopedia.
If you are even SLIGHTLY interested in cars or engines, the life and exploits of some of these early pioneers is absolutely fascinating, and worth the read, and I look at the topic in a bit more detail in the article, Evolution of the Engine'.
But any way; Otto was a boffin; and he built one of the earliest practicable internal combustion engines, all be it a stationary plant engine, that ran on 'city gas' from a fixed supply, and used a pilot light, just like your central heating boiler, as it's ignition system!
It was NOT a very useful engine for motorised transport, and the way that it worked is all a LITTLE bit difficult to explain.
As you can see, doesn't look much like an engine, and it doesn't have a crank-shaft and where's the piston & cylinder?!
actually, the cylinder is the pedestal that the thing looks like its resting on, and the piston is at the end of that long toothed rack running up the middle of it.
It worked 'upside down' to the engine's we are familiar with; combustion in the cylinder pushed the piston up, and then it fell, under the force of gravity, back down the bore, that rack turning a gear that spins the fly wheel and the rest of the mechanism's via an ingenious arrangement of one way roller clutches and cogs.
It is a fascinating device, but not much of a contribution to science or engineering! And, while it was an early internal combustion engine, it wasn't actually the first.
So, Otto's contribution to technology then, was his study of engines, and the principles and theories he derived from looking at them and building them, and ultimately, the mantra; 'Suck, Squash, Bang, Blow'! And cutting to the chase; he described and defined a four phase combustion cycle;
Charge (fuel & air) drawn into the engine
Charge put under pressure
Charge ignited, and allowed to burn, releasing energy
Burned charge expelled from the engine, ready for a fresh cycle to begin
It doesn't sound like much of a theory, does it? But, it it is the principle on which just about all internal combustion engines operate, and without the understanding that Otto provided into the topic, the pace of development that we have seen, and the degree of dependency that society has invested in the internal combustion engine would have been significantly handicapped.
And if Descartes can be vaunted as one of the greatest minds of all time for a theory that is condensed down to five words, 'I think, therefore I am', so Otto has got be be at least four fifths as great as Descartes, hasn't he!