Four-stroke engine

The four-stroke engine solved a fundamental problem that had limited earlier internal combustion designs: efficiency. Étienne Lenoir's 1860 engine drew in fuel and air during the power stroke itself, wasting energy by igniting an uncompressed mixture. Nikolaus Otto's insight was to compress the fuel-air mixture before ignition. Compressed gases burn more completely and expand more forcefully. This simple change doubled the efficiency of internal combustion and created the engine that would power the automobile age.

The adjacent possible for the four-stroke engine required the internal combustion concept to exist but also required Otto to recognize why existing designs were inefficient. The Lenoir engine worked—it powered small workshops and pumps—but consumed enormous amounts of fuel for the power it produced. Otto, a traveling salesman turned engineer, spent years experimenting before achieving the four-stroke cycle in 1876.

The cycle Otto developed has four distinct strokes: intake (fuel and air enter the cylinder as the piston descends), compression (the piston rises, compressing the mixture), power (ignition drives the piston down), and exhaust (the piston rises again, expelling burned gases). The compression stroke was the key innovation. By compressing the mixture before ignition, Otto extracted far more work from each combustion event.

Otto's engine was reliable, efficient, and relatively quiet—qualities that made it commercially viable. Within ten years, more than 30,000 Otto cycle engines were sold. The engine ran on coal gas, which limited its mobility, but the principle worked equally well with liquid fuels. Otto's company, Deutz AG, hired two engineers whose names would become synonymous with automobiles: Gottlieb Daimler and Wilhelm Maybach.

Daimler and Maybach left Deutz and adapted Otto's engine for transportation, increasing its speed and power density to make it practical for vehicles. Their 1885 motorcycle and 1886 automobile used modified Otto engines. Carl Benz, working independently, also built automobiles around the four-stroke design. The engine that Otto created for stationary industrial use became the engine that powered cars, trucks, motorcycles, boats, and aircraft.

The Otto cycle remains the principle behind virtually all gasoline engines today. Diesel engines use a variation with compression ignition rather than spark ignition, but the four-stroke rhythm persists. Every car on the road traces its engine's lineage back to Otto's 1876 design—intake, compression, power, exhaust, repeated thousands of times per minute.