In the 1860s Étienne Lenoir had built a gas-fuelled internal-combustion engine; in 1876 Nikolaus Otto perfected the four-stroke cycle that separates intake, compression, power, and exhaust. Although Otto's engine was stationary, heavy and gas-fed, it was thermodynamically far more efficient than the steam engine: it burned its fuel inside the cylinder rather than in an external boiler. The problem was to compress this cycle into an engine small, light, and fast enough to fit on a moving vehicle. In the mid-1880s two German teams reached very similar solutions independently of one another.

In Mannheim, Karl Benz placed a horizontal single-cylinder, water-cooled, petrol-fed engine at the rear of his three-wheeled vehicle designed in 1885. Unlike Otto's slow engine, Benz's used electric spark ignition, water cooling, and ran at several hundred revolutions per minute. On 3 July 1886 the first public road trial took place on the streets of Mannheim. Near Stuttgart at Cannstatt, Gottlieb Daimler and Wilhelm Maybach, after leaving Otto's company, independently designed a high-speed (600+ rpm) compact petrol engine and in late 1885 attached it to a wooden-framed motorcycle they called the "Reitwagen." In March 1886 they mounted a similar engine on a horse carriage. In the summer of 1888 Bertha Benz, without her husband's knowledge, drove the Patent-Motorwagen 106 km from Mannheim to Pforzheim with her two sons — the first long-distance test of the automobile and its first serious public encounter.

The engine quickly created an automobile market, but the real turning point came in 1908 when Henry Ford produced the Model T on a moving assembly line. The price of the car fell from 850 dollars in 1908 to 260 dollars by 1925; the car became reachable for the middle class. Petroleum began to displace coal as the primary energy source: refineries, pipelines, filling stations, and the great oil companies (Standard Oil, the Anglo-Persian forerunner of BP, later Aramco) built a global infrastructure. By the mid-twentieth century the car was reshaping the form of cities worldwide: suburbs in the United States, motorway networks in Europe, then car parks and traffic in every corner of the world.

The meaning of the internal-combustion engine lay less in speed than in independence. Without depending on the care of a horse, a railway route, or an electric wire, an individual could go in a direction and at an hour of their own choosing. The cost of that freedom was high: through the twentieth century traffic accidents took millions of lives, cities were cut off from the pedestrian, and more than a quarter of the rise in atmospheric CO₂ came from the transport sector. In the first quarter of the twenty-first century the electric motor has begun to displace the internal-combustion engine; yet there are still more than 1.5 billion internal-combustion vehicles in the world. The Otto–Benz–Daimler trio again reminds us that in modern technology, invention rarely settles on a single point.