Internal combustion engine

• 1. Introduction

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  Inside an internal combustion engine, a fuel burns with an oxidizer inside a chamber that sits directly in the path of the working fluid. The high-temperature, high-pressure gases produced by that combustion press straight against the engine's own parts. Sometimes the force lands on pistons. Sometimes it spins turbine blades, or pushes a rotor, or rushes out through a nozzle. Each time, chemical energy becomes motion. That motion drives most of the cars, aircraft and boats that move people around the planet. The first commercially successful versions of this machine appeared in the mid-19th century. The first modern one, the Otto engine, was designed in 1876 by the German engineer Nicolaus Otto. How did a controlled explosion become the steady heartbeat of a vehicle? Who fired the prototypes that came before Otto, and why do some of these engines burn fuel in sharp pulses while others burn it without ever pausing? And what does a machine like this leave behind in the air once the work is done?

• 2. A Crowded Race

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  In 1791, John Barber developed the gas turbine, long before any of these machines powered a vehicle. The path to a working engine was crowded with inventors who each added a piece. Thomas Mead patented a gas engine in 1794, the same year Robert Street patented an internal combustion engine that was the first to use liquid fuel. In 1798, John Stevens built the first American internal combustion engine.

  In 1807, the French engineers Nicephore Niepce and Claude Niepce ran a prototype that used controlled dust explosions, called the Pyreolophore. Napoleon Bonaparte granted it a patent, and the engine powered a boat on the Saone river in France. Nicephore Niepce later went on to invent photography. That same year, the Swiss engineer Francois Isaac de Rivaz invented a hydrogen-based engine ignited by an electric spark. In 1808, De Rivaz fitted it to a primitive vehicle, described as the world's first internal combustion powered automobile.

  In 1854, in the United Kingdom, the Italian inventors Eugenio Barsanti and Felice Matteucci obtained a certification titled "Obtaining Motive Power by the Explosion of Gases". The Great Seal Patent Office granted them patent No. 1655 in 1857, and they secured further patents in France, Belgium and Piedmont between 1857 and 1859. Belgian engineer Jean Joseph Etienne Lenoir produced a gas-fired engine in 1860, and Nicolaus Otto patented the first atmospheric gas engine in 1864.

  American George Brayton invented the first commercial liquid-fueled engine in 1872. By 1876, Otto was working with Gottlieb Daimler and Wilhelm Maybach when he patented the compressed charge, four-cycle engine. Karl Benz patented a reliable two-stroke gasoline engine in 1879, and in 1886 he began the first commercial production of motor vehicles built around such an engine. The thread continued for decades: Rudolf Diesel developed the first compression ignition engine in 1892, Robert Goddard launched the first liquid-fueled rocket in 1926, and the Heinkel He 178 became the world's first jet aircraft in 1939.

• 3. Pulse Versus Flame

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  The term internal combustion engine usually refers to a machine where combustion is intermittent, arriving in sharp bursts. The familiar two-stroke and four-stroke piston engines work this way, along with variants like the six-stroke piston engine and the Wankel rotary engine. In every case, the fire flares, does its work, and then the chamber resets for the next burst.

  Gas turbines, jet engines and most rocket engines belong to a second class entirely. These run on continuous combustion, where compression, burning and expansion happen at the same time in different places inside the machine. The flame never goes out while the engine runs. A gas turbine compresses air, raises its temperature by burning fuel, and lets the heated gases expand through turbine blades that spin a shaft.

  The contrast sharpens against external combustion engines like steam or Stirling engines. In those machines, energy reaches a working fluid that is never mixed with or contaminated by combustion products. The fluid might be air, hot water, pressurized water, or even boiler-heated liquid sodium. The defining line of the internal combustion engine is that the fire and the working fluid share the same chamber.

• 4. Anatomy Of A Piston Engine

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  The base of a reciprocating engine is the engine block, typically cast from iron for its wear resistance and low cost, or from aluminum. The block holds the cylinders, which in multi-cylinder engines line up in one row as a straight engine, in two rows as a boxer or V engine, or occasionally in three or four rows as a W engine. Single-cylinder engines, nicknamed thumpers, are common in motorcycles and light machinery.

  Pistons are short cylindrical parts that seal one end of the cylinder and slide continuously inside it. Smaller engines use aluminum pistons, while larger ones use cast iron, and performance applications reach for titanium or forged steel. The top surface, called the crown, is usually flat or concave. Rings fitted around each piston block gases from leaking into the crankcase and oil from creeping into the combustion chamber.

  The cylinder head bolts onto the block and seals the cylinders opposite the pistons. It carries the intake and exhaust ports, the valves that open and close them, and in spark ignition engines the spark plug. Often those valves are poppet valves, though rotary or sleeve valves appear too. One or several camshafts and springs control when the valves open, though some engines use a desmodromic mechanism that needs no springs.

  A connecting rod links each piston to an offset section of the crankshaft called a crankpin. The crankshaft sits below in the crankcase, held by main bearings, and it turns the up-and-down motion of the pistons into rotation. At the bottom, a sump collects falling oil so it can be cycled again. The exhaust gases leave through the exhaust manifold, may pass a catalytic converter and muffler, and finally exit at the tailpipe.

• 5. Two Strokes Or Four

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  Top dead center marks the piston's position nearest the valves, and bottom dead center is where it sits furthest from them. In a four-stroke engine, each piston makes two strokes for every turn of the crankshaft, cycling through intake, compression, power and exhaust. During intake, the piston drops and pulls in air or an air-fuel mix called the charge. During compression, both valves close and the piston rises until ignition begins just before top dead center.

  A two-stroke engine compresses the timeline, completing a full cycle every single crankshaft revolution. Because four jobs must fit into two strokes, no stroke belongs to just one task. The engine uses the last part of the power stroke and the first part of the compression stroke for combined intake and exhaust, a process called scavenging. The work to push out burned gas and bring in fresh charge comes from either the crankcase or a separate blower.

  Dugald Clerk developed the first two-cycle engine in 1879, using a separate cylinder as a pump to move the fuel mixture. In 1899, John Day simplified that design into the form of two-cycle engine still widely used today. Day cycle engines are crankcase scavenged and port timed, using the crankcase and the lower cylinder as a pump.

  Crankcase scavenged two-strokes carry real costs. They use a total-loss oiling system, where all the lubricating oil eventually burns with the fuel, so they release more polluting exhaust and run less efficiently than comparable four-strokes. In the United States, two-stroke engines were banned for road vehicles because of the pollution. Off-road motorcycles are still often two-stroke but are rarely road legal, while many thousands of two-stroke lawn maintenance engines remain in use.

• 6. Giants And Specialists

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  The largest reciprocating internal combustion engines are low-speed compression ignition engines that use blower scavenging and uniflow scavenging. Built for marine propulsion and electric power generation, they reach the highest thermal efficiencies of any internal combustion engine. The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel, used in large container ships, stands as the most efficient and powerful reciprocating internal combustion engine in the world, with a thermal efficiency over 50%. By comparison, the most efficient small four-stroke engines sit around 43%, and size is itself an advantage because of the rising ratio of volume to surface area.

  Multiple crankshaft configurations can skip the cylinder head entirely by placing a piston at each end of the cylinder, an opposed piston design. Thermal efficiency improves precisely because there are no cylinder heads. The Junkers Jumo 205 diesel aircraft engine used two crankshafts at either end of a single bank. The Napier Deltic diesel engine went further, using three crankshafts to serve three banks of double-ended cylinders arranged as an equilateral triangle with the crankshafts at the corners.

  The Wankel rotary engine throws out piston strokes altogether. It follows the Otto cycle but lets the phases happen in separate locations, so it may be thought of as a four-phase engine. The drive shaft rotates once during every power stroke instead of twice, giving it a greater power-to-weight ratio than piston engines. The Mazda RX-8, the earlier RX-7 and other models used it, and it also appears in unmanned aerial vehicles where small size and high power-to-weight ratio matter.

  Inventors have reached well beyond two and four strokes. Nicolaus Otto manufactured and sold a double expansion engine in 1879, though the owner returned it for poor performance. The six-stroke engine was invented in 1883, with variants like the Griffin, Bajulaz, Velozeta and Crower designs that fire every three crankshaft revolutions and capture the waste heat of the Otto cycle with an injection of air or water.

• 7. Spark, Heat, And Ignition

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  Every internal combustion engine needs its mixture ignited, either by spark ignition or compression ignition. Before reliable electrical methods existed, engineers used hot tube and flame methods, and experimental engines have even been built with laser ignition. When Bosch developed the magneto, it became the primary way to produce the electricity that energizes a spark plug, and many small engines still rely on magneto ignition.

  Gasoline engines draw in a mixture of air and gasoline and compress it as the piston climbs. Early engines ran at compression ratios of 6 to 1, but as those ratios rose, so did efficiency, and high-performance engines can now run reliably at 12 to 1. Charles Kettering developed a lead additive that allowed higher compression ratios, a practice progressively abandoned for automotive use from the 1970s onward, partly over lead poisoning concerns. The same Kettering developed the electric starter at Delco, ending the era when every gasoline automobile needed a hand crank.

  Compression ignition takes a different path. Diesel, PPC and HCCI engines rely solely on the high temperature and pressure of compression, usually twice or more that of a gasoline engine. A diesel engine takes in air only, then sprays a small quantity of fuel into the cylinder just before peak compression, where it ignites instantly. Because they depend on that heat, diesel and HCCI engines are more prone to cold-starting trouble, which is why light-duty diesels often use glowplugs to pre-heat the combustion chamber before starting.

  The voltage involved climbs high. A spark ignition system typically needs about 10,000 volts, supplied by an induction coil or transformer. Capacitor discharge ignition was developed to fix a high-RPM misfire problem, and its voltages can reach 60,000 volts. As early as 1900, Rudolf Diesel was running his engines on peanut oil, a reminder that the fuel feeding these sparks and heat has always been open to substitution.

• 8. Power, Limits, And Exhaust

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  Most engines, even when aided by turbochargers and stock efficiency aids, hold an average efficiency of only about 18 to 20%. The ceiling is set by physics. An internal combustion engine is a heat engine, and its theoretical efficiency cannot exceed that of the Carnot cycle, which depends on the gap between the engine's lower and upper operating temperatures. The upper temperature is bounded by the thermal limits of the materials and by the fuel's resistance to auto-ignition.

  Forced induction pushes against those limits by delivering compressed air to the intake, raising the pressure, temperature and density of the incoming air. A supercharger draws its power directly from the engine shaft, while a turbocharger runs off a turbine spun by the exhaust. The technique is especially valuable in aviation engines, which must operate at high altitude. The latest Formula One engines have pushed thermal efficiency past 50%, and a modern turbofan can operate as high as 48%.

  What leaves the tailpipe carries a cost. Reciprocating engines produce air pollution from incomplete combustion of carbonaceous fuel, with main derivatives of carbon dioxide, water, and soot called particulate matter. Studies in humans and animals link inhaled particulate matter to asthma, lung cancer, cardiovascular issues, and premature death. Combustion also releases nitrogen oxides, sulfur, uncombusted hydrocarbons, carbon monoxide, and hazardous pollutants like benzene and 1,3-butadiene.

  Incomplete combustion usually traces back to insufficient oxygen to reach the perfect stoichiometric ratio. The flame gets quenched by the relatively cool cylinder walls, leaving unreacted fuel that the exhaust expels. Because removing carbon dioxide from exhaust is impractical, interest is turning toward alternatives. Sustainable fuels such as biofuels and synfuels, along with electric motors powered by batteries, stand as the examples already on the table, and the question of whether hydrogen could one day replace fossil fuels in these engines remains open.