Engines

The word "engine" is a bit old fashioned. At one time, an engine was any kind of mechanical contrivance. For example...

• The original, artificial computer was a mechanical device called the difference engine. I had to add the word artificial in there since the word computer originally referred to people whose job it was to perform repetitive calculations very efficiently. The difference engine was an "artificial computer" as it was designed to perform the same operations that these "human computers" were performing.
• The decorative borders found on US currency are drawn (in part) by a mechanical device called a cycloidal engine; basically a lathe mounted on a pendulum that etched lines on to a metal plate mounted on another pendulum. When the periods of the two pendulums are incommensurate (have no common multiple) the line etched on the plate would trace out an attractive geometric pattern like the ones shown below.
• Prior to the use of gunpowder, large projectiles were launched during military attacks by means of a siege engine. The best known example of such a device (at least for speakers of English) is the catapult, but the term also includes such devices as the ballista, the mangonel, and the trêbuchet.
• Although not mechanical, a search engine is an electronic contrivance for sifting through the vast wasteland that is the Internet in search of an unusual word, a specific group of ordinary words, a phrase consisting of words in a particular order, or other such identifying strings of characters. Although I can't verify it, I believe that the contemporary "search engine" is a parody of the earlier "siege engine". The coincidence is just too striking given the nerdish associations between computers and Medieval fantasy role playing games common at about the time when the Internet moved from academic playground to pop cultural phenomenon.

With the exception of the last example, the original meaning of the word engine is now pretty much obsolete. This can be traced back to the development of the steam engine in the middle of the Nineteenth Century. Nowadays, an engine generally refers to a device that transforms heat into mechanical energy. Technically, such a device is a heat engine, but in the current era, the adjective "heat" is generally dropped.

Although the two devices are often confused, an engine is not the same as a motor. An electric motor (often just called a motor) is a device for transforming electrical energy into mechanical energy.

... What about compressed air motors?

This gives rise to several linguistic problems. Why are automobiles sometimes called "motorcars"? Many cars do have motors in them, but the device responsible for propelling them isn't a motor. It's an engine. At the dawn of the Twenty First Century cars use motors to drive the wiper blades, open and close the windows, adjust the seat and side view mirrors, and spin CDs; but they only use an engine to drive the wheels (although this is likely to change). The largest auto maker in the US may be called General Motors, but what they really sell are automobiles driven by engines.

The description of an engine is very simple, but it refers to a wide variety of different devices. Engines can be found in cars, trucks, motorcycles, planes, boats, ships, trains, lawnmowers, chain saws, model airplanes, portable generators, cranes, augers, drills, and rockets (to give but a few examples). Engines can be classified according to one or more of several schemes.

• by the motion of its parts
  • Reciprocating Engine
    pistons going up and down
    piston arrangements: V, W, inline/straight, flat, radial
  • Rotary Engine
    parts that spin like in a wankel, turbine, turbocharger, jet turbine, turbojet, fanjet
  • Rocket Engine
    need not have any moving parts, motion is strictly produce by action-reaction, hot gases are expelled backward, rocket goes forward, also known as reaction engines
  • Unknown
    ramjet, scramjet
• by where the fuel is burned
  • Internal Combustion
    fuel is burned inside the chamber containing the working fluid (air)
  • External Combustion
    fuel is used to heat the working fluid (air, liquid water, steam, molten sodium)
• by the manner in which fuel is burned
  • Continuous Combustion
    a steady flow of fuel burns in a steady flame
  • Intermittent Combustion
    discrete amounts of fuel are periodically burned
• by the cycle through which the operating gas is run
  • Otto Cycle
    Nicolaus Otto (1831-1891) Germany, conceived in 1861, built in 1876
    gasoline powered cars
  • Diesel Cycle
    Rudolf Diesel (1858-1916) France-England, patented in 1892, successfully built in 1897
    trucks, locomotives; intended to used powdered coal but usually burn diesel oil, but can also run on vegetable oil (biodiesel)
  • Rankine Cycle
    William Rankine (1820-1872) Scotland, described in 1859
    steam turbines, Watt's engine, Newcomb's engine
  • Brayton Cycle
    George Brayton (1830-1892) US, first proposed the concept in 1873
    gas turbines, jets
  • Miller Cycle
    Ralph Miller (1890-????) Denmark-US, first patent application in 1945 (abandoned), reapplied in 1949,51,52
    Variation on the forced induction Otto-cycle with asymmetric valve timing
    • early-closing intake valve, closes before BDC reducing pressure below atmospheric, cooled adiabatically as well, lower Tc for increased efficiency
    • late-closing exhaust valve, stays open while intake valve is open, effectively a pass through, delays onset of compression
    • late-closing intake valve, closes before BDC delaying onset of compression, some reverse flow into intake manifold, counteract this with a supercharger and an intercooler
  • Wankel Cycle
    Felix Wankel (1902-1988) Germany, designed 1954 tested 1957
    Mazda again, very few moving parts
  • Stirling Cycle
    Robert Stirling (1790-1878) Scotland, invented in 1816
    works on any temperature difference, high efficiency, low to no exhaust (OTEC: Ocean Thermal Energy Conversion), high manufacturing cost

Information Taken from: https://physics.info/engines/