My Favorite Engine Designs

Pavel Radzivilovsky (VisionMap ltd.)

Summary

My favorite engine designs

  • The original Otto cycle ICE. The power is limited by amount of oxygen during intake at the external pressure.
  • The Diesel cycle is an optimization. Ignition is done by adiabatic compression heat up. This involves higher compression ratios, and therefore higher temperatures and improved Karnot efficiency. On the other hand, higher pressures imply stronger, heavier and therefore slower-moving parts. The default choice for trucks, buses, ships, submarines and large stationary generators for long time, it was now successfully scaled down to cars - and even sport cars. Despite the violent explosions giving Diesel ICE the characteristic "taxi" noise, it now becomes increasingly popular.
  • The two-stroke ingenuity, is used in motorcycles and lawn-mowers. Any comment on this stunning picture is useless - this is the only way to do it. Saving the two strokes improves power-to-weight, but comes with a price of inefficiency and pollution. It has characteristic motorcycle noise, and is an excellent choice for emergency equipment and mobile generators.
  • The Wankel invention of mathematical precision reduces the number of moving parts from forty-three to one. All four strokes - intake, compression, work and exhaust are done by this rotor. This enables increased safety and faster operation, and consequently better power-to-weight ratio. Used in Mazda sport cars and some UAVs, it is not as efficient as four-stroke.
  • During intake and exhaust the ICE working chamber is connected to the outside environment. During compression and work it is sealed off, with an explosion in the middle. If work finishes at the same volume as compression started, same material and volume and different temperature would mean higher pressure than in the start of compression - which is obviously the atmospheric pressure. The Atkinson cycle fixes exactly that, by making each of the strokes to have different path length. Less powerful for given volume than Otto, the Atkinson Engine was first used by Toyota Prius - which can tune the engine to be more Atkinson-like or Otto-like according to the driver's power demand. Miller cycle is a different approach to the same problem: exhaust valve is kept open during part of the compression.
  • Gnome rotary is a funny ICE design for aircraft. It is famous for its safety due to built-in chamber redundancy.
  • Jet Engine is probably the most counter-intuitive, and even to seasoned physicists it often takes time to grok. In jet engine's simplest form, the inlet fan compresses air into the burning chamber, then fuel is injected and ignited on the way, and flies out on the other side. The inlet fan is turned by an axis connected to the outlet fan, which, in turn -by the rushing outlet air. I am literally not making this up. But why wouldn't it work the other way around, one would ask. Of course, this is taken care of by the design - the front fan is bigger and stronger! Exhaust gases are hotter thus having more volume, so a weaker (pressure-wise) fan can power the stronger one - without violating the conservation of energy. Grossly inefficient, the Jet Engine became the king of the skies due to highest power-to-weight ratio, outstanding safety record and low production and maintenance cost. It has very few moving parts, almost no solid-to-solid friction and can suck enormous amounts of air. Jet turbine can be used as-is, or connected by a shaft to turn a propeller (turbo-prop), a layer of fan blades (turbojet) or anything else, such as a helicopter rotor (turboshaft). It is often used in stationary applications as an emergency supply for power demand peaks.
  • Watt did not invent the steam engine out of the blue. The original idea was to use vapor to pump out water out of coal mines. All they had is water, fire and air. This shows a pretty advanced stage of what eventually inspired creating a general work-performing machine for the industry and transport.
  • On the opposite end of the scale from the Jet, we have the Stirling Cycle Engine, sometimes referred to an ECE - External Combustion Engine. It involves separation the working fluid from burning. Lighter gases (typically Helium) can be used, making it the champion of efficiency. Also, the power does not decline with lower air density at high altitudes, opening interesting opportunities for aviation. It was proposed for solar-thermal power generation as an alternative to expensive photovoltaic cells. High thermodynamical efficiency enables it for industrial waste-heat utilization at small temperature differences.
  • It was noticed that Stirling cycle is similar to a standing wave. This lead to another ingenuity - invention of the thermoaccoustic engine. There is no working chamber - the air parcel is confined by the standing sound wave itself. The loudest of all - this exotic engine design has zero (solid) moving parts, making it the cheapest to produce and maintenance-free. Efficient power extraction becomes a challenge due to the high frequencies. Typically used to power another thermoaccoustical device. Has a certain potential for solar-thermal energy gathering, where efficiency is not important (there is plenty of sunlight anyway) but the device cost per unit power is currently the highest of all power production methods.
  • The Peltier effect thermocouple allows making electrical work (as in Charge times Voltage) directly from the temperature difference, without passing through mechanical work on the way. It is often referred to as a Solid State Engine - as it produces work from heat and is therefore qualified for the engine title. Expensive and inefficient, consumer models are light, small and handy to work with. In spaceflight, it is used as a part of an RTG nuclear cell on the four man-made interstellar spaceships, where the Sun is too far to produce much sunlight. RTG is also used on some polar lighthouses. .
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