A vacuum engine refers to any kind of engine which derives its force from

air pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The Standard atmosphere (unit), standard atmosphere (symbol: atm) is a unit of pressure defined as , whi ...

against one side of the piston, while also having a

partial vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

on the other side of it. This pressure differential can be the result of heat transfer, or mechanically produced by an external source.

History

Some early

gas engine A gas engine is an internal combustion engine that runs on a fuel gas (a gaseous fuel), such as coal gas, producer gas, biogas, landfill gas, natural gas or hydrogen. In the United Kingdom and British English-speaking countries, the term is ...

s worked on the "vacuum" or "atmospheric" principle in a similar way to the

Newcomen steam engine The atmospheric engine was invented by Thomas Newcomen in 1712, and is sometimes referred to as the Newcomen fire engine (see below) or Newcomen engine. The engine was operated by condensing steam being drawn into the cylinder, thereby creating ...

. A mixture of gas and air was eaten by the cylinder and ignited; the mixture expanded and part of it escaped through the exhaust valve; the valve then closed, the mixture cooled and contracted, and atmospheric pressure pushed the piston in. Such engines were very inefficient and were superseded by engines working on the

Otto cycle An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what happ ...

Heat engines

(also called flame-licker engine, flame-engine, or flame-dancer) At the beginning of an intake stroke, a valve in the head of the cylinder opens and admits a charge of burning gas and air, which is trapped by the closing of the valve and expands. Towards the end of the stroke the charge comes into contact with a water- or air-cooled part of the cylinder and is chilled, causing a sudden drop in pressure sufficient to suck the piston – which is open towards the crank – back on the return stroke. The valve opens again in time for the piston to expel the burnt gases before the next stroke begins.

Vacuum motor

Sectional illustration of player piano interior action
12. is the Pneumatic motor powered by 1. Pedal In a vacuum motor, the partial vacuum is created by an external pump. These motors were commonly used to power

railway turntables A railway turntable or wheelhouse is a device for turning railway rolling stock, usually locomotives, to face a different direction. It is especially used in areas where economic considerations or a lack of sufficient space have served to weig ...

in the UK, using vacuum created by a

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

vacuum brake The vacuum brake is a brake, braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in countries influenced by British practice. Va ...

ejector A vacuum ejector, or simply ejector, or aspirator, is a type of vacuum pump, which produces vacuum by means of the Venturi effect. In an ejector, a working fluid (liquid or gaseous) flows through a jet nozzle into a tube that first narrows an ...

. The operating principle is similar to a steam engine – in both cases power is extracted from a difference in pressure. Small vacuum motors were also used to operate

windscreen wiper A windscreen wiper (Commonwealth English) or windshield wiper (American English) is a device used to remove rain, snow, ice, washer fluid, water, or other debris from a windscreen, vehicle's front window. Almost all motor vehicles, including ...

s in

automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...

s. In this case, the motors were powered by

manifold vacuum Manifold vacuum, or engine vacuum in a petrol engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere. Manifold vacuum is an effect of a piston's movement on the induction stroke and the airflow thr ...

. This arrangement was not very satisfactory because, if the

throttle A throttle is a mechanism by which fluid flow is managed by construction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ha ...

were wide open, the wipers would slow down, or even stop. Modern automobiles use electrically powered wipers. Modern automobiles still use a vacuum motor of a kind, however, the

vacuum servo A vacuum servo is a component used on motor vehicles in their brake, braking system, to provide assistance to the driver by decreasing the braking effort. In the US it is commonly called a brake booster. A vacuum servo, also known as a power bo ...

. Brakes are operated by a hydraulic system, but they use a ‘vacuum motor’ to amplify the force provided by the driver. Small vacuum motors were also used from the late 1960s to control

servomechanism In mechanical and control engineering, a servomechanism (also called servo system, or simply servo) is a control system for the position and its time derivatives, such as velocity, of a mechanical system. It often includes a servomotor, and ...

s such as door locks, heater controls or movable bonnet ventilation flaps. Some

Player piano A player piano is a self-playing piano with a pneumatic or electromechanical mechanism that operates the piano action using perforated paper or metallic rolls. Modern versions use MIDI. The player piano gained popularity as mass-produced home ...

use vacuum engines. You could say that the global Industrial Revolution arose because of a 'vacuum motor', because all the early steam engines, especially the pioneering Boulton and Watt engines, operated with almost atmospheric pressure steam. You can easily make a demonstration vacuum engine using a flywheel, simple plumbing parts and a few other simple components, as Neil A Downie shows in the reference. A vacuum system can be used for power transmission, although the maximum power that can be transmitted to a vacuum motor is less than conventional pneumatics. There is an optimum pressure for the operation of a vacuum power transmission system, of around 0.4 bar (8 psig), as Downie also shows. Although less efficient than pneumatics, it can be perfectly workable. For example, a pipe on vacuum can transmit as much power on 0.4 bar (8 psig) as a pipe on 8 bar (100 psig). The system is efficient enough that Boulton and Watt used vacuum power transmission in their factory. They called the vacuum main in the factory the ‘spirit pipe’.Smiles, Samuel, Lives of the Engineers, the Steam Engine, Boulton and Watt, London, John Murray (1878).

Ideal Thermodynamic Process

Unlike the ideal

engine, the vacuum engine relies on a constant heat source provided by burning fuel. As mentioned above, a valve allows an intake of heat into the piston chamber. Estimating the heat in or Qin is constant in the controlled volume space, the

ideal gas equation The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first sta ...

PV = nRT implies an increase in the pressure of the piston chamber. After the valve closes, the piston undergoes an adiabatic process during the downward stroke. Once the piston reaches the bottom of its stroke, the chamber is cooled either by the surrounding air or water, and the resulting Qout forces the pressure in the piston to decrease. The system then undergoes another adiabatic compression of the gas in the chamber, which is subsequently released by the valve at the top of the cylinder's stroke, while simultaneously allowing new heated gas to enter the chamber. One of the major issues that this engine encountered while being developed was that the efficiency of this model was extremely poor in real applications. Because the heat source is not contained to a specific area, only a small portion of the potential fuel is being consumed to power the engine. Because

Engine efficiency Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines- #Internal combustion (gasoline, die ...

is defined by the relationship between the amount of work done and the potential energy in the fuel consumed, it can be seen that in the vacuum engine only a small amount of the burning fuel is being used to power the engine. The rest of the fuel energy is lost to the surrounding atmosphere.

References

External links