A vacuum engine (also called flame-licker engine, flame-engine, flame-dancer) derives its force from

air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars ...

against one side of the piston, which has a

partial vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often d ...

on the other side of it. At the beginning of an outstroke, 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 outstroke begins.

History

Some early

gas engine A gas engine is an internal combustion engine that runs on a gaseous fuel, such as coal gas, producer gas, biogas, landfill gas or natural gas. In the United Kingdom, the term is unambiguous. In the United States, due to the widespread use of ...

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 often referred to as the Newcomen fire engine (see below) or simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creat ...

. 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 Such engines were very inefficient and were superseded by engines working on the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 hap ...

Vacuum motor

In a vacuum motor, the partial vacuum is created by an external pump. These motors were commonly used to power railway turntables 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, oil or, rarely, wood) to heat water in the loco ...

's vacuum brake

ejector An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...

. 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 wipers in

automobile A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...

s. In this case, the motors were powered by

manifold vacuum Manifold vacuum, or engine vacuum in an internal combustion 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 ...

. This arrangement was not very satisfactory because, if the

throttle A throttle is the mechanism by which fluid flow is managed by constriction 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' ...

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 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 booster o ...

. 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 control engineering a servomechanism, usually shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism. On displacement-controlled applications, it usually includes a built-in ...

s such as door locks, heater controls or movable bonnet ventilation flaps. 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 22 mm (7/8") pipe on vacuum can transmit as much power on 0.4 bar (8 psig) as a 6 mm (1/4") 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 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

{{commons category
Concept illustration
Engines

History

Vacuum motor

Ideal Thermodynamic Process

See also

References

External links