Elliott J. Stoddard invented and patented two versions of the Stoddard engine, the first in 1919 and the second in 1933. The general engine classification is an external combustion engine

External combustion engine

An external combustion engine is a heat engine where an working fluid is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work...

 with valves and single-phase gaseous working fluid (i.e. a "hot air engine"). The internal working fluid

Working fluid

A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder...

 was originally air, although in modern versions, other gases such as helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 or hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 may be used.

One potential thermodynamic

Thermodynamics

Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 advantage of using valves is to minimize the adverse effects of "unswept volume" in the heat exchangers (sometimes called "dead volume"), which is known to reduce engine efficiency and power output in the valveless Stirling engine

Stirling engine

A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....

.

The 1919 Stoddard Engine

The generalized thermodynamic processes of the 1919 Stoddard cycle are:

• 1. Adiabatic
  Adiabatic process
  In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
  
   compression
  Compression ratio
  The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
  
  
• 2. Isobaric
  Isobaric process
  An isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, , and barus,...
  
   heat-addition
• 3. Adiabatic expansion
  Thermal expansion
  Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
  
  
• 4. Isobaric heat-removal

The 1933 Stoddard Engine

In the 1933 design, Stoddard reduced the internal volume of the heat exchanger

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s maintaining the same generalized thermodynamic processes as the 1919 cycle.

Gallery