|The Stirling Cycle
A very simple a low frequency example of the Stirling cycle is given by the drinking bird.
Where the energy for the pulsing fluid is provided by a temperature difference dT between the two glass bulbs linked by a concentric glass tube.
The beak of the bird dips into a cold volume of water cooling the bulb of the left whilst the ambient warm air heats the glass bulb on the right creating the temperature gradient required to put energy into the system. The bird will continue to oscillate about its pivot so long as the temperature difference is maintained.
Note: This usually requires the water level to be topped up as it evaporates. Our bird has worked for over a year.
The drinking bird
|The principle elements of a stirling engine are:
An enclosed volume of working fluid or gas.
A hot chamber at one end and a cold at the other connected by a specific size of small bore tube or a matrix of finely spaced plates .
The gap between the plates and the diameter of the tube has to be such as to allow a certain working fluid boundary film thickness to be established at the surface. This boundary film helps absorb and regenerate the energy of the working fluid by creating a fluid film nozzle across the system that expands and compresses the working fluid as it oscillates between the hot and cold ends.
The original Stirling engine was patented in 1816 by Rev. Robert Stirling. Principally a closed system. This means that a working fluid is enclosed within the system and that energy is added externally. The heat energy required to drive the cycle can therefore be supplied by any fuel or heat source available.
The Stirling cycle is created when the temperature gradient across the system is sufficient to drive an oscillating fluid pulse between the hot and cold extremities of the system.
The speed and frequency of the cycle will vary with the density and pressure of the working fluid used and the temperature gradient driving the system. An integral part of the Stirling cycle is the regenerator which is found in the middle of the hot and cold volumes. The effeciency of which improves the overall thermal efficiency of the system.
Modern versions of the stirling engine are produced by Whispergen and are used in both marine and domestic applications.
One way to reduce pollution would be for everybody to install one of these in there home!
There needs to be a 900 (p/2) phase difference between hot and cold fluid displacement..
Its revolutionary yet based on an 1816 patent By Rev Robert Stirling> With a thermal efficiency greater than the modern SI and CI engines.
But then there is the government tax revenue shortfall/paradox.
|This illustration of a Stirling engine manufacture in 1842
shows a sectioned layout of the engine.
The enclosed vessel is heated from the bottom.
Heat input to the working fluid which in this case is air at above atmospheric pressure. Expands the air volume below the displacer piston D. Moving it upwards.
Which in turn displaces the cold air above the piston. Chilled by passing cold water through a matrix of cooling tubes C.
The cold and hot ends of the system are connected by a coaxial duct that has a larger diameter than the displacer cylinder
This cooler denser air then passes via an external coaxial duct that contains a large array of closely spaced steel plates. That form the regenerator R. With an air gap between plates of 0.78mm.
The oscillating fluid drives the displacer piston up and down guided by a central sliding shaft that has three pressure seals to conserve the cycle pressure.
|Closely packed radial plates in the coaxial cylindrical regenerator. 1842 Dundee Stirling engine.|
|This experimental engine was used to run a foundry in
Cylinder bore diameter 305mm (12ins )
Displacer piston stroke 610mm (24 ins.)
Speed 40 rpm (0,67c/s)
Coal consumption 22.7kgs/hr (50lbs/hr.) 1.43kg/kwhr.
Power output 15.82*103 W
|Note:I have temporarily used these images whilst I am preparing some drawings.|
Reverend Robert Stirling Doctor of Divinity (DD)
Some reference reading:
Engine regenerator design by Allan Organ University of Cambridge
Stirling air engines there history and development by Allan Organ and Theordor Finkelstien.
A life history by Robert Sier is informative but fails to reach the quality of LTC Rolt's biography of Robert Stephenson.
Rev Robert Stirling by Robert Sier ISBN 0 9526417 0 4
Andy Ross Stirling engines
One of the best books on the development of practical Stirling engines and the problems encountered with scaling designs.
"Making Stirling Engines" by Andy Ross
Stirling Engine with Ross yoke
|Andy Ross, a prominent Stirling engine
experimenter, invented the linkage illustrated here.3
The engine is identical in operation to the two cylinder
Stirling. In this illustration, the left cylinder is the hot
The linkage allows the engine to be more compact and reduces side loads on the pistons and connecting rods (since their travel is almost linear).
Copyright 2000, Matt Keveney. All rights reserved.
The D-90 Ross Yoke-drive Engine The Ideal Adiabatic Solution
The 90 cc D-90 engine is fully described in Andy Ross' fascinating book "Making Stirling Engines" (1993). At Ohio University we have a D-90 engine which forms part of the Senior Lab course - a required course for all Mechanical Engineering students. In this section we examine the results of performing an Ideal Adiabatic simulation of the D-90 engine under specific typical operating conditions as follows
In order to simulate the engine by means of the Ideal Adiabatic model equation set given previously, we require the equations for the Yoke-drive volume variations and derivatives Vc, Ve, dVc and dVe (all functions of crank angle q), as well as the void volumes of the heat exchangers Vk, Vr, and Vh.
The cyclic convergence behaviour of the Ideal Adiabatic model is extremely good, and using 360 increments over the cycle, the system effectively converges within 5 cycles. The convergence criterion chosen is that after a complete cycle both variable temperatures Te and Tc must be within one degree Kelvin of their initial values. We now consider the solution of the temperature variables Tc and Te, the heat energy variables Qk, Qr, Qh, and the work energy variables Wc, We, and the net work done W. These results are presented as plots showing the variation of these parameters with the crank angle q.
It is of interest to examine the two components, Wc and We, which
added together gives the net work done W. These are shown as dashed
lines in the following diagram.
Recall that for the Ideal Isothermal model
Furthermore the Ideal Adiabatic model in itself does not give results which are significantly different from those of the Ideal Isothermal model. The pressure-volume diagram is of similar form, and the power output and efficiency are quantitatively similar (albeit the efficiency of the Ideal Adiabatic model is about 10% lower for reasons described above). However the the behaviour of the Ideal Adiabatic model is more realistic, in that the various results are consistent with the expected limiting behaviour of real machines. Thus the heat exchangers become necessary components without which the engine will not function. The required differential equation approach to solution reveals the considerable amount of heat transferred in the regenerator, indicating its importance in the cycle, and provides a natural basis for extending the analysis to include non-ideal heat exchangers (Simple analysis). Thus the solution of the Ideal Adiabatic model equations is equivalent to a simulation of the engine behaviour in all respects, from setting up the initial conditions until convergence to cyclic steady state is attained. Throughout this process all the variables of the system are available as by-products of the simulation and can be used for extending the analysis. Thus for example the mass flow rates through all the heat exchangers can be used in order to evaluate the heat transfer and flow friction effects over the cycle.
On to the Simple Analysis
Back to the computer program function set 'adiab'
Back to the Stirling Cycle Machines home page
|American Stirlings MM-7 Stirling Engine|
The ultimate DT engine that will run on a temperature difference of 40C
Linear Stirling engines
NASA reasearch on the application of linear stirling engines to drive alternators and generate electric power.
The benefits of linear Stirling engines are that they remove the need for a mechanism to tranform linear diaplacement into rotary power and are simple in concept with fewer working parts.
Benefits of linear Stirling engines:
Important parameters for designing an efficient Stirling engine:
ŠJohn Gwynn and sons2003
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