The Manson Engine - A simple External Combustion Engine for the 21st Century
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Page 2
The Manson cycle engine was designed
back in 1952 by A.D. Manson  and first
published in Newnes Practical Mechanics
March 1952 p193.

Click here for
page 1 of the article   650k
Click here for
page 2 of the article   245k

Contains constructional notes and
drawings of the original Manson engine.
Caution these are big files!  If you prefer
to read off the screen, this text now fully
transcribed - scroll down to view text and
Click here for Steve Truscott's Manson
Click Here for Transferator Manson
What is a Manson Cycle Engine?

The Manson cycle is one of several
thermo-dynamic engine cycles which uses the
thermal expansion of air to do work. In this
respect it is similar to a hot-air engine or
Stirling engine.
When air is heated it expands and this
expansion can be used to make a piston move
out in a cylinder. Then the air is cooled and the
piston will move back.

When the piston is coupled to a crankshaft
and flywheel it will do rotary mechanical work
and this can be used for turning machinery,
pumping water or generating electricity.

The key feature of the Manson is that it uses
external combustion and therefore can use any
fuel which can be burnt. It could also be made
to run using the heat of focussed sunlight.
So to Sum up the Manson Engine.

It is an open cycle external combustion
hot-air engine, with the cycle controlled by a
piston operated valve. It takes a fresh
charge of cold air each time the piston
reaches TDC. It exhausts the heated air to
the atmosphere at the end of the expansion
stroke at BDC.

The displacer and piston move together as a
single component.

There is an expansive power stroke and a
power suction stroke making two power
strokes per cycle.

The engine, when started will run in either

The piston diameter should be
approximately 46% of the diameter of the

The engine is of simple construction and
may be very attractive to Developing World
production where only a few tens of watts
are required. It can be made easily on a
lathe and drilling machine using common
Figure 1 shows the first Manson Engine as
published in Newnes Practical Mechanics.

The spirit burner on the left, heats up the hot
cap, whilst the right hand side of the engine is
kept cool with a water jacket. A heat break
of asbestos washers and the firebox wall
helps to isolate the hot end of the engine from
the cold end.

With the engine in the position shown, top
dead centre (TDC) , the air inlet port has just
opened and the air from the outside rushes in
to equalise the pressure inside the engine with
atmospheric pressure. The flywheel inertia
carries the engine past TDC and the air inlet
port closes.

The piston and displacer now begins to move
outwards to the right and the relatively cold
air in the cylinder marked  A is displaced to
the hot end. It is squeezed down the annular
gap between the displacer and hot cap and
heats up rapidly. It begins to expand and the
internal pressure of the engine starts to rise.
above atmospheric pressure. As a result the
heated air exerts pressure on the piston and it
is accelerated to the right thus performing the
expansion stroke.

On reaching the end of the stroke, the pipe C
running down the centre of the engine, now
lines up with the exhaust port X on the
cylinder wall. Any remaining internal pressure
is vented to the atmosphere as the heated air
rushes down the exhaust pipe.
The internal pressure of the engine is now
restored to atmospheric pressure as the
exhaust port closes. The flywheel carries th
piston past BDC and the piston displacer
combination begins to move inwards to the

There will still be some residual hot air in the
hot end, but as the displacer returns, this is
displaced to the cold end where it cools and
contracts and the pressure rapidly falls below
atmospheric pressure. The pressure of the
atmospher outside, now pushes the piston to
the left performing the vacuum or suction
power stroke, untill all the internal air is at the
cold end.  The air inlet opens and the engine
cycle continues.

The pipe C, running down the centre of the
displacer was a means of conveying the hot
exhaust air down through the middle of the
engine without causing it to heat up the cold
end of the engine.

The pre-heated air coming from the exhaust
could be used to provide additional hot
draught to the fire - particularly if solid fuel
was being used.
One of the novel features of the Manson
engine is that it only requires one moving part
to do useful work. Other engines like the  
Steam or Stirling engine rely on separately
moving valves or displacers to control the
cycle, and this adds to engine complexity. The
Manson, I believe, is thus  unique, and this is
the attraction - a simple heat engine with
minimum moving parts.

The engine works on the principle of 2 power
strokes, firstly an expansion stroke which
pushes the piston outwards when the internal
pressure within the engine exceeds the
external (usually atmospheric) pressure.  This
expansion stroke is then followed by a
vacuum or suction stroke. At the end of each
stroke a port is opened to the atmosphere, by
the piston skirt, and thus the internal pressure
of the engine is restored to atmospheric
pressure so that the cycle may begin again.

Key Technical Points

The displacer in the Manson engine is
connected directly to the power piston. This
power piston needs to be of a given diameter
with respect to the displacer in order to get
the most work from the expanding air.  If the
piston diameter is too small then it will see
very little force, if it is too large a diameter,
then it will occupy too much volume in the
cold end leaving very little air left in the
remaining annular gap to displace to the hot
end and expand.  Obviously there must be
some optimum ratio between displacer
diameter and piston diameter.

Using Steve Truscott's Manson Calculator
this can be shown to be a piston diameter of
46% of the displacer diameter.

The annular gap between displacer and Hot
cap should be about 1/32" (0.8mm) for every
inch (25mm) of diameter.

Scaling up from Manson's original sketch
(Fig. 1 ) he seems to have got these figures
fairly correct!
Manson Engine Builders to Date

Steve Truscott - Australia

Julian Wood UK

Geoff Bartlett UK
a 7cc 0.5W engine 1000rpm.

Ken Boak UK   53mm bore 38mm stroke
Experimental Manson Transferator

Ole Berge USA a large engine similarto
Manson engine

E.Schmidt Germany
Rupp Heisluftmoteur - on sale,  see below
I am currently devising ways of making the
Manson engine more powerful and easier to
construct from everyday materials and parts.
Read this report for more information.
Digging Deeper
An XL Spreadsheet to calculate Manson
engine design parameters by
Steve Truscott.
Indicator diagram plots from Steve's first
Manson engine.  XL format.
For a 38k downloadable Word file
describing the cycle, some variations and
some recent work-
click here.
Click Here for latest photos
Email Ken to discuss Manson
A Novel Hot Air Engine; Constructional Details of an Experimental Double Acting Model  by A.D. Manson.
From Newnes Practical Mechanics 1952.
The hot air engine here described
works on a new principle devised by
the writer. As will be seen by the
sketch (Fig. 1.) it is simple and
therefore should not be difficult for the
average mechanic to construct.

The illustration shows the  piston,
which is tubular, to be directly
connected to the displacer so that they
move together as one piece.

The working cycle is as follows:
starting on the out stroke. As the piston
moves out the cold air contained in the
cold portion of the displacer chamber  
A, is displaced to the hot end and the
pressure gradually rises, so driving the
piston. At the end of the outstroke the  
piston comes to a position where the
two ports X register, and therefore all
the air above atmospheric
pressureescapes from the hot end. On
the return stroke the small quantity of
hot air remaining in the hot enf of the
displacer chamber is displaced to the
cold end and cooled, therefore, a
partial vacuum is formed, which
increases as the stroke continues and
the piston is thus forced in by the
pressure of the atmospher until near the
end of the stroke, when the air inlet
port registers with that of the working
cylinder. Air now rushes in to fill the
vacuum and the cycle of operations is
thus completed.

The following are the advantages of
this type of hot-air engine:

The heated air during the out stroke
remains in the hot end until the end of
the stroke and is discharged directly
from the hot end to the atmosphere
and thus heating of the cold end is
avoided to some extent. The incoming
air is always at atmospheric
temperatuer and pressure.
The engine is double acting and can be
reversed. There are few moving parts,
no additional mechanism being
required to drive the displacer.

Constructional Details.

Starting with the displacer chamber,
this is made from a piece of mild steel
tube, or it can be constructed from a
piece of sheet metal not more than
1/32"  (0.8mm) thick. One end is
closed by a piece which is cut a little
larger than the diameter of the cylinder.
It is then beaten out hollow or dished,
as in Fig 1. and fitted in the end of the
tube, being secured air tight by brazing
or welding.

A ring made from 1/4" square iron rod,
or cut from plate, is fitted on the
outside of the other end. It can be
brazed on or soft-soldered. The
chamber should then be placed in the
lathe and the ring trued up as it has to
carry the working cylinder.
Working Cylinder.

This cylinder is made from a piece of
mild steel pipe, the finished size being
1/1/8 " bore  (28.5mm) by 3.125"  
(79.5mm) long.The outside should be
turned bright and the step to receive
the flange should be cut for a distance
of 3/16"  (4.75mm) at one end.  
Before smoothing out the bore the
air-inlet ports and the exhaust port
should be drilled, care being taken to
ensure their correct positions.

The cylinder flange is made from a
piece of mild steel plate 3/16"
(4.75mm) thick. It should be turned
and a spigot of 1/32" (0.8mm) made
on the inside to fit the mouth of this
displacer chamber. A light groove
should be cut  on the otherside to mark
the position of the 8 fixing screws, the
holes for which should now be drilled.
The flange can now be used as a jig to
drill the tapping holes in the chamber
flange ring. The flange can now be
pressed or lightly driven onto the
cylinder and  solder should afterwards
be applied all around the joint. A thick
paper gasket rubbed with oil and
graphite will make an airtight joint
when the cylinder and chamber are
finally united together.

The Displacer.

The displacer can be made from a
piece of tube or from sheet metal abou
1/32" thick . The outside diameter
should be 1/16" (1.6mm) less than the
inside diameter of the chamber. One
end is dished like that of the chamber
whilst the other end is about 1/8"
(3.2mm) thick and flat. All joints on
this part should be brazed or welded
and airtight. A hole should be bored in
eache end at its centre to thake the
1/4" (6.35mm) bore  exhaust pipe C.

The Piston.

the piston which is in the form of a tube
can be brass or other anti-friction
metal. It should be 4.5"  (114.3mm) by
1.125" (28.5mm) outside diameter and
a fairly tight fit in the cylinder. A flange
3/16" (4.75mm) thick having four
equally pitched , countersunk  holes for
the 1/8" screws (3.5mm) which unite it
to the displacer , is fitted at one end. It
can be brazed or soldered to the piston
and afterwards this part should be
trued in the lathe and the outside
diameter of the flange made equal to
that of the displacer.
There is also  a disc soldered in the
piston tube flush with the flange at this
end. It is about 1/8" (3.2mm) thick and
it is bored at its centre to take that part
of  the exhaust pipe which is fitted
inside the piston. The air inlet pipe
should then be made and fitted, care is
necessary to fit them in their correct
positions. See Fig. 1.
A short piece of brass rod (Fig.2 )
having a slot cut to take the small end
of the connecting rod has also to be
fitted and soldered inside the piston.
The hole for the gudgeon pin is then
drilled square through the piston tube
and the pice of brass which thus forms
the bearing for the gudgeon pin. The
gudgeon pin is held in position by a
short screw passing through the
upperside of the top end  of the
connecting rod and for a short distance
into the pin itself. A hole in the piston
tube allows the pin to be fitted.

Having  got the piston to this stage it
should now be made a good sliding fit
inside the cylinder by lapping, using
metal polish as a grinding medium.

Two or three asbestos washers are
fitted between the piston flange and the
displacer to prevent as far as possible,
heat getting to the cold end. Bt varying
the number of the above mentioned
washers, the length of the displacer can
be finally adjusted.

When assembled the piston should
move freely the full stroke of the
engine, and allowing 1/16" (1.6mm) for
clearance the displacer should not rub
on the inside of the displacer cylinder.

The exhaust should be on the top and
the air inlet on the underside of the
cylinder. The connecting rod does not
need to be heavy. A disc or a double
web crank can be used. The
crankshaft may be 5/16" (8mm)
diameter and the flywheel about 5"
(127mm) in diameter.

Cooling Water Tank.

This tank can be constructed of
galvanised iron. It will have to be
soldered to the displacer chamber, and
the bottom, which should be 1/16"
(1.6mm) thick, will have to project
1/2" (12.7mm) at each side to take the
holding down screws as the thrust and
pull of the piston is taken on this part.
Key Dimensions/Cutting List

Displacer Diameter   2"     50mm
Displacer Length     2"     50mm
Piston Tube length   5.2"  132mm
Piston Tube diameter 1.125" 28mm
Cylinder Length      3.125" 79mm
Cylinder diameter    1.25"  32mm
Cylinder bore        1.125" 28mm
Con-rod length       3.5"   89mm
Flywheel Diameter    4.5"  114mm
Exhaust tube bore    1/4" 6.35mm
Crankshaft diameter  5/16"   8mm
Crankpin diameter    1/4" 6.35mm
Small end diameter   3/16"   5mm
The lamp for burning methylated spirit
is provided with a rectangular
reservoir, having a short length of 1/4"
(6.35mm) tubing soldered at the
bottom, on the other end  of which is
another piece of tubing fitted with three
3/8" (10mm)  diameter burners. (Fig.

The wicks are made of asbestos cord
twisted together. The lamp should be
placed centrally under the hot end of
the displacer chamber. The baseboard
may be of oak or other hardwood 1/2"
(12.7mm) or 5/8" (16mm) thick. It
should have two end pieces on  to
prevent warping, as shown in the
photograph of the completed model.
Manson Cycle indcator diagrams - measured from real engine by
Steve Truscott.   XL Spreadsheet format
Recent Manson Engine Developments

The following work has been done since
September 2000.

Steve Truscott has built a Manson engine,
simulated its operation in XL, and used a
pressure transducer, ADC, PIC micro and
PC to plot out indicator diagrams.

Geoff Bartlett (Birmingham UK)  has built a
7cc displacement Manson engine which
runs at 800rpm to 1000 rpm in either
direction. Geoff is now inverstigating the
control of the Manson cycle using better
valves. For this purpose he is constructing a
larger engine.

Julian Wood of Sterling Stirling  (UK) has
produced a couple of Manson models and
hopes to offer this type of engine for sale
at reasonable cost.

Ole Berge of Minnesota USA, has created
a large engine similar to the Manson and this
was displayed at the Lake Itasca show in
August last year. Details are sketchy at the
moment, but I am expecting further
information from the Arizona Flywheelers
Cottonwood Show.

Ken Boak (web-author) has begun work on
an experimental engine using a transferator
instead of the usual displacer.
See panel below.

He also has ideas for high performance
engines using a special variant of the basic
Manson cycle.  More on this in a later

See latest Transferator Engine Page
Early sketch of Steve Truscott's Manson Showing arrangement of ports
Here is a Rupp-Heis-Luft-Moteur by E.Schmidt of Germany. It's a type of Manson
Engine, modelled expertly in pyrex and ground glass. This engine can be bought from
Schmidt's website with many types of Stirling engine. You can also write to :
PBO. 2006, Koernerstr. 3,  D-61440 Oberursel, Germany.  The cost is DM 327
-about $150.
Fig. 2 The Heat-exchanger end of the Manson Scaled up to Drinks
Can Size.
A Modern Implementation of the Manson Engine using  Tin
Manson's 1952 design may be altered to use ready available
stainless steel containers in order to simplify the construction. It is
also possible to make a low cost version utilising drinks cans for
the displacer and a pet food tin for the hotcap/displacer chamber.
Although made from easily obtained items, the steel cans will not
have the same life expectancy as stainless steel containers.

I have scaled the original drawings on the photocopier and come
up with the following basic design parameters for each of these
two suggestions.

The best ratio between the displacer and the power piston is
2.175:1 (46%) although 2:1 will give perfectly reasonable

i) Using Insulated Mugs

I have come across a parallel sided stainless steel double walled
insulated mug - Geofff Bartlett put me on to these.  The mugs are
made from two deep drawn stainless steel cups, one placed inside
the other and welded together around the rim.  The two containers
may be separated by carefull grinding the weld away - leaving a
ready made displacer and hotcap.  The inner cup is 72mm inside
diameter an the outer is 77mm outside diameter.  The length of the
outer is 99mm. This should leave an annular gap of about 2mm,
taking into account the thickness of the material.   The correct size
for the power piston bore will be between 32 and 35 mm -
utilising whatever tubing is available close to this size.

ii) Using a drinks can.

These are a nominal 2.6" (66mm) in diameter and are a fairly good
displacer fit in a deep drawn (seamless) pet food tin which has a
71mm inside diameter.  The complete displacer chamber could be
made by soldering two pet food tins together to make a tube that
is 175mm long.  The soldered joint will be close to the water
jacket and will not see high temperatures.

It may be worth looking around for long cans  7" or 175mm from
which the displacer chamber may be made in one piece.  
Possibilities include jumbo beer cans, large aerosol spray cans and
cans from household products.  I have found a 600ml household
cleaner in a steel tube 66mm in diameter and a useable length of
230mm. The nominal 66mm diameter seems to be a standard size.
Although some aluminium aerosol cans can be found with a 59mm

Care must be taken when handling aerosol cans. The pressure
should be carefully released with extreme caution that the product
does not squirt towards the face or eyes. Common sense is key
here.  Cans should be thoroughly cleaned prior to incorporating in
engines. Some cans appear to have a bright internal finish but this
is a clear lacquer applied to make the steel suitable for food
containers. This lacquer needs to be taken off with abrasive
"emery" cloth prior to attempting to make soft soldered joints.  
Use a high wattage soldering  iron of at least 25W.

iii) A Combination of Stainless outer and steel can inner.

Some cookware containers used for flour shakers are available
with a 68mm diameter. You may find that these make an ideal
hot-cap fit when used with a drinks can or aerosol can displacer.
The length is 89mm and there is often a screw on lid which could
be used to advantage.

As is the case with all recycled engines - its just a matter of going
down to the store with an accurate rule or calipers, and pretending
that your are from the trading standards office. A white lab-coat
and clipboard may help in getting you thrown out quicker. Don't
mention Manson, or you're bound to be arrested;-)

Some Basic Design parameters.

Having found a suitable hot cap and displacer pair, the power
piston diameter may be calculated as 46% of the displacer

The stroke of the engine is fairlly long and a 1:1 relationship
between displacer bore and engine stroke is perfectly acceptable.
Shorter strokes can be used but the long stroke give a longer
period for the heat transfer to occur.

It is essential to keep the ratio of the hot cap heated area to the
displaced volume as large as possible.

Manson's original engine had a 2" troke and a 2" displacer
diameter. The power piston bore was 1.125".  Subtracting the
piston volume from the displacer swept volume gives about 66cc
with a hot cap area of about 107cm2. The ratio is 1.6. Increasing
the diameter of the hot cap and piston will reduce this ratio further
and lead to a less efficient engine. This is the reason why smaller
hot air engines a(with plain heaters) are generally better performers
than large engines. An engine of half the linear dimensions will have
twice the ratio of area to volume.

Having calculated the internal swept volume of the engine it is then
necessary to calculate the amount of cooler coil required.  In the
transferator design it is important that the space between the
transferator and piston cylinder is effectively filled with as much
cooler coil as possible - and a double layer coil of 8 turns  6mm
(1/4")  OD copper pipe will fill this otherwise dead volume quite
well. About 2.3 m (7.5')  of tube can be wound into this space.

Update. For larger engine sizes, disposable propane canisters  are
available with a 4.25"  (108mm) OD and 5" (127mm)  length.
Heavy duty propane cylinders fitted with a threaded torch
connection and a "Schraeder" style recessed valve,  sized 71mm
OD x 250mm tall are also available from automotive superstores.
(Halfords larger stores in the UK). It's an expensive way of buying
propane, but a cheap way of getting a useful pressure container.
Here is the original article from
updated to include metric
equivalents wherever possible.

Asbestos washers and sheet as
mentioned in the article should be
replaced with a more modern heat
insulation material. Ceramic board
and ceramic paper are possibilities.

Galvanised iron should be treated
with care as it emits toxic zinc
oxide fumes when heated during
welding or brazing.