Rider's hot air engine

The Rider hot air engine is a technological marvel born in America.

Rider Air Engine
Inventor Alexander K. Rider, of Walden, in the county of Orange and State of New York.
Country America
Year 1875
Patent Yes
Operation Closed cycle
Combustion External
Engine type Two cylinders
Working piston Single acting
Output 0.5 HP to 4 hp
Fuel consumption Coal, gaz, wood


The Rider air engine has been a very popular engine in America that came to Europe, and also Australia. It is one of the most widespread engine for little power.
It was found especially serviceable on railroads for filling water tanks; in public buildings, hotels, universities, asylums, city and country residences, and all descriptions of manufactories; for irrigation purposes; in short, wherever a small, neat, compact motor is required.

The engine was constructed with great care, and exhibiting superior workmanship, noiseless, simple, safe, economical, and efficient. It was built without the use of valves, springs, levers, or, in fact, any delicate parts whatever, the moving parts being reduced to the lowest possible number, namely, the pistons, shaft, and connections.

Engine Arrangement

This engine is of the class of the closed cycle engine in which the working fluid - in this case the atmospheric air - never comes in touch with the combustible of the furnace and is heated externally.
The same air is used continuously, as there is neither influx nor escape, the air being merely shifted from one cylinder to the other is alternately compressed, heated, expanded, and cooled.
Within such a closed cycle engine the mass of air remains always constant, its volume will vary according to the position of the pistons only, a,d its pressure will vary according to both the position of the pistons and the temperature.

The engine is symmetrical in appearance, has two separate and parallel cylinders, one on the cold side and the other on the hot side where the furnace is also to be found.
This strict separation of the two sides is of much advantage as heat and cold do not mix together like in other engines.

Rider Air Engine

In the vertical elevation A is the compression cylinder or cold cylinder, B the power cylinder or hot cylinder, C is the piston on the cold side or cold piston with its connections, D is the piston on the hot side or hot piston with its connection, H is a regenerator, E is a water cooler, and F is a heater located above the furnace.

The cooler E is made of a jacket that surrounds the lower portion of the compression cylinder A and in which circulates cold water. That way, the cylinder C is always kept cool.
On the other side, the lower portion of the power cylinder B is kept hot by the action of the fire below the heater F.

Their are two cranks I I which stand at an angle of about 45° though giving a phase shift to the pistons of half the height of each cylinder. The J J connecting rods, and K K packings are in duplicate for each cylinder. L is a simple check valve which supplies any slight leakage of air that may occur.

Between the compression and power cylinders A and B is situated the regenerator H. It is composed of a number of thin plates slightly thickened at their edges, which, while affording a free passage to the air, sub-divides it into thin sheets. It is so placed between the cylinders as to be traversed by the air in its passage each way between the hot and cold cylinders. Thus the heat is alternately abstracted from and returned to the air in its passage backwards and forwards through these plates.


The operation of the engine is as follows. The compression piston C first compresses the cold air in the lower part of the compression cylinder A into one half of its normal volume. The heat due to the compression is absorbed by the cooler and the compression is done isothermally.

During the completion of the down stroke of the compression piston C the hot piston D undergoes an upward motion in the hot cylinder and the compressed air is transferred from the compression cylinder A toward the heater F. During this transfer the air volume remains constant.

On its way to the heat F, the air passes through the regenerator H which - at this very moment of the engine cycle - is hot. The compressed cold air warms up when touching the thin metal plates of the regenerator and takes away the heat from them. The metal plates cool down until becoming cold as the air takes the heat away.

When the warmed up compressed air comes into the heater, it heats up even more and, the volume remaining constant, the air pressure increases strongly, corresponding to the increase of temperature. This great pressure impels the power piston up to the end of its stroke. This is the power stroke of the engine which is due to the expansion of the hot air.

It is followed by the downward motion of the hot piston D that pushes the expanded hot air from the hot cylinder B into the regenerator toward the cold cylinder C. When passing through the regenerator the warm expanded air leaves the greater portion of its heat in the cold regenerator plates, to be picked up and utilized on the return of the air toward the heater.
The air cools down and keeps on loosing pressure.

This air cooling down is completed by the cooler when the air arrives in the cold cylinder A as the cold piston C moves up to its highest level.
The air is now back in the cold cylinder A and is completely cooled, its pressure has fallen to its minimum, the power piston descends, and the compression again begins.

The cranks angle being 45°, the piston phase shift determines exactly the motion of the air within both cylinder and its volume.
This can be illustrated taking following assumptions:

  • The cold air in the engine is at atmospheric pressure: 1 Atm
  • The cold and hot cylinder have the same size and can contain 2 air volume units each (1 unit of air is equal to the half fo the cylinder volume)
  • The air is heated up to 300°C, enough to double the air pressure.

Volume pressure table

Compression piston Cold cylinder Working piston Hot cylinder Air volume units Air pressure Engine phase Impact of air
Up Full Middle Half empty 3 1 Atm Cooling down Constant volume
Middle Half full Down Empty 1 2 Atm Compression Isothermal
Down Empty Middle Half full 1 4 Atm Heating up Constant volume
Middle Half full Up Full 3 2 Atm Expansion Isothermal

Distinctive features

1 - Simplicity as an art
Rider is one of the few that reduced his air engine to the minimum. The whole construction has been done in such a way as to avoid all unecessary. Nothing is worse than a tortious piece of technology. It only adds losses, failure, risk and reduce life span. In engineering, simplicity gives birth to bright and solid technology.

2 - Separation from hot and cold
The arrangement in two cylinders was a discovery, or a re-discovery, as it is said it has been inspired by the Franchot air engine. But at the contrary of the Frenchman, Rider did a strict separation of the hot and cold sides. This was an important step that seldom took place in the engine construction.

Many air engine suffered from the hot air mixing with the cold one. As a result after a period of work the engine came to rest. For the inventors that succeeded in maintaining the engine working, the hot and cold mixing was nonetheless a loss of performance. Compression of hot air uses more energy than the compression of cold air. Therefore before compression, the air must be completely cooled down in order to use as less work as possible during the compression phase.
To do so, there are a few ways like using a regenerator and a cooler. Rider added his own way, the cylinder separation and pushed his logic to have each of them dedicated to one sole operation : cooling or heating.

3 - The regenerator
The regenerator has been very popular and most air engines before 1850 had one. Nevertheless due to its intricacies and practical issues it was slowly abandonned. At the Universal Exhibition of 1889 in Paris, the regenerator had almost disappeared from all air engines. Rider was an exception, and he did well.
The perfect theoretical regenerator is always a balance between maximum heat surface and minimum volume; which is a kind of a contradiction.
The perfect pratical regenerator has still not yet being invented. Apart from the maximum surface / minimum volume contradiction, all regenerators so far had huge problems with metal corrosion and resistance due to never ending heating up and cooling down of the material.
Rider addressed these issues in some ways. Instead of using meshes of thin metal parts, his regenerator was composed of thin metal plates. The metal surfaces was then limited compared to meshes, but has a much better resistance. Furthermore, they could be easily replaced. This mid-term compromise offered good practical results.

Rider went around another issue of the regenerator : the latter was equalizing temperatures within the engine and as such was adding to the problem of the mixing of hot and cold air to such an extend that it was becoming a hindrance to the air engine. In the Rider engine, the air cooling is not only done by the regenerator alone, but also by the cooler. With this added mean, the engine receives effectively and alternatively cold air and hot air and due to the separation of hot and cold side, the mixing is almost totally avoided.

4 - The pistons
In the Rider air engine the pistons have both two function. As in all engines, the two pistons close tightly the compression cylinder and the working cylinder. But in Rider engine the cold piston has also a cooling function and the warm piston has a heating function.
Rider adds two fundamental features to both pistons, without complication, just by making them longer.
The length of the cold piston forces the air to run all along it. Be it at the compression or at the cooling phase, this give to the air an increased cooling surface. More on it below.
The length of the warm piston allow to separate the piston sealing from the warm side and also gives rooms for increased heating surface. More on it below.
The warm piston is hollow and the air within it prevents the sealing from heating too much. Air is a good isolator.
For the most powerful engine, Rider added a water jacket to the top of the warm cylinder further preventing any injury to the piston joints. This small water jacket was linked the cooler water jacket and run at the same time.

5 - The air cooling
The air cooling has received careful thoughts from Rider, first with cylinder separation, second with the regenerator, third with the cooler. The water jacket surrounds 2/3 of the cold cylinder but the effective volume when the engine is working is only 1/3, the one located at the bottom. The bottom of the cold piston never goes above this third. In other words, the jacket covers double the need of the cylinder. This was done on purpose to increase the cooling time and is the reason why the piston is so long. As might be seen on the picture, the regenerator is positionned at the top of the cylinder and not at the bottom, at a level way above the piston when it is at its highest position.
Therefore after the expansion stroke, when the cold piston is moving upward and the air moves back to the cylinder, the air has to go down all along the water jacket and cools before entering the bottom of the cylinder and keeps on cooling has the piston keeps moving upward. The cooling time is greatly enhanced, leveraging enormously the cooler effect.
When the cold piston moves downwards the cooling is again at work during the compression phase. The air goes first down along the water jacket and then up along it ensuring an almost perfect isothermal compression. Another construction would have been to position the regenerator at the bottom of both cylinders as the communication between two cylinders ends is more intuitive. But this would have reduced the cooling effect especially during the move upward of the cold piston.
This is an example how Rider brought simplicity to an art. With a simple ideas, he took the technology to a new level.

6 - Heater
Heating time is crucial, and Rider solved this by increasing the heating surfaces. To do so he used several means.
First he increased the surface of the heater giving it a hollow shape or an inversed "U" shape so that the air is in touch with a greater heat surface. His next idea was to add a heating feature to the warm piston. He did so by making the warm piston matching the shape of the heater. The greater heating surface of both the heater and the warm piston give increased heat to the air.
At last Rider added a thin metal sheet or cylinder between the heater internal wall and the warm piston. That way, when the warm piston moves upwards and the air is coming out of the regenerator, it has to pass along the heater wall and this metal sheet undergoing a first heating. Then the air passes between this metal sheet and the hollow shape of the warm piston undergoing additional heating. During the remaining upward motion the air continues to heat up thank to the warm piston hollow shape.
The special care given to the air heating ensured nearly an isothermal expansion that was one of the best condition for transforming the heat energy into mechanical energy.

7 - Miscellanous
The Rider engine is one of the few that had a perfect symetry between the two cylinders and therefore an excellent balance for the flywheel and the engine as a whole. Having the cylinder in a vertical position, he neutralized the effect of gravity. Except from the friction lossed, the energy for lifting the piston weight when moving upward was counter balance by the gravity on the downward stroke.
At last the greasing of the pistons and the cranks was made easy.


1 - Air volume and engine performance
Closed cycle engine have a performance limitation due to the air volume variation. This will be best understood when considering the volume variations of the air during the different phase of the engine cycle. After the compression stroke, the cold piston has done half of its downward move and therefore the volume of the air in the cold cylinder is halved. After the expansion stroke, means after the power stroke, it is expected that the volume of the air that has been cut by two during the compression phase, comes back to its initial volume. This is not the case as can be seen from the above volume pressure table: the volume of the air after expansion is 1.5 the initial volume.

The reason for it is that the variation of the air volume do not depend on the heating (the temperature) but on the motion of the pistons. Those move according to the cranks that stand at an angle of 45°. As shown in the above volume pressure table, the air volume increases over the initial air volume by 50%. A change in the crank angle would not better this fact.
Finally after expansion the hot air volume has increased by 50% in the warm cylinder, and it be transferred to the cold cylinder.

All closed cycle engine are bound to this fact and the only way to get around it is to having double acting pistons in which the 50% increase in volume due to the air expansion is used for the decrease of an equal air volume during the corresponding compression stroke.
Having single acting pistons, Rider could not get around it.

There are three consequences on the engine efficiency:

First an increase of 50% of the total hot air volume means that the warm cylinder has not enough capacity to collect all of it. As a consequence, during the expansion, 50% of the hot air inevitably has to go back in the cold cylinder. In doing so it goes through the regenerator and the cooler and looses pressure instead of properly expanding in the warm cylinder. This is a tragic loss of efficiency knowing that this transfer to the cold side occurs during the exansion stroke. It is even more tragic considering that air, unlike steam, has a very short expansion burst and therefore such a leak toward the cold side, occuring right at the begining of the expansion stroke, is horribly nocious.
This flaw is inherent to the Rider engine arrangement, but is found in all closed cycle engine in similar ways.

Second, after the expansion, as the cold piston moves up, the hot air in the hot cylinder passes to the cold side where it cools down. The impact of the cooling is to decrease the air pressure, but as we have seen, its volume does not change. Therefore, after the air is transferred to the cold side its volume is till +50% over the cold cylinder capacity. The extra volume has to remain in the hot cylinder where it is still taking heat from the heater. In parallel the cold piston starts the compression stroke and both pistons, cold and warm, move down. Compression occurs in the cold side, but also in the hot side where the air is still heated up.
Compressing hot air ask much more energy than compressing cold air, and this is why there is another important loss in efficiency.

The last consequence is less damaging, but after having lost so much efficiency, it is a pity. The hot air volume being increase by 50%, its cooling requires 50% more surfaces. Rider, who has done an excellent work in doing so, has limited the damage. Nonetheless, what a waste! A lower volume would require less water to cool, less material to build the engine.

2 - Harmful volume
A theoretically optimized regenerator has great exchange surfaces for a minimal volume. In practice this does not work, and all regenerator that are efficient bear the drawback to add a harmful volume to the air engine.

As an example let's assume an air engine with a working cylinder and a compression cylinder having each with a capacity of 2 liters and a regenerator having a air volume of 1 liter. When the air in the engine is compressed, the pressure is the same in all the engine and its total volume is of 2 liters : 1 liter in the compression cylinder + 1 liter in the regenerator.
This volume is transferred to the hot cylinder where it is heated up. Le volume in the regenerator takes up less heat than the volume in the heater. Therefore the gain in pressure done by the air is an average between the gain realized in the regenerator and the one realized in the heater. This average is never as high as the maximum that is possible would the air be only located in the heater. As a result the expansion stroke is systematically of lower performance than the practical maximum.

There are symetrical consequences on the cold side. The compression stroke compresses the air within the cold cylinder but also the air within the regenerator. Added energy is needed for this volume and, knowing that the air in the regenerator has lost heat but is stil not cold, the extra needed energy is even higher.
The regenerator is a good idea, but the air volume contained within it is of no use, worse it creates the harm of a strong loss of performance.

Alexander Monski in Eilenburg - a German manufacturer building the Rider engine under licence - saw additional disadvantages lying in the construction of the regenerator. The passages between the thin cast iron plates in the regenerator at the entrance to the cooling cylinder are blocked by the injected and entrained lubricant, thus making the passage of the air more difficult and though more powerful firing is required.

Therefore, the regenerator should be arranged in a ring around the hot cylinder (working cylinder) so that it is adequately protected due to its distance from the cold cylinder. Both cylinders are connected by a flat, elliptical, double-walled tube, which also serves to cool the heated air.

3 - Poor use of heat
Another imperfection pointed out by Monski was evident in the poor use of heat and the need for a very heavy flywheel. Monski installs 2 fire pots and 2 hot cylinders in order to achieve a more even and quiet run.
The two fire pots next to each other (see picture) are heated by a joint firing, so that with a comparatively small grate surface, a lower fuel consumption and better efficiency can be achievable. To achieve a more even gear, the cranks of pistons B and B1 are offset by 180 ° to each other. The cold cylinder C, in order to supply for both hot cylinders, is a double-acting one and is moved by the crank K. The plunger of a pump is coupled to the guide slide of the cold cylinder piston rod, which the cooling water pushes between the double wall of the cold cylinder.
The Monski Rider Air Engine
The regenerators R and R1 are arranged in such a way that R for the working piston B is connected through the channel b to the space above the piston A, while R1 is connected through the channel d to the space below the piston A. The tongue H inserted between the two fire pots forces the fire gases to rinse and heat the pots evenly.

4 - Water cooling
This is a nice feature of the Rider engine. But it has its downside. Because of the need of water, the engine cannot be used everywhere. And nowadays as water is scarce, its use for cooling is rather a waste than an optimization.

5 - External firing
This is the lot of all external fired engine. The best furnaces still looses around 30% of the energy of the combustible that goes into the air in form of wasted gas.
Rider was aware of this and gave special attention to the furnace. Its internal walls are surrounded by bricks that isolate the metal from the fire. But hot gas still has to go into the air, though leaking energy into the atmosphere.

Another drawback of external fired engine is the loss due to the indirect heating of the working fluid. The air is not in touch directly with the fire, but only with the metal and therefore do not collect all the energy that is spent.

Other issue with the external firing is the resistance of the heater. But the Rider engine overstepped this issue.

Conclusion about the Rider hot air engine

Rider's air engine has been an important improvement in the field of the hot air engine.

The strong limitations described above are due primarily to the choice to build a closed-cycle engine and were shared by all other closed cycle engines. Some inventors got rid of the harmful volume by taking away the regenerator. But none had found the solution to get around the air volume issue.

This said, the Rider engine is certainly one of the best hot air engine ever made. Rider brought it to such a level that it could easily compare with the German Lehmann Air Expansion Machine, or the Hot Air Machine of Hock & Martin, Austria.

Manufacturers of the Rider hot air engine

  • Rider Engine Company in Walden.
  • DeLamater Iron Works in New York.
  • The Rider-Ericsson Engine Co., NY, which took over the Rider Engine Company and the Delamater Iron Works.
  • Hayward Tyler & Co, London - England.
  • Sächs Motorenfabrik Otto Böttger, Dresden-Löbtau, Saxony - Germany.
  • Eilenburger Eisengießerei u Maschinenfabrik Alexander Monski, Eilenburg, Sachsen Anhalt

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Archives for download
Smith & Winchester illustrated catalogue, with the Rider engine
The Popular Science Monthly - Vol XVIII, April 1881
Hayward Tyler & Co - International Exhibition of 1878 in Paris

Interesting pages
Coolspring Power Museum - Original Rider air engine
About Manufacturers of the Rider Air Engine
Grace's Guide - Hayward Tyler and Co
Vintage Machinery
National Museum of American History - Rider patent model
Robert Sier's page on Rider air engine
Rusty Iron - Improved Rider hot air pumping engine
Darrell's Hot Air Engines
Hot air engines Gallery
Andy Ross improved linkage
Video - Original Rider air engine
Video - Original Rider air engine
Picture - Original Rider air engine

Sources of information

Document Author Date Title
Scientific American 1876, January 29 The Rider compression engine
Polytechnische Journal, Band 222 (S. 401–419) Müller‐Melchiors 1876 Rider's calorische Maschine
Scientific American 1878, March 2 The Rider compression pumping engine
Annales et archives de l'industrie au XIX siècle Luchard 1878 Petits moteurs industriels
Die Motoren für das kleingewerbes Alfred Musil 1879 Die Heißluftmaschinen
Polytechnische Journal, Band 249 (S. 145–151) Anonymous 1883 Rider'sche Heißluftmaschine
Die Kraftmaschinen des kleingewerbes J.O. Knocke 1889 Heißluftmaschinen
La mécanique Exposition Universelle de 1900 1900 Moteur-pompe Rider