The compressed carbon dioxide motors were accompanying the very beginnings of the aircraft heavier than air. The first flying motor models were driven by compressed air and compressed carbon dioxide motors.

CO2 motors meant another development level after the compressed air motors. A great future was forecast to these motors during the First World War, although at the beginning they were heavier than compressed air motors. Unlike them, they were working for a longer time, but in a cold weather they were easy to get frozen.

The best known motors were again those made by Max. Braune - a successful manufacturer of compressed air motors. Max. Braune introduced production of start-type three-, five- and eight-cylinder motors. The motors were running also on compressed hydrogen, however best on carbon dioxide. They were equipped with a pressure controller for controlling the gas flow and speed. The filling device connected to the carbon dioxide cylinder was fitted with a pressure relief valve limiting the pressure in the model tank to 5000 kP. It was possible to borrow the cylinders from any beer tapping facility. Not even with a carbon dioxide motor was, however, the model able to take off with due to the fast pressure drop in the tank. After opening the gas inlet to the motor and initiating the propeller rotation with a cord actuator it was necessary to launch such a model from the hand.

The first carbon dioxide motor was assembled by Marcellus Martin from France. The five-cylinder star-type motor stated a power output of 0.6 kW; its tank accepted as much as 800 g of carbon dioxide. The last type of this designer was a start-type three-cylinder motor "PRIMA" with cylinder bore 16 mm, stroke 18 mm, and a total weight of 250 g.

The eight-cylinder motor "CÉTONIA" from 1911 featured a gas distribution system operating with the help of sliding valves controlled by an eccentric unit. It achieved a power output capacity of 0.736 kW, its weight was 1000 g. The price of the motor amounting to FRF 200 was very high for the time before the First World War.

A motor of this type was used in the model of the design prepared by P. du Motel, with a span of 2.5 meters and a length of 3 meters. The model carried 1.5 kg of carbon dioxide, its take-off weight was 17.5 kg. At the Paris airport it achieved a flight lasting for 92 seconds on May 23, 1911, which made it possible to fly over a distance of 1178 meters, given the average speed of 46 km/h. With the same type of motor, H. d. Villers achieved even a speed of 80 km/h. His model featured a span of 2.5 meters, length of 2.75 meters, the load-bearing area was 1.10 m2. The motor achieved a speed of 1400 rpm with a propeller of diam. 650 mm and a pitch of 900 mm. The take-off weight of the model was 14 kg.

An interesting design could be registered in the case of the motor "IDEAL" with one piston moving in an aluminum pipe with a length of 1000 mm, forming at the same time the fuselage of the model. The piston movement was transferred to the propeller featuring a diameter and a pitch of 350 mm with the help of cone transmissions. The time of run of this motor was 40 to 50 seconds, given an average speed of 1800 rpm.

Fiex miniature rotary three-cylinder motor featuring a weight of only 48 g was giving 0.07 kW for a period of 90 seconds.

The RADIGUET-MASSIOT motors were made in both rotary and firm arrangement with two, three and five cylinders. They could work on compressed air, carbon dioxide or steam. The last two types had also generators, i.e. evaporation equipment increasing the working pressure of the gas up to 8000 kP.

I have already mentioned the first motors created at the beginning of the last century. The motors were running on liquefied carbon dioxide supplied from siphon cylinders. It was Bill Brown that reappeared this drive method for modelers after the Second World War. Bill Brown was already well known thanks to his design of the first model builder's motor with a sparking plug - Brown Junior (more than fifty thousand motors of this first type really produced in series were produced before the Second World War in several versions).

At the beginning of the 1940s Bill Brown was making the first tests with CO2 motors. His first success with these motors came in 1942. I have rare photographs from Bill Brown, which show his first CO2 motor, which was running, as evidenced by his own note on the reverse side of the photograph.

The first Brown carbon dioxide motor featuring a capacity of 0.29 cm3 (the bore to stroke ratio was - 7/7.52 mm) was made in series from 1947 by Herkimer Tool and Model Works in the New York State. The motor called O.K. CO2 was a result of several years of development of CO2 motors performed by Bill Brown. Unlike today's CO2 motors which feature always more and more plastic parts, this motor had all parts made of metal - in terms of processing it corresponded to low-volume motors with a glow plug "O.K. Cub" of the same firm.

The capacity of the O.K. CO2 motor was necessarily adapted to the higher weight of the entire set (64.5 g), caused by the use of a standard siphon cylinder, attached to a light-weight holder. The motor still did not have the light-weight tank which is typical today and into which the gas is conducted from the siphon cylinder. And yet it became very popular among modelers. With a 200/75 mm propeller it was suitable for aircraft models featuring a span from 900 to 1000 mm. The speed of the O.K. CO2 motor was set up in the same way as for all other Brown motors by screwing the cylinder into the crankcase. The motor was made by Herkimer until the 1960s, when they started to offer it as built in a cartmodel (dragster).

Another American motor, Buzz CO2, with a substantially lower capacity of 0,09 cm3 and at the same price level (O.K. CO2 cost USD 4.95) was offered in the 1950s. This motor was equipped with a light-weight pipe tank featuring a diameter of about 7 mm and a length of almost 100 mm. At that time it was already clear that it is better to conduct the gas from the gas cylinder to the tank, whereby the drive unit weight reduces significantly, which in turn facilitates a reduction of the motor capacity. The volume of a standard gas cylinder was sufficient for two or three fillings of the tank.

After further experiments with pipe tanks W. Brown designed another motor in 1947 (Campus A-I00) with a capacity of 0.025 cm3 only. The weight of the miniature motor was less than 7 grams. The motor capacity was increased in 1970 to 0.08 cm3 (the bore to stroke ratio -4.6/5 mm) and under the name MJ-70 it is produced up to now (together with other types) in the small Pennsylvania factory "Brown Junior Motors Inc."

The MJ-70 motor has a steel cylinder and a plastic piston, while the crankcase with three footings is cast of electron (light metal). Including the very light tank, the weight of the motor is 7.5 grams; with a propeller of 100 mm (diameter) it still belongs among the best CO2 motors in the world.

A miniature flat twin cylinder motor (MJ-140V) was designed in 1973 with the use of certain parts from the previous motors. The motor featuring a weight of 10.9 grams (without a propeller), which can be attached to the model with two bolts, is an ideal driving unit for small flying dummies.

Another masterpiece made by Bill Brown and coming from 1981, when this outstanding designer celebrated his seventieth birthday, is his smallest CO2 motor known as Campus A-23. The motor capacity is 0.023 cm3 (bore to stroke ratio - 3/3.2 mm) and with a tank of a volume of 3 cm3 and a propeller of a diameter of 89 mm it runs on one filling for more than two minutes. The new motor reminds the former Brown motor "Campus A-I00" with its appearance, but there were made some treatments following an extension of the motor lifetime. The main parts of the motor are made of the same materials as in the case of the MJ-70 and MJ-140 motors. A very interesting feature is the use of a capillary tube made of an aluminum alloy for gas inlet, which is only glued in the terminal parts. Despite the miniature dimensions, the motor is attached at the crankcase footings with three bolts. Thanks to the total weight (with a propeller of 6.75 grams), the motor is an ideal driving unit for the Peanut category. The price of this excellent motor corresponds to its unusual nature - in 1981 it was 40 dollars.

In 1991, on the occasion of his eightieth birthday, thanks to K.J. Hammerschmidt's initiatives, Bill Brown and his wife Dorothy visited Europe and were guests at modelers in Germany, Austria, Hungary and Czechoslovakia. During our meeting in my house we prepared a design of our small common motor Ga¹parín - Brown GB12.

The GB12 motor with a bore of 2.5mm and a stroke of 2.5mm is the smallest motor with a metal re-bore in the world, and it proves excellent skills and experience of Bill Brown in spite of his age.

Bill designed and produced also a twin-cylinder version of the GB12 motor, known as GB12T. The GB12 and GB12T motors are the last Bill Brown motors.

For Brown CO2 motors it is possible to use also tanks featuring a volume of 2; 6; 10 and 20 cm3, which means that thus it is possible to achieve a runtime of as much as 12 minutes! With the largest tanks these motors were suitable also for driving the miniature attached and RC models.

All Bill Brown CO2 motors are a fantastic example of simplicity and perfect performance capacities. I always admired this remarkable feature of his motors very much.