I grew up with a fascination with steam engines, loving everything from huge locomotives to steam powered cars. I spent many hours in our little garage machine shop constructing working models. Reading one day, I came across an idea for a steam powered radio controlled model airplane. I have been obsessed with making this idea a reality ever since.

During my free time as a mechanical engineering student I have designed an engine that would be somewhat suitable for such an application. Weight, as well as power issues have been overcome yet one particular problem has long perplexed me. I could not figure out how to mechanically vary the RPM of the engine quickly and easily. This is a must for the demands of flying a model aircraft.

Not knowing the first thing about microprocessors, I decided to find out if I could use one to control the RPM of a steam engine. I solved my problems by incorporating a Zilog Z183 microprocessor into my project. Similar to fuel injection in a gasoline engine, steam is "injected" into the appropriate cylinder of a steam engine at the appropriate time. Signals, which are calculated and produced by the microprocessor, turn on and off steam valves. These valves are actuated by solenoids. The speed of the steam engine is selected using a variable resistor. The Z183's A/D converter reads the voltage across the resistor. This information is then used to calculate the appropriate time delays and solenoid signals. The following diagram illustrates the cycle of the engine.

Diagram 1 Engine Cycle

Solenoid 1 is on allowing steam to enter cylinder 1. This pushes piston 1 to the left, driving the crankshaft clockwise. Solenoid 2 is off allowing the air to escape from cylinder 2. The photo gate which is on the other side of the engine is not tripped.

Solenoid 1 is on allowing steam to enter cylinder 1. The engine remains in this position for a set time delay. The photo gate is tripped which will, after the time delay, permit solenoid 2 to be turned on.

Solenoid 2 is on allowing steam to enter cylinder 2. This pushes piston 2 to the right, driving the crankshaft clockwise. Solenoid 1 is off allowing the air to escape from cylinder 1. The photo gate is not tripped.

The functions of the microprocessor are outlined in the following block diagram. Two signals are produced to control each solenoid-actuated valve. The Z183's A/D converter reads the voltage across the variable resistor. A photo gate is also read. The photo gate is used to insure that steam is not prematurely injected into the opposing cylinder.

Diagram 2. System Block Diagram

Now that it is possible to control the speed of a steam engine I plan to further expand the project. With my new found microprocessor knowledge I have plans to include a temperature probe and a microprocessor controlled fuel regulation device for the steam engines boiler. This will ensure that the steam produced by the boiler will be at a constant temperature/pressure. Also a pressure transducer and microprocessor controlled pressure bleed off valve could be attached to the steam supply line. This again will help deliver a steadier supply of steam to the engine.