Engine: How to Compound Steam Engines?

High-pressure steam is used in a steam engine for greater efficiency and less space per horsepower.

Now with the use of this high-pressure the range of expansion is correspondingly increased.

If full expansion takes place in the single-cylinder the walls of the cylinder are to be made very thick.

This way engine withstands the high pressure but also required a heavy flywheel.

All this made a simple engine very heavy in construction, with large variations of temperature and more leakage past the piston.

These are some basic engineering difficulties mostly observed while designing the machine.

To overcome these difficulties, the expansion of steam is made in two or more cylinders.

This is called “Compounding the expansion of steam”.

steam engine compounding

So, There are three different methods of compounding steam engines,

  • Tandem Compounding

In this type, the cylinders are arranged on a common axis and two pistons have a common piston rod, connecting rod, and crankshaft.

The exhaust steam from the high-pressure cylinder passes directly into a low-pressure cylinder where it further expands.

The turning moment in this type of engine is not uniform due to the common crank and piston rod.

Because all the torque acts at the same time.

This is the basic disadvantage of such types of engines and requires a large flywheel.

The Tandem arrangement gives the constructional economy because of fewer components of the engine.

  • Woolf Compounding

In this type, two cylinders are arranged side by side.

While the two cranks are placed at 180degreese to each other.

The pistons of high pressure and low-pressure cylinders begin and end their strokes together respectively.

The exhaust of steam from a high-pressure cylinder coincides with the suction in a low-pressure cylinder.

In this case, also exhaust steam from a high-pressure cylinder passes directly into a low-pressure cylinder where it further expands.

Since the two cranks are at 180 degrees to each other, the two cycles are in phase.

But this causes a large variation in the turning moment of the crankshaft and necessitates the use of a heavy flywheel.

  • Receiver Type Compounding

In this type, two cranks are mounted at 90 degrees to each other.

Hence steam from the high-pressure cylinder cannot exhaust directly into the low-pressure cylinder.

A container, known as a receiver is used to collect steam from the high-pressure cylinder.

This low-pressure cylinder draws steam from the receiver for further expansion.

The turning moment in receiver-type compounding is more uniform therefore a lighter flywheel can be used.

With the three-cylinder engine, the cranks are arranged at 120 degrees to each other but still, small turning moment variation can be achieved.

Besides this information, you are suggested to read something more from below engineering books

Further recommended readings are A Brief History and Stationary Engines
To get the more details about the topic, I further recommended reading

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