Tuning a 15 HP Reid Type A for Line Shaft Duty

The heart of a blacksmith shop

| June 2006

Editor's note: The following is Part 2 of a two-part article on the 15 HP Reid Type A that iron sculptor and blacksmith Joel Sanderson uses to run the line shaft in his blacksmith shop. Read Part 1

This is a 15 HP Type A running on propane with hot tube ignition. For my shop to run smoothly, the engine driving it must also run smoothly. I can expect this engine to fire every revolution and to maintain a speed with no greater than 5 RPM variation.

Hot tube theory
One of the first things I had to figure out was hot tube ignition and its principles. I don't believe this ignition system is as simple as it first appears. There are many variables affecting a hot tube's performance, and many of those vary while the engine is in operation. The three primary factors affecting the hot tube are its length, temperature and the amount of compression into the hot tube, as generated by the main cylinder. However, the temperature of the engine's water jacket and the gas-to-air ratio also significantly affect the hot tube and the engine's timing.

Each firing in the cylinder leaves burned gases in the hot tube, which fail to exhaust out of the engine. The longer the hot tube, the easier it is to compress these gases in order to expose the new charge to enough area in the hot tube to induce combustion. In other words, the longer the hot tube, the less pressure (by compression) is necessary for ignition and the earlier in the cycle that ignition will occur.

The temperature of the hot tube also affects the timing. The hotter the tube, the easier the new gas mixture ignites, requiring less heat from compression to induce combustion. This means, as the temperature is increased, the ignition will occur earlier in the stroke.

Compression generates heat. This heat, when combined with the heat from the hot tube, affects when in the cycle the ignition will occur. This is important to remember, because as the governor opens and admits more air and gas, more heat will be generated by compression, causing the ignition to occur earlier in the stroke. Compression also determines how fast a fresh mixture is forced into the hot tube. More volume in the cylinder, when compressed, means the remaining spent charge in the hot tube (from the previous firing) is forced further into the tube. Just as with a longer hot tube, where more area induces earlier ignition, with greater compression, more area of the tube is exposed to the new charge.

Knowing how a Reid's timing is affected, it is understood that - due to the workload on the engine - each engine is running with no load and the mixer valves are nearly closed. Thus, they prevent the charging cylinder from drawing in a full breath of gas and air. This partial charge is then introduced to the main cylinder, which, not having been given a full charge, will not make full compression. This mixture is still of proper ratio to fire, and if a spark were introduced it would, but hot tube ignition requires sufficient compression of the gasses into the hot tube to ignite. In order for this to happen with this partial compression at no load, the timing of the hot tube would have to be advanced to the point where it would ignite the charge under low pressure. This is done by either lengthening the hot tube or heating it to a higher temperature.


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