Compound engines

[historydavid]

Have come across a merchant ship which was described as having "compound engines."

Does anyone kwow what these are/were?

Best wishes

David

[ionia]

Have come across a merchant ship which was described as having "compound engines."

Does anyone kwow what these are/were?

Best wishes

David

A compound engine utilizes the expansion of steam in two stages: in its simplest form, first in a High Pressure Cylinder and then the steam is exausted into a Low Pressure cylinder and used again. Some engines used two LP cylinders with one HP cylinder. The first patent was taken out in the late 18th century by Jonathan Hornblower but the system did not come into general maritime use until the Pacific Steam Navigation Company introduced it in their paddle steamers VALPARAISO and INCA in the 1860's and within a few years their entire fleet had been modified. The Admiralty introduced them into the RN in the 1870s. Although compound engines meant an increase in machinery weight of between 5% and 10% they gave an increased fuel economy of about 30%. A later development was the triple expansion engine which interposed a third cylinder between the HP and LP cylinders and so used the steam three times. There were variations over the years, including heating the steam between cylinders.

[FAAAEd]

A compound engine utilizes the expansion of steam in two stages: in its simplest form, ... A later development was the triple expansion engine which interposed a third cylinder between the HP and LP cylinders and so used the steam three times. There were variations over the years, including heating the steam between cylinders.

A very good description of compound engine operation, which will be familiar to any Railway Buff particular those into LNWR and Midland Railway loco's.

With triple expansion I think it worth pointing out that these engines often had 4 cylinders with 2 large diameter low pressure ones as seen on Titanic and Olympic.

Most of the pre-Dreadnought battleships had triple-expansion engines as did many units of the turtle-back classes of destroyers. Some of these latter had locomotive type boilers they being distinguished by having funnels in pairs close together.

[J T Gray]

If you want to see a working triple-expansion marine engine, the SS "Sir Walter Scott" on Loch Katrine in Scotland is still powered by its original engine, and the engineer is more than happy to answer questions if you ask nicely!

I was delighted, as my great-grandfather served his apprenticeship in Dundee at the same time as Sir Walter's engines were being built in Glasgow and no doubt he'd have been all to familiar with the things!

ADrian

[bob lembke]

The physics of the multiple stage engine dictates that the lower-pressure cylinders have to have a greater volume/cylinder piston area in order to work efficiently, as the volume of the steam increases as its pressure is stepped down through successive expansions. Just dreaming up numbers, the first expansion might drop the pressure from 60 psi to 40 psi, the second expansion from 40 psi to 20 psi. In older single expansion engines, the steam might be expanded once to 40 psi and then dumped, wasting a lot of the energy in the steam.

(The above numbers are just off the top of my head, I am a mechanical engineer, but have never had much to do with steam engines. In a well-designed single expansion engine it probably would be designed to try to expand the steam from, say, 60 psi to 30 psi, before venting.)

Bob Lembke

[FAAAEd]

The physics of the multiple stage engine dictates that the lower-pressure cylinders have to have a greater volume/cylinder piston area in order to work efficiently,

Bob Lembke

Indeed, that is why I mentioned Titanic and Olympic as illustrations of details of these ships have been much published of late.

[historydavid]

Thank you all for your comprehensive and clearly understood reponses.

Best wishes

David