User:Musiconeologist/draft

Probably mainly for trying out rewordings etc. that I'd rather not do in the actual article until I'm sure I prefer them.

For Capuchon (chimney) article:

Original
A moving locomotive can suffer a problem whereby the stream of air flowing over the front of the chimney meets the inside of the back rim and is diverted down the chimney instead of passing above it. This downdraught increases the pressure in the engine's smokebox, interfering with the process of drawing exhaust through from the firebox.

Capuchons were initially added to raise the airstream slightly and keep it clear of the back rim, thereby preventing the downdraught.

Edited
If the stream of air over the front of a moving locomotive's chimney meets the inside of the back rim and is diverted down the chimney, the resulting downdraught increases the pressure in the engine's smokebox and interferes with the process of drawing exhaust through from the firebox.

Capuchons were initially added to raise the airstream slightly and keep it clear of the back rim, thereby preventing the downdraught.

Third version
A moving locomotive can suffer the problem of air being diverted down the chimney when it meets the back rim, rather than simply flowing past. This downdraught increases the pressure in the engine's smokebox, interfering with the process of drawing exhaust through from the firebox.

Capuchons were initially added to raise the airstream slightly and keep it clear of the back rim of the chimney, thereby preventing the downdraught.

Smoke flow near the locomotive
It was found that a capuchon also helped deflect the stream of smoke upwards and away from the locomotive. This was desirable because of the way air flows around a moving locomotive.

As the locomotive advances, the air immediately in front is pushed radially outwards. This outflow creates a ring of low pressure immediately to its rear: that is, around the front part of the locomotive. This in turn causes a ring of eddies to form.

Smoke which is too close to the locomotive is caught in the eddies and swept back down, potentially obscuring the line of sight of the crew. A capuchon helped to keep the smoke away from the eddies.

For later locomotives a capuchon was less effective. A larger boiler left less height available for the chimney, and improved engine efficiency reduced the pressure of the blasts of steam used to expel the smoke. The result was gentler expulsion through a shorter chimney. A capuchon alone was no longer adequate and additional measures were needed, such as the use of large metal plates to affect air flow.