Chuff-Synchronisation - Now what's that all about?

Richard Nixon, drawings' custodian of the Sentinel Drivers' Club and Isebrook's carer, emailed me recently with a technical query about double-engined Sentinels.

His concern was that, with two engines, if they were not coupled together so that each chuffed up their respective chimneys at the same time, instead of air being drawn up through the fire, would each engine merely suck air back down the other engine's chimney?

To understand the point, a bit more needs to be known about how a double engined loco works. In particular, each engine has its own double chimney mounted immediately above the firebox.

Twin double chimneys

Plan view of boiler showing the twin double chimneys

Single exhaust pipe divides to feed two chimneys

Blasting exhaust steam through a nozzle in each chimney tube draws air upwards through the chimney. The air should be drawn up from below through the firegrate.

This all works fine if both engines chuff at the same time; however, if they don't then air can be drawn down the un-chuffed chimney instead of up through the fire (probably being the path of least resistance). The result would be a poorer fire in times of need.

So are 7109's two engines chuff-synchronised?

I had originally thought this was true and that the two engines on 7109 were gear locked together.

A gear case on the end of each engine crankshaft
with the larger gear case below

From the above picture, it looks as if there would be a gear for each engine in its upper gear case (black, like a pair of eyebrows) meshed with the larger gear in the rusty/grey gear case below. (There is the same situation on the other side). If all these gears were in place as described, then the two engines could have been chuff-synchronised together.

However; to my surprise, when I first undid the oil filler cap in the front engine's off side gear case (the one to the right in the photo), I found that the gear case was empty. On the near side, the rear engine's case was also empty.

At first, I thought we'd been diddled but then it dawned on me. There is a sprocket for a chain to drive the front axle from each lower gear; the two gears are not linked and rotate on a fixed, 'dead' axle. Thus the engines rely on the chains being set appropriately and tightly enough to achieve chuff-synchronisation.

Twin sprockets for chain drive to front axle

It's doubtful that chuff-synchronisation was achieved particularly well in both directions due to slack in the chains.

This situation evolved in later double geared locos such as Sentinel 9622.

Sentinel 9622's drive chains

9622's chains are double width, presumably for extra strength. These were most likely adopted because of the double gearing. Depending on whether low or high gear is selected, the drive will be transferred to the front axle by one chain or the other but not both. Hence all the driving force has to go via a single chain in contrast to 7109 where both are used at the same time.

To make the double gearing work, both engines are locked together at the end of the crankshaft where the gear is engaged. This happens by having a gear in each upper casing meshed with the gear in the lower casing.

However, there is still more to the picture: double geared engines have a neutral setting in which neither high nor low gear is engaged and the engines can be run with neither load nor locking together. This enables better warming through of the cylinders prior to loco movement.

Front view showing different sized gears at each side
(Diagram derived from 1930 A2 drawing, courtesy Richard Nixon)

Side view of upper and lower gear casings
(Diagram derived from 1930 A2 drawing, courtesy Richard Nixon)

Of course, the asynchronous rotation of the two engines in neutral means that when they are put into gear, they could be in any relative state of rotation. As such, sometimes they might be chuff synchronised and sometimes not in an unpredictable manner.

So Sentinel 7109, 9622 and other double engined locos have the same problem.

There is yet another consideration and that is to do with the resonant frequency of the chimney system. This may not be relevant if the chuffing frequency never gets high enough to reach resonance and I am sticking my neck out here due to lack of in-depth knowledge of this type of system. (Ask a designer of resonant, high-performance, car exhaust systems!).

Below resonance, a chuff up one pair of chimneys could easily pull air down the other pair if out of sync. At resonance, I do not know what happens. Above resonance, the chuffing will be so fast that sucking air back in won't take place and each chimney will behave as if sealed. (I'm assuming that the chimney system behaves like a ported loudspeaker enclosure).

So the conclusion to all this? I think we'd better just try it and hope for the best!