Donkey Engine

[Richard Goodwin]

So, if I remember correctly, the donkey engine was a separate engine to the main engine and it provided power (more efficiently than the main engine) for the onboard electrical s when the main engine was off similar to an APU on an aircraft?

So, some questions to our Chieftain veterans:

1. What was the engine; a desiel?
2. What did it sound like.......any pointers - youtube and such like so I can hear it would be appreciated?
3. Did it run at constant speed and was it noisy?
4. was it running all the time even when the main engine was on?

Many thanks in advance

Richard

[Mark Heaps]

The Auxiliary Engine or "Donkey engine" as it was commonly termed was a diesel 2-stroke Coventry Climax H30 with 23 HP.
The speed was controlled by a hand operated lever. It was normal to give it a couple of minutes to warm up before moving the lever to full speed and then putting the generator under load.

The generator was rated at 150 Amps so helped assist the batteries when starting the main engine especially in cold conditions, and also was sufficient to supply the systems when in an overwatch situation and the main engine could be shut down to preserve fuel, or when static to run the BV or recharge the radio batteries.
The generator on the main engine was rated at 350 Amps. Normally the Aux Engine was left on when the Main Engine was running, just in emergency case of the Main Engine Generator stopping functioning.

Edit - And if I remember correctly, the Generator Panels were so designed that when both generators were working, the total electrical load was proportionately shared between the two

[Stephen White]

More properly the "Generating Unit Engine" or GUE, sometimes called the "Genny". The H30 Mk 4 No 10A was introduced on the Mk3 and was one of the few major components on Chieftain which didn't require much modification throughout the life of the tank. There was a complicated and unreliable hydraulic start system for the main engine which was rarely used. The ME wasn't started on hull batteries, so the GUE was invariable started in order to provide ME start. Thereafter as Mark says, the electrical power generation was shared between the GUE and ME generator. The GUE exhaust was moved to the upper right of the exhaust box on the Mk 2 and it produced a very distinctive and separate sound to the ME exhaust. At idle, the GUE sounded like any other small diesel but under load, the exhaust note was a constant but raucous howl. I would like to see the Armortek model offer a separate special sound for GUE start but if it's not offered, I'll be looking to create a bespoke set of sound files as I did for the Centurion.

The GUE exhaust also produced a much less obvious smoke signature compared with the ME. If you expected to be in a position of observation for some time, it was usual to scan using GUE power only, in order to keep the gun power laying kit and radios running, which allowed you to communicate, observe and fire with minimum signature. In some circumstances, a "silent watch" was required and then, with no power generation on line, you had to monitor the battery conditions carefully and switch off unnecessary loads. The searchlight projector was rated at 100A and required the GUE to be online. But by using white light, you were no longer on silent watch and it was acceptable to run engines.

Under operational conditions, we would expect to make the "battle link" which provided a means to connect the hull and turret batteries in parallel. You could then run all the kit and maintain a silent watch but the risk of flattening the batteries was increased. Making the battle link would have been one of the formal steps taken to increase readiness during an escalation to hostilities. Time to change underwear.

[Mark Heaps]

Maybe not applicable on the early marks, but the later marks were capable of being started from battery power alone if the GUE was not functional. The hydraulic start was an emergency measure still available.
In peace time, relays connecting the hull batteries to the turret batteries, and other relays allowing gun kit, NBC pack, BV, and other systems to work, were supplied by secondary outputs on the generators, generator pilot lines.
"Throwing" the battle link fed those relays directly from the hull batteries, thereby allowing those systems to operate even if both generators were off-line, but required the crew to monitor battery voltage and prevent them being discharged too much to get the GUE started. IIRC, the loader was the only one capable of monitoring it with voltage guages on the radios ( at least with Clansman, I do not know whether they were present on Larkspur ).
As a final resort, there was a lever on the Turret Junction Box, called the OMC (Overriding Master Control ) which when operated would physically connect the two sets of batteries, operate the relays and theoretically enable the crew to fire a few more rounds from the now disabled tank before abandoning it. We termed it the "Oh My Christ switch"

[Mark Heaps]

In a battle situation , the only thing that could completely stop the main gun from firing was insufficient voltage to trigger the vent tube. I cannot remember whether Chiefy had it, but Challenger 1 had an emergency battery under the gun which could be switched in and fed only the vent tube circuit to get a few more rounds down range even if all other battery power was lost.
Mark

[Stephen White]

Good recall, Mark:

[Richard Goodwin]

Gents,

Many thanks for the informative and detailed replies; I've learn't a significant amount as I'm am sure others have too. I wonder how many other vehicles and aircraft had an equivalent of the 'Oh My Christ' switch

From what you've told me, I thought I'd found a video where the Chieftain was being started in the cold with the potential sound of a GUE prior to ME start but sadly, it was being jumped by 432 so back to the drawing board

Richard