The Accucraft live steam Garratt is a lovely model, but one of the problems that many owners have identified is that the rear engine often hesitates while running. Put a stock Accucraft Garratt on a running stand and it’s really apparent that the rear engine does not run as fast as the front one, and appears to have a regular bind in it.
Having done some research on this, I learned that the problem relates to back pressure in the exhaust line from the rear engine to the smokebox.
The Garratt’s steam and exhaust lines are plumbed as follows:
– The steam line runs from the boiler, through the throttle/regulator in the cab then through the displacement lubricator
– The steam line then travels through one of the fire tubes where the hot exhaust gases heat the boiler (and keep the steam nice and hot), and into the smokebox
– Here, the steam line runs through a T (Steam T)
So far, so good. Now, here’s where the problems start:
– From the Steam T, one line runs forward to the cylinders on the front engine. An exhaust line runs back from these cylinders to a second T (Exhaust T) in the smokebox, and then exhausts through the chimney. These runs are quite short.
– Meantime, a second line from the Steam T runs underneath the boiler and cab to the cylinders on the rear engine. An exhaust line runs back from these cylinders – again, under the cab and boiler – to the Exhaust T in the smokebox, and then exhausts through the chimney. These runs are quite long.
Since the lines to the rear engine are longer than those to the front engine, the steam in the rear lines has more time to cool down – both as it rushes to the cylinders and then as it returns to the chimney. The problem is exacerbated by the fact that the lines are slung outside the engine, where they’re exposed to the cooling effects of the air. Even on a hot day, the air is going to be a lot cooler than the steam in these lines.
As the steam cools, of course, some of it condenses back into water. And when this water-logged steam from the rear engine arrives at the Exhaust T, it hits the hotter, drier steam from the front engine – and it’s like walking into a sudden gust of wind. All of this steam – from both engines – then has to force its way up the very small diameter exhaust pipe in the chimney.
The exhaust pipe and the higher-pressure (hotter, drier) steam from the front engine act on the (cooler, wetter) steam from the rear engine, and that pressure runs back up the rear exhaust line. This increases the force required by fresh steam to move the rear cylinders. So, the rear engine runs more slowly, and hesitates as the pressure in the rear exhaust line fluctuates.
People have discovered that the Garratt actually runs better with the front cylinder drains open. This reduces the pressure in the front exhaust line. In effect, the rear exhaust now owns the route up the chimney. But it also means the front engine is exhausting some of its steam before it has acted on the piston in the cylinder: That’s no good.
In discussing this with friends, I decided that incorporating a condensation tank into the rear exhaust line would help matters. The theory is, wet steam enters the tank, which is of larger diameter than the steam line. The water drops to the bottom of the tank, where it can drain away – either through a valve, or through a small pipe that constantly bleeds a bit of the exhaust out of the tank. Most of the exhaust – now drier – continues through the exhaust line to the chimney.
In examining the Garratt, I realized that one of the two tanks that form part of the cosmetic brake system on the locomotive could be used as a condensing tank. The tanks are held to the frame with straps and bolts.
I unbolted a tank and then drilled the ends.
The exhaust line enters the tank near the top edge of the tank end – and exits directly across from the entrance.
Under the inlet, I drilled a second hole near the bottom edge of the tank end for a drain pipe.
I formed appropriate plumbing from brass tube and silver soldered it in place. (It’s low pressure, so it does not have to be silver soldered, but it was a good opportunity to practice the technique.)
I ran the drain line into the centre of the space below the boiler, just ahead of the ash pan. I crimped the end of the line, and drilled a line of small holes in the wall of the pipe to allow it to exhaust captured water from the tank. Steam pressure forces out the water, so the holes can be quite tiny.
When the Garratt is in steam, the steam emerging from underneath the machine looks like a leaky joint – something quite common on the prototypes. Notice the amount of steam emerging from under the cab in this video:
Accucraft Garratt owners have also discovered that removing the exhaust pipe in the chimney helps matters greatly, in two respects:
– It shortens the length of pipe through which the exhaust from both engines must travel, so it reduces the back pressure.
– It eliminates the discharge of oily water out the chimney and over the top of the locomotive. This discharge now happens in the smokebox, which then drains onto the tracks.
These two modifications greatly improved the performance of the rear engine on my Garratt. I’m glad I did them.