Category Archives: Garratt

Garratt Back Pressure

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.

Here’s how it looks:
Garratt-Condenser Tank photo Garratt-CondenserTank_zpsd408cc44.jpg

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.

This is fairly straightforward, so I’ve done this as well. All that’s required is a suitable wrench to reach in through the smokebox door:
 photo Garratt-ChimneyModification_zpsc241efd1.jpg

These two modifications greatly improved the performance of the rear engine on my Garratt. I’m glad I did them.

Garratt modifications

Pan shot photo Garratt-01.jpg

I’ve made a number of modifications to my Accucraft Garratt, which I thought I’d detail here. Most of these are in the cab, so let’s start there:
Crowded cab photo Garratt-02.jpg

On the right side, I’ve added a resonator whistle created specifically for this model by DJB Engineering. Installation required rebending the pipe that feeds the pressure gauge (making sure to retain the loop to act as a water trap). The gauge used to be located in the upper right corner of the cab, with the dial read through the window on the front wall. I’ve tucked it inside the cab at an angle that makes it easy to read when the cab roof is open, which is handy while raising steam before a run.

The resonator itself lives in the cosmetic ash pan under the firebox:
Garratt-DJB Resonator photo Garratt-DJB-Resonator_zps885f76c0.jpg

With the cab roof in place, the whistle lever can be operated by sticking a finger through the door – unless there’s a driver in the way:
Freddie al Fresco photo GR-Garratt-Driver-Forward.jpg

As the cab interior photo shows, I’ve added a swing-out driver’s seat patterned on the seats that these Garratts had when they worked on the two-foot gauge lines in South Africa. I wrote an article about this modification that appeared in the January, 2012 issue of Garden Rail magazine. More on the Garratt Driver here.

To the left of the seat and below the pressure gauge, I’ve added a fuel tank water bath warmer. Since I live in a cold country (Canada), the air can be quite cool in the spring and fall. Since the fuel tank is carried in the rear tank – away from the boiler – the cool air can reduce the pressure from the butane fuel tank, which reduces performance. The solution, shown in the photo below, is a bath warmer – in this case, from DJB Engineering. The valve allows one to draw some steam out of the boiler and feed it into the water bath around the fuel tank, to heat up the bath if needed:
Garratt cab - labelled photo LS-Garratt-WhistleBath-02_zps9e97f0b4.jpg

Let’s have another look at the first cab photo:
Crowded cab photo Garratt-02.jpg

I’ve replaced the stock water fill valve with a bayonet-style, which allows one to connect a water bottle for refilling on the go. I believe I picked up the new valve from Chuffed 2 Bits, although I can’t find it on their website.

In the lower left corner of the cab is another customization I’ve done – adding a blowdown valve to the water gauge. After several runs, I determined that the gauge easily becomes blocked with a trapped air bubble, which means it can’t provide an accurate reading of the water level in the boiler. I added this valve to the bottom fitting of the gauge, and routed the drain pipe through the floor of the cab. A quick twist of the valve shoots the air (plus water and steam) out of the gauge. Closing it up again provides a more accurate reading. The valve came from DJB Engineering.

To make it easier to read the gauge, I’ve fitted a brass plate between the water glass fittings. I temporarily added diagonal stripes to it with a black marker, but will go back and paint the plate white then add black stripes. Here’s a look:
A new regulator handle photo Garratt-03.jpg

The above photo also shows a replacement regulator handle (throttle, in North American parlance). I didn’t like the look of the plastic knob provided by Accucraft. The handle only rotates about 120 degrees before hitting something in the cab, but that’s plenty to provide steam power. I picked this up from Milton Locomotive Works.

Finally, one modification that’s not in the cab: I bought a resin casting of a coal load designed to replace the metal plate that covers the bunker. This also came from Chuffed 2 Bits, although it no longer appears on their website. I painted it myself and I’m quite pleased with how it turned out:
Garratt - coal load photo LS-Garratt-Coal-01_zpseb5fbcd7.jpg

Garratt Driver

Roger Reverse photo IMG_4466.jpg

A couple of years ago, I added a swing-out driver’s seat to my Accucraft Garratt model. I based this on photos and drawings of the seats that these Garratts had when they worked on the two-foot gauge lines in South Africa. I wrote an article about this modification that appeared in the January, 2012 issue of Garden Rail magazine.

This photo shows how I created a bracket to support the seat. It fits over the handrail posts on the inside of the cab, and is held in place by the handrail nuts:
Garratt cab seat photo Garratt-Seat-01_zpsa99d52f7.jpg

Here’s the seat stowed away inside the cab – which keeps it safe during transport:
Garratt cab seat - in photo Garratt-Seat-03_zps02365c2f.jpg

Here’s the seat deployed and ready for a driver:
Garratt cab seat - out photo Garratt-Seat-02_zpsabb64e54.jpg

I commissioned Rob Bennett at Busy Bodies to create a pair of drivers for me – one for forward operation (l), the other for reverse running (r):
Garratt driver (front) photo Garratt-Driver-01_zpsc9da250e.jpg

I had to send Rob some photos of me in the pose I wanted, as well as a labelled photo of the Garratt so that he could build a mockup of the cab and seat, to make sure his figures fit through the doorway and onto the seat:
Garratt Driver - Seat Dimensions photo Garratt-Driver-Dimensions_zps7bb5a1fb.jpg

Rob did a great job, and the drivers fit without any issue.

I did make one modification to the figures – not for the squeamish:
 photo Garratt-Driver-02_zpsce8336dd.jpg

As the above photo shows, I drilled each driver’s, um, seat, and glued a small rare earth magnet in place. The top of the seat has a steel nailhead embedded in it, and the magnet holds the driver to this nailhead so he doesn’t fall out of the Garratt while it’s in motion.

Spring is coming

This week I’ve been dusting off a couple of my live steam locomotives and related projects.

Last year, I was able to run my Isle of Man Peveril… but my Welsh Highland Railway Garratt and my Leek and Manifold Valley Light Railway ER Calthrop were Shelf Queens.

In fact, the ER Calthrop has never turned a wheel under steam. This summer, I’d really like to right that wrong!

Freddie al Fresco photo GR-Garratt-Driver-Forward.jpg

Chocolate steam photo LMV-01.jpg

On time for Douglas photo Peveril-03.jpg