Percentage for Performance

It’s what sports coaches talk about a lot. However, when I decided recently to improve my steam locomotive driving in Railworks Train Simulator my lessons definitely came down to understanding percentages and the pressures to achieve performance.

When I first got involved in driving trains on the pc I did try the steam engines that came with the Train Simulator program. I found it very difficult to get the locomotives to climb the banks on the supplied Somerset and Dorset route. The train might stagger over the top at 5mph or stall before I got there! I knew I was doing something wrong but at the time I couldn’t be bothered largely because I was more interested in driving the modern diesels and electrics. In general, using the expert controls, they are a more straight forward proposition to drive though they each have their foibles and as the more advanced computer models of types have appeared they have required more skill and understanding to get the best from them.

44179 at StratfordIn Microsoft Train Simulator and more recently in Train Simulator 2016, there are tuition sections. The most recent is the Academy which includes how to use the main controls of the steam Locomotive to start and stop. It is a short lesson driving a Fowler 4F locomotive on a flat piece of track without a train. To Summarise the lesson: Set the Reverser to full forward; release the Brakes; open the Throttle a bit; once moving adjust the Reverser to around ¼ forward; open the Throttle fully. Simple isn’t it? Well… No it’s not!

Let me ask a question – What’s the difference between a Diesel locomotive and a Steamer? We’ll ignore Electrics here because technically they’re not locomotives as they don’t generate their own power (and won’t until someone makes a viable Quantum Singularity electricity generator). There is a clue in that sentence though. It’s in the method of power generation and how it is transmitted to the rails. That answer may seem obvious but a little more in-depth analysis will serve to explain the difference and give greater understanding – for me if not for anyone else 😉

Taking the Diesel first, it has – surprise; surprise – a diesel engine that has a rated maximum power output. That is connected to the driving wheels either via a generator which supplies electrical power to electric motors mounted on the axles or mechanically via either a hydraulic torque converter or a gearbox like that used in a bus or truck. Mainline diesel locomotives tend to be either diesel-electric or diesel-hydraulic. The key point is that the diesel engine has a maximum power that you will get whenever you open the throttle fully. You adjust the throttle to achieve the amount of power at the rails that you require for the task in hand.

The Steam Locomotive has an external combustion engine connected to its driving wheels. There is an interesting point I’d like to make here. I think you can view the power generation section; the steam producing boiler – and the drive section; the cylinders transmitting the power to the wheels as separate items rather than a whole. My reason for this assertion is that both require human control and that the requirements of the transmission section can be at odds with the needs of the power generation area. And that is a key difference to the Diesel where the whole process of turning fuel into effort at the rails is controlled by the throttle – a necessary simplification to make the point.

First let’s look at the power generator of the Steam Locomotive – the Boiler. The Boiler is a dual-fuel machine – it requires coal for the fire and water to heat in order to produce steam for the drive section of the locomotive to use. Every boiler has a rated maximum pressure in LB’s per Sqinch (pounds per square inch or psi). Taking one of the commonest mixed traffic Steam Locomotives in the UK, the Stanier Black 5, we find that the boiler pressure was rated at 225psi. On the Somerset and Dorset line the 7F 2-8-0 locos had a boiler rated at 190psi. I’m going to ignore other things like superheating so as not to complicate matters further! Anyway, the boiler requires the fireman to supply coal to the firebox in an amount judged to maintain the correct heat in the fire for maximum steam generation and to add additional water to the boiler as the steam is produced and used. It’s worth noting that maintaining the boiler pressure is easier if the water level is kept at a high level (90% full) as that leaves less room for the steam and therefore causes it to reach the desired pressure quickly.

The driver controls the Throttle and Reverser – the throttle is usually a lever but the reverser can be of a lever or a screw type. Just a note on terminology here – In the UK the Throttle is also called the Regulator and I understand that the Reverser is known as the Johnson Bar in the US. I’ll stick with Throttle and Reverser here to avoid confusion. These controls both work on the way that the steam produced by the boiler is used by the pistons in the cylinders to drive the wheels. The Throttle opens the top-feed in the boiler allowing steam to access the Steam Chest of each cylinder. The Reverser controls what percentage of the steam that is available in the steam chest to access the cylinder to drive the piston at each stroke – the lower the percentage the less steam is allowed in.

This is starting to get a bit more technical than I intended. There are key points that I think need to be understood though. Firstly, the size of the cylinders is designed to work with steam at the pressure supplied by the boiler. It is the expansion of the steam within the cylinder that does the work. The driver has to control the combination of available steam from the Throttle opening with a percentage setting on the Reverser that ideally balances the two for efficient use of the steam at the boiler pressure. This can be done at a given speed by more than one combination of settings. For example – Full Throttle and 15% Reverser or 75% Throttle and 30% Reverser. On a specific type of locomotive both could produce the same level of speed performance and steam economy. In general, assuming full throttle, the percentage of steam required per stroke diminishes as speed increases as the steam spends less time in the cylinder to do its work.

That’s fine on the flat but what about when you hit that steep incline? I know… Open the Throttle or increase the Reverser percentage! Wrong!!! Oh, you can get away with it on a short, sharp, incline between flat stretches of track but not on a long climb. Remember that bit about the cylinders being designed to work with the rated boiler pressure? Either of those two options will start to drain the boiler of pressure because you will be taking more steam than it’s capable of producing. If the boiler pressure drops then the steam is less pressurised and so cannot expand as much when it gets to the cylinder and you will be into a downward spiral where you demand ever more steam from the boiler. Sooner or later it will be at such a low pressure that the loco can no longer maintain forward momentum and stalls.

So, what should you do? Maintain the boiler pressure at all costs! You have to adjust the Throttle and Reverser combination to ensure that the rate of steam generation is never exceeded. Instead, you allow the speed to drop to what I refer to as the balancing speed. Every loco has a balancing speed for a given weight of train and incline – the speed at which the horsepower the locomotive can put to the rails will maintain steady momentum. So, if you ensure that you maintain the pressure of the boiler at all times the locomotive will take the train up and over the climb at that balancing speed – assuming that the weight of train is not beyond the locomotive’s actual capabilities. Let me give an example of the difference between this method and using too much steam to try and maintain speed. With a Black 5 and increasing the Reverser Percentage in an attempt to maintain speed up a 1 in 50 climb – passed the summit at 8mph and was down to just over 5mph near the top. Same climb with same train but maintaining boiler pressure – minimum speed 22mph and passed the summit at 27mph with the Throttle wide open and the Reverser at 13%.

60009 accelerating away from ShildonIt’s not perfect, I’m still learning. Ironically, I had a lot of clues available to me in a very old book that I own called Top Link Locomotives by Norman McKillop. As a Top Link Driver himself he knows what he’s writing about. It was through his book that I first met Nine (60009 – Union of South Africa). I read through her run again on the Queen of Scots Pullman and noted that at no time did the Reverser go beyond 30% for any part of the journey. In fact most of the run was in the 15-20% area. And that pointed me to a solution to my Steam Driver woes. Further research took me to an excellent web article by an American author – Al Krug – which explains some of the differences between Steam and Diesel traction very well. You can read it here. I also researched some logs of runs on the Somerset and Dorset line and even the professionals could get it wrong on those grades – from which I draw some comfort 🙂

So I’ll leave you with a couple of shots of Railworks Steamers in action – please enjoy 🙂


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