TRACTION AND MARINE ENGINES
The demands of either traction or marine engines differ considerably from those of steam locomotives, where gradients and speeds change frequently. In addition, compounding to make better economical use of the steam has less design strictures than in the locomotive chassis. As a consequence their engine unit developments have taken different paths.
Although several valve gears have been invented for traction engine use, including the original Baker gear, Stephenson’s is the one most applied. Since this gear is so capable of first class steam distribution equality it is disconcerting to find its suspension so poor in most traction engine designs. Most are end-suspended with the usual poor results and any semblance of smooth running left for the heavy flywheel to accommodate.
It is rare to see a weighshaft and lifting arm capable of rendering good events even in full gear. The proximity of these components to the boiler may place restrictions on their position, but because they constitute the only means of error correction they deserve better attention. It seems to have been assumed that a square layout of the reversing mechanism will automatically give equal favour to both forward and reverse gears, which is far from the case. Care in placing the weighshaft and adjusting the length of the lifting arm can dramatically improve all the events and do more justice to the Stephenson’s design.
This is particularly so where a launch link is used. The reasons for its use are that eccentric diameters are reduced and the overall working height minimised in a situation so close to the boiler. Employing a launch link in direct drive to a slide valve gives rise to a knuckle joint and upsets valve symmetry, and the end-suspension so common is ill-equipped to deal with the resultant errors. Placement of the weighshaft and the length of the lifting arm become critical, and few traction engine gear designs are able to take full advantage of Stephenson’s inherently good features in respect of valve symmetry.
Compounding therefore brings an advantage probably unrecognised, as it divides the expansion between cylinder bore ratio and gear, thereby relieving the expansion link of a responsibility it can ill perform, rather in the same manner as marine engines.
Overall, three things will directly affect the design of valve gear for marine engines. As they are predominately vertical engines with a requirement to keep the overall height within bounds the connecting rods will be short compared with the stroke and angularity problems therefore arise.
In marine applications most engines are compounded to make maximum use of the properties of steam and the valve gear will of necessity work mainly in full gear. The relativity of cut off between each stage of expansion is adjusted to balance the power produced by each cylinder, provision usually being incorporated in the lifting arms.
A third faculty arises as vertical engines increase in size: the fact that gravity plays a part necessitates different treatment of the up and down strokes, at least in the provision of lead steam.
The first of these conditions does not affect the efficacy of the cylinder itself, but because the crankshaft forms the drive for the valve gear the angularity of the connecting rod is passed on to the gear to upset the cut offs unevenly, even at the full gear cut offs in the region of 75%. Most well regulated valve gears, especially in full gear, tend towards a longer cut off at the top port and in considering the aid of gravity on the down stroke this is the reverse of what is required. It is fairly easy to reverse this cut off tendency, but not without seriously affecting the other events.
Irregularities on the exhaust side can often be attended independently from the other events, since the other edge of the valve only is concerned. Small positive and negative exhaust laps will effect equalisation, but in general if the cut offs are practically equal the exhausts should follow. Unequal laps and greater lead at the bottom port counter the gravity effects and reduce pounding, but it should be appreciated that pounding is not always a sign of insufficient lead - inequalities of cut off can induce the same symptoms.
Solid or two bar expansion links are frequently used in marine engines. The latter allow direct alignment of the eccentric rod end with the die. Furthermore, suspension may be anywhere on the link. This is deceptive: both end-suspension and central suspension require modification if reasonable cut off equality is to be maintained, since these are the only design-adjustable links in the kinematic chain. The performance is quite unlike that of a locomotive application.
Another seemingly innocuous factor presents itself where the HP valve is an inside admission piston valve and the other cylinders are furnished with outside admission slide valves. Far from being a straightforward directional reversal the corrective measures to equalise events suffer a reversal that the suspension elements may not be able to accomplish well. Each case will be different and the issues ought carefully to be addressed at the design stage. The cut off adjusters incorporated in order to balance the expansion levels of a triple engine have a specific task and cannot address event equality issues. Similarly, the shims normally placed between the eccentric rods and their straps are only capable of a little fine tuning.
In all applications, and marine engines are no exception, it is folly to think that putting the suspension and reversing elements on the other side of the engine, ie. a mirror image in elevation, is at all practical unless the rotation of the crankshaft is also reversed. The corrective devices to equalise the valve events render this impossible without upset and need reworking.
In addition, the common advice for end-suspension is to use the forward or ahead end away from the weighshaft in order to use the smoother arc of a longer lifting link. Depending on the layout this advice may prove foolish. Similarly, the textbook advice to space the expansion link pins at 2.5 – 3 x valve travel is emphatically misleading. The effect on increased lead and compression levels notched up even slightly, and also on the corrective measures in the suspension, can be counter-productive to say the least. If full gear running only is envisaged no harm may ensue but the correct locomotive proportions should be used in all engines requiring the full range of the expansion link – expansion link is a misnomer if compounding.
Stationary steam engines vary considerably in layout and type, often work at constant speeds and are less restricted in the space available. They display innumerable methods of driving many different valves and by means ill-adapted to other applications.
All the engines types on this page appear to have been developed in some isolation of each other save for basic steam engine principles. Strong traditions are evident, sometimes without subsequent enquiry into their veracity. None have been subject to computer simulation at the design stages and there is much scope for enhancement in the popular modelling area. Very often a valve gear of considerably better performance remains dimensionally close to an original design and is therefore unlikely to offend the most fastidious prototype modeller.