The New Zealand Railways Magazine, Volume 9, Issue 8 (November 1, 1934)
Locomotive Development in New Zealand
Locomotive Development in New Zealand
IN the previous article of this series the genesis of the “Pacific” type of locomotive was traced to a New Zealand designer, and it was mentioned that the Baldwin Company had, in the course of discussion leading up to the design of the first “Pacific,” suggested a curiosity in the shape of a six-coupled engine with a “Wooten” type of boiler. This engine got no further than the suggestion, and—to quote from the previous article—“unfortunately time was too pressing to allow the New Zealand engineers to fully satisfy their curiosity as to how the Company proposed to fill up the extraordinarily long rigid wheel-base of 16ft. 8ins.”
Curiously enough, a reader has probably solved the mystery. In the Delaware and Hudson Railway Bulletin for September, 1934, just received, is a historical picture showing the D. and H. locomotive No. 64 as photographed in March, 1887. The heading to the picture reads: “Brand New from Dickson Locomotive Works,” and the type of locomotive shown would certainly meet the sketchy specification given by the Baldwin Company for the suggested engine for New Zealand, in 1900. The “Wooten” type of boiler is clearly shown, as is also the long rigid wheel-base. There is little doubt that a non-technical reader has solved the mystery that “Progress” was too lazy to elucidate, and the long arm of coincidence is apparent in the fact that the Delaware and Hudson Magazine arrived in the office in the same week as the copy of the “New Zealand Railways Magazine” that mentioned the problem!
It is clear that such a type of engine would be quite unsuitable for the curved track in New Zealand, and, while the driver would have a comfortable ride, one wonders how the fireman would fare—riding on the bumping tender and aiming at the swinging firehole door opening on the long overhanging boiler! The “Pacific” type was evidently evolved just in time to save the development of a new type of acrobatic, india-rubber fireman! The evolution of this “Pacific” type of locomotive left one vexed historic question of locomotive design still in dispute—the question of compound versus simple engines.
In the simple engine the steam is passed through one cylinder in its working path from the boiler to the exhaust. This cylinder is perforce exposed to great variations of temperature and, especially at low speeds, a considerable proportion of the entering hot steam is condensed on the walls of the cylinder—which have just been cooled through contact with the colder exhausting steam. The greater portion of this condensed steam clings, as water, to the walls of the cylinder during the working stroke, but is evaporated during the exhaust stroke and escapes up the funnel. Much of the heat of the steam is, therefore, not employed usefully, but is lost. Further, a similar action goes on in the valve chamber and there is, as well, considerable leakage through the valve and piston rings at high pressure.
The compound engine is one means of counteracting these losses. In the compound engine the steam is passed through two cylinders in its working path. The variation in temperature is, naturally, now much smaller in each cylinder and less harmful condensation occurs in each. Further, all leakage of the high pressure steam through, and all condensation in the first smaller valve and cylinder, is entrapped and gives up at least some of its useful force in the second larger low pressure cylinder. So far all is gain and in relatively slow running engines, say up to 150 revolutions per minute, the gain is pronounced. At high speeds in a locomotive, however, the number of revolutions per minute is over 300. At this speed, the steam is exposed for such a small fraction of a second to the influence of the cylinder walls that very little, if any heat interchange and loss (certainly not enough to justify the extra complication and friction of two cylinders in place of one) takes place.
A further reason exists why compounding is not so favourable in a locomotive as in the ordinary marine engine. The draught for the locomotive furnace is obtained from the forcible emission up the funnel into the atmosphere of the exhausted steam. This steam is perforce at fairly high pressure and exerts a negative, or “back pressure” force upon the piston that has to drive it out into the atmosphere. In a compound engine the low pressure piston, which does this driving, is about 50 per cent, greater in area than the piston of a simple engine of corresponding power. The harmful back pressure is, therefore, much greater in a compound locomotive and, at high speed, this entails a serious loss.
The truth, as we can easily see it today, is that in the locomotive, the compound engine is, per se, more economical than the simple engine at speeds up to (say) thirty miles per hour and less economical at higher speeds. Even this partial economy is gained at the expense of extra complication, wear, expense and unreliability. The compound locomotive should therefore only be considered for those rare circumstances where a low speed locomotive is necessary, or can be advantageously used.
The issue was complicated by the opinion, which is not yet always recognised as a fallacy, that a more uniform turning movement than is given by a two-cylinder simple engine is desirable in a locomotive. It is still held by many that the more even turning movement minimises slipping and allows greater power to be developed with any fixed adhesive weight. Unfortunately, this is one of the “selling points” in the two extraordinary advertising campaigns of modern railway times—electrification and the internal combustion engine—so it is unlikely that the truth will gain general acceptance quickly. However, that may be, De Glehn firmly believed, with all locomotive designers of his time, that the undoubtedly more even torque given by his four-cylinder balanced compound locomotives was a great point in their favour.
In general, however, traffic requirements are best met by a standardised locomotive suitable for all-round work and the simple “Pacific” type of locomotive is outstanding in this respect This fact was realised twenty years ago by locomotive designers in New Zealand, but passenger and freight locomotives are still distinguishable elsewhere. A recent day's work of one “Ab” engine—Wellington to Palmerston North, on the fast non-stop Limited, Palmerston North to Marton, on the fast through goods, Marton to Palmerston North, on the slow local goods, ten hours on the ballast train, and off again on the Limited to Taihape—surely emphasises the point that Sir Isaac Newton was wrong when he cut two holes in his study door, a big one for the cat and a little one for the kitten. Our artist shows amusingly well what actually happens to the designer of special-service holes and locomotives.