I borrowed a 1927 locomotive cyclopedia ( Truly amazing book) and was looking at trailing truck and tender booster engines. What I had noticed was that there was no way of disengagement of the driving engine from the wheels. When the booster is working steam is introduced to the cylinders just like the main drivers. When steam is cut the wheels and reciprocating machinery is still moving as long as the wheels are turning. Wouldn’t that be a tremendous drag on the locomotive? You would think there would be some sort of clutch or sliding gear set to disengage the engine? Looking at the parts breakdown there appears to be nothing to keep the piston and rods from moving while the wheels turn. I can see why they were not very popular.
IIRC this same question came up maybe a year ago, you could do a search of the forums to see. Anyway, seems to me the concensus was that no, it had virtually no drag effect. Apparently the relative unpopularity had more to do with maintenance problems.
A steam locomotive that wasn’t under power would roll pretty freely despite having cylinders and siderods. I recall reading somewhere that when roller bearings came along engines became so free-wheeling that 2-3 men could move even a big engine just by pushing on it. Frank King in one of his books noted that the DM&IR almost lost one of their first Yellowstones into a swamp, because it started to roll down a track that everyone had always thought was flat so they didn’t bother to tie it down. Turns out it had a tiny grade to it that was enough to get the big engine rolling.
I’d say it’s a safe bet trailer-truck boosters always had the idler gear, so they could be cut out. The 1924 Franklin booklet so describes them. Dunno whether that applies to tender boosters as well-- maybe any engine that had a tender booster was thereby limited to 20 mph?
Boosters did mechanically disconnect from the trailing truck wheel to eliminate unneeded wear and tear. In certain applications boosters made a lot of sense. Look at the typical 4-8-4. There you have an engine with a large, powerful boiler, but limited adhesion due to only 4 drive axles. At low speeds, it would be impossible to apply the boilers full output to the rail. The higher the boiler pressure, the worse it got at low speed because you are applying more force to the wheels with no increase in adhesive weight.
I know Ross Rowland was a huge fan of boosters, first with the 2101 and later with the 614. He told me that the booster more than doubled the DBHP on the 614 below 20 MPH. This was played out during a 614 Chessie steam trip where he marched a fully loaded 26 car passenger train (with a lot of older heavy weight equipment in the consist) right up the infamous Sand Patch Grade with no helpers and no slipping of the main drivers. Any other 4-8-4 with out a booster wouldn’t have made it halfway up that grade with that much tonnage in tow.
This point is not to be taken lightly. The horsepower produced by the typical booster engine was quite modest, but it was produced at a very low speed. (The horsepower of a 4-8-4 at low speed was also quite modest.) So even a few hundred horsepower, geared down to be applied at a speed of five miles per hour, supplied a substantial boost in tractive effort.
It’s also the reason the booster had to be disconnected above a very slow speed. As the locomotive speed increased, the effectiveness of the booster rapidly decreased. The tractive effort boost provided dropped off rapidly because the engine simply couldn’t be supplied with steam fast enough, nor could it withstand the high revs and piston speeds. The locomotive did worse with the booster connected above a certain very low speed.
Think about a loaded tractor trailer lugging up a steep grade. In low gear, the diesel behaves much
My understanding has been that the booster was meant to add to the tractive effort in order to get the bearings down the length of the train moving, and thence to get the train up to a minimal speed such that the trailing tonnage could be propelled to higher sustained speeds solely by the horsepower developed by in the cylinder saddle available at the time of cutting off the booster. Even if a typical booster only provided another 10k lbs of tractive effort, and the Y’s did much better, that isn’t something to decline if you know the booster will get a revenue train to the point where the engine can ‘take it from there’.
Can anyone imagine a single Genesis unit attempting that. Can you say “French-fried traction motors”? I knew you could!
I don’t know where the claim of doubling the draw-bar HP comes from – the main cylinders are driving the 4 main wheels at pretty much the traction limit, and a booster engine is driving one trailing axle on the rear truck at a somewhat lower tractive effort per axle (they had to do it that way, otherwise that booster axle would be slipping all the time on account of the single-axle vs average-of-coupled-axles adhesion limits), and at whatever speed, HP = tractive effort in pounds X speed in feet per second X conversion of foot-pounds per second into HP. That means at any speed, the contribution of the booster is in proportion to tractive effort on the boosted axle, and I can’t see it being close to half the total tractive effort.
You are right about a single Genesis, but just about any single DC six axle could do it. I figure about 80-90,000# TE is needed to get a 26 car passenger train over Sandpatch. At 20 mph, that’s 4500 drawbar HP or so. At 10 mph, thats 2250 HP. At these low speeds, the boiler can make much more HP than the steam engine can use, so, if I increase the number of driving axles by 75% (from 4 to 7) by employing a booster, I should be able to increase my drawbar HP by the same amount. “Double” and “below 20 mph” are smallish exaggerations, I think.
A booster would be far more useful if it could power an additional 3 axles. My understanding of them, however, is that they were placed cylinders-pointed-rearward off the last axle of the trailing truck, and as such, they powered at most one axle.
Some tender boosters had siderods, although it had been remarked that siderods on the smallish tender truck wheels limited them pretty much to yard or transfer service on account of speed restrictions.
The remark that “boosters ate steam” earlier in this thread is one I believe. If the knowledge of locomotive firemen has been passed down to our generation through oral tradition, and if they guys whose job it was to keep steam up had experience with boosters, those things must have contributed to high steam consumption under heavy load.
A person has to figure that when a steam locomotive is at max tractive effort, the efficiency with which steam is used goes way down because one is probably operating close to max cutoff, not doing expansive working of the steam in the cylinders to optimize efficiency. A Northern at max tractive effort and low speed could be close to the steaming capacity of the boiler even if the DBHP was at the left-hand shoulder of the curve, and then to throw on the booster, that could keep the fireman on his toes.
A booster may even be set to a fixed and perhaps steam-eating high cutoff as it is only used sparingly at high tractive effort situations anyway. But as to a “booster eating steam”, I am also wondering if adding the booster was a “straw that broke the camel’s back” s
Yes, there were steam locomotives where every axle was drived by what amounted to a booster engine. One example was the shay. It could pull like crazy with the limitation that speed was limited to less than 15 mph, and that only if one wanted to overhaul the cylinders soon. More like 8 or nine mph was a realistic operating limit.
As far as boosters increasing the number of driven axles, a trailing truck never had more than one axle driven by the booster. This was true for a Northern type as well. So for a Northern, the weight on drivers might increase by 20%, but typically the increase would be less, more like 10 to 15% because the weight on the trailing truck axles wouldn’t be as great as that on the drivers. A typical booster engine would increase tractive effort by about 10,000 to 12,000 lbs, and that only at low speed.
Now for a Northern, tractive effort produced by the main engine could remain at a peak much higher than the 5 miles per hour of a booster’s peak tractive effort, precisely because the large drivers and valves of the main engine didn’t restrict steam flow at higher speeds, and the boiler could produce all the steam the engine needed. The booster did eat steam at high speeds and for diminishing returns, so it would be cut out pretty early.
Remember that one characteristic of a steam locomotive is that, unlike a diesel-electric, it can be overloaded and produce more than it’s rated power. The boiler can be overfired and produce more steam than it would at its best efficiency point. Lots of black smoke and unburned carbon up the stack, but lots of steam. A diesel couldn’t do that. The locomotive had a horsepower limit set by the ability of the prime mover to convert diesel fuel to ponies. Hence the old adage that a steam locomotive could pull any train it could start, but a diesel could start a train it couldn’t necessarily pull (being
When railroads tested their engines’ drawbar horsepowers, and drew a power vs speed graph that peaked at 5000 dbhp at 40 mph, did that mean the engine could actually produce 6000? 7000? 8000? dbhp at 40 mph? If so, shouldn’t that info appear on the graph somewhere? Seems relevant.
I haven’t read a 1927 Locomotive Cyclopedia, but I do have a 1925 Locomotive Cyclopedia and it contains a detailed description of steam locomotive boosters.
On page 547: "The Locomotive Booster is a simple two-cylinder steam engine mounted on a cast steel bed plate. This engine is geared to the trailer axle through an idler gear. This (idler) gear is automatically engaged as required (for booster operation) and (automatically) disengaged when the power of the Booster is no longer needed". The answer then is that the booster steam engine “turns” only when it’s power is needed, otherwise it is automatically disenegaged and, as today’s truck drivers might say, it is in neutral and not turning.
Figure 1245 in the 1925 Locomotive Cyclopedia: “The Locomotive Booster” (p. 547) is an excellent cut-away view of a typical locomotive booster with all major components identified, including the Clutch Cylinder, the Idler Gear Rocker and the Idler Gear. From this is it fairly obvious how the engaging and disengaging is accomplished. And yes, the exhaust steam from the booster engine was directed to the smoke box at the front, where it joined the exhaust steam from the engine’s (main) cylinders in the nozzle, thus adding to the draft.
Certain of the statements regarding the power available from a booster seem a bit ambitious. Turning to the Model Railroader Cylopedia I, Steam Locomotives, p. 199 the New York Central class J3a Hudsons, it reports that these steam locomotives could develop 53,960# T.E. without booster and 66,060# T.E. with booster. .That would put each of the three driving axles at 17,987# T.E. and the one axle powered by the booster engine at 12,100# T.E., or about 22% of the total T.E.