thumb|300px|Booster engine with the cover removed to show the mechanism. The driven axle is on the right; the booster normally hangs behind it.
thumb|300px|Diagram showing how a booster is installed and connected.
A booster engine for steam locomotives is a small supplementary two-cylinder steam engine back-gear-connected to the trailing truck axle on the locomotive or one of the trucks on the tender. It was invented in 1918 by Howard L. Ingersoll, assistant to the president of the New York Central Railroad.
A rocking idler gear permits the booster engine to be put into operation by the engineer (driver). A geared booster engine drives one axle only and can be non-reversible, with one idler gear, or reversible, with two idler gears. There were variations built by the Franklin company which utilized side rods to transmit tractive force to all axles of the booster truck. These rod boosters were predominately used on the leading truck of the tender, though there is an example of a Lehigh Valley 4-8-4 using it as a trailing tender truck.
A booster engine is used to start a heavy train or maintain low speed under demanding conditions. Rated at about at speeds from , it can be cut in while moving at speeds under and is semi-automatically cut out via the engineer notching back the reverse gear or manually through knocking down the control latch up to a speed between , depending on the model and gearing of the booster. A tractive effort rating of was common, although ratings of up to around were possible.
Tender boosters are equipped with side-rods connecting axles on the lead truck. Such small side-rods restrict speed and are therefore confined mostly to switching locomotives, often used in transfer services between yards. Tender boosters were far less common than engine boosters; the inherent weight of the tenders would decrease as coal and water were consumed during operation, effectively lowering the adhesion of the booster-powered truck.
Advantages/disadvantages
thumb|300px|Franklin Type-E locomotive booster affixed to [[Reading 2102|Reading T-1 no. 2102]]
Advantages
The booster was intended to make up for fundamental flaws in the design of the standard steam locomotive. Most steam locomotives do not provide power to all wheels. The amount of force that can be applied to the rail depends on the weight on the driven wheels and the factor of adhesion of the wheels against the track. Unpowered wheels are generally needed to provide stability at speed, but at low speed offer no advantages and just add weight. The application of a booster engine to a previously unpowered axle meant that overall starting tractive effort was increased without penalty to the adhesion levels of the main engine.
As the "gearing" of a steam locomotive is fixed (because its pistons are linked directly to the wheels via rods and cranks), a compromise must be struck between ability to exert high tractive effort at low speed and the ability to run fast (without either inducing excessive piston speeds or the exhaustion of steam). At low speeds a steam locomotive is not able to use all the power the boiler is capable of producing; a booster engine puts that wasted potential to use.
The increased starting tractive effort provided by the booster meant that, in some instances, railroads were able to reduce the number of, or eliminate the use of additional helper locomotives on heavier trains. This resulted in lower operating and maintenance costs, higher locomotive availability and productivity (ton-miles), and ultimately, greater profitability.
Disadvantages
Boosters were costly to maintain, with their flexible steam and exhaust pipes, idler gear, etc. Improper operation could also result in undesirable drops in boiler pressure and/or damage to the booster. The booster and its associated components also added several tons of weight to the locomotive which would be considered "dead weight" at speeds above which it could be used. Additionally, if the booster suffered a failure where the idler gear could not be disengaged, the entire locomotive would be speed restricted to or less until it could be taken out of service to facilitate repairs, decreasing locomotive availability.
Performance
Tractive effort
A rough calculation of booster tractive effort could be made with the following formula:
<math display=block>t = \frac {cd^2spr} {w},</math>
where
- t is tractive effort in pound-force
- c is the coefficient representing mean effective pressure, normally set to 0.80
- d is the piston diameter in inches (bore)
- s is the piston stroke in inches
- p is the working pressure in pounds per square inch
- r is the booster gear ratio
- w is the diameter of the trailing wheels to which the booster is geared in inches
Operating speeds
The typical locomotive booster employed a pair of by cylinders. Available gear ratios and associated operating speeds for both the Franklin type C and type E booster models are detailed in the table below.
{| class=wikitable style=text-align:center
|+ Locomotive booster gear ratios and associated operating speeds both of the conversions from class C7 to class C9 (in 1931), which were given boosters of J. Stone & Co Type C3; and one of class S1 (in 1932). The remaining four were all fitted to new locomotives: the two P1 2-8-2 locomotives, built in 1925 with the Franklin Type C-1 booster; and two class S1 locomotives built in 1932. By ordering five boosters from Stones at the same time, the LNER was able to obtain a price of £875 each, whilst the three bought from Franklin had cost more than £1,100 each. The boosters were removed between 1935 and 1938, apart from those on class S1 which were retained until 1943.