What are Poppet Valves ... and why do they matter?

In order to understand what poppet valves are and why they were used on the L-1 and L-2a Hudsons, it is first necessary to understand the conventional piston valve and how it works.2

PISTON VS. POPPET VALVES

With conventional piston valves, the valve chamber is on top of the cylinder, and contains the spool shaped piston valve. Steam is admitted into the center of the valve chamber, and exhaust ports at each end of the valve chamber lead to the exhaust nozzle in the smoke box. Steam passages connect each end of the valve chamber to the corresponding ends of the cylinder. The valve gear (usually Walschaerts or Baker on C&O locomotives) causes the piston valve to oscillate within the valve chamber, alternately admitting steam into one end of the cylinder or allowing steam to exhaust from that end of the cylinder. Because the design of the valve is symmetrical, the same admissionexhaust sequence occurs at the other end of the cylinder as well, but precisely out of phase with the first end: while steam is admitted at one end it is exhausting at the other, and while steam is exhausting from the first end, it is being admitted into the other.

The point where the moving valve closes off admission of steam into the cylinder is called "cut off," and when it occurs with respect to the position of the piston is adjustable by the engineer. When starting, the engineer will typically use a long cut off, allowing steam to be admitted into the cylinder for as much as 80% of the piston stroke. As the speed is increased, the engineer shortens the cut off, admitting steam during less and less of the piston stroke until at high speeds steam is only admitted during perhaps 10% of the piston stroke.

The dashed curve in Figure 1 (right) shows the amount the valve is open both for steam admission and exhaust as the driving wheels turn through slightly more than one revolution. Here, crank angle rather than piston stroke is shown, where 0 degrees is when the crank pin is closest to the cylinder, straight ahead of the driver axle. As the cutoff is shortened, both the valve area and the duration of the valve opening become less, so that at high speeds, where very short cutoff is used, both the admission and the exhaust are very restricted. This restriction on the admission is desirable, since it causes less steam to be used, and allows the steam more time to expand, but restriction on the exhaust is highly undesirable. When the exhaust is restricted, some of the steam is left in the cylinder and compressed as the piston makes its return stroke, and this compression requires energy that would otherwise be applied to the rails as tractive effort. As the locomotive speed increases, this effect becomes more and more pronounced, resulting in a marked decline in horsepower, as shown in the dashed curve on Figure 2.

The poppet valve essentially eliminates this problem by opening and closing very rapidly, providing very high valve area regardless of cutoff, and making the operation of the exhaust valve independent of the intake valve. In operation, the poppet valve is similar to the valves in an internal combustion engine, although they are constructed differently.

The solid curve in Figure 1 shows the valve area for the poppet valve compared to piston valves. Note that the valve opening area is greater with the poppet valves, and that the exhaust port for the poppet valve is open longer than for the piston valve. The solid curve on Figure 2 shows the effect of these changes on the horsepower vs. speed curve for a poppet valve locomotive. Note that there is essentially no fall off of power as the speed increases, indicating that the poppet valve is doing its job effectively. Note too that the use of poppet valves may allow the railroad to use a less powerful locomotive in a given service than would otherwise be required with a piston valve locomotive, if the intended running speed falls within the range where poppet valves give significantly more power than piston valves.

THE L-1's FRANKLIN TYPE A POPPET VALVES

The poppet valves installed on the C&O's Hudsons were supplied by the Franklin Railway Supply Company, a subsidiary of the Lima Locomotive Works. Franklin's poppet valve division was formed in 1937 to develop and commercialize the European Dabag poppet valve system, and upgrade it to make it more suitable for use on American locomotives. Franklin's first version of the poppet valve was the Type A, which used oscillating cams to drive the valves. Later, they developed the Type B, which used a rotating cam to drive the valves. The C&O used both types, the Type A on the L-1s and the Type B on the L-2a's.

In construction, the poppet valve has dual seats and is hollow, so that when it is open, steam can flow through the valve as well as around it, as shown in Figures 3 and 5.

The valves are arranged horizontally above the cylinder, in two sets, one for each end of the cylinder. Type A poppet valves, as were installed on the L-1, had two inlet valves above two exhaust valves on each end, for a total of eight valves per cylinder, as shown in Figures 4 and 5.