What are Poppet Valves … and why do they matter?

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


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 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.

The Type A poppet valve system has four valves at each end of cylinder, driven by cams that oscillate (rock back and forth) in a cam box above the cylinder and between the valve chests, and a valve gear box on the frame along the center line of the locomotive, located in the case of the C&O L-1 behind the cylinder saddle. The valve gear box contains four miniature Walschaerts valve gears, one each for the intake and exhaust valves for each side. Cutoff is controlled in the usual manner, with a miniature liking link that moves the radius bar up and down in the expansion link, all done with a miniature power reverse. The oscillating motion driving the valve gear box is derived from links on the cross heads on each side of the locomotive.

Generally speaking, once properly set up, poppet valves gave good service, although their smaller and therefore more fragile construction compared to conventional valve gear did require a light hand when adjustments were required. In one case on the PRR, a Franklin technician was asked to adjust the poppet valves on K4 Pacific No. 5399. He asked the local shop crew to bring their tools, and he would show them how to perform the adjustment. When they arrived, tool cart in tow, he was dismayed to see that the smallest tool that they had brought was a three-inch wrench, and retrieved his own set of smaller tools from his car. In another case on the PRR, the famous T1 duplexes were giving trouble in regular service on the western part of the PRR system, with many broken valves. Finally, Franklin sent out a technician to watch how the locomotives were being operated in an effort to determine the cause of the failures. The technician returned to Baltimore and simply handed his supervisor his notebook and stopwatch, telling him to check the mile-post timings. It seemed that the trains were taking longer than scheduled on station stops, and the PRR enginemen were running at speeds up to 140 mph between stops to make up the time. The poppet valves were simply not designed to be strong enough to stand up to that kind of service.3

On the C&O L-1s, the Franklin Type A poppet valves gave very little trouble, and were not noted to be a problem by the railroad’s motive power people.


As noted above, after developing the Type A poppet valve, with its rather complicated drive system located on the locomotive center line, Franklin developed the Type B system, which used rotating cams to actuate the valves and a simple gear drive from the drivers to rotate the cams.

As shown in Figure 7, the Type B poppet valve has three valves at each end of cylinder, driven by a rotating cam in the cam box between the valve chests. The cam has three separate sections, one for each of the three valves on each end of the cylinder (the same section controls the valves at both ends of the cylinder.) The cam profile in each section varies over the length of the section, and cutoff is controlled by moving the cam transversely. The cam is driven from a crank on the main driver, coupled to the cam box by a long drive shaft with universal joints.

On the C&O, three of the L-2a class gave excellent performance, but the other two initially gave the poppet valves a bad reputation, constantly giving trouble and frequently breaking the valve gear. During their first 50,000-mile shopping, the source of the problem was found to be the three driver sets being .25 inch out of quarter, making it impossible to properly adjust the valve timing on the two sides of the locomotive. Once this problem was corrected, these two locomotives then gave service equivalent to their three sisters.


I have drawn on many sources in putting this article together. First and foremost was C&O Power, by Phil Shuster, Gene Huddleston, and Al Stauffer. Gene Huddleston’s Chesapeake & Ohio Superpower Steam Locomotives also provided much valuable material. Tom Dixon’s short article “The C&O’s Streamlined Hudsons” and H. Reid’s “My Memories of the Streamlined Hudsons,” both published in the March 1972 issue of the C&O Historical Newsletter, were also invaluable. Bill Withuhn’s series of articles on poppet valves, published in The Railroad Enthusiasts Bulletin in 1975 and 1976, which he provided to Tom Dixon, who in turn passed them on to me, were essential to gaining an understanding of the operation of poppet valves and their benefits. I have re-used many of Bill’s illustrations, and while I have drastically paraphrased his words, without them, the section on poppet valves could not have been written. Tom Dixon very generously gave me access to all of the research material he has accumulated over the years. Finally, Jim Boyd, Editor Emeritus of Railfan & Railroad magazine, kindly discussed these engines with me, helping me to understand the impact of their various features on the overall performance of the locomotive.

For those who might be interested in seeing how poppet valves operate, I have placed an animation on my web site at .

2: This section relies very heavily on two sources: a. series of articles by Bill Withuhn published in The Railroad Enthusiasts Bulletin in 1975 and 1976, and Franklin Railway Supply Co.Bulletin No. 25, The Franklin System of Steam Distribution.

3: Bill Withuhn relates this incident based on his interviews with Franklin Railway Supply officials Julius J. Kirchhof, Raymond P. Delano, and J. Edgar (Jim) Smith.

Copyright Chesapeake and Ohio Historical Society, Inc. May 2006

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