As is usually the case when getting down and dirty constructing something from scratch, you have your preferences on how things should be built and what pieces should be utilized. Hobbyists of steam engines are no different, and many have their own takes as to how to properly assemble them and why. Building from the ground up is meticulous, so deciding on your parts ahead of time is advantageous. A common question that arises between two separate schools of thought are which is better between a slide valve or piston valve during assembly? To better decide on which is the more fruitful option, it is wise to fully understand each valve’s pros and cons. Though not everything you need to know about model steam engine kits, valve type makes up the framework of construction.
Of the two, slide valves have an older history with engines, primarily because they were/are less expensive when needed to help power locomotives. Rectilinear in shape, slide valves are a device that regulates the admission of steam into a steam engine, as well as the emission of the exhaust. Most engine connoisseurs know this of course, but not everyone is well versed in the subtle differences between this valve and a piston. Slide valves have less steam blow-by loss which is favorable in maintaining its force consistency.
One of the main drawbacks to the usage of a slide valve is that because they cannot be lubricated they are susceptible to valve failure. When used in the past, this was obviously of major concern, for engines were generating super heated steam, and it was obviously not ideal for this to adversely affect each slide valve. Super heated steam almost made incorporating slide valves counter-intuitive because it warmed water to the point that it was nothing but a gas. The gas’s temperature ran so hot and produced so much power that the friction was literally destroying the valve itself. Slide valves are notorious for wearing ‘in,’ meaning this pressure was generating force on the valve to press against the cylinder. As such, the steam caused slide valves to wear over time, and this was just one of the reasons piston valves began to be ushered in for the manufacturing of new engines. The seal that is being created is problematic for substantial airflow, which naturally will eventually reduce the power of the locomotive.
Since you are likely building a small steam engine for your own personal collection, slide valves still offer a fluid solution to the flow of air. It is a myth that because some problems arose from the usage of slide valves on larger scales that they were made completely obsolete. Like most pieces of technology, a more logically crafted unit followed in the piston, but slide valves are still worth their weight in gold for cost purposes, particularly in hobby creations.
For people participating in the recreation of engine building, visual appeal is very important to them as well. On that front, the operation and execution of slide valves in motion are a very pleasant aesthetic image to take in, and something to keep in mind for your next model.
Though hotly debated, comparing the valves is part of the fun of having a model steam engine, and the fact that slide valves are still used frequently today despite some drawbacks prove they have stood the test of time in remaining efficient.
As super heated steam was a major deterrent for the proper execution of slide valves, piston valves were/are a more practical design as far as air flow because all the aforementioned pressure created is better equalized with its use. Whereas this steam would press the slide valves against the cylinder causing them surface harm, piston valves can level out the pressure more ideally because it is moving up and down. Pistons rely on ‘inside admission,’ and as slide valves wear ‘in,’ piston valves wear ‘out.’ What eventually is negatively manipulated for piston valves is the bore.
A mark in the ‘pro’ corner for piston valves is that when used they can reverse the engine if the flow of air is reversed. Additionally, construction of piston valves have rings on them making them easier to lubricate as opposed to slide valves. Piston rings on the valve are critical in preventing blow-by. Steam passages can be constructed shorter with piston valves which is integral to reducing steam flow.
A frustration among engine enthusiasts is that piston valves are extremely difficult to seal, and even if this feat is managed, leakage and wear are typically still inevitable. The sounds created in using piston valves is more or less in the eye of the beholder; some find it annoying while others feel it is all part of the steam engine experience.
What the debate between slide valves and piston valves really comes down to is what type of vision you have for your steam engine. Are budget and appearance important to you? Are you trying to construct a duplicate of the originals (usually slide valves), or are you re-imagining the way you put together an engine? Unless you plan on running your steam engine repeatedly for long periods of time, lubrication is obviously not going to be an issue. Even though pistons were introduced after slide valves, some feel they are more true to a train’s DNA because they were used for so long in conjunction with railroads. The building of steam engines is a time consuming and delicate process, and relies on patience to be satisfied with the result. Piston valves may make more sense for how they were innovative to movement of the engine, but drawbacks mentioned previously like the noise or sealing may give you pause. Conversely, slide valves present a rather inexpensive path to creation, but do not deliver the advancement piston valves displayed. Each selection has its pros and cons, but both can successfully run your steam powered engine. Regardless of which valve you choose to include, either one will have to be ‘timed’ to make sure the engine runs smoothly. This means that all the parts are moving in unison so the engine itself can ‘take over’ and be self-sufficient. If torn, a complete buying guide to steam engines can help you make up your mind on which avenue to take.