At its most basic, a slide-valve is like a sliding patio door; move it one way and a passage is opened to allow free transfer of intake air or exhaust, move it the other and the flow of air or exhaust is blocked.
Although slide-valve gas engines seem exotic today, when Nikolaus Otto patented the 4-cycle engine in 1876, slide-valves represented mainstream technology. Slide-valves were used extensively in steam engines, then the dominant form of power, for the admission and exhaust of steam, and it was only natural for Otto to incorporate slide-valves into his early engines. Otto might have been a mechanical visionary, but he was also a practical man.
The simplest slide-valves were really no more than flat blocks of machined metal. Easy to manufacture, they had the added benefit of being easy to maintain. Although basic poppet valve technology is at least as old as slide-valve technology, poor metallurgy meant that early poppet valves weren’t robust enough to stand up to the rigors of sustained use. Even when poppet valves became the industry standard they still required a relatively high degree of maintenance, requiring frequent cutting, cleaning or reseating to maintain compression.
However, the slide-valve’s limitations, including high friction and poor sealing against high pressure, pushed engineers to perfect the poppet valve, still the technology of choice in today’s gas- and diesel-powered engines.
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