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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