This partial calculator compares selected valve half-travel, outside lap and port width to determine maximum geometric uncovering. It does not calculate valve events, steam flow, cutoff, lead, exhaust, pressure loss or engine performance.
Steam Engine Valve Chest - Selected Port-Opening Ledger
Explore selected valve half-travel, outside lap and port-width geometry.
Equation Used
- Travel, outside lap and port width are entered linear dimensions.
- Travel is symmetric, so maximum displacement from center is T/2.
- Effective open width cannot exceed entered port width.
- Signed excess is geometric and is not a flow margin.
- Timing, lead, exhaust, thermodynamics, leakage and safety are excluded.
The secondary figure is an abstract port-uncovering ledger, not a complete valve chest design.
Selected port-opening ledger
At maximum selected displacement, geometric uncovering beyond outside lap is max(0,T/2-L). Effective open width is capped at selected port width. Coverage is effective width divided by port width, and signed excess is uncapped uncovering minus port width.
What the calculator determines
Use it to compare three entered linear dimensions. It cannot establish actual valve motion, timing or steam capacity.
Selected relationships
u=max(0,T/2-L); e=min(u,W); coverage=100e/W; signed excess=u-W.
Selected-input example
At 18 mm travel, 6 mm outside lap and 10 mm port width, maximum geometric uncovering is 3 mm, effective open width is 3 mm, coverage is 30% and signed excess is -7 mm.
Not a valve-event or flow model
Eccentric phase, lead, inside lap, exhaust geometry, linkage, valve speed, pressure, density, losses, leakage, cutoff and engine demand are excluded.
Questions
Does 100% coverage predict full steam flow?
No. It only means selected geometric uncovering reaches the entered port width.
What does negative signed excess mean?
The selected maximum uncovering is smaller than the entered port width by that magnitude.
Does this set valve timing?
No. Crank angle and eccentric phase are not inputs.
Authoritative references
- University of Illinois digitized Practical Locomotive Operating — defines outside lap, lead and valve travel.
- Smithsonian slide-valve demonstration model — documents reciprocating slide-valve port admission and exhaust.
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