A marine trunk engine attaches a large hollow trunk to the piston and pins the connecting rod inside it. The trunk passes through a stuffing box, reducing the working area on that cylinder face.
Trunk Engine Interactive Calculator
Enter cylinder bore, trunk outside diameter, one assumed equal mean effective pressure and mean piston speed. Results are ideal one-cylinder arithmetic—not a steam-cycle, valve, friction, output or design model.
Equation Used
- One double-acting cylinder is represented.
- The same entered mean effective pressure is applied to both piston faces.
- Cylinder and trunk are ideal concentric circles.
- Entered piston speed is mean piston speed.
- Valve events, expansion, condensation, pressure variation, friction, leakage, gland loading, connecting-rod geometry, inertia, cylinder count, shaft output and structural limits are omitted.
Hollow Trunk, Internal Pin and Stuffing Box
Sennett and Oram's 1899 marine-engine text describes the trunk as a large hollow member cast or bolted to the piston and working through a steam-tight stuffing box. The connecting rod attaches directly to a journal or gudgeon in the piston.
This compact arrangement removes the separate piston rod and crosshead. The calculator isolates only its unequal circular working areas.
What This Calculator Represents
The tool computes full circular piston area, subtracts the trunk cross-section from the trunk-side face, and applies one entered MEP equally to both faces. It reports one-cylinder ideal indicated power from entered mean piston speed.
It does not predict a historic engine's operating point or shaft output.
Ideal Area and Indicated-Power Arithmetic
net trunk-side area = max(0, full area - pi d²/4)
ideal one-cylinder indicated power = p × v × (Afull + Anet)/2
trunk/full area ratio = min(100%, d²/D² × 100%)
Worked Arithmetic Example
For D=1000 mm and d=500 mm, full-face area is about 7854 cm² and net trunk-side area about 5890 cm². With entered MEP 200 kPa and mean piston speed 3.5 m/s, ideal one-cylinder indicated power is about 481.1 kW. The trunk/full area ratio is 25%.
Model Boundary
- Calculated: two ideal circular areas, their ratio and one pressure-area-speed result.
- Not calculated: stroke, rpm, cylinder count, valve timing, cutoff, expansion, steam consumption, condensation, friction, gland force or heat, rod obliquity, inertia, bearing loads, leakage, shaft power, efficiency, materials, clearances, strength or safe operation.
Frequently Asked Questions
Why is the trunk-side area smaller?
The trunk cross-section occupies part of that piston face.
Is mean pressure predicted?
No. One equal value is entered for both faces.
Is this shaft power?
No. It is ideal indicated arithmetic before mechanical losses.
Does piston speed determine rpm?
Not without stroke.
What does trunk OD greater than bore mean?
Only an arithmetic clamp to zero net area; it is not physical engine geometry.
Can the calculator size or diagnose an engine?
No.
References
- Richard Sennett and Henry J. Oram, The Marine Steam Engine (1899): trunk, stuffing-box and connecting-rod arrangement.
- N. P. Burgh, Modern Marine Engineering (1881), chapter VI: Penn cylinder and double-trunk piston construction and dimensions.
- U.S. Patent 797,880: connecting rod and transversely disposed pin within a trunk piston.
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