Hiscox’s diagonal twin-screw engine places two inclined cylinders above low-set shafts so their connecting rods cross in the transverse view. The crossed arrangement is its defining feature. Follow the two separate slider-crank trains, then calculate the horizontal and vertical projections of a selected cylinder-axis length.
Diagonal Twin-Screw Engine Interactive Calculator
Two inclined cylinders drive separate crankshafts through connecting rods that cross in this transverse view. Adjust the inclination and axis length. The working animation closes both slider-crank trains; the calculator reports centre-line projections, not the total installed engine height or width.
Equation Used
- Both axes have the entered inclination; the shown dimensions scale together with L.
- Illustrative proportions: shaft spacing 0.44L, crank radius 0.08L, connecting rod 0.50L and piston rod 0.33L.
- The two rods cross only in projection and occupy separate fore-aft planes. There is no joint between them.
- Counter-rotation, equal animation speed and starting phase are chosen for display, not mechanically imposed synchronization.
- Cylinder casing thickness, bearings, auxiliaries, clearances and access space are excluded from the projection outputs.
L is measured along one cylinder axis from its driven crankshaft centre to the outer cap. The cylinder, bedplate and crank radius extend beyond this line. The displayed projections are not a full installation envelope.
Each cylinder drives the opposite low-set shaft
The piston moves along its fixed inclined bore. A rigid piston rod carries that motion to a crosshead constrained by parallel guide bars. A connecting rod joins the crosshead to a revolving crank pin. The left-hand cylinder reaches across to the right-hand shaft, and the right-hand cylinder reaches across to the left-hand shaft.
The rods are drawn in different fore-aft planes. The pale edge on the front rod makes the projected crossing clear: it is not a pin, a scissor joint or a collision. The source illustrates this crossed arrangement as a way to economize space.
The animation solves each rod’s length rather than moving the piston and crank independently. Its illustrative proportions are listed beside the calculator. The valves and steam passages are omitted, so the animation demonstrates kinematics rather than a thermodynamic cycle.
A historical marine-engine arrangement
The source identifies the machine as a twin-screw engine: two screw-propeller shaft lines are the intended application. The compact crossed layout differs from simply placing two inclined cylinders side by side over their nearest shafts.
The drawing does not establish a particular vessel, operating speed, power rating or control arrangement. Those details require records for the actual engine. Use engine-specific records to establish the vessel, shaft phasing and operating ratings.
Projection and slider-crank closure
For an axis segment of length L at inclination θ above horizontal, the vertical component is L sin θ and the horizontal component is L cos θ. Compared with the same segment standing vertically, the reduction of its vertical projection is 100(1 − sin θ)%.
These quantities describe a line, not the outside of the engine. Cylinder radius adds to the upper envelope; shaft position, crank radius, bedplate and ancillary equipment add other dimensions. The horizontal projection of one axis is not the width of the complete twin-shaft bed.
In local coordinates along each cylinder axis, the crank pin is at (r cos φ, r sin φ). A crosshead at (q, 0) must stay distance b from that pin. The outward solution is q = r cos φ + √(b² − r² sin² φ). The selected b/r ratio leaves that square root real throughout every revolution. Piston travel is twice the crank radius.
A one-metre axis at 40°
With L = 1 m and θ = 40°, the vertical axis projection is approximately 0.643 m and the horizontal projection 0.766 m. The vertical fraction is 64.3%, or a 35.7% reduction relative to a one-metre vertical line.
For the chosen animation proportions, the crank radius is 0.08 m and connecting rod length 0.50 m. Crosshead travel is 0.16 m. These dimensions demonstrate the motion; they are not measured dimensions of the historical engine.
Compactness needs more than a centre line
Inclining an axis trades vertical projection for horizontal projection. Crossing the rods changes how the two trains can be placed in relation to the shaft lines. Both are geometric packaging choices, but neither alone establishes the complete clearance envelope.
Separate rod planes, crosshead guidance, bearing loads and structural support must be provided in a real arrangement. This model does not infer durability, lubrication performance, engine balance or manufacturing tolerances from the chosen angle.
Diagonal twin-screw engine questions
Do the rods connect where they cross?
No. They pass in separate fore-aft planes and drive different shafts.
Why are the cranks shown with a phase difference?
The starting positions are chosen to make both trains easy to follow. The illustration does not claim that independent shafts must maintain that phase.
Is the height result the space needed in an engine room?
No. It is the vertical projection of the chosen axis segment. Cylinder diameter, crank and bed geometry, auxiliaries and access clearances must be added for a real installation.
Does changing L change the rod length?
Yes. This particular illustration scales its fixed proportions with L. The listed proportions define this illustration; they are not measured dimensions of the historical engine.
Does the animation calculate steam power?
No. Its prescribed crank motion and geometric closure do not include pressure, valve events, speed or heat transfer.
Reference
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), movement 223, printed page 68. The original engraving and description show the crossed connecting rods of the diagonal twin-screw arrangement.
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