The Forrester rotary engine described by Gardner Dexter Hiscox uses a cylindrical block and guard wing carried on a shaft eccentric. The guard wing slides within a two-port rotary valve, whose outer shell can be moved by linkage to reverse the engine. The calculator below is a separate ideal force-and-lever-arm comparison; it does not calculate the historical engine’s valve timing or measured output.
Forrester Rotary Engine Interactive Calculator
Vary effective pressure difference, loaded area, assumed perpendicular lever arm and speed to compare ideal force, torque and power.
Equation Used
The comparison assumes F = P × A acts tangentially at a constant perpendicular lever arm e. It converts that assumed torque and speed to ideal power. An eccentric offset alone does not establish the effective lever arm throughout a real engine cycle.
- Pressure is the effective difference across the loaded face, not boiler pressure alone.
- Area is the effective steam-loaded piston face area.
- For this arithmetic, e is assumed to be the perpendicular force lever arm. Actual cycle geometry must establish whether that assumption applies.
- Friction, leakage, cutoff, expansion and valve timing are not modeled. The moving illustration is a conceptual eccentric view, not a valve-motion solution or a real-time RPM display.
Block, guard wing and reversing valve
Hiscox’s figure 344 identifies a block and guard wing that swing on an eccentric. The wing also slides in the rotary valve and operates its ports. External levers and connecting rods move the valve’s outer shell for reversal.
That arrangement couples working-chamber motion to steam admission and exhaust. A complete analysis requires the chamber dimensions, wing constraints, port openings and pressure over shaft angle.
The simple moving view on this page shows eccentric motion and a conceptual chamber divider. It does not reproduce that valve linkage. Its colors are illustrative regions, not calculated steam pressures or sealing boundaries.
Studying direct rotary steam mechanisms
The Forrester arrangement is a historical example of combining an eccentric working member with steam distribution and reversal. It is useful for comparing different ways of obtaining rotation from fluid pressure.
The reference description does not establish a universal pressure rating, service life, speed range or installation at a particular mill or museum. Matching a surviving engine to the drawing requires its own records.
Ideal force and torque comparison
For effective pressure difference P in psi and loaded area A in square inches, F = PA gives force in lbf. If that force acts perpendicular to a constant lever arm e in inches, T = Fe gives lbf·in. Divide by 12 for lbf·ft.
At entered speed N in RPM, ideal horsepower is HP = TftN/5252. This assumes the entered force and arm represent the torque being maintained. It is not a prediction of average historical-engine torque.
For an actual pressure-driven mechanism, instantaneous torque depends on force direction and chamber geometry. Average indicated work requires pressure and volume throughout the cycle; brake power also includes mechanical losses. Neither a swept-volume formula nor an efficiency factor is present in this calculator.
Example: an assumed constant force arm
Enter 60 psi pressure difference, 20 in² effective area, a 1.5 in perpendicular lever arm and 300 RPM. Force is 1,200 lbf. Torque is 1,800 lbf·in, or 150 lbf·ft, and ideal power is 8.57 hp.
Changing speed to 150 RPM leaves the assumed torque unchanged and halves power to 4.28 hp. Halving pressure instead also halves force, torque and power. These comparisons follow the entered assumptions; they do not predict leakage, port choking, steam consumption or a safe engine operating point.
What the comparison leaves out
An eccentric mechanism, a reciprocating engine and a turbine convert fluid energy through different geometries. Their performance cannot be ranked from pressure and nominal offset alone.
For the Forrester arrangement, the guard wing, rotary valve, sealing surfaces and reversing linkage all matter. This calculation does not establish clearances, spring settings, flywheel inertia, restoration costs or overhaul intervals.
Operating and maintenance decisions require the specific engine’s condition, drawings and approved pressure limits. The entered pressure range is an arithmetic control range, not permission to increase boiler or engine pressure.
Frequently asked questions
What makes the historical Forrester arrangement distinctive?
Hiscox describes a block and guard wing on an eccentric, with the wing sliding in a two-port rotary valve. Linkage moves the valve shell to reverse the engine.
Does P mean boiler pressure?
No. For F = PA it must represent the effective pressure difference across the loaded area. Supply pressure alone does not establish that value during a cycle.
Is eccentricity always the torque arm?
No. Torque uses the perpendicular distance from the shaft axis to the force line. The calculator assumes that distance equals the entered e; it does not derive it from chamber geometry.
Why does speed change power without changing torque?
Force and lever arm are held constant by the model. Power then increases in proportion to speed. Real pressure and losses may change with speed.
Does ideal power equal delivered shaft power?
No. Valve timing, changing chamber pressure, leakage and mechanical losses are omitted. The result is an assumed constant-torque comparison.
Does the animation show the reversing linkage?
No. It is a conceptual eccentric illustration. It does not solve Hiscox’s guard-wing and rotary-valve constraints.
Can these values set operating pressure or clearances?
No. Use the specific engine’s documented ratings and condition assessment. This calculator does not establish operating or repair specifications.
Historical reference
Gardner Dexter Hiscox, Mechanical Movements, Powers, Devices and Appliances, figure 344: “Forrester” rotary engine. The entry identifies the block, guard wing, rotary valve and reversal linkage; it does not provide the performance ratings of a particular engine.
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