A Franchot rotary engine is a double-acting steam engine in which the cylinder itself rotates about a fixed trunnion that carries the inlet and exhaust ports, with the piston rod driving a crank on the output shaft. It solves the problem of converting reciprocating piston motion into rotation without a separate slide valve or eccentric — the rotating cylinder is its own valve. The trunnion ports admit live steam to one side of the piston while the other side exhausts, switching twice per revolution. Patented by Henri Franchot in the 1840s, the layout produced compact, low-vibration plants used in light marine and workshop service at 150–300 RPM.
Operating Principle of the Franchot Rotary Engine
The Franchot layout takes a single cylinder, mounts it in two trunnion bearings, and lets the whole cylinder swing — actually rotate continuously — around the trunnion axis as the crank turns. The piston rod is hinged to the crankpin. As the crank rotates, the piston reciprocates inside the cylinder while the cylinder itself rotates about its trunnions. That rotation is what makes the mechanism work as a valve: the trunnion is hollow and carries two crescent-shaped ports cut into its bearing surface, one for live steam and one for exhaust. As the cylinder rotates, the corresponding port in the cylinder's trunnion bush passes alternately over inlet and exhaust, admitting steam to one end of the bore and releasing it from the other.
Because the cylinder is double-acting, you get a power stroke every half revolution — same as a conventional double-acting engine — but with no slide valve, no eccentric, no D-valve gear, and no piston rod gland to seal against high pressure. The seal is between the cylinder trunnion and its fixed bush, and that's where the engine lives or dies. The trunnion port crescent must lead the dead-centres by a few degrees of advance — typically 4° to 8° depending on speed — to give the steam time to enter the bore before the piston starts its working stroke. Get the advance wrong and you lose mean effective pressure; the engine runs but pulls less indicated horsepower than it should.
If the trunnion-to-bush clearance opens up beyond about 0.05 mm radial, you get cross-leakage between live and exhaust ports — steam blows straight from inlet to exhaust without doing work, the exhaust runs hot, and steam consumption climbs without any output to show for it. The other classic failure is wear on the crankpin hinge of the piston rod. The rod oscillates relative to the cylinder as the cylinder rotates, so that hinge sees a full reversal every revolution. A worn hinge bush will let the piston cock in the bore, scoring the cylinder wall, and you'll hear it as a soft knock at half engine speed.
Key Components
- Rotating Cylinder: Cast iron or bronze cylinder with bore typically 50–150 mm, mounted on two coaxial trunnions. It rotates with the output shaft and acts as both working chamber and valve body. Bore-to-piston clearance held to 0.05–0.10 mm with cast iron rings.
- Fixed Trunnion Bushes: Stationary bronze bushes carrying the live steam supply on one trunnion and the exhaust on the other. The bush face contains crescent ports cut to give the required steam lap and lead �� usually 4° to 8° of angular advance ahead of dead-centre.
- Piston and Rod: Conventional double-acting piston with two ring grooves. The rod terminates in a hinged eye at the crankpin rather than a crosshead, so the rod itself swings as the cylinder rotates.
- Crankshaft and Crankpin: Single-throw crank with throw equal to half the piston stroke. The crankpin carries the rod hinge bush. Crankpin bush must hold radial clearance under 0.04 mm to avoid the half-speed knock characteristic of this engine.
- Trunnion Steam Passages: Drilled passages through each hollow trunnion connecting the bush ports to each end of the cylinder bore. These passages set the dead volume of the engine and want to be kept short — long passages drop indicated mean effective pressure noticeably below 200 RPM.
Where the Franchot Rotary Engine Is Used
The Franchot rotary engine never reached the scale of conventional reciprocating practice, but it found a niche wherever compactness, low vibration, and the absence of valve gear mattered more than thermal efficiency. You see it in light marine launches, small workshop drives, and a handful of demonstration and heritage installations where the visual drama of a rotating cylinder is itself the point. Modern preservation work tends to focus on Forrester, Fielding & Platt, and a small group of French builders who carried the Franchot layout into the late 19th century.
- Heritage Marine: Single-cylinder Franchot engines fitted to small Edwardian launches in the 5–12 IHP range, typically running 60 psi saturated steam at 200–250 RPM.
- Industrial Heritage Demonstration: Forrester rotary engines preserved at gasworks and pumping museums in the UK Midlands, driving auxiliary fans or display line shafts.
- Workshop Drives (Historical): Late-19th-century light machine shop drives where the rotating cylinder eliminated the visual nuisance of an exposed slide-valve eccentric.
- Educational Steam Models: Stuart Models and similar live-steam kit suppliers offer simplified Franchot-pattern rotary engines in the 0.25–1.0 cubic-inch displacement range for model boats.
- Steam Fairs and Static Displays: Operating Franchot engines at events such as the Great Dorset Steam Fair, run on auxiliary boilers at low pressure for visual demonstration rather than useful work.
- Stationary Pump Drives (Historical): Small Franchot-pattern engines used in late-Victorian gasworks for purifier-bed exhauster fans, where 2–4 IHP at 250 RPM was sufficient.
The Formula Behind the Franchot Rotary Engine
What you actually want from a Franchot engine is indicated horsepower — the power the steam develops on the piston before mechanical losses. The standard PLAN equation applies, but with one wrinkle specific to this layout: the effective mean effective pressure drops faster with speed than in a slide-valve engine, because the rotating-cylinder port timing has fixed cutoff and no expansion linkage. At the low end of the typical operating range — say 120 RPM on a small launch engine — you're getting close to full boiler pressure on the piston for most of the stroke, but you're turning slowly. At the nominal 200–250 RPM you hit the design sweet spot. Push past 350 RPM and port throttling drops your effective MEP by 20–30%, so doubling the speed does not double the power.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| IHP | Indicated horsepower (double-acting, both strokes) | kW (× 0.7457) | hp |
| Pm | Mean effective pressure on the piston over the stroke | kPa | psi |
| L | Piston stroke length | m | ft |
| A | Piston cross-sectional area | m<sup>2</sup> | in<sup>2</sup> |
| N | Engine speed | rev/min | RPM |
| 33,000 | Conversion constant from ft·lbf/min to horsepower | — | ft·lbf/min per hp |
Worked Example: Franchot Rotary Engine in a heritage shingle-mill demonstration drive
You are computing the indicated horsepower of a recommissioned 1891 Franchot rotary engine being returned to demonstration running at a heritage shingle-mill museum on Vancouver Island, where it will drive a short countershaft for a single circular shingle saw display off saturated steam at 70 psi inlet exhausting to atmosphere. The engine has a 4 inch bore, 5 inch stroke, and target running speed of 220 RPM with measured mean effective pressure of 48 psi at the indicator card.
Given
- Bore = 4 in
- L = 5 in (0.4167 ft)
- Pm = 48 psi
- N (nominal) = 220 RPM
- Inlet pressure = 70 psi
Solution
Step 1 — compute the piston area from the 4 inch bore:
Step 2 — at the nominal 220 RPM with measured MEP of 48 psi, plug into PLAN with the factor of 2 for double-acting operation:
That is honest indicator-card horsepower for a small heritage engine — enough to pull a single 24 inch circular shingle saw through softwood with a light feed. The countershaft will turn over with visible authority but you would not load it like a working production engine.
Step 3 — at the low end of the typical operating range, drop to 140 RPM. With the longer dwell over the ports, MEP holds closer to 54 psi:
The engine sounds slower and more deliberate, the saw will only handle the lightest cuts, but steam consumption per IHP-hour is actually better here because port throttling losses are minimal.
Step 4 — at the high end, push to 320 RPM. Port throttling now bites hard and measured MEP collapses to roughly 36 psi:
You gained barely 9% more power for a 45% increase in speed, with the engine now running noticeably hot at the exhaust trunnion and shaking the bedplate. This is exactly why these engines were rated at 200–250 RPM and not pushed harder.
Result
Nominal indicated horsepower at 220 RPM is 3. 35 hp. To a visitor watching the engine run, that translates to a steady, unhurried beat with a single soft exhaust puff per half-revolution and just enough torque to keep the shingle saw cutting cleanly through cedar. Comparing operating points: 2.40 hp at 140 RPM (low end, most efficient steam use), 3.35 hp at the 220 RPM sweet spot, and a disappointing 3.66 hp at 320 RPM where port throttling has eaten most of the gain — the curve flattens hard above 280 RPM. If your indicator card shows MEP 20% below the predicted 48 psi at nominal speed, look first at trunnion port lead — a Franchot with port advance set to 2° instead of the correct 6° will lose 10–15% MEP. Second, check the trunnion-to-bush radial clearance with a feeler; anything above 0.05 mm produces cross-leakage from inlet crescent to exhaust crescent that you can confirm by a hot exhaust trunnion bush. Third, a tired piston ring set will drop compression on the return stroke and pull the diagram down on one side only, which an indicator card will show as visible asymmetry between top and bottom power strokes.
Franchot Rotary Engine vs Alternatives
The Franchot rotary engine sits in an awkward spot between the conventional slide-valve engine and the oscillating-cylinder engine. It is mechanically clever but thermodynamically compromised — and you only choose it when the visual or packaging case outweighs the efficiency case. Here's how it stacks up against the two layouts a heritage engineer would actually consider as alternatives.
| Property | Franchot Rotary Engine | Slide-Valve Reciprocating Engine | Oscillating Cylinder Engine |
|---|---|---|---|
| Typical operating speed | 150–300 RPM | 100–600 RPM | 100–400 RPM |
| Indicated thermal efficiency | 6–9% (no expansion control) | 10–15% (variable cutoff) | 5–8% |
| Valve gear complexity | None — cylinder is the valve | Eccentric, slide valve, linkage | None — cylinder pivot acts as valve |
| Steam consumption per IHP-hr | ~50–70 lb/IHP-hr | ~25–40 lb/IHP-hr | ~55–75 lb/IHP-hr |
| Vibration character | Low — rotating mass balanced | Moderate — reciprocating rod and crosshead | High — whole cylinder swings |
| Capital cost (heritage rebuild) | Moderate — trunnion bushes are specialist | Low — parts and skills widely available | Low — simplest of the three |
| Best application fit | Display, light launch, low-vibration drives | Working heritage plant, line shafts | Toys, models, simplest demonstration use |
| Sensitivity to clearance wear | High — trunnion clearance is critical | Moderate — adjustable valve gear compensates | Moderate — pivot wear self-aligns somewhat |
Frequently Asked Questions About Franchot Rotary Engine
The trunnion port crescents have fixed geometry — there is no variable cutoff. As speed climbs, the time available for steam to flow through the port falls linearly, but the port area is constant. Above roughly 280 RPM on a typical small Franchot, the inlet port acts as a throttle and live steam never reaches full bore pressure before the piston is already partway down the stroke.
You can confirm this with an indicator card: the top of the diagram will show a rounded, sloping admission line instead of a sharp pressure rise. The fix is either to run the engine at design speed, or to enlarge the inlet crescent — but enlarging it past about 25° of arc starts to overlap exhaust and you lose more to short-circuiting than you gain in admission.
Lead angle interacts with running speed and steam pressure. The 6° figure assumes design pressure and design speed. If you're running lower pressure than the original (common in heritage demonstration where you cap the boiler at 60 psi instead of 90), the steam takes longer to fill the dead volume and you want more lead — try 8° to 10°.
If you're running a stiffer modern packing or a tighter ring set than original, friction at startup is higher and you may want even more lead at low speed but back to nominal at running speed. The honest answer is to take indicator cards at three lead settings and pick the one that gives the squarest diagram top.
For a working launch that needs to make passages, choose the slide-valve compound every time. The compound's variable cutoff lets you cruise economically and open up for headway when needed; steam consumption will be roughly half the Franchot's at the same IHP. The Franchot has no expansion control — you're running full pressure to cutoff every stroke.
The Franchot only wins if the launch is a static display or short-run demonstrator where the visual interest of the rotating cylinder is the actual product. For 90 minutes of harbour cruising on a single boiler fill, the compound is the right answer.
That half-speed knock is almost always the rod hinge bush at the crankpin. The rod oscillates through a substantial arc every revolution because the cylinder rotates while the crank turns — so the hinge bush sees one full reversal per revolution, not per stroke. As the bush warms up, thermal expansion of the steel pin into a bronze bush eats clearance unevenly and the rod cocks under load reversal.
Pull the rod and measure the hinge bush bore. If clearance has opened past about 0.04 mm radial when hot, replace the bush. Don't just shim it — the oscillating motion will work any shim out within an hour of running.
Slightly hotter is normal — the exhaust trunnion sees steam that has done work but is still warm. Visibly hotter, to the point of discolouring the bronze, usually means cross-leakage from the inlet port crescent to the exhaust port crescent through worn trunnion-to-bush clearance. Live steam is short-circuiting straight to exhaust without going through the bore.
Confirm by closing the throttle and watching the exhaust trunnion temperature: it should drop quickly. If it stays hot for a long time, you have leakage past the trunnion seal. The bush needs re-machining or replacement; running it as-is wastes steam and accelerates wear on both faces.
You cannot fit a conventional cutoff governor because there is no valve gear to act on — the port timing is fixed by the trunnion crescent geometry. What you can fit is a throttling governor on the steam supply line upstream of the trunnion, which is how nearly all surviving Franchots were originally controlled.
A simple Pickering or Hartnell-pattern throttling governor on the inlet line will hold speed within about ±5% of setpoint under varying load. That's loose by mill-engine standards but fine for a demonstration drive or a launch. If you need tighter regulation, the Franchot is the wrong engine for the job.
References & Further Reading
- Wikipedia contributors. Steam engine. Wikipedia
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