The Union Model Gas Engine is a single-cylinder, four-stroke stationary internal combustion engine built by the Union Gas Engine Company of San Francisco from the 1890s into the early 1900s. It runs on the Otto cycle — intake, compression, power, exhaust — with hit-and-miss governing that holds the exhaust valve open whenever the engine overspeeds, skipping fuel charges until RPM drops back into band. Union built these engines to drive shop line shafts, irrigation pumps, and small marine craft. Surviving 6-25 HP units still run at heritage shows today.
Union Model Gas Engine Interactive Calculator
Vary bore, stroke, mean effective pressure, RPM, and hit-and-miss firing interval to see brake horsepower, firing rate, and crank torque.
Equation Used
This calculator applies the classic PLAN horsepower relationship to a single-cylinder four-stroke Union gas engine. Mean effective pressure, swept volume, and the hit-and-miss firing interval set indicated power; a fixed 85% mechanical efficiency converts it to brake horsepower at the crank.
- Imperial PLAN horsepower form is used.
- Stroke is converted from inches to feet.
- Mechanical efficiency eta_m is fixed at 0.85.
- rev_per_fire represents the hit-and-miss governor firing interval.
How the Union Model Gas Engine Actually Works
A Union Model Gas Engine fires once every four strokes — two crankshaft revolutions per power event. The piston draws a charge of gasoline or naphtha vapour mixed with air through an atmospheric inlet valve, the rising piston compresses it to roughly 60-70 psi, ignition lights it off, and the expanding gas drives the piston down. The exhaust stroke pushes the burnt charge out through a mechanically lifted exhaust valve. That is the Otto cycle four stroke sequence in plain terms, and Union built theirs around a heavy flywheel — typically 2 to 3 times the mass you'd expect on a modern engine of the same bore — because the flywheel inertia is what carries the piston through the three non-firing strokes between power events.
Governing works on the hit-and-miss principle. A flyball or pendulum governor watches engine RPM. When the engine reaches set speed, a latch catches the exhaust valve and holds it open. With the exhaust valve stuck open, the cylinder cannot pull a fresh charge or build compression — it just pumps air. The engine coasts on flywheel energy until RPM drops back below the trip point, the latch releases, and the next compression stroke fires normally. You hear it as the classic "pop... whoosh, whoosh, whoosh... pop" cadence at every heritage engine show. Ignition on early Union engines was hot tube ignition — a platinum or nickel-alloy tube screwed into the combustion chamber and heated to dull red by an external gasoline burner. Later Union models switched to low tension make and break ignition with an igniter trip inside the cylinder.
Tolerances matter more than people expect. The hot tube must glow at the correct point in the compression stroke — if the burner flame drifts or the tube cools below roughly 700 °C, you get late ignition, kickback, or no fire at all. The exhaust valve latch geometry is unforgiving too: a worn latch surface that lets the valve drop 0.5 mm too early causes the engine to fire when it should be coasting and overspeed in seconds. Common failure modes on a restored Union are cracked hot tubes from thermal cycling, stretched governor springs that drift the trip RPM down by 30-50 RPM, and worn igniter trip rollers on the make-and-break versions.
Key Components
- Cylinder and Piston: Cast iron cylinder, typically 4.5 to 8 inch bore on Union's 6-25 HP range, with a single piston running in a water-cooled jacket. Bore-to-piston clearance held to about 0.005 inch on a fresh build. The piston carries 3-4 cast iron rings.
- Flywheel(s): Twin flywheels on most Union models, often 36 to 54 inches in diameter. The flywheels store enough rotational energy to carry the engine through three idle strokes between firings and to absorb the impulse of each power stroke without speed flutter above ±5%.
- Hot Tube Ignition Assembly: Platinum or nickel-iron alloy tube threaded into the head, heated externally by a gasoline torch to roughly 700-800 °C. Timing is controlled by a sliding chimney that exposes the tube to compressed charge at the correct crank angle. Replaced by make and break ignition on later units.
- Hit and Miss Governor: Flyball or pendulum governor mechanically linked to a latch on the exhaust valve pushrod. Trip speed factory set around 240-300 RPM depending on horsepower rating. A 1 mm shift in latch engagement geometry moves the governed speed by roughly 15 RPM.
- Atmospheric Inlet Valve: Spring-loaded poppet valve with no cam — opens purely on the vacuum produced by the descending piston. Spring tension set so the valve cracks at about 1-2 psi below atmospheric. Too stiff a spring and the engine starves; too soft and it back-fires through the carb.
- Mixer / Carburetor: Simple float-feed or wick-feed mixer drawing gasoline or naphtha fuel. Air-fuel ratio set by a needle valve, typically tuned 13:1 to 14:1 by ear and exhaust colour.
- Crankshaft and Connecting Rod: Forged steel crankshaft running in babbitt-poured main bearings. Big-end on the connecting rod is a bolted strap with shim adjustment — clearance set to 0.002-0.003 inch when fresh.
Where the Union Model Gas Engine Is Used
Union built these engines for the working economy of the American West before grid electricity reached small towns and farms. You'll find original Union engines in three broad service categories — shop power, water pumping, and small marine — and surviving examples now serve a fourth: heritage exhibition. The Union Gas Engine Company sat in San Francisco and shipped engines up and down the Pacific coast, so a disproportionate share of survivors turn up in California, Oregon, and Washington collections.
- Agricultural Irrigation: A 10 HP Union horizontal driving a centrifugal irrigation pump on a Sacramento Valley orchard, lifting water from a slough into flood ditches at roughly 400 GPM.
- Small Marine Propulsion: A 6 HP Union marine variant powering a 28-foot fishing launch out of Monterey, swinging a two-blade bronze prop at 250 RPM through a 2:1 reduction.
- Shop Line Shaft Power: A 15 HP Union driving the overhead line shaft at a small machine shop in Stockton, California, running a lathe, drill press, and shaper through flat-belt takeoffs.
- Heritage Exhibition: A restored 8 HP Union running daily at the Western Antique Aircraft and Automobile Museum's engine shed during summer events, hand-started on naphtha fuel.
- Mining Hoist Power: A 25 HP Union historically used to power a small ore hoist at a Sierra Nevada gold operation, replaced by electric in the 1920s but preserved at a county historical society.
- Sawmill Auxiliary: A 12 HP Union driving an edger at a small redwood mill in Mendocino County before grid power arrived in the 1930s.
The Formula Behind the Union Model Gas Engine
The single number that tells you whether a Union Model Gas Engine will actually pull its rated load is brake horsepower derived from indicated mean effective pressure, swept volume, and firing rate. On a hit-and-miss engine the firing rate is the awkward variable — the engine doesn't fire every cycle, it fires only when the governor lets it. At light load the engine might fire 1 power stroke in 8 crank revolutions; at full load it fires every 2 revolutions like a continuous-running engine. The low end of the typical operating band is roughly 10-20% of nameplate, where the engine loafs and skips most cycles. Nominal cruise sits around 60-75% of rated load with regular firing every 4-6 revolutions. The high end is sustained full load where the governor stays untripped and every compression stroke fires — and that is where you find out whether your bearings, your hot tube, and your cooling jacket are all up to spec.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| BHP | Brake horsepower delivered at the crankshaft | kW (× 0.746) | hp |
| Pmep | Mean effective pressure during the power stroke | kPa | psi |
| L | Piston stroke length | m | ft |
| A | Piston area (π × bore² / 4) | m² | in² |
| Nfire | Actual firing rate — power strokes per minute under load | 1/min | 1/min |
| ηm | Mechanical efficiency — typically 0.75-0.82 on these engines | dimensionless | dimensionless |
Worked Example: Union Model Gas Engine in a restored 8 HP Union horizontal driving a feed grinder
You are recommissioning an 8 HP Union Model horizontal engine — 6.5 inch bore, 10 inch stroke, factory rated speed 280 RPM — that a Sonoma County heritage cider mill wants to put back to work driving a small flat-belt feed grinder during their fall apple-pressing weekends. You want to predict brake horsepower at three operating points: light loafing load, the nominal grind load, and a brief full-load surge when an apple jams the grinder feed throat.
Given
- Bore = 6.5 in
- Stroke (L) = 10 in
- Rated RPM = 280 1/min
- Pmep nominal = 70 psi
- ηm = 0.78 dimensionless
Solution
Step 1 — compute piston area from bore. This is fixed for the life of the engine and sets how much force every psi of Pmep turns into:
Step 2 — at the nominal load, the governor lets the engine fire roughly every 4 crank revolutions, so firing rate at 280 RPM is 280 / 4 = 70 power strokes per minute. Plug into the BHP formula and apply mechanical efficiency:
That sits at about 40% of nameplate — exactly where a hit-and-miss engine likes to live for steady-state shop work.
Step 3 — at the low end, light loafing with no real load on the belt, the governor only lets it fire every 12 revolutions. Firing rate drops to 280 / 12 ≈ 23 power strokes per minute:
You can hear this in the exhaust note — long silent gaps between pops, maybe one fire every couple of seconds. The engine is barely working, just spinning the flywheels and overcoming friction.
Step 4 — high end, full-load surge when an apple wedges the grinder. Governor stays untripped, every compression stroke fires, firing rate hits 280 / 2 = 140 power strokes per minute:
Steady continuous firing — the classic "every-stroke chuff" that tells you the engine is fully loaded. The engine has roughly 1.6 hp of headroom before it bogs.
Result
Nominal brake horsepower at the cider mill's normal grind load comes out to about 3. 2 hp — well within the engine's 8 hp rating and right in the sweet spot where a hit-and-miss Union runs cool, fires evenly, and idles cleanly between pops. The low-end loafing figure of 1.05 hp gives you the long-silence cadence, while the 6.4 hp full-load surge is what you hear when an apple jams and the engine briefly fires every stroke before the operator clears the jam. If you measure noticeably less power on a dynamometer brake — say 2.0 hp at the nominal point instead of 3.2 — the three most likely culprits are: hot tube cooling below the 700 °C ignition threshold causing late or skipped fires, a stretched governor spring tripping the latch 30-50 RPM low and starving the engine of firing opportunities, or a leaking exhaust valve seat (look for sooty bluing on the seat face) bleeding compression and dropping Pmep by 15-20 psi.
Union Model Gas Engine vs Alternatives
A Union Model is one of three broad approaches to early stationary power. Stack it against a contemporary throttle-governed engine like a Fairbanks-Morse vertical and a much later high-speed engine like a Briggs-style flathead, and the engineering tradeoffs jump out fast.
| Property | Union Model (hit-and-miss) | Throttle-governed stationary (e.g. Fairbanks-Morse Z) | Modern small four-stroke (e.g. Honda GX240) |
|---|---|---|---|
| Operating speed (RPM) | 240-300 | 400-600 | 1800-3600 |
| Power-to-weight ratio | ~0.02 hp/lb | ~0.04 hp/lb | ~0.4 hp/lb |
| Speed regulation under varying load | ±5-8% (skip-fire flutter) | ±2-3% (smooth) | ±1% (electronic on EFI) |
| Fuel flexibility | Gasoline, naphtha, kerosene with hot tube swap | Gasoline or kerosene | Gasoline only (most variants) |
| Typical lifespan between rebuilds | 20,000+ hours, often 50+ years | 10,000-15,000 hours | 1,000-3,000 hours |
| Starting method | Hand-rolled flywheel + lit hot tube burner | Hand crank + battery igniter | Recoil pull or electric |
| Replacement parts availability | Hand-fabricated or specialist heritage suppliers | Some NOS, mostly fabricated | Off the shelf at any small engine shop |
| Cost today (running condition) | $2,000-$15,000 (collector market) | $1,500-$8,000 | $300-$800 new |
Frequently Asked Questions About Union Model Gas Engine
Bright orange means your tube is hotter than 900 °C — actually too hot. At that temperature the platinum or nickel alloy ignites the charge well before TDC, which either kicks the engine backwards or the pre-ignition pulse vents back through the inlet and you get no useful power stroke. Pull the burner back so the tube glows dull cherry red, around 700-750 °C.
The other common cause is timing chimney position. The sliding chimney that exposes the tube to compressed charge has to open at the right crank angle. If the chimney linkage is worn or someone reassembled it one tooth off on the cam gear, the tube sees the charge either too early (pre-ignition) or after the piston is already past TDC (no fire at all). Mark TDC on the flywheel and verify chimney opening lines up within ±5° of the factory timing mark.
If the engine will run more than a few hours per show, make-and-break is the practical choice. Hot tubes crack from thermal cycling — typical service life is 50-200 firing hours before the tube splits at the threaded shoulder. You'll spend more time maintaining the burner and replacing tubes than running the engine.
Hot tube stays right for short demonstration runs and originality-judged shows. Many later Union models actually left the factory with make-and-break, so the conversion is period-correct rather than a modernisation. Keep the original hot tube assembly in a box for show-and-tell and run the make-and-break for the working hours.
Almost always cooling jacket scale or low water level. These engines were built before pressurised cooling — most ran on an open hopper or thermosiphon tank. As the water heats and evaporates, the level drops below the upper jacket passages and localised hot spots form near the exhaust valve. That heat soaks into the head, raises charge temperature on the inlet stroke, and you get partial pre-ignition that confuses the governor latch timing.
Check water level every 30 minutes during a run. If the engine has lived 100 years on hard water, also pull the hopper plug and look for limescale buildup — anything more than 3-4 mm of scale on the jacket walls cuts heat transfer enough to cause exactly this symptom.
Hunting like that points at the governor flyball linkage, not the engine itself. The flyball arms ride on small bronze bushings that wear oval over time. When the bushings have 0.5 mm of slop, the latch trip point moves 20-30 RPM each cycle and the governor can't settle — it overshoots high, trips the latch, undershoots low, releases, and repeats.
Pull the governor head, check the bushings against the spindle, and replace them if they show any visible play. Also check the governor spring free length against the factory spec stamped on the spring perch — a spring that has lost 5% of its free length will hunt even with perfect bushings.
Modern 87-octane pump gasoline works on most Unions but you'll fight ethanol issues. The ethanol attacks shellac on old float bowls, swells natural rubber fuel lines, and corrodes the brass mixer needles. Use ethanol-free gasoline (REC-90 or marine-grade) if you can get it.
Naphtha (Coleman fuel / white gas) actually runs cleaner in a hot tube engine than gasoline — lower volatility means less vapour lock on a hot day and the hot tube ignites it more reliably. Many heritage operators run naphtha exclusively for that reason. Do not run anything above about 91 octane — high octane resists hot tube ignition and you'll get late, weak fires.
You're sizing for cyclic speed variation, not average power. The rule of thumb on hit-and-miss engines is the flywheel pair must store at least 100 times the work done in a single power stroke at rated load. For an 8 HP Union firing every 4 revolutions at 280 RPM, that's roughly 1,100 ft-lb per stroke times 100 = 110,000 ft-lb of stored kinetic energy in the flywheels.
Solve for required moment of inertia using KE = ½ × I × ω² with ω at rated speed and you'll land on roughly 90-110 lb-ft² per flywheel for an 8 HP engine — which matches the original Union castings within 5%. Undersized flywheels show up as visible RPM flutter on every fire and a rough belt drive that whips and slaps under load.
References & Further Reading
- Wikipedia contributors. Hit-and-miss engine. Wikipedia
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