A Z-crank engine is a piston engine whose crankshaft uses a Z-shaped offset between two crankpins so opposing pistons can share a single crank throw while travelling on parallel but laterally offset axes. Unlike a conventional inline crank where pistons share a common bore line, the Z-crank stacks them side-by-side to shorten engine length. The geometry trades a small secondary couple for a much more compact package. Designers reach for it in tight installations like UAV powerplants and portable gensets where total length matters more than perfect balance.
Z-crank Engine Interactive Calculator
Vary reciprocating mass, stroke, rod ratio, crankpin offset, and RPM to see the peak second-order rocking couple in a Z-crank engine.
Equation Used
The calculator estimates the peak second-order rocking couple caused by the lateral separation of the opposed reciprocating masses. Increasing mass, stroke, RPM, or Z-crank offset raises the couple; increasing rod ratio lowers the second-order term.
- Two equal opposed pistons with primary inertia forces cancelling.
- Peak rocking couple is estimated from the dominant second-order slider-crank inertia term.
- Rod ratio is connecting rod length divided by stroke.
- Offset is the lateral bore-to-bore crankpin separation.
Inside the Z-crank Engine
The Z-crank takes its name from the shape of the crankshaft when you look at it end-on — two crankpins offset by a short web that puts them on parallel but laterally separated centrelines. Each piston runs in its own bore, parallel to its neighbour, and the connecting rods reach across the offset to grab their respective crankpins. As one piston descends, the other rises, exactly like a boxer or opposed-piston layout, but the cylinders sit beside each other rather than facing each other. That single change is what makes the engine short.
Why build it this way? Compact ICE packaging. In a conventional flat-twin you need enough room for two cylinder heads pointing in opposite directions, plus pushrods, plus exhaust. The Z-crank collapses that footprint into something close to the width of two bores. The cost is a secondary inertia couple — because the two reciprocating masses do not share a single plane of action, you get a small rocking moment around the vertical axis at twice crankshaft frequency. You can damp it with a balance shaft or just live with it on small displacements under 200 cc.
If the crankpin offset tolerance drifts — say the web grinds 0.1 mm short of spec — primary balance forces stop cancelling and you'll feel it as a buzz through the mounting feet that gets worse with RPM. Common failure modes are crankweb fatigue cracking at the offset radius, big-end bearing wear from the asymmetric load path, and on two-stroke variants, scavenging losses if the port timing was set assuming pure opposed motion. The Z-crank does not behave exactly like an opposed crank — the lateral offset shifts the gas-pressure couple slightly, and that matters for port design.
Key Components
- Z-shaped crankshaft: The defining part — a single shaft with two crankpins joined by an offset web, typically 30 to 80 mm of lateral offset on small engines. The offset radius must be generous (R ≥ 1.5 × pin diameter) to keep stress concentrations below the fatigue limit of the forging.
- Parallel cylinder block: Houses two bores on parallel axes separated by the crank offset distance. Bore-to-bore spacing is fixed by crankshaft geometry — you cannot move bores closer without re-grinding the crank. Typical bore spacing on a 50 cc Z-twin is 70 mm centre-to-centre.
- Connecting rods: Standard I-beam or H-beam rods, but each rod operates with a small lateral angle component because the crankpin sits offset from the bore centreline. Rod length-to-stroke ratio of 1.8 or higher keeps that side-thrust within acceptable piston-skirt loading.
- Balance shaft (optional): Counter-rotating shaft running at crank speed, sized to cancel the secondary couple from the offset reciprocating masses. Required on engines above ~150 cc; below that the residual vibration is small enough to handle with rubber mounts.
- Crankcase scavenge port set: On two-stroke Z-cranks, the transfer ports must account for the phase relationship between the two pistons. The lateral offset means one cylinder's BDC happens a few crank degrees away from a true mirror of the other — port timing windows are usually within ±2° of the conventional opposed-twin values.
Who Uses the Z-crank Engine
Z-crank engines show up wherever total engine length is the limiting dimension and the design team can accept a small balance penalty. They are not common in mass-production automotive use — inline-fours and V-twins won that market on cost. The Z-crank lives in specialist niches: UAV propulsion, portable equipment, and a handful of historical experimental designs. Reliability depends almost entirely on getting the crankweb fatigue life right; engine vibration analysis on prototype runs catches most issues before they become field failures.
- UAV propulsion: Compact 2-cylinder Z-crank two-strokes around 50-100 cc used in tactical drones where the airframe nose section dictates a short engine length
- Portable power: Small Z-twin gensets in the 1-3 kW range marketed for boondocking and trade-show use, where the engine sits inside a 350 mm-deep enclosure
- Marine outboards: Experimental short-stack outboards where reduced engine length lets the powerhead sit closer to the transom for better weight distribution on small craft
- Karting and small racing: Specialty 125 cc kart engines built around Z-crank geometry to lower the crank-to-axle distance for chassis packaging
- Historical aviation: Pre-WWII experimental aero engines like some Lutz designs used Z-style crank arrangements to shorten the cowling on light aircraft
- Industrial pumps and compressors: Refrigeration compressors with twin pistons sharing a Z-crank to fit inside hermetic shells under 200 mm tall
The Formula Behind the Z-crank Engine
The most useful first-principles calculation for a Z-crank is the residual secondary couple — the rocking moment you cannot cancel with simple counterweights because the two reciprocating masses do not share a plane. At the low end of typical operation (say 1500 RPM on a stationary genset) this couple is small and rubber mounts handle it. At the design sweet spot of 3000-4500 RPM for portable two-strokes you'll start to see it as a measurable shake at the operator handle. Push past 7000 RPM in a kart application and the couple grows with the square of speed, which is when balance shafts become non-negotiable.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Msec | Secondary rocking couple about the vertical axis | N·m | lbf·ft |
| mrecip | Reciprocating mass per cylinder (piston, rings, pin, small end of rod) | kg | lb |
| ω | Crankshaft angular velocity | rad/s | rad/s |
| r | Crank throw radius (half of stroke) | m | in |
| Loffset | Lateral offset between the two crankpin centrelines | m | in |
| θ | Crank angle from TDC of cylinder 1 | rad | rad |
Worked Example: Z-crank Engine in a 90 cc Z-twin two-stroke for a fixed-wing surveillance UAV
Sasanami Aerospace is prototyping a 90 cc Z-twin two-stroke as the powerplant for a 25 kg fixed-wing surveillance UAV being built in Nagoya. They want the residual secondary couple at three operating points — idle at 2500 RPM, cruise at 5500 RPM, and full-power climb at 7500 RPM — so the airframe team can decide whether the firewall mount needs a balance shaft or whether tuned rubber isolators will handle the vibration into the avionics bay.
Given
- mrecip = 0.085 kg per cylinder
- r = 0.018 m (36 mm stroke)
- Loffset = 0.045 m
- Peak cos(2θ) = 1.0 dimensionless
- Bore = 0.040 m
Solution
Step 1 — convert each operating speed from RPM to angular velocity in rad/s. Cruise at 5500 RPM is the design point:
Step 2 — compute the peak secondary couple at cruise. The cos(2θ) term peaks at 1.0 twice per revolution:
That 11.4 N·m is what the airframe team will feel as a rocking buzz at twice crank frequency — about 183 Hz at cruise. It is enough to need attention in the mount design but not enough to demand a balance shaft on a UAV this size.
Step 3 — at the low-end idle case, 2500 RPM:
At idle the couple drops to 2.4 N·m — barely noticeable through the mounts. The pilot would describe it as a smooth idle. This is why most Z-twins feel fine on the ground and only start protesting once you push the throttle.
Step 4 — at the high-end climb case, 7500 RPM:
21 N·m is roughly double the cruise figure because the couple scales with ω2. At 7500 RPM the firewall starts to transmit visible motion to the camera gimbal — and that is the speed at which Sasanami's video team will start complaining about jitter. The sweet spot for this engine is around 5000-5800 RPM where couple and power output both sit in usable territory.
Result
Peak secondary couple at the 5500 RPM cruise design point is 11. 4 N·m, hitting twice per revolution at 183 Hz. At idle (2500 RPM) the couple falls to 2.4 N·m — a smooth feel through the mounts — while at 7500 RPM climb power it climbs to 21.2 N·m, doubling the cruise number because of the ω2 scaling, and that is where camera gimbal jitter sets in. If your bench measurement reads higher than 11.4 N·m at cruise, suspect: (1) the reciprocating mass figure — many builders forget to add the small end of the rod, which is typically 25-30% of total reciprocating mass, (2) crankpin offset machined long, since 0.5 mm of extra Loffset adds about 11% to the couple, or (3) phase error between the two crankpins, where even 2° of angular drift turns the cos(2θ) curve from a clean cancelling pair into a partly additive one.
Choosing the Z-crank Engine: Pros and Cons
The Z-crank competes mainly with the conventional opposed-twin (boxer) and the inline-twin on the same displacement. Each layout has a place — the question is which axis you cannot afford to grow. Pick the Z-crank when length is fixed and you can spend a little weight and complexity to get there.
| Property | Z-crank twin | Opposed (boxer) twin | Inline twin |
|---|---|---|---|
| Engine length (per 100 cc displacement) | ~110 mm | ~260 mm | ~180 mm |
| Primary balance | Good (forces cancel) | Excellent (forces and couple cancel) | Poor (requires balance shaft above 250 cc) |
| Secondary couple at 6000 RPM (typical 100 cc) | 12-18 N·m | 0 N·m (cancelled) | 8-12 N·m |
| Crankshaft manufacturing cost (relative) | 1.4× | 1.2× | 1.0× (baseline) |
| Maintenance interval to first big-end inspection | ~300 hours | ~500 hours | ~500 hours |
| Typical RPM ceiling (small displacement) | 8000 RPM | 9000 RPM | 10000 RPM |
| Best application fit | UAV, compact gensets, hermetic compressors | Aircraft, motorcycles, light vehicles | Karts, scooters, small marine |
| Service complexity | High — Z-crank requires special jigs to true | Medium | Low |
Frequently Asked Questions About Z-crank Engine
Almost always a phase issue between the two crankpins or a mass mismatch between the two reciprocating assemblies. The secondary couple formula assumes the two cylinders are exactly 180° out of phase and have identical reciprocating mass. In practice, even 1-2° of crankpin angular drift — well within typical forging tolerances — converts what should be a cancelling primary force pair into a partly additive one, and that primary residual is much larger than the secondary couple you calculated.
Weigh both pistons, pins, rings and rod small ends together. A 3 g mismatch on an 85 g assembly is enough to put a measurable buzz into the mount feet. Then put the crank in V-blocks with a dial indicator on each pin and confirm the angular index is within ±0.5°.
No, and this is where a lot of homebuilt Z-crank prototypes lose 10-15% on their power curve. The lateral offset between bores means the gas-pressure events in cylinder 1 and cylinder 2 are not perfect mirror images — the exhaust pulse from one cylinder reaches the transfer ports of the other a few crank degrees off from where the boxer-twin map expects.
Re-time the transfer port openings 2-4° later relative to BDC compared to the equivalent boxer engine. If you instrument both exhaust ports with pressure transducers you will see the asymmetry clearly on the scope.
Below 100 cc and below 6000 RPM cruise, tuned rubber isolators usually win. A balance shaft adds 8-12% parasitic loss, weight equivalent to maybe 30 minutes of fuel, and another bearing set to fail. The decision rule we use: if peak secondary couple at cruise exceeds 15 N·m or your sensitive payload (camera, IMU) is mounted within 200 mm of the engine, fit the balance shaft. Otherwise spend the engineering effort on getting the rubber mount natural frequency at least 2.5× below the 2× crank frequency excitation.
This is the classic Z-crank wear signature and it is rarely an oil problem. The lateral offset causes a small but persistent side-load component on each rod that is opposite in direction between the two cylinders. If the crankcase oil pickup or the splash feed favours one side of the case, one bearing runs effectively starved while the other gets normal flow.
Check the oil mist distribution under a strobe at idle. On two-strokes also check that the reed valve cage is centred — a 1 mm offset in reed cage position can bias mixture distribution between cylinders, and the leaner cylinder's bearing runs hotter, accelerating its wear.
Cost and crankshaft fatigue, mostly. The Z-crank's offset web is a stress riser that needs careful radius work and ideally shot peening, which adds maybe 40% to crankshaft cost compared to a flat-plane inline crank. On automotive duty cycles — 200,000+ km expected life at variable load — that fatigue margin gets tight. The packaging benefit only matters when length is the constrained axis, and in cars width and height are usually tighter than length. So the engineering tradeoff goes the other way for road vehicles but flips for UAVs and gensets.
About 11% increase in peak secondary couple at any given RPM, because Msec scales linearly with Loffset. On the 90 cc UAV example that takes cruise couple from 11.4 N·m up to roughly 12.7 N·m. Probably acceptable for a one-off prototype. The bigger concern is fatigue — a longer offset means a longer bending moment arm on the crankweb, and the stress at the fillet radius scales with the offset too. If you lengthen the offset, increase the fillet radius proportionally or you'll halve the crankshaft's expected fatigue life.
References & Further Reading
- Wikipedia contributors. Crankshaft. Wikipedia
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