A Quasiturbine is a rotary internal combustion engine built around a four-blade articulated rhomboidal rotor that flexes inside a near-oval stator, completing four full thermodynamic strokes per shaft revolution without a crankshaft or eccentric shaft. It replaces the triangular Wankel rotor with a deformable chain of four pivoted blades, which lets the chamber volume change at a more uniform rate. The design exists to give a smooth high-frequency cycle with low vibration in a compact package. Saint-Hilaire's QT75-SC prototype produced roughly 75 kW from a 1.7 L footprint.
Quasiturbine Cycle Timing Interactive Calculator
Vary revolution count, cycle timing, strokes, and chambers to compare Quasiturbine stroke completion against a conventional piston four-stroke.
Equation Used
This calculator follows the worked comparison: a Quasiturbine completes the four-stroke sequence in one shaft revolution, while a conventional piston four-stroke needs two revolutions. The outputs show how many stroke steps each chamber completes over the selected shaft revolutions and the resulting cycle-rate advantage.
- Ideal kinematic timing only; combustion pressure, leakage, and efficiency are not included.
- Each chamber follows the same stroke sequence.
- The comparison uses four-stroke logic completed in one Quasiturbine shaft revolution versus two piston-engine revolutions.
How the Quasiturbine Works
The Quasiturbine, sometimes written QT, swaps the Wankel's rigid triangular rotor for four blades joined at four pivots — a closed rhomboidal chain that flexes as it tracks the inner profile of the stator. As the shaft turns, opposite corners of the rhombus stretch and compress, pinching off four chambers between blade, stator wall, and the carriage rollers (or carriage-less variants in newer Saint-Hilaire designs). Each chamber sees intake, compression, combustion, and exhaust ports cut into the stator wall — same four-stroke logic as a piston engine, but completed in one shaft turn instead of two.
The stator profile is not a true ellipse. It's a Saint-Hilaire epitrochoid-like curve calculated so that the blade pivots stay in continuous contact without slamming. If the profile is wrong by even 0.1 mm at the apex, you get blade chatter at the top of compression and the seal strips eat themselves inside 50 hours. Pivot pin clearance is the other killer — anything above 0.05 mm radial play and the rotor hunts under load, which shows up as combustion knock the timing system can't predict because the chamber volume is no longer where the geometry says it should be.
The big functional difference from a Wankel is that there's no eccentric shaft. The rotor's centre of mass stays close to the geometric centre of the stator, so first-order vibration is near zero. That's the whole reason the photo-detonation cycle research community is interested — you can run very fast pressure rises (detonation rather than deflagration) without shaking the engine off its mounts. The tradeoff is that you now have eight high-pressure seal interfaces per rotor instead of three, and every one of them has to hold without an oil film if you want the engine to swallow hydrogen or run pure-oxygen photo-detonation modes.
Key Components
- Rhomboidal rotor (4 blades, 4 pivots): Four steel or titanium blades articulated at four pivot pins form the deformable rhombus. Each blade typically runs 60-90 mm long in prototype scales, with pivot pin diameters held to ±0.01 mm to keep chamber volume repeatable across all four positions.
- Stator (Saint-Hilaire profile housing): The inner bore is machined to a non-elliptical curve that keeps the four blade pivots in continuous rolling contact. Surface finish must be Ra ≤ 0.4 µm — coarser than that and the apex seals score the wall within the first 20 hours of running.
- Carriage rollers: On carriage-equipped variants, four rollers ride between the blade pivots and the stator wall, transferring side load and acting as the seal carrier. They're the highest-wear part on the engine — typical service life 200-500 hours depending on combustion pressure.
- Apex and side seals: Each blade-pivot junction needs an apex seal plus two side seals to contain combustion pressure. Eight apex seals total per rotor, versus three on a Wankel — this is why seal cost dominates QT manufacturing economics.
- Stator porting (intake, compression, combustion, exhaust): Ports are cut directly into the stator wall at angular positions matching the four chamber phases. Port timing is fixed by geometry — you cannot retard or advance it without re-machining the housing, which is why most QT prototypes run wasted-spark or laser-triggered ignition.
- Output shaft: Driven directly by the rotor centre via a yoke or splined hub. Because there's no eccentric, shaft balance is straightforward — typical residual imbalance below 2 g·mm at the bearing journals.
Industries That Rely on the Quasiturbine
The Quasiturbine has not displaced the piston engine in any volume application, but it has earned a foothold in a handful of research and niche-power roles where its specific properties — high power density, low vibration, ability to run unconventional thermodynamic cycles — beat the alternatives. Most production-quantity examples are still tied to Saint-Hilaire's own laboratory in Quebec or licensed research builds.
- Aerospace / UAV propulsion: Saint-Hilaire QT75 demonstrator targeted at long-endurance UAV airframes where the near-zero first-order vibration lets the airframe carry sensitive optical payloads without isolation mounts.
- Combustion research: Photo-detonation cycle research at Université de Sherbrooke using QT-pattern rotors to study constant-volume detonation combustion, which a piston engine cannot survive structurally.
- Stationary power generation: Compact 30-50 kW gen-set prototypes built by independent licensees, running on natural gas with the QT operating as a pneumatic expander upstream of the combustion stage.
- Hybrid vehicle range extenders: Demonstrator installs in concept hybrid cars where the QT's 1.7 L equivalent footprint and balanced shaft output let the engine bolt directly to a generator without a flywheel damper.
- Pneumatic and steam expansion: Compressed-air and steam variants used as zero-emission expanders in industrial waste-heat recovery, exploiting the QT's good low-rpm torque (the rotor produces useful work down to 50 RPM, where a Wankel stalls).
- Marine auxiliary power: Experimental small-craft auxiliary units where the absence of crankshaft means the engine can be mounted in any orientation without re-plumbing the oil system.
The Formula Behind the Quasiturbine
What every QT designer needs to know first is the swept volume per shaft revolution, because that's what sets the air consumption, the fuel rate, and the brake power ceiling. The number changes dramatically across the operating range — at the low end of typical QT speeds (around 1,000 RPM) you're working with low chamber turbulence and combustion is sluggish, so volumetric efficiency drops to 70%. At the nominal design point (3,000-4,000 RPM) the chamber refilling lines up with the port timing and you hit the geometric maximum. Push past 6,000 RPM and seal flutter starts to leak the chambers across each other, so effective swept volume falls again. The sweet spot for most QT prototypes sits at 3,500 RPM.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Vswept | Total swept volume per shaft revolution (sum across all 4 chambers) | cm³ | cu in |
| Amax | Maximum chamber cross-sectional area (rotor at expanded position) | cm² | sq in |
| Amin | Minimum chamber cross-sectional area (rotor at compressed position) | cm² | sq in |
| Lblade | Blade axial length (depth of the rotor into the stator) | cm | in |
Worked Example: Quasiturbine in a 50 kW natural-gas microCHP unit
You are sizing the swept volume and expected brake power of a Quasiturbine QT-pattern rotor for a 50 kW natural-gas microCHP (combined heat and power) unit destined for a small dairy operation. The rotor uses Amax = 22 cm², Amin = 6 cm², and a blade axial length of 8 cm. You need to know what the engine produces at idle (1,000 RPM), at the design point (3,500 RPM), and at the redline (6,000 RPM) so you can size the generator coupling and the heat-recovery exchanger.
Given
- Amax = 22 cm²
- Amin = 6 cm²
- Lblade = 8 cm
- BMEP (assumed natural gas) = 9 bar
Solution
Step 1 — compute the swept volume per shaft revolution from the geometry:
So every shaft turn pumps 512 cm³ through the chambers — roughly the displacement of a 500 cc motorcycle single, but completed in one revolution instead of two, which is the QT's defining trick.
Step 2 — at the design point of 3,500 RPM, compute air mass flow and brake power using BMEP:
That's short of the 50 kW target on a single rotor — which is why most QT microCHP designs stack two rotors on a common shaft. At 3,500 RPM the chamber refilling matches the stator port timing and volumetric efficiency runs around 92%, so the number above is realistic.
Step 3 — at the low end of the operating range, 1,000 RPM:
At 1,000 RPM the engine barely keeps the generator excited — the chambers fill slowly, combustion is sluggish, and you'd hear it as a lumpy uneven exhaust note. This is the regime where photo-detonation research engines actually run, but for a microCHP it's basically idle.
Step 4 — at the high end, 6,000 RPM, the geometry says:
In theory you're nearly at target on a single rotor. In practice the apex seals start to flutter above roughly 5,500 RPM in a typical 8 cm blade build, leaking pressure between chambers, and real-world output flattens around 38-40 kW. The sweet spot sits at 3,500-4,000 RPM where seal life and power density both make sense.
Result
Nominal brake power at the 3,500 RPM design point comes out to 26. 9 kW from a single QT rotor, which means a two-rotor stack delivers the 50 kW microCHP target with margin. At 1,000 RPM the same rotor only makes 7.7 kW — useful for warm-up but not enough to support full generator load — while pushing to 6,000 RPM theoretically reaches 46 kW but in practice plateaus near 39 kW once seal flutter starts leaking chamber pressure. If you measure 20 kW or less at 3,500 RPM instead of the predicted 26.9 kW, the most likely causes are: (1) apex seal preload below 8 N which lets combustion pressure blow past into the next chamber, (2) stator surface finish coarser than Ra 0.4 µm causing seal scoring within the first 20 hours, or (3) port timing offset because the stator was clocked wrong by 2-3° at assembly. Check seal blow-by with a leak-down test before you start re-tuning the spark.
When to Use a Quasiturbine and When Not To
The Quasiturbine competes mainly with the Wankel rotary and the conventional reciprocating piston engine in the same power class. Each has a distinct fingerprint on power density, vibration, and serviceability, and the choice almost always comes down to whether you need the QT's specific combustion-cycle flexibility or you'd be happier with a mature off-the-shelf piston unit.
| Property | Quasiturbine | Wankel rotary | Reciprocating piston (4-stroke) |
|---|---|---|---|
| Strokes per shaft revolution | 4 (one full Otto cycle per turn) | 1 per rotor face, 3 per shaft turn via 3:1 gearing | 0.5 (4-stroke needs 2 turns) |
| Typical operating speed range | 1,000-6,000 RPM, sweet spot 3,500 | 2,000-9,000 RPM | 600-7,000 RPM |
| First-order vibration | Near zero (balanced rhomboidal rotor) | Low (eccentric mass requires counterweight) | Moderate to high (reciprocating mass) |
| Apex seal count per rotor | 8 | 3 | N/A (piston rings instead) |
| Power density (kW/L) | ~30-45 (prototype) | ~75-100 (production) | ~50-80 (production) |
| Typical seal/ring service life | 200-500 hours (prototype builds) | 1,500-3,000 hours | 5,000-10,000 hours |
| Cycle flexibility (detonation, Stirling, steam) | High — supports photo-detonation and steam expansion | Low — Otto only | Low — Otto or Diesel only |
| Manufacturing maturity / parts availability | Research / prototype scale | Limited (Mazda, niche aerospace) | Mature, global supply chain |
| Cost per kW (small batch) | Very high — custom seals dominate cost | High — limited supplier base | Low — commodity |
Frequently Asked Questions About Quasiturbine
That's almost certainly apex seal flutter. Above roughly 5,000 RPM the inertial load on the seal strip exceeds the spring preload that holds it against the stator wall, so the seal lifts for a few microseconds during each chamber transition. Combustion gas blows past into the adjacent chamber and you lose the pressure differential that drives torque.
Quick diagnostic: run a motored leak-down at the suspect RPM with the spark plugs out and an air supply on one chamber. If leak rate jumps by more than 2× compared to the cranking-speed value, the seals are flying. Fix is heavier seal springs (typical preload 8-12 N) or — better — switch to a labyrinth-backed seal design.
Yes, and that's the main reason the research community keeps funding QT work. The rhomboidal rotor's centre of mass barely moves, so the shock load from a constant-volume detonation event doesn't translate into a hammering force on the bearings the way it does in a piston engine. Saint-Hilaire's photo-detonation prototypes have run pulse rates above 600 Hz without bearing failure.
The catch is that the seals see roughly 3× the peak pressure of a deflagration cycle, so apex seal life drops from a few hundred hours to under 50. Detonation operation is a research mode, not a production duty cycle.
For a UAV, no — pick the Wankel. The QT's vibration advantage is real but a UAV isn't bothered by Wankel-class first-order vibration once the engine is rubber-mounted, and the Wankel's seal life (1,500+ hours) and supply chain (Aixro, AIE, UAV Engines Ltd) are decades ahead of anything QT.
The QT only wins if you're running a non-Otto cycle — pneumatic expansion, steam, or photo-detonation — or if your payload genuinely cannot tolerate any vibration isolator mass (some optical SAR payloads). For a vanilla gasoline range extender, the Wankel is the lower-risk choice.
Because it isn't tuneable. Port timing is set by the angular position of the holes machined into the stator wall — there's no camshaft, no VVT, no way to retard or advance anything once the housing is bored. If you want different timing you re-machine the stator or build a new one.
Most QT builders compensate by tuning ignition timing aggressively (laser-triggered systems are common) and by changing fuel injection phasing. If your engine runs lean at high RPM, the answer is almost never the ports — it's the injector duty cycle hitting its limit, because the QT fires four times per turn and you run out of injector open-time fast.
Two suspects, in order of likelihood. First, the rotor pivot pins have radial clearance above 0.05 mm, which lets the rhomboid collapse slightly under combustion pressure and reduces Amax. Measure pin-to-bushing clearance with a dial indicator while you load the rotor — if you see more than 0.05 mm of float, that's your volume loss right there.
Second, the stator profile may be machined slightly undersize at the apex regions. Saint-Hilaire's curve is unforgiving — a 0.1 mm error at the major axis costs you 3-4% of swept volume because that's where Amax is set. Coordinate-measure the stator bore at 12 angular positions and compare to the design CAD before you blame anything else.
Because the QT is fundamentally a positive-displacement machine with continuous chamber sealing — the seals hold pressure even when the rotor is barely moving. A Wankel's apex seals depend on centrifugal force to load them against the housing, and below about 800 RPM that force drops below the spring preload and the seals leak.
This is why QT variants get used as steam and compressed-air expanders. At 50-100 RPM the engine still produces clean torque pulses, which makes it useful in waste-heat recovery where the heat source can't drive a high-RPM cycle.
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