Making the Wave Disc Engine Practical: A Technical Roadmap
Making the Wave Disc Engine Practical: A Technical Roadmap
The following content has been AI cogenerated, this is a devops take perspective on potential engine improvement.
‘How modern manufacturing, oil-free bearings, and plasma ignition could finally solve the engineering challenges identified by Michigan State University’s pioneering research
The Promise That Stalled
Between 2009 and 2013, a team at Michigan State University led by Dr. Norbert Müller built something remarkable: a combustion engine that used shock waves instead of pistons. Funded by a $2.5 million ARPA-E grant, the Wave Disc Engine project demonstrated working hardware achieving quasi-constant-volume combustion—a thermodynamic cycle that theoretical analysis suggested could reach 60% thermal efficiency, nearly double conventional engines.
The prototype worked. In 2011, the team demonstrated 1 kW output at 7,000 rpm to Department of Energy officials. Dr. Müller’s research group, collaborating with Dr. Janusz Piechna from Warsaw University of Technology and researchers from Purdue, Czech Technical University, and University of Tokyo, had proven the fundamental concept viable.
Then the funding ended, and the Wave Disc Engine largely disappeared from public view.
The WDE wasn’t abandoned because the physics failed. It stalled because three engineering problems proved exceptionally difficult with the manufacturing and materials technologies available at the time: thermal management, precision blade manufacturing, and lubrication in extreme-temperature environments.
Fourteen years later, those problems have potential solutions. Technologies that were experimental or prohibitively expensive in 2011 are now commercially proven. This analysis examines how modern engineering approaches could address the challenges MSU’s team identified—building on their foundational work rather than reinventing it.
Understanding the WDE Architecture
Before examining solutions, it helps to understand what makes this engine fundamentally different.
A conventional piston engine compresses fuel-air mixture gradually, burns it, and extracts work through expansion—all in the same cylinder, sequentially. The crankshaft, connecting rods, valves, and camshaft coordinate this dance. Friction losses, incomplete combustion, and throttling waste roughly 60-65% of the fuel’s energy.
The Wave Disc Engine operates on different principles. A spinning disc contains radial channels that alternately align with intake and exhaust ports in stationary end plates. When a channel rotates past a closing port, the sudden blockage creates a compression shock wave that travels down the channel, compressing the fresh charge almost instantaneously. Combustion occurs, and expansion waves extract work as the channel approaches the opening exhaust port.
This achieves something piston engines cannot: quasi-constant-volume combustion. The compression happens so fast that heat transfer losses during compression become negligible. The theoretical efficiency advantage over constant-pressure combustion (conventional engines) is substantial.
MSU’s 2006 review paper in the ASME Journal of Engineering for Gas Turbines and Power—which became the journal’s most downloaded paper—laid out the thermodynamic case comprehensively [1]. The team’s subsequent experimental work validated that the concept could produce useful power.
The challenges they encountered were practical, not theoretical.
Challenge One: Thermal Management
Shock-wave compression generates intense localized heating. Combustion temperatures required for high efficiency would destroy most engineering metals. The MSU team identified thermal management of the disc and end plates as a critical challenge.
The solution may lie in wave rotor physics itself.
Wave rotors possess an inherent self-cooling property that wasn’t fully appreciated in early development. Each channel alternately passes cool compressed air and hot expanded gas dozens of times per second. This frequency far exceeds the thermal response time of the channel walls. The metal never reaches thermal equilibrium with either stream—temperatures average to something significantly below peak combustion temperature.
NASA research on wave rotor topping cycles for gas turbines confirmed this effect enables operation at combustion temperatures that would destroy conventional turbine blades without active cooling systems. The physics suggests a wave disc could potentially operate without liquid cooling circuits, eliminating a major source of complexity and weight.
For applications pushing maximum performance, two material technologies extend the thermal envelope:
Thermal Barrier Coatings using yttria-stabilized zirconia provide 150-300°C temperature reduction across coatings only 0.2-0.4mm thick. This mature technology, standard in jet engines and high-performance automotive applications, allows hotter combustion while keeping substrate metal within survivable limits.
Ceramic Matrix Composites—silicon carbide fiber in silicon carbide matrix—operate continuously at 1200-1400°C while weighing 30-50% less than nickel superalloys. GE’s LEAP engine uses CMC turbine shrouds in commercial aviation today, demonstrating production readiness. For future WDE variants targeting maximum efficiency, CMC rotors could eliminate thermal constraints entirely.
The practical approach: leverage inherent self-cooling with conventional superalloys and TBC coatings for initial development. Reserve CMC for high-performance variants where the cost premium delivers proportional capability.
Challenge Two: Precision Manufacturing
Wave disc channels must maintain precise dimensions to control shock wave timing. The MSU team’s experimental facility featured sophisticated instrumentation precisely because small geometric variations significantly affect wave propagation. In 2011, achieving required tolerances meant expensive multi-axis CNC machining of complex internal geometries.
Metal additive manufacturing has fundamentally changed the economics.
Laser Powder Bed Fusion now achieves dimensional tolerances of ±0.1-0.2mm with minimum feature sizes around 300 micrometers. Surface roughness of 5-15 micrometers Ra as-printed improves below 1 micrometer with post-processing techniques like MMP (Micro Machining Process).
Inconel 718, the workhorse nickel superalloy for aerospace additive manufacturing, offers excellent printability with mechanical properties exceeding wrought material after heat treatment: ultimate tensile strength above 1,247 MPa, operating temperatures to 650-700°C. Supply chains and processing parameters are thoroughly characterized.
For applications requiring lighter weight, Electron Beam Melting enables crack-free processing of gamma titanium aluminide at half the density of nickel superalloys. GE Aviation prints thousands of TiAl turbine blades annually for commercial jet engines.
The manufacturing process chain for a wave disc rotor:
- Design with 0.2-0.5mm machining allowance on sealing surfaces
- Print via LPBF (Inconel) or EBM (TiAl)
- Hot Isostatic Press at 1120°C/100+ MPa to eliminate porosity
- Machine sealing surfaces to final dimension
- Apply thermal barrier coating to channel walls
- CT scan for defect inspection
This process produces geometries impossible to manufacture conventionally—optimized channel shapes, internal cooling passages, integrated features—at costs that decrease with volume rather than requiring expensive tooling amortization.
Challenge Three: Lubrication
The MSU team explicitly identified seal lubrication as a critical unsolved problem. The wave disc spins at thousands of RPM with sealing surfaces exposed to combustion temperatures where conventional lubricants carbonize. Oil contamination would foul combustion; oil-free operation seemed to require materials that didn’t exist.
The microturbine industry solved this problem two decades ago.
Capstone Green Energy has shipped over 9,000 microturbines using patented foil air bearings—completely oil-free. Their 30 kW units operate at 96,000 RPM with demonstrated mean time between failure exceeding 100,000 hours. No scheduled bearing maintenance. No oil changes. No contamination.
Foil air bearings create hydrodynamic lift through a compliant foil structure. As the shaft spins, it drags air into a converging gap, generating pressure that supports the load without contact. At operating speed, a thin air film separates all surfaces.
The engineering challenge is start/stop wear before the air film establishes. NASA developed PS304 coating—chromium oxide for wear resistance, silver for low-temperature lubrication, barium fluoride/calcium fluoride eutectic for high-temperature operation. This coating survives thousands of start-stop cycles while enabling continuous operation above 650°C.
For wave disc sealing surfaces (distinct from shaft bearings), hybrid labyrinth-brush configurations reduce leakage 50-80% versus labyrinth seals alone while maintaining non-contact operation. High-temperature brush seals using cobalt superalloy bristles operate reliably at temperatures and speeds consistent with WDE requirements.
The result: a completely oil-free engine architecture. No lubricant reservoir, pump, filter, or cooler. Maintenance intervals measured in tens of thousands of hours rather than thousands of miles.
Advanced Ignition: Beyond Spark Plugs
The original MSU prototypes used conventional spark ignition, but the WDE’s architecture creates unique demands. Shock-wave compression occurs in microseconds. Each channel fires dozens of times per second. Ignition timing precision measured in microseconds affects efficiency significantly.
Transient Plasma Ignition offers capabilities matched to these requirements.
TPI delivers 25,000-volt pulses in 12 nanoseconds, creating plasma through both thermal and chemical pathways. The plasma generates reactive radicals—atomic oxygen and hydrogen—that accelerate combustion beyond what thermal ignition alone achieves.
Testing on conventional engines demonstrated 6% fuel consumption reduction. However, those engines operate at roughly 38% efficiency. The WDE’s 60% baseline leaves less headroom for improvement—gains from better ignition are partially already captured by the efficient thermodynamic cycle.
A realistic estimate: plasma ignition contributes 2-3% relative efficiency improvement in a WDE, not the 6% seen in conventional engines. Still meaningful, but the larger benefit may be enabling leaner combustion. Plasma ignition extends flammability limits, allowing operation at air-fuel ratios where conventional spark fails. Lean operation improves efficiency and reduces NOx formation.
Transient Plasma Systems has advanced this technology to near-commercial readiness. Integration requires replacing spark plugs with TPI units and pulse generation electronics—a manageable engineering task.
Laser ignition presents an alternative with compelling characteristics: no electrodes to erode, flexible positioning within the combustion chamber, nanosecond timing precision. Miniaturized laser igniters now approach spark plug dimensions. However, commercial readiness lags plasma systems, and window fouling remains an active development challenge. For near-term WDE development, plasma ignition represents lower technical risk.
Heat Recovery: Capturing the Remaining 40%
Even at 60% efficiency, the WDE rejects substantial energy in exhaust heat. Recovering a fraction of this energy pushes system efficiency higher.
Electric turbocompounding fits hybrid vehicle applications particularly well. An exhaust turbine drives a high-speed generator, converting exhaust enthalpy directly to electricity. Commercial systems demonstrate 4-7% fuel consumption reduction across millions of operating hours.
For hybrid range extenders, this electricity feeds directly into the battery—no mechanical coupling complications. The turbocompound unit adds one rotating assembly using the same foil air bearing technology, maintaining oil-free architecture throughout.
Realistic efficiency contribution: 4-6 percentage points added to system efficiency. This is genuinely additive—exhaust waste heat exists regardless of how efficient the base cycle is.
Thermoelectric generators offer simplicity (no moving parts) but currently achieve only 3-5% conversion efficiency. Their practical role: supplementary power for control electronics and auxiliaries, reducing parasitic loads on main output. A compact TEG producing 200-500W requires no maintenance and adds minimal complexity.
CO2 Emissions: Fuel Options for the Improved WDE
A practical range extender must address not just efficiency but emissions. The WDE’s high thermal efficiency translates directly to reduced CO2 per kilometer driven—but fuel choice matters enormously. Here’s what to expect from a 25-30 kW wave disc range extender across different fuel options.
Calculation basis: 65% WDE thermal efficiency, 95% generator efficiency (61.75% fuel-to-electricity), typical EV consumption of 18 kWh per 100 km. This requires approximately 105 MJ of fuel energy per 100 km of driving.
Gasoline
Gasoline remains the most accessible fuel with established infrastructure. Each liter contains approximately 32.4 MJ (9 kWh) of energy and produces 2.31 kg CO2 when burned.
| Metric | Improved WDE | Conventional Range Extender |
|---|---|---|
| System efficiency | 61.75% | 36% |
| Electrical output per liter | 5.56 kWh | 3.24 kWh |
| CO2 per kWh generated | 416 g | 713 g |
| CO2 per 100 km driven | 75 g/km | 128 g/km |
The improved WDE achieves 41% lower CO2 emissions than a conventional range extender on gasoline—comparable to the best conventional hybrids but in a simpler, lighter package. For context, EU 2025 fleet targets require 93.6 g CO2/km; the WDE range extender operating on gasoline already beats this target.
E85 (85% Ethanol)
E85 presents a compelling near-term pathway to lower lifecycle emissions. While ethanol produces CO2 when burned, the carbon was recently captured from the atmosphere by the source crops—making it largely carbon-neutral on a lifecycle basis.
Direct combustion emissions run approximately 1.65 kg CO2 per liter, but lifecycle accounting changes the picture dramatically:
| Ethanol Source | Lifecycle CO2 Reduction vs Gasoline |
|---|---|
| Corn (US average) | 40-50% |
| Sugarcane (Brazilian) | 70-90% |
| Cellulosic (agricultural waste) | 85-95% |
E85’s lower energy density (25 MJ/L vs 32.4 MJ/L) means 30% more fuel volume consumed, but the WDE’s efficiency advantage partially compensates:
| Metric | WDE on E85 | WDE on Gasoline |
|---|---|---|
| Fuel consumption per 100 km | 3.9 L | 3.2 L |
| Direct CO2 per 100 km | 64 g/km | 75 g/km |
| Lifecycle CO2 (corn ethanol) | ~35-40 g/km | 75 g/km |
| Lifecycle CO2 (sugarcane) | ~10-20 g/km | 75 g/km |
With cellulosic ethanol from agricultural waste, lifecycle emissions approach 5-10 g CO2/km—over 90% reduction from gasoline baseline.
Natural Gas (CNG/LNG)
Natural gas offers the lowest CO2 emissions of any fossil fuel due to methane’s favorable hydrogen-to-carbon ratio. Each kilogram contains approximately 50 MJ and produces 2.75 kg CO2—roughly 55 g CO2 per MJ versus gasoline’s 73 g CO2 per MJ.
| Metric | WDE on Natural Gas | WDE on Gasoline |
|---|---|---|
| CO2 per MJ fuel | 55 g | 73 g |
| CO2 per kWh generated | 320 g | 416 g |
| CO2 per 100 km driven | 58 g/km | 75 g/km |
Natural gas delivers 23% lower CO2 than gasoline in the same WDE, achieving emissions competitive with battery electric vehicles charged from average European grid electricity.
The practical challenge: compressed natural gas tanks are bulky. A range extender application—where the tank only needs to provide emergency/extended range rather than primary fuel storage—may tolerate this better than a primary-fuel vehicle.
Biogas
Biogas is chemically similar to natural gas (primarily methane) but derives from anaerobic decomposition of organic waste—food waste, agricultural residues, sewage, slaughterhouse waste. Sweden provides excellent real-world data on biogas vehicle fuel performance, with vehicle gas (fordonsgas) consisting of 96% biogas as of 2023.
Understanding biogas emissions requires distinguishing between two calculation methods:
The HBK method (used for EU Renewable Energy Directive compliance) measures direct production chain emissions. The ISO method (system expansion) additionally credits avoided emissions—what would have happened to the feedstock otherwise.
| Calculation Method | Typical Biogas | Manure-Based Biogas |
|---|---|---|
| HBK method (regulatory) | 2-20 g CO2-eq/MJ | 5-15 g CO2-eq/MJ |
| ISO method (system expansion) | ~0.6 g CO2-eq/MJ | -20 g CO2-eq/MJ |
| Biogas Product | Reported Emissions | Source |
|---|---|---|
| Average Swedish CBG (compressed) | 2.3-7.4 g CO2-eq/MJ | Svensk Biogas, Miljöfordon Sverige |
| Average Swedish LBG (liquefied) | 5.4 g CO2-eq/MJ | Svensk Biogas |
| Swedish fordonsgas average | 7.4 g CO2-eq/MJ | Miljöfordon Sverige (2023) |
For a WDE range extender running on typical Swedish biogas (7.4 g CO2-eq/MJ):
| Metric | WDE on Biogas | WDE on Gasoline |
|---|---|---|
| Fuel energy per 100 km | 105 MJ | 105 MJ |
| CO2 per 100 km | ~8 g/km | 75 g/km |
| Reduction vs gasoline | ~90% | baseline |
This aligns with Swedish industry claims of “up to 90% reduction” for biogas vehicles and represents a 92% climate benefit according to Miljöfordon Sverige.
Can biogas achieve negative emissions?
Using the ISO method with manure-based biogas, emissions can be calculated as negative (-20 g CO2-eq/MJ) because:
- The carbon released was recently captured from atmosphere by feed crops
- Capturing manure for biogas prevents methane release that would otherwise occur during conventional manure handling
- Methane has 80× the short-term warming potential of CO2
At -20 g CO2-eq/MJ, a WDE would achieve approximately -21 g CO2/km—genuinely carbon negative. However, this represents the best-case scenario (manure feedstock, ISO accounting), not typical operation.
Realistic biogas emissions range for WDE:
| Scenario | CO2 per 100 km | Reduction vs Gasoline |
|---|---|---|
| Typical biogas (HBK method) | 5-10 g/km | 87-93% |
| Best case manure biogas (ISO method) | -15 to -25 g/km | >100% (negative) |
The conservative estimate of 5-10 g CO2/km for typical biogas operation already represents exceptional performance—comparable to battery electric vehicles charged from renewable electricity.
Hydrogen
Hydrogen produces zero CO2 at the point of combustion—only water vapor. However, lifecycle emissions depend entirely on how the hydrogen was produced:
| Production Method | Lifecycle CO2 per kg H2 | WDE CO2 per 100 km |
|---|---|---|
| Grey (natural gas reforming) | 9-12 kg | 45-60 g/km |
| Blue (reforming + carbon capture) | 2-4 kg | 10-20 g/km |
| Green (renewable electrolysis) | 0.5-2 kg | 2-10 g/km |
Hydrogen’s energy density (120 MJ/kg, roughly 33 kWh/kg) means excellent range per kilogram, but storage remains challenging. At 700 bar compression, hydrogen tanks achieve approximately 5% gravimetric efficiency (5 kg H2 per 100 kg tank system).
For a range extender carrying 2 kg hydrogen (reasonable for emergency/extended range):
- Energy content: 66 kWh
- Electrical output at 61.75% efficiency: 41 kWh
- Extended range: ~225 km
With green hydrogen, those 225 km produce approximately 4-20 g CO2 total lifecycle emissions—essentially zero-emission driving from an internal combustion architecture.
The combustion advantage over fuel cells: The WDE can potentially run on hydrogen without precious metal catalysts, operates at higher temperatures (better for hydrogen’s fast flame speed), and tolerates fuel impurities that would poison fuel cell membranes. For hydrogen produced from variable renewable sources with less-than-perfect purity, a WDE range extender may prove more robust than fuel cell alternatives.
Emissions Summary Table
| Fuel | CO2 per 100 km | vs. Conventional Gasoline RE |
|---|---|---|
| Gasoline | 75 g/km | -41% |
| E85 (corn ethanol) | 35-40 g/km | -69% |
| E85 (sugarcane) | 10-20 g/km | -85% |
| Natural gas | 58 g/km | -55% |
| Biogas (typical) | 5-10 g/km | -92% |
| Biogas (manure, ISO method) | -15 to -25 g/km | >100% (negative) |
| Hydrogen (grey) | 45-60 g/km | -53% |
| Hydrogen (blue) | 10-20 g/km | -85% |
| Hydrogen (green) | 2-10 g/km | -95% |
The Practical Path
For near-term deployment, gasoline compatibility ensures the WDE range extender works with existing infrastructure while still delivering 41% emissions reduction through efficiency alone.
E85 capability (requiring only minor fuel system modifications) opens the door to 70-90% reductions where ethanol infrastructure exists—particularly Brazil, parts of the US Midwest, and expanding European markets.
Biogas offers the most compelling near-term pathway to near-zero emissions. Sweden demonstrates this is practical today: 96% of vehicle gas sold is biogas, with over 40 public filling stations and established supply chains. At 5-10 g CO2/km, a biogas WDE matches battery electric vehicles charged from mixed-grid electricity—while retaining the flexibility to run on other fuels when biogas isn’t available.
Hydrogen readiness future-proofs the architecture for eventual green hydrogen infrastructure, achieving near-zero emissions without abandoning internal combustion’s advantages in robustness, power density, and manufacturing simplicity.
The improved WDE doesn’t require choosing one fuel pathway. The same basic engine architecture can accommodate multiple fuels with relatively minor modifications—hedging against uncertainty in which low-carbon fuel infrastructure ultimately prevails.
Why Now?
The Wave Disc Engine concept dates to the 1980s. MSU’s intensive development occurred over a decade ago. Why would this architecture succeed now when earlier efforts stalled?
Three things have changed fundamentally.
Additive manufacturing reached production maturity. In 2011, metal 3D printing was a prototyping curiosity. Today, aerospace companies print certified flight hardware by the thousands. The precision, repeatability, and economics now support WDE production.
Oil-free bearings proved themselves at scale. Capstone’s 100,000+ hour demonstrated reliability across 9,000+ units transforms foil air bearings from research project to proven commercial technology with established supply chains.
The application landscape shifted. Hybrid and electric vehicles create demand for efficient range extenders that didn’t exist in 2011. A range extender doesn’t need throttle response or variable RPM—it runs at constant speed, optimal efficiency, charging batteries. The WDE’s characteristics align perfectly with this use case.
Conclusion
The Wave Disc Engine doesn’t require breakthrough inventions to become practical. It requires careful integration of technologies that have matured since MSU’s pioneering work ended: aerospace-grade additive manufacturing, proven oil-free bearing systems, advanced ignition technology, and established heat recovery approaches.
Dr. Müller’s team demonstrated the thermodynamics work. They identified the engineering challenges clearly. The solutions they needed either didn’t exist or weren’t economically viable in 2011.
Those constraints have changed.
Realistic performance targets 63-66% thermal efficiency in a package dramatically simpler and lighter than piston engine generators. No oil system. Minimal maintenance. Direct electrical output compatible with hybrid powertrains.
The WDE won’t replace piston engines for applications requiring throttle response and variable speed. But for the specific task of efficiently converting fuel to electricity in a hybrid vehicle—running at constant speed, optimized conditions, maximum efficiency—the engineering case has strengthened considerably since MSU’s prototype last ran.
Perhaps it’s time for shock waves to have their moment.
References
[1] Akbari, P., Nalim, M.R., and Müller, N. “A Review of Wave Rotor Technology and Its Applications,” ASME Journal of Engineering for Gas Turbines and Power, Vol. 128, pp. 717-735, October 2006.
[2] Piechna, J., Akbari, P., Iancu, F., and Müller, N. “Radial-Flow Wave Rotor Concepts, Unconventional Designs and Applications,” ASME Paper IMECE2004-59022, 2004.
[3] “Development of a Wave Disk Engine Experimental Facility,” AIAA 2012-3703, 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 2012.
[4] Piechna, J. and Dyntar, D. “Numerical Investigation of the Wave Disc Micro-Engine Concept,” International Journal of Gas Turbine, Propulsion and Power Systems, December 2008.
[5] DellaCorte, C. and Bruckner, R.J. “Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery,” NASA/TM—2010-216762, 2010.
[6] Transient Plasma Systems, “Engine Test Results Press Release,” August 2022.
This analysis builds upon the foundational research conducted at Michigan State University under Dr. Norbert Müller’s leadership, funded by ARPA-E grant DE-AR0000004. The engineering solutions proposed utilize commercially available technologies from the aerospace, microturbine, and advanced ignition industries.