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Case study · Ooni Karu 2

Combustion and door design for a multi-fuel oven, with a clear view of the fire.

The Karu 2 is Ooni's multi-fuel outdoor oven, burning wood, charcoal and gas. I was performance lead and design engineer for the door and the combustion system: the door assembly and its high-temperature magnetic latch, the patent-pending ClearView™ anti-sooting geometry, the fuel tray, chimney baffle and primary air inlet, and pre-compliance. It won an iF Design Award and a German Design Award in 2024, and its door architecture carried onto the iF-winning Karu 2 Pro in 2025.

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−27%
solid fuel consumption vs the previous generation
−70%
door BOM cost, with a larger window
<1 min
door assembly time, from over 24 hours
4.7★
across 1,744 customer reviews
The design process

How the project ran, end to end

The Karu 2 gained a large glass door where the previous generation had insulated steel, which made the oven harder to heat before any improvement could start. The door and the combustion system had to be developed side by side, because every change to one moved the other. This is how I worked through it.

  1. Set the targets and the constraints

    A 500 °C stone with a 60-second cook, an even flame, lower fuel use than before, stable performance in wind and rain, and touch-temperature and emissions compliance. The door also had a strict industrial design intent, with no visible fixings.

    Requirements · DVTP · industrial design intent

  2. Model the air path

    A thermodynamic model and CFD told me where the primary air should come in, how much, and how the chimney should draw, before I started cutting metal.

    Thermodynamic modelling · CFD

  3. Test systematically

    Blanking plates over the inlets let me change one variable at a time, with instrumented combustion and touch-temperature testing on every configuration.

    Test rig design · instrumentation · combustion testing

  4. Hand-prototype the door

    Sheet metal and borosilicate doors, built by hand and run through full wood cooks on older ovens, so the anti-sooting idea was proven before any design went to tooling.

    Rapid prototyping · iteration · invention

  5. Size the critical parts by hand

    The baffle handle was sized as a fin and the magnetic latch from a moment balance, so simulation and testing confirmed a design that was already close.

    Hand calculation · heat transfer · mechanism design

  6. DFMA under strict design intent

    Cast and pressed parts designed for one line of draw, two hidden fixings, and the gasket designed out altogether.

    DFM & DFA · die casting · pressing · tolerancing

  7. Pre-compliance, then production

    I owned pre-compliance and DVTP testing end to end, including the documentation, and worked with the supplier through tooling and production.

    Pre-compliance · design verification · supplier liaison
2.1 · Combustion & airflow 2.2 · ClearView™ door 2.3 · Door DFMA & latch
Skills used

What this project called on

Engineering
Combustion & airflowThermodynamic modellingHeat transferHand calculationInvention & patentingMechanism designDFM & DFATolerancingMaterials selectionPre-compliance & DVTPUser testingIndustrial design intent
Tools
SolidWorksSolidWorks Flow Simulation (CFD)SolidWorks Simulation (FEA)Instrumented temperature testingMulti-channel data logging
Manufacturing
Pressure die castingSheet metal pressingBorosilicate glazingHigh-temperature magnetsTooling supportIncoming part inspection
Case study 2.1 · Combustion & airflow

Optimising combustion and cooking performance across multiple fuels

Problem

A multi-fuel oven has to burn wood and gas well with the same airflow and combustion setup, and the two want almost opposite things. Solid fuel wants a deep bed, primary air from underneath and a long residence time, while gas needs controlled entrainment for a stable flame front.

On top of that, the Karu 2 was gaining a large glass window where the previous generation had insulated steel. Glass radiates and conducts heat straight out of the cavity, so the combustion system had to win back that loss before it could improve anything at all.

Targets: 500 °C stone with a 60-second cook · even flame across the chamber · lower solid fuel use than the previous generation · stable in wind and rain · touch temperature and emissions compliance.
Fig. 38, Flame geometry across the chamber
Fig. 39, Combustion optimisation
Fig. 40, Blanking plates for systematic testing
Process

A thermodynamic model and CFD for the air path, and a handle sized by hand before FEA

  1. 01Model the air path. I used a thermodynamic model and CFD to study the area and position of the primary air inlet and the chimney, optimising the airflow for fast heat-up and for holding low temperatures, and accounting for wind and rain ingress.
  2. 02Test one variable at a time. Blanking plates over the inlet let me step the airflow systematically, with combustion testing on both solid fuel and gas, and a round of gas valve optimisation.
  3. 03Redesign the fuel system. The results fed a new fuel tray, 45% larger for a broader, more even flame, a new chimney baffle and a new primary air inlet.
  4. 04Size the baffle handle by hand first. The handle is a rod running from a 500 °C chamber to a surface the user has to grip. Before simulating anything I sized it as a fin, one-dimensional conduction with convective loss along its length, where the tip temperature Ttip = T∞ + (Tbase − T∞) / (cosh mL + (h/mk) sinh mL) falls as mL grows. That set the length, thickness and position of the shaft: a thin stainless shaft with a deliberate thermal break, and the grip placed inside the standard's touch-temperature limit.
  5. 05Verify for compliance. I ran instrumented touch-temperature testing and owned pre-compliance and DVTP testing end to end, documentation included.
Fig. 41, Combustion testing
Fig. 42, Gas valve optimisation
Fig. 43, Instrumented touch-temperature testing
Fig. 44, Combustion simulation
Results

A more efficient oven, even with a glass door

−27%solid fuel use
+45%fuel tray area
500 °Cwith a 60-second cook

The combustion work cut solid fuel consumption to 27% less than the previous generation, and the larger tray gave a broader, more even flame with better coverage of the stone. The oven still reaches 500 °C with a 60-second cook despite losing heat through a large glass window, and it burns compliantly on every gas type.

The new baffle handle passes touch temperature with margin, where the previous model's handle ran hot, and the final baffle design was re-used on the larger Karu 2 Pro.

Fig. 45, Compliant combustion with all gas types
Fig. 46, Performance with solid fuels
Fig. 47, Final baffle handle design
Case study 2.2 · ClearView™ door invention

Designing and integrating a glass door to view cooking food

Problem

The original plan was a small peephole in the Karu 2 door, so a user could glimpse the pizza without a large window sooting up. The airwash technology from the flagship Karu 16 was out of budget, it was unreliable, and on this model it would have covered most of the available viewing area anyway.

Soot sticks to glass when unburned carbon particles reach a surface cool enough to hold them. My aim was to keep that particle-laden gas off the glass without complex parts, and the old architecture showed what the alternative cost: 30% of the glass blocked by the anti-soot structure, over 30 parts and 40 fixings in the door, 24 hours of assembly and cure time, and soot still an issue.

Constraints on the door: no added components · no added assembly time · no reduction in window area · must work with wood, charcoal and gas · must survive 500 °C and thermal cycling · strict design intent on visible surfaces · lower budget.
Fig. 49, The problem: soot on the previous generation door
Fig. 50, The original proposal: a peephole in the door
Process

Recessing the glass and letting passive cool air create a turbulent boundary layer

  1. 01Understand why soot sticks. Soot reaches the glass by thermophoresis and diffusion across the boundary layer. A thick, still layer lets particles drift in and settle, so tripping the flow along the inside face of the glass thins that layer and sweeps the particles back into the hot gas, where they burn.
  2. 02Make the geometry do the work. Recessing the glass and bringing in passive cool air creates that turbulent layer with no fans, no extra parts and nothing added to the line.
  3. 03Hand-prototype and test for real. I built sheet metal and borosilicate doors by hand and tested them on older ovens before committing any design to tooling. Every iteration was run through a full wood cook and judged on the glass.
  4. 04Iterate until it holds across the whole window. Early prototypes still sooted: the air path was in the right place but the aim and the gap were wrong. Each round moved the entry, changed the angle of attack along the glass and closed the gap, until the flow stayed attached across the whole window rather than just its lower half.
  5. 05Prove it tooled and protect it. Tooled prototypes confirmed the result, and the geometry was filed as a patent.
Fig. 51, Early hand-built prototype, still sooting
Fig. 53, Hand-built prototypes nailing the geometry
Fig. 52, The final anti-sooting geometry
Results

Clear glass, a larger window, and a cheaper BOM and running cost

0added components or assembly steps
2products shipped on the architecture
iF 2025for the Karu 2 Pro

The new geometry solved the problem outright. On tooled prototypes the soot lands on the surrounding metal while the glass stays clear through a full wood cook. Because the whole mechanism is geometry, nothing was added to the bill of materials or the line, the window ended up larger than the original brief allowed, and the peephole was never needed.

There was a second win on gas: the flame stays visible through the glass, so the door stays shut. The previous door had to come off to check the flame, which cost a lot of the oven's efficiency every time. The architecture then scaled onto the Karu 2 Pro, a larger oven with a larger window, with the BOM and assembly time reduced again at the bigger size.

Fig. 54, Tooled prototype: soot on the metal, clear on the glass
Fig. 55, Karu 2 Pro, inheriting the architecture
Case study 2.3 · Door DFMA & magnetic latch

The door performing in high-performance conditions, including magnets at 500 °C

Problem

The previous door took over 24 hours per unit to assemble, mostly waiting for a gasket to cure, and used over 70 parts and fasteners. The new door had to be cheaper and faster to build, and keep the industrial design intent on every visible surface.

It also had to stay shut. The door is held by magnets that must hold at temperature and still release by hand, and a magnet's strength drops as it heats up.

Process

DFMA for the door, and a latch sized for its working temperature

  1. 01Design the cast parts for the tool. Uniform wall thickness, with draft and release direction resolved on one line of draw.
  2. 02Design the pressed parts for the press. Generous flow radii, and features pressed rather than welded.
  3. 03Cut the fixings to two, both hidden. That kept the visible surfaces clean for the industrial design intent.
  4. 04Protect the glass. Tabbed glass brackets are located by the tab and deliberately compliant, so the assembly can't over-constrain the glass and crack it.
  5. 05Delete the gasket. Compressive high-temperature parts replaced it, which removed the cure time entirely.
  6. 06Size the latch for the hot case. I placed the magnets where the door runs coolest and reduced the conduction path into them, then sized the hold from a moment balance about the hinge line, Fhold·dm ≥ SF (W·dxg + Fw·dw), using the derated pull at the magnet's working temperature and a wind-gust term. I then measured pull-off hot, and the first magnet chosen was the last. The magnets are replaceable without destroying the door, which matters on a product with a five-year life outdoors.
  7. 07Cover the rest of the environment. Material selection for extreme heat, UV and rain, heat shields verified in simulation, drop testing, and a removable door that was designed, user tested and patented.
Fig. 56, Magnet placement and moment calculations
Fig. 57, Shipped door detail
Fig. 58, Incoming part inspection
Results

A cheaper door, built in under a minute

−70%door BOM cost
<1 minassembly, from 24 h+
7parts and 2 fixings

The door BOM came down by over 70% with a larger window, and assembly went from over 24 hours to under one minute. Both improved again on the larger Karu 2 Pro, which shows the architecture scales.

How I could help you

The same process, on your product

Understand the physics, prototype fast and cheaply, then design every part for how it will be made. That's the heart of Manufacturing Preparation and Design Verification. If you need a part to perform in harsh conditions and still be cheap and quick to build, it's exactly the kind of problem I like.

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