# TM-180 / TM-250 — Module Specification Sheet

**v1.0.0 — companion to `TM-250-HANDOFF.md`**
Two build variants of the opposed free-piston linear generator. Same architecture, scaled.

Scaling law: **`P ∝ k²`**, mass `∝ k³`, frequency `∝ 1/k` — mean piston speed held constant at 13 m/s.

---

## 1. Efficiency build-up

Where the efficiency comes from, and what each feature is worth. Percentages are **fuel-to-electricity at the DC bus**, not shaft BTE.

### Mechanical stack

| Step | Δ | Cumulative | Why |
|---|---|---|---|
| Baseline: modern 4-stroke HD diesel at best point | — | **42.0%** | Reference (Cummins X15, DD15 class) |
| **Opposed piston** — no cylinder head | **+3.0** | 45.0% | ~30% less heat-loss surface area. The head is the hottest, leakiest part of any engine |
| **Long stroke** — high expansion ratio | **+1.5** | 46.5% | Extracts more work before the exhaust port opens. Same lever marine engines use to reach 54% |
| **Free piston** — no crank, rod, valvetrain | **+1.5** | 48.0% | Deletes piston side thrust (~45% of engine friction), big-end and main bearings, valvetrain |
| **Fixed-point operation** | **+0.5** | **48.5%** | No throttling, no transients, no cold-cycling |

### Electrical stack

| Step | Δ | Cumulative |
|---|---|---|
| Linear alternator (95%) | −2.4 | 46.1% |
| Active rectifier (97%) | −1.4 | 44.7% |
| Scavenge blower parasitic (−4%) | −1.8 | **42.9%** |

**→ Baseline module: 42.9% fuel-to-electricity.**

### Optional additions

| Addition | Δ | Cumulative | Notes |
|---|---|---|---|
| **HCCI / lean premixed combustion** | **+2.5** | 45.4% | Near-constant-volume burn; peak temp under ~1,800 K so NOx and PM both fall 90–99%. Only viable because CR is controllable per cycle |
| **Software-variable compression ratio** | **+1.0** | 46.4% | No crank fixes TDC. Optimal CR per load and per fuel. No crank engine can do this |
| **Electric turbocompound (MGU-H)** | **+3.5** | **49.9%** | +1.8 from deleting the standalone blower, +1.7 net exhaust recovery. See §5 |

**→ Fully equipped module: ~49.9% fuel-to-electricity.**

> **Honesty note.** 42.9% is defensible from published OP-diesel results. 49.9% is a *target* assembled from individually-demonstrated effects that have never been combined in one machine. Treat the baseline as the design case and the additions as the development programme.

---

## 2. TM-180 — 180 kW variant

**Use case:** primary module for a Ram 2500-class conversion. Two of these = 360 kW, matching a 4-motor 360 kW continuous drivetrain. Also the recommended prototype scale.

### Geometry

| Parameter | Value |
|---|---|
| Bore | **143 mm** |
| Piston travel (each) | ±80.5 mm (161 mm) |
| Gas-column stroke | 322 mm |
| Bore area | 0.01606 m² |
| Swept volume | **5.17 L** |
| Bore/stroke | 0.44 |
| Compression ratio | 20:1 nominal, software-variable 14–24 |
| Clearance volume | 272 cm³ |
| Bowl | 114 mm dia, 22.7 mm deep, 2 mm squish |

### Operating point

| Parameter | Value |
|---|---|
| Cycle frequency, max | **40.5 Hz** |
| Cycle frequency, design point | 33 Hz (peak efficiency, ~82% of max) |
| Mean piston speed | 13.0 m/s |
| Peak piston velocity | 20.5 m/s |
| Peak acceleration | 5,214 m/s² (**531 g**) |
| BMEP | 10 bar |
| Mover mass | 9.4 kg |
| Peak inertial force | 49 kN per mover |

### Power and fuel

| | Baseline (42.9%) | Full spec (49.9%) |
|---|---|---|
| Fuel input | 420 kW | **361 kW** |
| Mechanical work on movers | 195 kW (**262 hp**) | 195 kW |
| **Electrical to DC bus** | **180 kW** | **180 kW** |
| Diesel consumption | 42.3 L/h | **36.3 L/h** |
| Specific consumption | 235 g/kWh | **202 g/kWh** |

### Electrical machine

| Parameter | Value |
|---|---|
| Topology | Tubular PM, dual airgap, quasi-Halbach |
| Pole pitch | 24.3 mm |
| Poles per mover | 12 |
| Magnet stack | 292 mm |
| Stator length | 453 mm |
| Magnet wall | 3.4 mm |
| Airgap | 1.5 mm each side |
| Airgap flux | 1.0 T |
| Electrical frequency | ~270 Hz mean, 422 Hz peak |
| DC bus | 800 V |
| Bus current at rated | 225 A |
| Magnet material | Sm₂Co₁₇ |

### Package

| Parameter | Value |
|---|---|
| Overall length | **1,109 mm** |
| Diameter over housing | 148 mm |
| Mass | **212 kg** |
| Specific power | 0.85 kW/kg |

---

## 3. TM-250 — 250 kW variant

**Use case:** single-module Ram 3500-class conversion, or paired for 500 kW. This is the geometry in `tm250.py` / `TM-250-assembly.step`.

### Geometry

| Parameter | Value |
|---|---|
| Bore | **170 mm** |
| Piston travel (each) | ±96 mm (192 mm) |
| Gas-column stroke | 384 mm |
| Bore area | 0.02270 m² |
| Swept volume | **8.72 L** |
| Bore/stroke | 0.44 |
| Compression ratio | 20:1 nominal, software-variable 14–24 |
| Clearance volume | 459 cm³ |
| Bowl | 136 mm dia, 27 mm deep, 2 mm squish |

### Operating point

| Parameter | Value |
|---|---|
| Cycle frequency, max | **34 Hz** |
| Cycle frequency, design point | 28 Hz |
| Mean piston speed | 13.1 m/s |
| Peak piston velocity | 20.5 m/s |
| Peak acceleration | 4,380 m/s² (**446 g**) |
| BMEP | 10 bar |
| Mover mass | 16 kg |
| Peak inertial force | 70 kN per mover |

### Power and fuel

| | Baseline (42.9%) | Full spec (49.9%) |
|---|---|---|
| Fuel input | 583 kW | **501 kW** |
| Mechanical work on movers | 271 kW (**364 hp**) | 271 kW |
| **Electrical to DC bus** | **250 kW** | **250 kW** |
| Diesel consumption | 58.7 L/h | **50.4 L/h** |
| Specific consumption | 235 g/kWh | **202 g/kWh** |

### Electrical machine

| Parameter | Value |
|---|---|
| Topology | Tubular PM, dual airgap, quasi-Halbach |
| Pole pitch | 29 mm |
| Poles per mover | 12 |
| Magnet stack | 348 mm |
| Stator length | 540 mm |
| Magnet wall | 4 mm |
| Airgap | 1.5 mm each side |
| Airgap flux | 1.0 T |
| Electrical frequency | ~225 Hz mean, 354 Hz peak |
| DC bus | 800 V |
| Bus current at rated | 313 A |
| Magnet material | Sm₂Co₁₇ |

### Package

| Parameter | Value |
|---|---|
| Overall length | **1,322 mm** |
| Diameter over housing | 176 mm |
| Mass | **360 kg** |
| Specific power | 0.69 kW/kg |

---

## 4. Side by side

| | TM-180 | TM-250 |
|---|---|---|
| Bore | 143 mm | 170 mm |
| Travel | ±80.5 mm | ±96 mm |
| Displacement | 5.17 L | 8.72 L |
| Frequency (max) | 40.5 Hz | 34 Hz |
| Peak acceleration | 531 g | 446 g |
| Electrical output | 180 kW | 250 kW |
| Mechanical equivalent | 262 hp | 364 hp |
| Fuel at full output | 36.3 L/h | 50.4 L/h |
| Length | 1,109 mm | 1,322 mm |
| Mass | 212 kg | 360 kg |
| **kg per kW** | **1.18** | **1.44** |
| Cycles to 20,000 h | 2.9 billion | 2.4 billion |

**The trade:** TM-180 is lighter per kW (mass ∝ k³, power ∝ k²) and packages more easily. TM-250 accumulates fatigue cycles more slowly, has better surface-to-volume so slightly better real-world efficiency, and needs half the control channels per kW.

**Recommendation: 2 × TM-180.** Lighter for the same total power, redundant, fits a Ram 2500 rail gap side by side, and 143 mm bore is a more forgiving prototype scale.

---

## 5. Addition detail

### HCCI / lean premixed combustion — +2.5 pts

Premixed charge, whole-charge autoignition, no flame front. Near-constant-volume combustion approaches the ideal Otto cycle. Lean operation keeps peak temperature below ~1,800 K, so NOx never forms and premixing means no soot — **both fall 90–99% simultaneously**, which conventional engines cannot do.

Conventional engines can't run HCCI reliably because ignition timing is only indirectly controllable. **This machine can: compression ratio is adjustable per cycle via alternator braking force.** Toyota's free-piston prototype reached 42% specifically using premixed compression ignition.

### Software-variable compression ratio — +1.0 pt

Nothing mechanically fixes TDC. The controller sets how far the movers travel.

- Optimal CR per load: higher CR at light load for more expansion
- Optimal CR per fuel: diesel 20:1, methanol 16–17:1, LPG with spark assist, ammonia, hydrogen — **one hardware set, all fuels**
- Enables the HCCI timing control above

This is the single capability no crankshaft engine has, and the primary commercial argument for the architecture (cf. Mainspring Energy).

### Electric turbocompound / MGU-H — +3.5 pts

Bidirectional electric machine on the turbo shaft. **No mechanical supercharger is possible — there's no crankshaft to belt off**, so boost is exhaust-driven or electric only.

| Mode | Condition | Behaviour |
|---|---|---|
| Motoring | Startup, low load | Drives compressor electrically. Zero lag. **Replaces the standalone blower** |
| Generating | High load | Surplus exhaust energy → DC bus |

**Control law: extract only the surplus after driving the compressor and maintaining scavenge pressure differential.** Uniflow scavenging requires intake pressure > exhaust pressure. Over-extract and the cylinder won't clear.

**Why it fits here:** every other application must invent power electronics and a DC bus to use an MGU-H. This design has both already.

**Costs:** 100,000–200,000 rpm shaft needing foil or air bearings; exhaust at 400–600 °C so use induction or switched-reluctance, **not** permanent magnets; and boost alters gas spring stiffness, which shifts resonant frequency — a coupling unique to free pistons.

**Phase 2.** A second hard control problem on top of the primary one. Base module must run first.

---

## 6. What is NOT in these numbers

Further levers, deliberately excluded from the 49.9% figure:

| Lever | Potential | Why excluded |
|---|---|---|
| **ORC bottoming cycle** | +5 to +10 pts | Needs a condenser and significant volume. Ideal for stationary/CHP, marginal on a truck |
| **Lower piston speed** (8 m/s, bigger bore) | +2 to +3 pts | Trades directly against mass and package |
| **Ceramic thermal barrier coatings** | +1 to +2 pts | Unproven durability at 2.4 billion cycles |
| **Larger scale** | +2 to +4 pts | Surface/volume improves as 1/k. Locomotive scale reaches 48–50% with no other change |

Combined ceiling is roughly 55–58% — the same territory as large marine two-strokes, reached by different means.

---

## 7. Benchmarks

| Machine | Fuel-to-electricity |
|---|---|
| Small portable genset (5–30 kW) | 25–32% |
| Industrial genset (100–500 kW) | 35–40% |
| Li Auto 1.5T range extender | 40.5% (**measured, in 500,000+ vehicles**) |
| **TM-180 / TM-250 baseline** | **42.9%** (designed) |
| Large genset (1–3 MW) | 40–43% |
| **TM-180 / TM-250 full spec** | **49.9%** (target) |
| Wärtsilä 31 | ~50% (measured) |
| Mercedes F1 power unit | ~52% (measured) |
| Marine two-stroke | 51–54% (measured) |

**Positioning:** the baseline beats every production genset in its size class. The full spec would match multi-megawatt marine plant at 1/1000th the displacement — which is exactly why it should be treated as a target, not a specification.