Chapter 51 · Boost Control & Thermal Management

EGT Model & Component Protection

The differential EGT observer, Bauteileschutz, and why dyno tuning breaks it.

Calibration symbols

TABGBTS TABG_W DLBTS LAMFA G235 ATR BTS

ECUs covered

06A906032LP

Size

7 symbols · 1 diagram · ~998 words

The differential EGT observer, Bauteileschutz, and why dyno tuning breaks it.

On high-specific-output turbocharged engines like the VAG 1.8T 20V, exhaust gas temperature (EGT) is the single most critical boundary constraint governing engine longevity. Sustained temperatures exceeding 950\text{°C} cause rapid plastic deformation of austenitic turbine housings, blade creep on Inconel 713C turbine wheels, and thermal melting of catalytic converters.

While high-output transverse platforms (such as the Audi TT 225 PS and Audi S3 equipped with engine code BAM) utilize an active physical K-type thermocouple probe (G235) threaded directly into the exhaust housing of the BorgWarner K04-023 turbocharger, standard transverse 1.8T applications (AWP, AWW, AUQ) feature no physical EGT probe. Instead, Bosch ME7.5 executes a real-time thermodynamic simulation known as the **Exhaust Gas Temperature Model (Abgastemperaturmodell - ATR). When modeled temperature exceeds physical limits, the ECU initiates Component Thermal Protection (Bauteileschutz - BTS)**, dumping massive volumes of excess fuel to quench the exhaust stream.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   BOSCH ME7.5 EXHAUST GAS TEMPERATURE (ATR) MODEL PIPELINE                      │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Engine Speed: N_mot ] ───┐                                                                   │
│   [ Relative Load: rl ]  ────┴─► [ Steady-State Base Map: KFNBST ] ──► T_base (750°C - 980°C)    │
│                                                                           │                      │
│   [ Spark Retard: ZW_opt - ZW_act ] ─────────────────────────► [ dZw ] ──┼──► (+) Summation     │
│   (Every 1° of retard adds ~10°C - 15°C of exhaust heat)                  │                      │
│                                                                           │                      │
│   [ Mixture Air/Fuel Ratio: Lambda ] ────────────────────────► [ dLam ] ─┼──► (-) Evap Cooling  │
│   (Enrichment λ < 0.85 extracts heat via vaporization)                    │                      │
│                                                                           │                      │
│   [ Camshaft Overlap / VVT State: B_nws ] ───────────────────► [ dNWS ] ─┘                      │
│                                                                           │                      │
│                                                                           ▼                      │
│                                         ┌───────────────────────────────────────────────────┐    │
│                                         │ Low-Pass Thermal Inertia Filter (PT1 Delay)       │    │
│                                         │ (Models cast iron exhaust manifold thermal mass)  │    │
│                                         └─────────────────────────┬─────────────────────────┘    │
│                                                                   │                              │
│                                                                   ▼ Calculated EGT: TABG         │
│   ┌─────────────────────────────────────────────────────────────────────────────────────────┐    │
│   │ Component Thermal Protection Threshold Comparison: IF (TABG > TABGBTS)                 │    │
│   │ → Activate Bauteileschutz: B_bts = 1                                                    │    │
│   │ → Override Driver Demand Lambda (LAMFA) with Rich Quench Fueling: KFFDLBTS (λ ≈ 0.72)   │    │
│   └─────────────────────────────────────────────────────────────────────────────────────────┘    │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

51.1. Mathematical Physics of the Differential EGT Observer#

The instantaneous temperature of gas exiting the exhaust ports into the manifold runner (T_{\text{exhaust}}) is calculated from four coupled thermodynamic components:

$T_{\text{target}} = T_{\text{basic}}(N_{\text{mot}}, rl) + \Delta T_{\text{zw}} + \Delta T_{\lambda} + \Delta T_{\text{vvt}}$

Base Thermodynamic Combustion State (T_{\text{basic}}): Derived from the 2D surface KFNBST indexed by engine RPM and relative engine load. This represents the steady-state thermal equilibrium under stoichiometric combustion (\lambda = 1.000) at optimal mean best torque (MBT) spark timing.
Ignition Retard Thermal Shift (\Delta T_{\text{zw}}): When ignition timing is retarded away from MBT (due to knock control retard dwk_w or torque intervention), the combustion event is delayed deeper into the power stroke. Peak pressure occurs later, and burning gases expand directly through the opening exhaust valves into the manifold runners: $\Delta T_{\text{zw}} = k_{\text{zw}}(N_{\text{mot}}, rl) \cdot (ZW_{\text{opt}} - ZW_{\text{act}})$ In practice, an aggressive timing pull of -8\text{° KW} increases pre-turbine exhaust temperature by 80\text{°C}\dots 120\text{°C} in fewer than 300\text{ milliseconds}!
Lambda Vaporization Cooling (\Delta T_{\lambda}): Excess liquid fuel injected during enriched operation (\lambda < 0.85) does not undergo complete stoichiometric combustion. Instead, unburned hydrocarbon droplets vaporize inside the cylinder and exhaust runners, extracting latent heat of vaporization (h_{\text{vap}} \approx 350\text{ kJ/kg} for pump gasoline) from the gas mass: $\Delta T_{\lambda} = k_{\lambda}(N_{\text{mot}}, rl) \cdot (1.000 - \lambda)$
Thermal Inertia Low-Pass Filter (PT1): The cast-iron 06A exhaust manifold and ductile iron turbine housing possess substantial physical heat capacity. The temperature displayed to the ECU control loops (tabg_w) lags the instantaneous combustion gas temperature via a first-order differential time constant: $\frac{dT_{\text{exhaust}}}{dt} = \frac{T_{\text{target}} - T_{\text{exhaust}}}{\tau_{\text{manifold}}}$ Where \tau_{\text{manifold}} is dynamically computed from exhaust mass flow rate (msabg_w). Under high mass flow (WOT), \tau \approx 1.5\text{ s}; under light cruise, \tau \approx 8.0\text{ s}.

51.2. The Component Protection Subsystem (`BTS` / Bauteileschutz)#

When the modeled exhaust temperature tabg_w (or measured thermocouple temperature tg_w on BAM codebases) exceeds the calibration threshold TABGBTS (factory default: 920\text{°C}\dots 950\text{°C}):

The internal status flag B_bts transitions to 1.
The closed-loop fuel controller immediately drops out of driver-demand lambda mapping (LAMFA) and calculates the Target Protection Lambda (LAMBTS): $\lambda_{\text{target}} = \text{MIN}\left[ \text{LAMFA} ,\; 1.00 - \Delta\lambda_{\text{bts}}(N_{\text{mot}}, T_{\text{exhaust}}) \right]$
The ECU rapidly enriches the fuel mixture from \lambda = 0.85 down to \lambda = 0.72\dots 0.75 (10.5:1\dots 11.0:1 AFR).
This deluge of latent fuel vaporization halts further thermal rise and stabilizes pre-turbine temperatures below the catastrophic failure boundary of 980\text{°C}.

51.3. Core Calibration Maps & Relocation Table in `06A906032LP`#

Map / Symbol	Flash Offset (032LP)	Dimensions	Axes	Units	Description
`TABGBTS`	`0x0113A6`	1 \times 1	-	\text{°C}	Abgastemperaturschwelle für Bauteileschutz (EGT threshold to activate component protection, default 920\text{°C})
`KFNBST`	`0x0163C8`	12 \times 12	RPM \times Load	\text{°C}	Kennfeld der Grundabgastemperatur (Base steady-state modeled exhaust gas temperature)
`KFFDLBTS`	`0x0187B0`	8 \times 8	RPM \times Temp	Factor	Delta-Lambda für Bauteileschutz (Enrichment delta applied when BTS is active)
`FBSTABG`	`0x01878E`	6 \times 1	Temp	Factor	Multiplier curve scaling BTS enrichment based on calculated EGT
`DLBTS`	`0x0187A0`	1 \times 1	-	Factor	Absolute maximum allowable lambda enrichment clamp for Bauteileschutz (0.70)

51.4. Tuner Pitfalls: Disabling BTS for Dyno Numbers#

One of the most dangerous amateur calibration practices on the 1.8T platform is intentionally neutralizing Bauteileschutz to prevent the vehicle from "dumping fuel" and leaning out top-end AFR for inflated dynamometer numbers:

The Shortcut: Raising TABGBTS to 1100\text{°C} or zeroing KFFDLBTS forces the engine to run at \lambda = 0.85 (12.5:1 AFR) across prolonged 4th-gear dyno or highway pulls.
The Physical Consequence: In fewer than 15 seconds of sustained boost on a stock K03 or K04-001 with high backpressure (p_3 / p_2 > 2.0), actual exhaust gas temperatures exceed 1050\text{°C}.
The Failure Mode: Austenitic turbine housings crack across the wastegate seat, exhaust turbine blade tips erode from supersonic gas scouring, and exhaust valve heads stretch and burn, leading to catastrophic cylinder compression loss.
Proper Calibration Standard: Leave TABGBTS at 920\text{°C}\dots 940\text{°C} and calibrate KFFDLBTS smoothly so that BTS provides progressive, controlled thermal management without sharp, jerky rich stumble transitions.