EGR & Backpressure
Internal EGR thermodynamics, the BGMS backpressure model, and big-turbo retuning.
In this page
- 47.1. The Thermodynamics of Internal Residual Gas Fraction (x_r)
- 47.2. Bosch Exhaust Backpressure Simulation Model (BGMS)
- Factory K03 vs. High-Performance Turbine Backpressure Comparison
- 47.3. Core Calibration Maps & Relocation Table
- 47.4. Big-Turbo Upgrade Recalibration Guidelines (K04-001, G25, EFR)
- Related chapters
In modern internal combustion engines, Exhaust Gas Recirculation (EGR / Abgasrückführung - AGR) is employed to suppress peak combustion temperatures and minimize nitrogen oxide (NOx) formation. While naturally aspirated gasoline and diesel engines employ external EGR pipes, coolers, and electric EGR valves (N18), the transverse VAG 1.8T 20V platform features no external EGR plumbing whatsoever.
Instead, Bosch ME7.5 accomplishes emissions-compliant EGR entirely through **Internal Residual Gas Trapping (Innere Restgasmenge)** utilizing dynamic valve overlap controlled by Variable Valve Timing (NWST). In parallel, ME7.5 continuously calculates an internal mathematical simulation of pre-turbine exhaust backpressure (p3) to model the physical scavenging efficiency of each cylinder charge cycle.
┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│ INTERNAL EGR & SCAVENGING PRESSURE GRADIENT PHYSICS │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ │
│ Intake Manifold (Boost p2) Exhaust Manifold (Pre-Turbine p3) │
│ ┌───────────────────────────┐ ┌───────────────────────────┐ │
│ │ Pressure: p2 │ │ Pressure: p3 │ │
│ │ Fresh Air Charge (rl) │ │ Exhaust Backpressure │ │
│ └─────────────┬─────────────┘ └─────────────┬─────────────┘ │
│ │ Intake Valve Open │ Exhaust Valve Open │
│ ▼ ▼ │
│ ══════════════════════════════════════════════════════════════════ │
│ ║ CYLINDER COMBUSTION CHAMBER ║ │
│ ║ ║ │
│ ║ [ VALVE OVERLAP WINDOW: Intake Opens BEFORE Exhaust Closes ] ║ │
│ ║ ║ │
│ ║ Case 1: Boost > Backpressure (p2 > p3) ║ │
│ ║ → Positive Pressure Gradient: Fresh charge scavenges cylinder║ │
│ ║ → Zero residual gas, high volumetric efficiency (High VE) ║ │
│ ║ ║ │
│ ║ Case 2: Backpressure > Boost (p3 > p2) ║ │
│ ║ → Negative Pressure Gradient: Hot exhaust gas flows BACK ║ │
│ ║ into intake runner and combustion chamber ║ │
│ ║ → High Internal Residual Gas Fraction (xr > 18%) ║ │
│ ║ → Cylinder dilution, elevated knock risk, VE penalty ║ │
│ ══════════════════════════════════════════════════════════════════ │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘47.1. The Thermodynamics of Internal Residual Gas Fraction (xr)
In the Bosch Motronic calculation pipeline, the actual fresh air trapped inside the cylinder (relative cylinder filling - rl) does not equal the gross air drawn past the intake valve. A portion of the combustion chamber volume is always occupied by hot residual combustion gases from the prior stroke:
The residual gas fraction is governed by three primary physical variables:
- Valve Overlap Geometry: Defined by the mechanical camshaft lobe separation angle (LSA) and the dynamic VVT phase state (
B_nws). When VVT advances the intake camshaft by +22° crank angle, intake valve opening shifts from 14° ATDC to 8° BTDC. This expands the valve overlap window from 4° to 26° KW, dramatically increasing residual gas retention during part-load. - Pressure Ratio (Πe = p3 / p2): If the pre-turbine exhaust backpressure (p3) exceeds the intake manifold pressure (p2), exhaust gas expands backwards into the cylinder and intake runner during the overlap window.
- Engine Speed (Nmot): At low RPM (1500 rpm), the physical duration of the overlap window in milliseconds is long (toverlap≈ 2.9 ms), maximizing internal dilution. At high RPM (6500 rpm), the window contracts (toverlap≈ 0.66 ms), minimizing diffusion time.
47.2. Bosch Exhaust Backpressure Simulation Model (BGMS)
Because ME7.5 lacks a physical exhaust manifold pressure sensor, the firmware executes a real-time thermodynamic flow simulation to estimate pre-turbine exhaust pressure p3:
p3 = pambient + Δ pcat + Δ pturbine(mexhaust, T3, wg_pos)Factory K03 vs. High-Performance Turbine Backpressure Comparison
On the factory BorgWarner K03/K03s turbocharger equipped with an extremely restrictive turbine housing (A/R ≈ 0.35, exducer diameter 38 mm), exhaust backpressure skyrockets at elevated engine speeds:
| Engine Speed (RPM) | Boost Pressure p2 | Turbine Backpressure p3 (K03s) | Pressure Ratio p3 / p2 | Scavenging State | Residual Gas xr |
|---|---|---|---|---|---|
| 2000 RPM | 1450 hPa (0.45 bar) | 1500 hPa (0.50 bar) | 1.03 | Neutral Scavenging | 12.5% |
| 3500 RPM | 2050 hPa (1.05 bar) | 2350 hPa (1.35 bar) | 1.15 | Mild Backpressure | 8.2% |
| 5500 RPM | 1800 hPa (0.80 bar) | 2850 hPa (1.85 bar) | 1.58 | Severe Restriction | 11.0% |
| 6500 RPM | 1650 hPa (0.65 bar) | 3400 hPa (2.40 bar) | 2.06 | Extreme Reversion | 16.8% |
When p3 / p2 exceeds 1.50, the high backpressure halts scavenging. Hot exhaust gas trapped in the combustion chamber elevates mixture temperatures at the start of compression (TIVC), leading directly to catastrophic high-RPM knock and exhaust gas temperature spikes (T3 > 980°C).
47.3. Core Calibration Maps & Relocation Table
The residual gas calculation and filling correction maps in 06A906032LP include:
| Map / Symbol | Flash Offset (032LP) | Dimensions | X-Axis | Y-Axis | Physical Units | Function in Firmware |
|---|---|---|---|---|---|---|
KFPBRK | 0x0162E8 | 12 × 8 | RPM | Engine Load | Factor (0.0… 1.5) | Relative cylinder filling correction factor for internal residual gas |
FAGR | 0x016142 | 1 × 1 | - | - | Factor | Master residual gas scaling multiplier |
VNWS | 0x014B10 | 1 × 1 | - | - | RPM | Engine speed threshold for VVT overlap deactivation |
NNWSE | 0x014B14 | 1 × 1 | - | - | RPM | Engine speed cutoff for VVT engagement |
47.4. Big-Turbo Upgrade Recalibration Guidelines (K04-001, G25, EFR)
When upgrading from the restrictive stock K03 to an aftermarket turbocharger equipped with a high-flow turbine housing (A/R ≥ 0.63):
- Backpressure Collapse: Pre-turbine pressure drops from p3 / p2≈ 2.1 down to p3 / p2 ≤ 1.15 at 6500 rpm.
- Volumetric Efficiency Increase: Without exhaust gas reversion, high-RPM cylinder filling increases by 15… 25%.
KFPBRKNormalization: Flatten the high-RPM cells inKFPBRKtoward 1.00. In factory files,KFPBRKapplies aggressive negative filling penalties above 5500 rpm to account for K03 choking; retaining these stock penalties on a big turbo causes the ECU to miscalculate actual in-cylinder air mass, resulting in lean fueling errors and incorrect calculated torque.- VVT De-Overlap Optimization: With lower turbine backpressure, the intake camshaft advance can be safely extended higher into the rev range (up to 4500 rpm instead of the factory 3600 rpm cutoff
VNWS), generating dramatic spool and midrange torque improvements without reversion risk.