Chapter 47 · Emissions Systems, Diagnostics & Deletes

EGR & Backpressure

Internal EGR thermodynamics, the BGMS backpressure model, and big-turbo retuning.

Calibration symbols

NWST

ECUs covered

06A906032LP

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1 symbol · 1 diagram · ~854 words

Internal EGR thermodynamics, the BGMS backpressure model, and big-turbo retuning.

In modern internal combustion engines, Exhaust Gas Recirculation (EGR / Abgasrückführung - AGR) is employed to suppress peak combustion temperatures and minimize nitrogen oxide (\text{NO}_x) 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 (p_3) 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 (x_r)#

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:

$x_r =\frac{m_{\text{residual}}}{m_{\text{total}}} =\frac{m_{\text{residual}}}{m_{\text{fresh}} + m_{\text{residual}}}$

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\text{°} crank angle, intake valve opening shifts from 14\text{° ATDC} to 8\text{° BTDC}. This expands the valve overlap window from 4\text{°} to 26\text{° KW}, dramatically increasing residual gas retention during part-load.
Pressure Ratio (\Pi_e = p_3 / p_2): If the pre-turbine exhaust backpressure (p_3) exceeds the intake manifold pressure (p_2), exhaust gas expands backwards into the cylinder and intake runner during the overlap window.
Engine Speed (N_{\text{mot}}): At low RPM (1500\text{ rpm}), the physical duration of the overlap window in milliseconds is long (t_{\text{overlap}}\approx 2.9\text{ ms}), maximizing internal dilution. At high RPM (6500\text{ rpm}), the window contracts (t_{\text{overlap}}\approx 0.66\text{ 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 p_3:

$p_3 = p_{\text{ambient}} + \Delta p_{\text{cat}} + \Delta p_{\text{turbine}}(m_{\text{exhaust}}, T_3, \text{wg\_pos})$

Factory K03 vs. High-Performance Turbine Backpressure Comparison#

On the factory BorgWarner K03/K03s turbocharger equipped with an extremely restrictive turbine housing (A/R \approx 0.35, exducer diameter 38\text{ mm}), exhaust backpressure skyrockets at elevated engine speeds:

Engine Speed (RPM)	Boost Pressure p_2	Turbine Backpressure p_3 (K03s)	Pressure Ratio p_3 / p_2	Scavenging State	Residual Gas x_r
2000 RPM	1450\text{ hPa} (0.45\text{ bar})	1500\text{ hPa} (0.50\text{ bar})	1.03	Neutral Scavenging	12.5\%
3500 RPM	2050\text{ hPa} (1.05\text{ bar})	2350\text{ hPa} (1.35\text{ bar})	1.15	Mild Backpressure	8.2\%
5500 RPM	1800\text{ hPa} (0.80\text{ bar})	2850\text{ hPa} (1.85\text{ bar})	1.58	Severe Restriction	11.0\%
6500 RPM	1650\text{ hPa} (0.65\text{ bar})	3400\text{ hPa} (2.40\text{ bar})	2.06	Extreme Reversion	16.8\%

When p_3 / p_2 exceeds 1.50, the high backpressure halts scavenging. Hot exhaust gas trapped in the combustion chamber elevates mixture temperatures at the start of compression (T_{\text{IVC}}), leading directly to catastrophic high-RPM knock and exhaust gas temperature spikes (T_3 > 980\text{°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 \times 8	RPM	Engine Load	Factor (0.0\dots 1.5)	Relative cylinder filling correction factor for internal residual gas
`FAGR`	`0x016142`	1 \times 1	-	-	Factor	Master residual gas scaling multiplier
`VNWS`	`0x014B10`	1 \times 1	-	-	RPM	Engine speed threshold for VVT overlap deactivation
`NNWSE`	`0x014B14`	1 \times 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 \ge 0.63):

Backpressure Collapse: Pre-turbine pressure drops from p_3 / p_2\approx 2.1 down to p_3 / p_2 \le 1.15 at 6500\text{ rpm}.
Volumetric Efficiency Increase: Without exhaust gas reversion, high-RPM cylinder filling increases by 15\dots 25\%.
KFPBRK Normalization: Flatten the high-RPM cells in KFPBRK toward 1.00. In factory files, KFPBRK applies aggressive negative filling penalties above 5500\text{ 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\text{ rpm} instead of the factory 3600\text{ rpm} cutoff VNWS), generating dramatic spool and midrange torque improvements without reversion risk.