Wall Wetting & Transients
Phase-equilibrium wall-film physics and retuning transient fueling.
In this page
- 42.1. The Physics of Phase Equilibrium & Manifold Pressure Transitions
- 42.2. The Bosch ME7.5 Dynamic Wall Film Model
- 42.3. Recalibrating Transient Fueling for Large Injectors & Short-Runner Manifolds
- 1. The Large Injector Scaling Pitfall
- 2. Short-Runner Sheetmetal Intake Manifolds
- Related chapters
In steady-state dyno testing, tuning an engine to target air-fuel ratios is relatively straightforward. However, during real-world driving, rapid throttle transitions present a severe fuel metering challenge. When a driver stomps on the accelerator pedal, the engine does not hesitate because of mechanical lag—it hesitates because of **intake port wall wetting dynamics (Wandfilmdynamik)**.
In port-injected engines like the VAG 1.8T 20V, a substantial portion of the fuel sprayed by the fuel injector does not enter the combustion chamber as airborne vapor. Instead, it collides with the cold runner walls, port floor, and intake valve tulips, forming a static liquid puddle (Wandfilm). In Bosch Motronic ME7.5, transient air-fuel ratio stability is governed by a physical mathematical simulation of this fuel puddle.
┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│ BOSCH ME7.5 INTAKE RUNNER WALL WETTING & TRANSIENT FUELING │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ [ Fuel Injector Spray ] │
│ │ │
│ ┌───────────────────────┴───────────────────────┐ │
│ ▼ Direct Vapor Fraction (1 - Alpha) ▼ Wall Deposition (Alpha) │
│ Enters cylinder directly as airborne mist Condenses on runner walls and valves │
│ │ forming liquid fuel film (m_film) │
│ │ │ │
│ │ ▼ Evaporation Rate (Beta) │
│ │ Boils off into airstream based on │
│ │ manifold vacuum & runner temperature │
│ │ │ │
│ └───────────────────────┬───────────────────────┘ │
│ ▼ │
│ [ Total Combustible Cylinder Charge ] │
│ │ │
│ ┌──────────────────────────────────────┴──────────────────────────────────────┐ │
│ ▼ Sudden Throttle Tip-In (Pressure Rises) ▼ Sudden Throttle Lift-Off (Vacuum Rises) │
│ • Evaporation rate plummets • Fuel film instantly flash-boils into gas │
│ • Extra fuel condenses on walls • Cylinder receives excess unmetered fuel │
│ • Threat: SEVERE LEAN STUMBLE (λ > 1.30) • Threat: RICH OVERRUN SPIKE (λ < 0.70) │
│ • ECU Action: Commands Transient Enrichment • ECU Action: Cuts Injection (Enleanment) │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘42.1. The Physics of Phase Equilibrium & Manifold Pressure Transitions
- The Tip-In Acceleration Lean Stumble:
- When lifting off throttle, manifold pressure drops instantly from boost back into deep vacuum.
- The sudden pressure drop lowers the boiling point of the gasoline, causing the entire liquid wall puddle to flash-boil into vapor.
- The cylinder ingests this unmetered fuel vapor, resulting in a rich spike (λ < 0.70), black smoke, and catalyst overheating.
- The Tip-Out Deceleration Rich Bog:
42.2. The Bosch ME7.5 Dynamic Wall Film Model
To eliminate transient lean and rich spikes, the C167 operating system continuously calculates the net change in fuel film mass (dmfilmdt) on every intake stroke:
mfuel_injected = mfuel_target + α · mfuel_target - β · mfilmWhere:
- α (Wetting Factor / Transition Deposition Fraction): The fraction of injected fuel that drops out of the air stream and deposits onto the intake port walls. Plotted as a function of engine coolant temperature (
tmot) and intake manifold absolute pressure (pvdks_w). - β (Evaporation Rate / Film Decay Time Constant): The rate at which the existing fuel film evaporates into the cylinder per engine revolution. Highly temperature-dependent (hot cylinder heads evaporate fuel film four times faster than a cold engine).
42.3. Recalibrating Transient Fueling for Large Injectors & Short-Runner Manifolds
1. The Large Injector Scaling Pitfall
When upgrading to large high-flow injectors (e.g. Bosch EV14 550cc or 1000cc), calibrating only KRKTE scales the baseline fueling correctly. However, in factory software, certain transient acceleration enrichment multipliers do not scale purely with KRKTE.
- Symptom: Stomping the throttle causes an immediate rich bog (λ = 0.68), followed by sluggish spool-up until the engine clears the excess fuel.
- The Calibration Fix: Reduce the acceleration enrichment transition multipliers by 20% to 35% to reflect the immense mass of fuel delivered per millisecond by large injector nozzles.
2. Short-Runner Sheetmetal Intake Manifolds
Factory 1.8T intake manifolds feature long, curved cast-aluminum runners (L ≈ 280 mm) with substantial surface area that stores a large liquid fuel film.
- Aftermarket performance manifolds (e.g. SEM Motorsports, Integrated Engineering, or custom sheetmetal plenums) feature short, straight, high-velocity runners (L ≈ 130 mm) with polished internal finishes.
- Result: The surface area available to store a liquid fuel puddle is reduced by over 40%.
- The Calibration Fix: Reduce the wall wetting factor (α) and increase the evaporation rate parameter (β). This prevents the ECU from over-enriching on tip-in, delivering instantaneous, crisp throttle response under rapid pedal modulation.