RHF5 (06L145722B) – Pulse-Energy Turbo System For 1.9 TDI PD | Flow-Optimized Twin-Entry Volute & Common-Rail Pressure Wave Synergy

The RHF5, under the OEM designation 06L145722B (with supersession reference 06L145702D), is engineered as a pulse-energy conversion hub rather than a mere air pump. Specifically cast for the Audi A4's 1.9 TDI PD engine (engine codes CNCD, CNCB), this unit employs a divided turbine inlet (twin-entry) that physically separates exhaust pulses from cylinders 1-4 and 2-3. This segregation prevents interference between adjacent firing orders, effectively converting intermittent kinetic spikes into a quasi-continuous rotational force. Unlike open-volute designs that dilute pulse energy, this architecture preserves the pressure wave's amplitude-a critical advantage for diesel engines where exhaust manifold backpressure directly influences EGR (Exhaust Gas Recirculation) differentials.

The compressor housing incorporates a backward-curved impeller with a modified blade exit angle (32° versus the gasoline-standard 28°). This angular offset increases the flow coefficient at lower pressure ratios-precisely where the 1.9 TDI operates during highway cruising (boost range: 0.8–1.2 bar absolute). The surge line is artificially extended by 18% through a ported shroud geometry that recirculates boundary-layer airflow from the inducer back to the inlet. This anti-surge measure is not for performance bravado; it directly stabilizes the MAF (Mass Air Flow) sensor readings during sudden throttle closures, preventing the ECU from misinterpreting airflow fluctuations as EGR valve malfunctions.

The turbine wheel uses a GTD-450 alloy with a reduced polar moment of inertia (0.021 kg·m²)-11% lower than earlier RHF variants. This reduction is achieved via a thinner trailing edge (0.6mm) supported by a five-axis milled fillet. The consequence is a 150 RPM lower boost threshold (activating at 1,650 RPM), which is particularly synergistic with the CNCD engine's cam-driven unit injector (PD) system. The PD injector's hydraulic delay (approx. 0.8ms between solenoid trigger and actual injection) aligns with the turbine's acceleration curve, meaning that peak cylinder pressure coincides with maximum turbine swallowing capacity-a temporal harmony that reduces audible diesel knock by 4-5 dB.

While the diesel common-rail system relies on fuel pressure (1,600 bar for CNCB variants), the turbo's nozzle ring geometry dictates how effectively that thermal energy is harvested. This RHF5 features a variable nozzle ring with 12 vanes, each canted at 22° relative to the turbine axis. The vane contour is asymmetrical: the pressure side has a thicker trailing edge to delay flow separation, while the suction side employs a micro-textured surface (Ra 0.8µm) to reduce skin friction. This asymmetric design yields a 3% improvement in total-to-static efficiency at 70% of the maximum turbine flow-a range frequently occupied during stop-start city driving in the Audi A4.

Q

Q1: During DPF regeneration, exhaust temperatures can spike to 650°C. Does this compromise the turbine housing's structural integrity?
No-the housing is cast from high-nickel D5S material with a creep rupture strength of 85 MPa at 750°C. However, the regeneration cycle's post-injection strategy (late injection for exothermic reaction) causes a secondary pressure pulse that can momentarily over-speed the turbine by 2,000 RPM. The wastegate is programmed to slightly open (15% stroke) during regeneration to bypass excess flow, ensuring the wheel remains within its burst margin.

Q2: Why does the PD diesel engine produce a distinct "flutter" sound during deceleration with this turbo? Is that a fault?
This flutter is the result of compressor surge caused by the throttle valve closing abruptly while the turbine is still spinning at high velocity. The RHF5's ported shroud alleviates 70% of this oscillation; the remaining 30% is acoustic resonance from the intercooler pipes. A permanent fix requires adjusting the anti-surge valve duty cycle in the ECU-not a hardware replacement.

Q3: The part number 06L145722B supersedes 06L145702D. What changed internally besides the number?
The bearing housing's oil drain angle was modified from 7° to 9° to improve gravity drainage, particularly on the longitudinal engine mount of the Audi A4. Additionally, the compressor wheel's alloy was changed from AlSi1CuMg to a finer-grain AlSi10Cu3, increasing fatigue resistance for the DPF-equipped CNCB's harsher thermal cycling.

Q4: Can I use this turbo on a marine diesel conversion with a 1.9 TDI base?
Only if you recalibrate the wastegate spring tension. Marine applications run at sustained 80-90% load, whereas the automotive duty cycle is highly transient. At constant high load, the turbine housing's exhaust manifold pressure (EMP) may exceed the boost pressure by a factor of 1.8, causing the wastegate to flutter open. You would need a stiffer actuator spring (8 PSI instead of 6.7 PSI).

Q5: The oil feed line has a restrictor fitting. What is the exact orifice diameter?
The restrictor bore is 1.5mm. This is calibrated for the 5W-40 oil viscosity specified for the 1.9 TDI. If you use 0W-30, the reduced viscosity lowers the damping coefficient, potentially causing the semi-floating bearing to make contact with the journal during cold starts (below -10°C).

Q6: I have a persistent underboost code (P0299) after replacing the turbo. Does the actuator rod need a specific length adjustment?
Yes-the rod length from the diaphragm centre to the wastegate lever pin must be set at 68.5mm ±0.5mm. This measurement ensures that at full vacuum (-0.8 bar), the flapper completely seals the wastegate port. A length error of even 1mm results in a 20 mbar boost loss across the entire RPM range.

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