Turbocharger A/R Number Explained

Key Concepts: A/R Number

• A/R Number: Area over ratio; size of the compressor and turbine housing.
  • Larger A/R number = larger housings = bigger turbo.
  • Found printed on compressor or turbine housing, typically as 0.x or 1.x.
• Measurements:
  • Radius measured, and area calculated as π * radius².
  • Cross-section radius measured from center of turbo axis to compressor wheel.*

Performance Implications

• Smaller A/R Number:
  • Increases gas velocity, better turbo spool up at low RPM.
  • Can choke engine at high RPM due to limited gas volume pass-through.
• Larger A/R Number:
  • Greater volume of gas at high RPM, but reduced gas velocity.
  • Hence, it's common in racing applications.

Boost Terms and Concepts

• Boost Threshold:
  • RPM at which turbo starts producing significant boost.
  • Smaller A/R housing reduces boost threshold.
• Boost Lag:
  • Delay in torque after throttle opened.
  • Smaller A/R housing doesn’t significantly affect boost lag.
  • Smaller/lighter turbine and compressor wheels help reduce boost lag.

Bearings in Turbochargers

• Journal Bearings:
  • Longer-lasting but need a constant supply of clean oil.
• Ball Bearings:
  • Slightly reduced lifespan, more expensive.
  • Reduce both boost lag and threshold, require less oil.

CHRA (Center Housing Rotating Assembly)

• Core of the turbocharger minus the compressor and turbine housings.
• Replacing CHRA is fast and cost-effective means to rebuild a turbo.

Compressor Wheel

• Inducer: Part that bites air.
• Exducer: Part that spits air out.
• Increasing Inducer size:
  • Increases max air flow and peak power but slows spool-up.
• Increasing Exducer size:
  • Increases gas speeds and boosts sooner.
  • Supports higher boost pressures.
• Trim: Inducer diameter ÷ Exducer diameter, squared and multiplied by 100.
  • Higher trim = increased max air flow capacity.

Blade Design

• Splitter blades improve efficiency at high RPM, increase peak boost pressure.
• More main blades (without splitter) speed up spool but reduce peak power.
• Modern designs often have extended tips for better spool-up without adding mass.
• Super back design: Improves strength without much weight increase.
• Modern wheels: Often machined from a solid billet for lighter, more efficient designs.

Turbine Wheel

• Advanced materials (super alloys) improve performance and temperature resistance.
• Diesel engines: Generally lower exhaust temps compared to gasoline engines.

Turbo Connections

• Flanges (T2, T3, T4): Different sizes for various turbos.
• V-band Connections: Popular for ease of use and compact design.

Pressure Ratio

• Shows the amount of boost a turbo generates, accounting for atmospheric pressure.
• PR = (Boost Pressure + Atmospheric Pressure) ÷ Atmospheric Pressure.

Compressor Surge

• When turbo provides more air than engine can handle, reverse pressure waves can cause damage.
• Solutions: Smaller turbo or lower boost, blowoff valve or anti-surge housing.

Compressor Maps

• Essential for choosing the right turbo.
• X-axis: Airflow (convert to potential HP for simplicity).
• Y-axis: Pressure ratio (convert to boost).
• Efficiency islands indicate optimal performance ranges.

Twin-scroll Turbo

• Separates exhaust pulses to prevent interference, increasing efficiency and spool-up.
• Requires special exhaust manifold and turbine housing.

Wastegates

• Internal Wastegate: Trapdoor that opens to prevent overboost.
• External Wastegate: Separate from the turbine housing, allowing precise boost control.
• VGT/VNT (Variable Geometry Turbo): Uses adjustable veins instead of trapdoors to vary passage size for optimal performance.
• Not common in gasoline engines due to higher exhaust gas temperatures and cost of materials.

Summary

• In conclusion, understanding the various components and concepts related to turbochargers is crucial for optimizing engine performance. From A/R numbers to turbine wheel materials, each aspect plays a role in enhancing and tailoring turbo functionality according to specific needs and applications.