3.4 Forced Induction Systems (Turbochargers & Superchargers)
Key Takeaways
- Turbochargers utilize waste exhaust gas enthalpy to drive a turbine and compressor wheel, while superchargers are mechanically driven by the engine crankshaft belt or gears.
- Wastegate actuators (pneumatic or electronic EWGA) control turbocharger boost pressure by bypassing excess exhaust gas around the turbine wheel.
- Charge Air Coolers (CAC / intercoolers) lower compressed air temperature to increase air density, improving volumetric efficiency and suppressing engine knock.
- Compressor Bypass Valves (BPV) or Blow-Off Valves (BOV) vent air pressure spikes when the throttle closes suddenly, preventing destructive compressor surge.
Forced Induction Systems (Turbochargers & Superchargers)
ASE task C.14 covers inspecting, testing, cleaning, and repairing or replacing the turbocharger, supercharger, and related system components. Boost-related complaints reach the shop as power loss, over-boost codes, or oil consumption, and separating a mechanical turbine fault from a boost-control fault is the tested skill.
Forced induction increases engine power density by forcing compressed air into the engine cylinders at pressures above atmospheric pressure (boost pressure). Increasing intake air density allows the PCM to inject proportionately more fuel, dramatically improving engine torque and horsepower without increasing engine displacement.
Forced Induction Mechanics: Turbochargers vs. Superchargers
TURBOCHARGER FLOW:
[Exhaust Gas] --> (Turbine Wheel) --> [Exhaust Out]
|
(Shaft / CHRA)
|
[Ambient Air] --> (Compressor Wheel) --> [Charge Air Cooler] --> [Throttle Body]
SUPERCHARGER FLOW:
[Crankshaft Belt] --> (Mechanical Blower/Rotors) --> [Charge Air Cooler] --> [Engine Intake]
| Feature | Turbocharger | Supercharger (Roots / Twin-Screw / Centrifugal) |
|---|---|---|
| Drive Source | Engine Exhaust Gas Energy (Enthalpy) | Engine Crankshaft via Accessory Belt or Gear Drive |
| Parasitic Drag | Minimal (Exhaust backpressure tradeoff) | High parasitic horsepower draw on crankshaft |
| Operating Speed | 100,000 to 250,000+ RPM | 10,000 to 20,000 RPM (Engine RPM step-up ratio) |
| Boost Lag | Present (Turbo lag while exhaust pressure builds) | Instantaneous boost response from idle |
| Thermal Management | Extreme heat (1,200°F–1,700°F exhaust gas) | Lower operating temp, but compresses intake air |
| Lubrication | Engine oil pressure feeds CHRA bearings | Self-contained oil reservoir or engine oil feed |
Turbocharger Architecture & Bearing Assemblies
A turbocharger consists of a Turbine Housing (exhaust side), a Compressor Housing (intake side), and a Center Housing Rotating Assembly (CHRA).
- CHRA Bearings: The shaft rotates on floating brass journal bearings or ceramic ball bearings. Full engine oil pressure feeds the CHRA under pressure. Engine coolant often circulates through the CHRA housing to prevent oil coking (thermal carbonization of oil) when the engine is shut off hot.
- Shaft Clearance Specifications:
- Radial (Side-to-Side) Play: Small clearance allowed for oil film (typically 0.003 to 0.006 in.). Excessive play allows compressor/turbine wheel fins to scrape housing walls.
- Axial (End) Play: Strict tolerance (0.001 to 0.003 in.). Any perceptible endplay indicates worn thrust bearings, causing oil to push past dynamic shaft seals into the intercooler or exhaust.
Boost Pressure Control Systems
Unregulated turbochargers would build excessive boost as engine RPM increases, causing engine detonation or catastrophic mechanical failure. Boost control is accomplished via wastegates and bypass valves.
BOOST CONTROL SCHEMATIC:
[PCM Solenoid / EWGA] --> (Wastegate Actuator Arm) --> [Wastegate Valve]
|
(Bypasses Exhaust around Turbine Wheel)
Wastegates (Mechanical, EWGA, & VGT)
- Pneumatic Wastegate: Uses a diaphragm actuator referenced to manifold/boost pressure. When boost reaches a calibrated spring limit (e.g., 8 PSI), pressure pushes the diaphragm, opening a wastegate valve to bypass exhaust gas directly into the downpipe.
- Electronic Wastegate Actuator (EWGA): Uses an electric DC servo motor with a position sensor. The PCM regulates wastegate opening down to fractions of a millimeter, enabling fast spool-up during acceleration and opening the wastegate completely during cruising to eliminate exhaust backpressure.
- Variable Geometry Turbochargers (VGT/VNT): Utilizes movable guide vanes surrounding the turbine wheel. At low RPM, vanes narrow the exhaust passage, accelerating gas velocity to eliminate lag. At high RPM, vanes open wide to maximize flow.
Compressor Bypass Valves (BPV) / Blow-Off Valves (BOV)
When the driver suddenly releases the accelerator pedal at high boost, the throttle blade snaps shut. Compressed air bouncing off the closed throttle blade creates a severe pressure shockwave (compressor surge) back against the spinning compressor wheel.
- A vacuum-operated or PCM-controlled solenoid Compressor Bypass Valve (BPV) opens instantly during deceleration, recirculating air back to the turbo inlet.
- Atmospheric Blow-Off Valves (BOV) vent excess air to the atmosphere (common in aftermarket applications).
Charge Air Cooling & Diagnostic Troubleshooting
Compressing air heats it significantly (Boyle's Law: T2 > T1 as pressure increases). Intake air temperatures leaving a turbo compressor can exceed 250°F (120°C).
- Charge Air Coolers (CAC / Intercoolers): Air-to-air or air-to-liquid heat exchangers cool compressed intake air down to within 20°F–40°F of ambient temperature.
- Density Gain: Cooling charge air increases air density, suppressing engine knock and allowing increased ignition timing advance.
Forced Induction Diagnostic Fault Matrix
| Diagnostic Code / Symptom | Possible Root Causes | Verification & Diagnostic Test |
|---|---|---|
| DTC P0299 (Underboost) | Split CAC hose, stuck-open wastegate, damaged compressor fins, leaking BPV valve. | Perform intake smoke test / pressure leak check up to 15 PSI. Inspect wastegate arm linkage. |
| DTC P0234 (Overboost) | Stuck-closed wastegate valve, failed EWGA motor, pinched wastegate signal line. | Inspect wastegate linkage for binding; test actuator diaphragm with vacuum/pressure pump. |
| Blue Smoke from Exhaust | Blown turbo shaft oil seals, restricted CHRA oil drain tube, coked CHRA passages. | Check intake/exhaust pipes for oil pooling; measure shaft axial endplay with dial indicator. |
| Screeching / Whining Noise | Compressor wheel contact with housing; boost leak under pressure. | Remove intake duct; inspect turbine/compressor wheel edges for scoring and radial clearance. |
A turbocharged vehicle sets DTC P0299 (Turbocharger Underboost Condition) and experiences sluggish acceleration. During inspection, the technician hears a loud whistling noise under heavy load. A visual check reveals oil mist pooling around a rubber duct connection at the Charge Air Cooler (CAC). Which diagnostic test will confirm the leak?
A technician inspects a turbocharger for suspected internal oil leakage causing blue exhaust smoke upon deceleration. After removing the intake inlet pipe, the technician grasps the compressor shaft and measures 0.012 inches of axial (end-to-end) play using a dial indicator (spec is 0.001 to 0.003 inches). What corrective action is required?
What is the primary function of a PCM-controlled Compressor Bypass Valve (BPV) in a turbocharged engine air induction system?