Episode 191

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Published on:

2nd Jul 2025

Why Do We Really Use EGR? [E191]

Thanks to our Partner, NAPA Autotech Training

In this technical deep-dive, Matt Fanslow tackles the misconceptions surrounding Exhaust Gas Recirculation (EGR) in modern engines. Far from just a NOx-reduction tool, EGR plays a critical role in thermal efficiency, throttling losses, and combustion control. Matt dismantles common myths (like "lean burns hotter") and explains why engineers use EGR—even as technology evolves.

Key Topics Covered

EGR’s Real Purpose

  • Beyond NOx reduction: How inert exhaust gases slow flame fronts, improve thermal efficiency, and reduce throttling losses.
  • Why lean air/fuel ratios don’t burn hotter—but can still cook exhaust valves.

Throttling Losses & Efficiency

  • How EGR allows wider throttle openings, reducing engine workload and boosting fuel economy.
  • The link between EGR, Atkinson/Miller cycles, and extended combustion push.

Internal EGR & Valve Timing

  • Modern engines use cam phasing to trap exhaust gases, creating insulating "pockets" that reduce heat loss to cylinder walls.
  • SAE paper highlights: HCCI engines, controlled auto-ignition, and residual gas effects.

Why This Matters for Technicians

  • Understanding EGR helps diagnose drivability issues, software updates, and emission failures.
  • Matt’s rabbit-hole warning: Complexity is growing, but so are diagnostic opportunities.

Notable Quotes

  • "Lean air/fuel ratios burn longer, not hotter—that’s why exhaust valves fry."
  • "EGR isn’t just about emissions; it’s about making the engine work smarter, not harder."
  • "The more you know why engineers do something, the better you’ll diagnose it."

Resources & References

  • SAE Papers (Available at sae.org):
  • Lean Burn SI Engines: NOx Control via Air/Fuel Ratio Modulation (2017)
  • Impact of Valve Timing on Cold Start Emissions in GDI Engines (2019)
  • Effects of Valve Timing on Residual Gas, Combustion, and Heat Transfer (2009)

Thanks to our Partner, NAPA Autotech Training

NAPA Autotech’s team of ASE Master Certified Instructors are conducting over 1,200 classes covering 28 automotive topics. To see a selection, go to napaautotech.com for more details.

Contact Information

The Aftermarket Radio Network: https://aftermarketradionetwork.com/

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About the Podcast

Diagnosing the Aftermarket A to Z
From Automotive Diagnostics to Metallica and Mental Health
Matt Fanslow's Diagnosing the Aftermarket A to Z Podcast is a wide-open perspective on all aspects of the automotive aftermarket from a working diagnosticians' point of view. All topics and issues will be on the table.