Are EGR valves only on diesels?

by Alex

Expert answer:

Quick answer

No. Many petrol (gasoline) engines also use EGR to reduce NOx and prevent knock. Diesels rely on it heavily due to higher combustion temps, but EGR is common in spark-ignition engines too.

Detailed answer

While diesel engines do rely heavily on EGR, it’s not exclusive to them. Petrol (gasoline) engines also often feature EGR valves. Here’s how and why EGR is found across various engine types:

1. Diesel Emphasis
Yes, diesels get most of the attention because of their high compression ratios, which result in high combustion temperatures and lots of NOx. EGR is crucial there to recirculate some exhaust, cooling the burn. But that same NOx problem, albeit to a lesser degree, also affects many petrol engines.

2. Petrol/Gasoline Use
Modern spark-ignition engines often incorporate EGR to reduce knock under certain conditions and help meet tight emission standards. By reintroducing inert exhaust gas, they lower peak combustion temperature, preventing detonation and cutting NOx. While the effect might be less dramatic than in diesels, it’s still beneficial for emission compliance.

3. Presence in Older vs. Newer Models
Older petrol engines sometimes skipped EGR if they could control emissions adequately with a catalytic converter and ignition timing. But as regulations tightened, more brands added EGR to supplement those measures. Meanwhile, older diesels often omitted EGR entirely but had higher NOx emissions. Over the last few decades, both engine types have integrated EGR more extensively.

4. EGR Differences
– Diesel: Heavier soot, requiring robust valves and often EGR coolers.
– Petrol: Generally less soot, no or smaller coolers, but the principle stays the same—dilute the mixture for lower temperatures.

5. Key Motivation
The prime driver is always NOx reduction. Diesel engines produce more NOx, so EGR feels more “essential” there. But gasoline engines can also benefit, particularly at part-throttle cruising or certain mid-range loads, where NOx can spike.

Conclusion
No, EGR valves aren’t exclusive to diesels. Many petrol engines also employ them, though the design and extent of use may differ. The goal remains consistent across engine types: lower combustion temperatures to reduce NOx and stay within legal emission limits. So the next time someone mentions EGR as a purely diesel phenomenon, remember that petrol engines get in on the action too, though often with less fanfare.

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