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Old 12-24-2021, 10:45 AM   #28 (permalink)
EcoModding Lurker
Join Date: Dec 2014
Location: Alberta
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Merc - '97 Mercedes V230 TD Fashion
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Originally Posted by Blue Angel View Post
One thing I'm curious about is cold weather operation. A gas car runs rich to warm up the catalyst, which can take quite a while with light-footed driving, or while idling, and can result in fuel contaminating the oil. I don't believe diesels have this issue and would be much more efficient in the winter because of that, no? I didn't notice much change in fuel economy in the cold last winter and I'm wondering if that's why?
With the possible exception of throttle-fitted diesels, mentioned by another poster but completely unknown to me, a diesel engine is controlling idle by metering the fuel. Air flow is unrestricted. When the engine is cold, more of the fuel passes through unburnt (you may see white smoke on a cold start). This results in a less efficient burn and thus more fuel is required to maintain the idle. This natural feedback loop results in a richer AFR entering the cylinder, but this is not the direct result of any direct AFR control on the part of the ECM.

Some newer diesels have an exhaust choke that activates when cold to increase backpressure and thus temperature, to help warm the engine. This creates a very distinct and noticeable sound on some Powerstroke engines especially that sounds like a jet turbine even at idle.

As for cold idling, diesels are very efficient at idle, and this can lead to problems such as slobbering and glazing due to insufficient heat in the cylinder. This is why many bigger diesels are equipped with high-idle controls. Check out an informative video on YT by Adept Ape called "Is Idling Your Engine Bad? Does Idling Hurt Your Engine?". I'm not allowed to post links.
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