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      01-11-2013, 11:42 AM   #24
HighlandPete
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Quote:
Originally Posted by orcomma View Post
cold starts arent the only thing that affects economy in the winter - the denser air requires the ECU to inject more fuel to keep the same fuel/air ratio. The result is worse economy but (theoretically) slightly more power!
Here's a thought... If there was more power it would move the car faster, and we'd lift off the throttle. Also remember air to fuel ratio is not fixed in the diesel, it can lean out to something like 60:1 in normal use.

There is another factor which often gets left out when we discuss diesel engines in the cold, that is IAT, it can be too low for complete combustion. The cylinders just don't heat enough for optimum efficiency, and can take much longer to achieve that state.

When running the VW TDI engines, it was clear when driving for maximum economy, (in the 50 - 60mpg range), very little boost was necessary and therefore the intercooler was over efficient in colder weather.

At least 3 of us experimented with our engines and kept data. We all found that peak mpg was attained in the 12 - 16°C ambient temperature range. Below and above that range, efficiency dropped off. Mike Fishwick (long term VW enthusiast and often published in Volkswagen Driver) kept more data and experimented with blocking off his intercooler at lower ambient temperatures. He was still covering part of the intercooler in his Golf PD TDI at ambient temperatures towards 12 - 15°C, to get peak mpg.

He found he could increase the efficiency window which was best between 12 and 20°C in those engines, (I second that temperature range). In fact he claimed to improve his mpg by up to 5mpg when using the cardboard to block off the intercooler at the lower temperatures. The only change was warmer air entering the engine.

To clarify, these comments apply when driving light, particularly driving at constant speed. If you drive harder, then the boost required will heat the air and the intercooler cooler will be less critical to the above comments.

But in sub-zero temperatures even a harder worked engine can suffer from AIT chilling, hence the AIT heaters that get used on diesel engines in some industries and on HGV rigs running in cold climates.

HighlandPete
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