If you've never heard of an APU, you may not be a long-haul trucker.
The acronym stands for auxiliary power unit, in this case a generator or other form of generating electricity to run auxiliary services like air conditioning in the sleeper cabs of Class 8 over-the-road sleeper tractors while drivers are taking rest breaks during long trips.
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It turns out that adding APUs to semis can save an enormous amount of diesel fuel as opposed to the historic practice of keeping the truck's main diesel engine idling all night to generate power.
Now a recent blog post by Navigant Research points out that APUs offer the potential of saving 500 million gallons of diesel fuel each year.
Some APUs use small two-cylinder diesel generators to produce electricty; others store energy in glass-mat lead-acid batteries that power air conditioners.
The Navigant analysis suggests that there may now be 300,000 APUs on the road, perhaps one quarter of them all-electric.
And the savings from those units during 2013, it calculates, are roughly half a billion gallons of diesel fuel over the alternative of running the main diesel engines used to move trucks weighing tens of thousands of pounds for hours at a time.
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APUs likely aren't something that most car owners think about, unless they happen to fuel up at highway rest stops close enough to the truck parking area that they hear engines running at idle or generators in operation.
It all goes to point out, once more, that improving the efficiency of very low-efficiency engines--especially those that cover 80,000 miles or more a year, as do long-haul trucks--can save a ferocious amount of fuel compared to improving the gas mileage of already-efficient personal cars.