This is an opinion from Cummins
We publish and state some simple guidelines, but we have no official policy for diesel engine idling. Basically, engines are made to work and not idle. Excessive idle shortens oil drain intervals, is classified as "severe duty" service, and - under the worst of conditions - can lead to engine damage.
One issue with excessive idling is injector carboning which can lead to premature failure or increased injector maintenance. Excessive idling can also cause carbon build up on pistons and piston rings, which can lead to excess oil consumption.
Anything more than 30-40% idle is considered severe duty service, but how that percent idle is attained may be worse than severe. We generally don't like to see engines idled for more than 10 minutes. If the idle speed is increased to 1,100 -1,200 rpm then internal engine temperatures are warm enough to prevent the worst case damage and also pollute the air less.
Low oil sump temperatures, combined with low ambient air temps and low speed idle cause incomplete combustion, producing sludge, carbon deposits on injectors and internal engine components, weak organic acids in the oil - and, ultimately lead to valve sticking and bent pushtubes/pushrods.
There are options. Idling up the RPM as stated above is one. "Smart" systems which turn the engine off/on (while maintaining adequate coolant temperature) are another. Cummins has a system called ICON which does just that. Sometimes though, the engine needs to run - not just for the comfort, but for the safety of the operator. Arctic
operations are an example.
Generally, people have held the traditional belief that diesels should just "hammer away" at idle, and this is sort of a "diesel thing." This is an old belief that needs to be dispelled.
Operators and drivers prefer to keep the engine running for warmth in the winter and for A/C in the summer. Actual idle times are higher than one would expect, except for disciplined fleets which either reward for low idle times or provide other incentives.
In past studies, Cummins has determined that low speed idle can contaminate the oil at between 1-1/2 to over twice the normal rate;
this is one of the main reasons for recommending "severe duty" maintenance intervals for engines operating with more than 40% idle time. Another reason is that, if sump temperatures are not sufficiently high, the water (produced by combustion, let alone condensation) will not be heated enough to evaporate from the oil.
In the Dodge/Ram Cummins midrange engine application, both Dodge and Cummins have recommended owners to limit idle time to 5 minutes and have also offered a software upgrade "idle up" feature, which increases the idle speed, warming up the engine enough to limit incomplete combustion
There may be some balance in stop-and-go driving and urban delivery, wherein one would trade off the benefits to be gained from reducing idle time against the fact that shutting the engine down (every time it's not pulling a load) necessarily creates a "starting event." If the "off time" is long enough, there is a transient starting event time where the oil pressure is not fully established and the engine is running. Accelerated wear can occur during these conditions. In hot temperatures, when the oil is thinner, the oil drains away quickly so some degree of idle may be preferred to starting/stopping and restating the engine fifty times a day.
Common sense may be the best guideline.
Common sense would include an appreciation of the above and factor in outside temperature, whether or not the engine will be run for a long enough time later in the day (to burn off carbon and water), how long the idle is, whether or not the A/C needs to be run, etc. Again, one of the parameters defining "severe duty" is excessive idle time; running at idle for more than ten (10) minutes probably doesn't make sense; the "smart idle" ICON-type devices will help; balancing the cost of excessive idle vs. the utility/necessity/option of idle, etc.... all of these considerations should be factored in."
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Most truckers today use a small genny such as the Yamaha or Honda 2000 to run a heater in their cabs. Much less fuel used. less noise and it saves their money-maker for better things.