when is a rebuild normally due?

twolane · Mar 13, 2012

Burky said: ↑

First, there are too many other factors that go into making up the life of an engine to solely say that idling at night will reduce the lifespan of an engine by any set percentage. There may well be a percentage and some statistics that back that up, but I am not aware of where they might be, and I think that too many other things come into play to make a general statement.

I could easily make a very strong argument that the best thing that can happen to your engine in the search for long life is idling. (Bear with me here, and remember that this is a hypothetical argument, not what I am saying!) In idling, in purely mechanical terms, you have a relatively large displacement engine, heavily built, running at reduced rpms and producing only a fraction of it's potential horsepower. Considered in terms of pure metal, that's a pretty good formula for longevity. All of the parts are built for much higher loading than they are encountering at that time, there are few stresses placed on the engine due to the rpm it is working at, such as valve float or the forces involved in a connecting rod changing the direction of travel of a piston. And, the engine is only producing a small portion of the power it can develop, maybe 10% or less, which means that there will be very few stresses in the engine related to that. The power to cubic inch ratio is about 2 cid for every hp at power, and about 20 cid to each hp at idle.

(Put a 1 pound weight in your hand and throw your arm out to full extension and feel it. Now imagine a piston on top of a connecting rod, traveling upswards at full speed. Now imagine the stress on the connecting rod as it has to stop the piston and reverse it's direction of travel at full speed.)

So you could easily make a sustainable argument that idling is the best thing that could happen to an engine. I have a D4 Cat out in the pole barn, and it's engine meets that same requirement. It has a relatively large displacement engine for the power it puts out, runs a fairly low rpm, and as near as I can tell, has never been taken apart. The last time anyone looked at the internal parts of that engine, it was at the factory, and Admiral Yamamoto had not even come up with the plan to attack Pearl Harbor, the Battle of Britian had yet to be fought, and FDR was in office and in good health.

What that argument doesn't take into account is the temps that the engine runs at while idling and the effect the reduced temps have on the operation of the diesel engine, and the products within. A diesel operates without any form of spark, relying solely on the compression in the engine to fire off the fuel. Basically, the air is compressed until it reaches a temp of between 925-975 degrees, and at this point the comparatively low grade fuel is injected in a mist and lights off from the heat of the compressed air. The correct mechanical term is a "compression fired ignition", and therein lies the problem.

Since the cooling system has to be built to handle the maximum load the engine can be expected to see, it it way overbuilt for idling. And as a result, the engine cools much more during prolonged idling than is ideal for best operating conditions. As the engine cools, the areas surrounding the piston, like the cylinder heads and other masssive metal parts, do not retain their heat properly, and the engine cools down, which results in lower cylinder temps and misfires. When the engine misfires, the fuel does not ignite fully, and as a result you get unburned fuel in the engine. This leads to smoking, and soot and unburned particulates that clog up the path of the airflow. And with many newer engines using recirculation systems that run exhaust back through the engine for recombustion, the problem is going to be worse with the newer engines.

The second issue that occurs its that the engine oil is formulated to work at specific tempratures, and when it falls below it's optimum temps, it starts to lose it's lubricating qualities. Acids form, and you can even get condensation as the engine cools, which means that the water vapor that naturally gets into the engine as humidity in the intake air, is not seeing enough heat to remove it, and that moisture gets deposited into the engine oil and internally in the engine. So prolonged idling can cause all of these things to happen, and can cause harm to the engine. Not because of the specifics of idling the engine, but because the fuels and oils and even the coolants are designed to work best at a specific temperature, which the engine never sees while idling.

Bottom line, since there are only so many hours of life in an angine before it needs to be overhauled, I strongly prefer to ensure that those hours are spent bringing in income. If I don;t have to idle the engine on a truck, then I don't. Whether it is a company truck or one of my own, I just don't believe in excessive idling.

Sometimes I don't have a choice. LAst winter I had 6 nights that I was out in the truck and it was too cold to simply rely on my sleeping bag, and I idled the engine. In summer, I sometimes have no choice. In the last 10 weeks, I have idled the new truck 4 full nights (before the heat wave broke about 4 weeks ago), and 2 nights that I idled for about 2-3 hours due to a rainstorm forcing me to raise the windows or humidity that forced me to run until it cooled off some. I have a Webasto bunk heater in the new truck, so I don't plan to ever idle in winter now.

I also find that I sleep better without the idle. I don't wake up in the middle of the night and read the guages, and simply find it a better nights sleep. And a byproduct of that is that I am in a new truck, and some people that idle much more aren't. One of the numbers the company tracks is idle time, and while they don;t complain about it, I think it is one of the factors why there are people that have been there longer than I have still driving older trucks. Part of it is my productivity, but part is the weay I manitain and use the truck.
Click to expand...

You guys really oughta read that again!

What I don't tell most that don't ask, is that in addition to being an ASE certified Master Heavy Vehicle mechanic, Cummins engine company certified mechanic/machineist, & Caterpillar certified, I was shop foreman at the "Vegas Truck Center" for 5 years.

With all that said, I can assure you that Burky knows what he's talking about.

Well done Burky! Thanks for taking the time it took for you to type that.

EDIT; Burky I drive an older truck by choice, not because I have to.

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when is a rebuild normally due?

twolane Medium Load Member

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twolane Medium Load Member

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