Basic diesel engine question

[Jens]

I would like to run this up the flag pole and see what happens .....

When you run a diesel engine (and probably gassers too), common wisdom holds that you warm up the engine before applying a heavy load and you unload the engine for a while before shutting it down.

Why exactly are we doing that ?

My truck manual tells me to idle the engine before shutting it down after a heavy load but this is done to allow the turbocharger to cool down as far as I know (excessive heat would coke the oil galleries of the turbocharger and potentially seize the entire thing)

When we are talking lister(oid) I can see giving a short warmup ( maybe 30 seconds) to allow splash lubrication to get distributed throughout the machine and to build up a good oil layer on all sliding surfaces before loading them heavily. Why would I want to go beyond that ... or would I ?

When shutting down a Listeroid under heavy load, We have a fairly toasty exhaust system but I see nothing else being any different in temperature. Why would I unload the engine first? There is the common thread about the ST generators not liking to be spun down under load but this is not something that seems to affect my marathon generator head.

At the moment, I run Thumper at somewhere around 95% power and my shutoff system is based on engine heat. When the coolant temperature rises above about 220F or so, the engine shuts down. It does this from full load. Assuming that I have only resistive loads on the generator and neither the generator nor any of it's attached electrical devices complain about the sudden voltage/frequency drop, why would I want to go through a no load idle stage before shutting down ? How long should I unload the engine (if at all) before shutting down ?

Jens

[AdeV]

In a "normal" road engine, it's good to keep the load down until the oil is warm and, thus, thin enough to get everywhere it needs to get, in sufficient quantity. But that's a pressurised system, not splash lube. Similarly, the cool off is either for the turbo, or even to ensure there's no hot-spots in the engine if you've just given it a beating. Remember, the oil probably shifts as much heat as the water, in an vehicle engine.

Unless you've turbo'd Thumper, I don't believe any of the above apply particularly; and your brief "splash warmup" is probably quite sufficient. How hot does the sump oil get? I'd be willing to bet it's not that warm, even after an extended run.

[billswan]

jens

I will agree with what ADeV posted, except for the part where he says "the oil probably shifts as much heat as the water, in an vehicle engine". That I will disagree with respect, oil is a poor coolant it holds very few BTU's compared to water. It is much slower to absorb heat and carries less and then releases slowly. That is what I have experienced. Now don't get me wrong I know turbo charged diesels have oil cooling nozzles for the bottoms of pistons and coolers for the oil but if a test was conducted to see how many % of the engine heat went out the water coolers verses the oil coolers my best GUESS would be the water wins hands down........

Billswan

[mobile_bob]

here is my take on the subject

you allow warm up time so that the parts of the engine are allowed to come to the shape
they will be in at operating temp.

at operating temp is where the engine will be running most of the time, and where the rings
and everything else have broken in to operate at.

basically every part of an engiine will change shape a bit from cold to ~200degrees F
the roundness of the cylinder will change which probably is most important.

if the cylinder is a bit out of round when cold the rings will not seat as well as they will if hot
so if you put full load on too soon you get some blowby past the rings
which can have some negative effect on performance and lifespan.

so generally it is always best to allow some time to warm up before throwing on full load.

as for cooldown before shutdown

about the only thing i could see that might be beneficial to our engine's is allowing a bit of idle
time so that the valves can cool a bit, particularly the exhaust valve. also there might be something to
be said for allowing the engine to cool down some so that there is not as much thermal shifting of major casting
such as the head vs the cylinder, this should extend the life of the head gskt.
now obviously this would be most important to an engine that relies on a waterpump and not thermal cooling
where cooling continues normally after shutdown.

bob g

[mobile_bob]

i would like to add one more reason to allow warmup time
while it is unlikely on a 6/1 it might happen on higher hp versions of the engine:

if you don't allow enough warmup time, under heavy load the piston expands faster than
the cylinder liner, in so doing there is the risk of scoring the liner when the piston gets too big
for the hole.

usually this is more of a problem with very cold engines, and higher hp versions  where
more fuel is injected that is typical of lower power density engines.

So in conclusion seeing how you are looking to incorporate automation, perhaps it might be
prudent to set the programming to allow for a few minutes of no load operation on a cold
engine, followed by a light to moderate load until the engine reaches operating temps
and then switch in full load,, that is easy enough for a microcontroller to handle i would think.

on shutdown, have the micro drop the load, and allow the engine to run for maybe a minute
with no load before shutdown, can't hurt anything in my opinion.

bob g

[veggie]

I agree with Bob on the clearance issue.
There is definately a difference between Running clearances and Cold clearances.
In the "old" days when manufacturers had much less choice about metalurgy, greater "cold" clearances were set which then tightened up as things expanded at different rates. Loading a cold engine with too much "slap" was a concern.

There is also the philosophy that running a slow speed diesel engine unloaded at startup does not provide enough heat and keeps things "rattling around" longer than necessary (which may add to undue wear at startup).
So..... the philosophy in this case is to load the engine lightly (eg: 20%) at startup to facilitate a quicker warm up without undue load on the cold parts.
IMHO the lightly loaded warm up is the way to go.

Commercial emergency generators (Hospitals, Military etc...) get around this issue by keeping the lube oil and water jackets warm and ready to go. Pre-lube pumps circulate warm oil before fire-up. Then it's "balls out" right from the start.

Cheers,
Veggie

[RogerAS]

Hey All,

My older brother, now past on, was the shop manager for a large fleet of trucks. He was trained at the university level for this and knew his "stuff". He was also the crew chief for a top fuel drag team. He told me that oil is barely better than water for lubrication until the oil comes up in temperature. The molecular structure of the long chains become more sticky at operating temperature and hold their bonds better. This effect has an upper limit and the polymers start to break at the upper limits. The film of oil between metal parts has to be within the shear limits of the oil and this limit is at optimum within a given temperature range.

I run a Kubota 300 series driving via V belts a large frame (160 amp) automotive alternator. I usually allow my engine 10 minutes in the summer and 20 in the winter for the warming period. As far as cooling down I think the oil will be better protected from cooking if the engine is allowed about half the warm up time. I'd like to add a oil temp sensor/gauge to my setup but a good water temp gauge will suffice. I use this engine to keep my off grid battery bank up to snuff when the wind and sun aren't getting the job done.

RS

[mobile_bob]

Hello Roger and welcome to our little group of mad researchers

(ok some of us are just angry, some crazy, most a combination of both)

anyway, i have been thinking of your assertions as it relates to the need for the oil to be up to temperature
to afford its protections, this i might buy into in theory but
it is unlikely this has a marked effect on engines of low power density and ample sized brg dimension as the
lister/oids do.
it is also fairly obvious that most of these engine's barely get the oil warm (probably around 110 degree's F)

there may well be more concern in high output engine's of marginal design as is typical of some racecars, and
other lightweight high hp engine's where the oil must be allowed to warm up to its engineered temp range in
order for it to provide its full engineered protection and work as designed.

as for your warm up period of 20 minutes, certainly it could do no harm and all engine's would benefit from being allowed
to come up to operating temperatures before being heavily loaded. The problem with warmup as i see it is one of compromise
usually when one needs the power it is dark and he needs power fairly quickly, as in cases where he might have some medical
equipment to provide for or somesuch. In those cases i suspect the engine is started, brought up to speed and the generator
contactor closes onto load in a fairly rapid chain of events. That sort of thing certainly is very hard on an engine and cannot be
avoided in some cases.

bottom line to me, and something i would question is the lack of heat in the oil of a typical listeroid, there is not even enough heat
in most cases to drive off condensation much less get the oil up to some specific design/engineered temp range.

from following the listeroid boys and their engine's it would appear (thankfully) that having the oil up to proper temperature has not
been shown to be the cause of any premature failures that i am aware of.

bob g