Starting Diesels for short periods of time

[DarkBlack]

I thought the rev up was to meet emission standards by consuming unburned fuel, or perhaps to warm the catalytic converter. Or suck fumes out of the charcoal canister? At any rate, emissions related. If this helped preserve the engine it would have been recommended in the Owner's Manuals for the past 75 years, no?

Think about it this way - assuming the statement that most wear occurs upon starting, then its a good idea to pressurize the crankshaft clearances before the first compression stroke impact. Especially on seasonal farm equipment that hasn't run for months. High rpm and combustion impact on a dry crankshaft can't help preserve it.

You are correct. High initial idle in autos is for emissions, not for oiling. You are also correct that pre-spinning with a compression release gets oil between the rod's upper bearing half, and the crank, before the force of compression occurs.:thumbsup:

 

[Midniteoyl]

Google sucks nowadays.. always comes up with paid crap BS listings first, but you can search for why the engine revs at startup. Its an oil thing..

 

[DarkBlack]

I don't need to google, I'm pretty sure . High start idle in autos is determined by temperatures, not the fact that it is a startup. This is done for emission reasons, not to try to force thick oil through a filters bypass. That's why on real cold days the computer controlled idle speed is increased.

 

[Midniteoyl]

Oh, I agree about cold start modes, certainly...Get the oil flowing (it IS modern, low viscosity oil, correct?), heat the catalytic, etc.. However, did you notice that even when warm your engine revs higher than normal idle? Even for a sec?

 

[RaydaKub]

You just turned the motor over at a lower speed for a longer time with less oil than if you simply started it.
Engines nowadays start and REV UP for a few seconds immediately to get oil around..

You have metal on metal contact, but with no compression and no firing, it is simple mechanical motion until the oil is there. Without this compression release, you have the same metal on metal contact, but violent explosions taking place while there is no oil. Which is worse?

As for the revving, oil pumps are purely mechanical, aren't they, that is, the oil pump revolutions are in direct geared ratio to the crankshaft? (numbers completely made up here) So 1 revolution of the engine gets oil pushed 20% of the path through the engine. The 2nd revolution adds another 20%, the 3rd revolution adds another 20%, so it takes 5 revolutions to get oil all the way through the engine. Does it really help to do it any faster?

Let's assume it takes 2 revolutions to actually start. If it takes 5 revolutions to get the oil through the engine, 5 dry revolutions with no impact plus 2 lubricated revolutions with compression to start OR 2 dry revolutions under 22:1 compression plus 3 revolutions with cylinders firing violently. I dunno, but I think there's a case to be made that 5 and 2 is less wear than 5.

 

[MHarryE]

All this modern stuff. My old Case CC - 1928 - has a grease fitting for the valve train. Give it a shot of grease once a day, cold or hot weather. Crank bearings I believe are fed by dippers on the bottoms of the rods that dip into the oil at the bottom of the stroke, just like the '51 Chev my pappy bought me to learn how to drive with.

Today with emissions requirements all of the internal components have been improved to prevent wear. The emissions systems are required to meet specs for such a long period that cylinder wall leakage past the rings has been virtually eliminated by improved metals and coatings. What was true in the past no longer is true today, at least not to the same extent. That is what our engine guys tell me anyway.

 

[pwright]

I would hope that the manufacturers are building these CUTs with the actual usage in mind. These aren't industrial or ag tractors that are likely to get used for multiple hours straight after each start. I would expect to them to see a fair amount of usage that is for very short periods of time such as to load/unload something off a trailer or truck. Total time a couple minutes maybe.

 

[john_bud]

Multi starting an engine in cold weather and not running it hard to burn off moisture is the main concern (imho).

If you have to start then stop after a few moments, just plan on once every 2-4 weeks, run it hard for an hour at least. That will help a lot.

I'm not overly worried about cold start wear. Just keep the air filter clean and the oil changed. Good to go.

 

[RalphVa]

Think you guys are wrong about the high idle speeds at startup. Could be something to do with emissions, but, mostly, it's to ensure that the engines stay running. Guess you guys don't remember back in the olden days of carburetors in cars and pickups. They had a fast idle setting. The intent was to ensure that the engines would start and keep running. I've had cases in those olden times when the idle speed cam thingie wasn't working correctly. The engine would just start and die, start and die. Same thing happened on old diesels (not that old, pre 2004 ones) if you didn't give them 1/3 to 1/2 throttle when starting them. They'd just start and die, start and die until enough heat was built up in the combustion chambers to keep them running.

Ralph

 

[DarkBlack]

Ralph, I'm old enough to have worked with the tech you speak of. I know it well, but times have changed. multipoint injection, computer control.
In fact, My new truck, on a cold start, will lower the high idle speed instantly when it senses the switch that you put it in gear. That of course would be counter productive, if the high idle reason was to keep it going, since putting an old cold engine in gear was more likely to stall it.
What they are after is plenty of rich exhaust stream flowing to the cats, to get them going. Keeps the epa off their backs