How Often Do You SDtart Your Motor.

[MapisM]

In fact, their advice is to wrap a dustbin liner around the intake filters when the engines are not in use.
...
The big problem with this procedure is that you MUST remember to take them off

That's exactly what I meant when I said "sealing filters and exhausts".
In fact, you're actually missing the and exhausts bit: the salty air problem you mention is even worse on the exhaust side, because that remains full of salt water, unless fully winterised.
Re. how to remember, when winterising the engines it's common practice to stick a memotak on the ignition key cylinders.

MTU also recommended cranking the engines without fireing them.

Aaargh!!! Now I must reconsider!
Looking forward to Andy comments on this one.

PS: yes, on diesels you can keep the stop button pressed wihle cranking, to cut the fuel supply and avoid firing them.
On most smaller gasoline boats you can use the kill switch.

 

[misterg]

To reply to various points:

1. ...But let's say 10%, so that's 2cc, which is half a teaspoon. Now, my sump has 50 litres of oil. So my one minute of running puts 40ppm of water into the oil (by volume), and if done 4 times over the winter then 160ppm. I honestly don't think that's significant. I mean, elinminating that tiny quantity of water in the oil doesn't justify, I suggest, running an engine cold for a total of say 1-2 hours over the winter, and inflicting the resulting frictional wear. What do you think?

The numbers sound reasonable (without checking them). The only statement I'd quibble with is the one about frictional wear from 1-2 hours running - I doubt there would be anything appreciable, but we may have to agree to differ on this.

I'm not sure that your water would be evenly distributed through your 50l of oil, if it would just sit as a pool in the bottom of the sump, or if it would form an emulsion with the oil. - I suspect the latter. It would be pretty acidic, too, so you (I, anyway) wouldn't want it left sitting in the bearings. I don't know how much water you need for it to start being a problem.

2. There is no significant load on the bearings and hence no significant wear until the engine fires. (Ok, there will be a higher load on the crank bearings that happen to be pushing a piston on its compression cycle, say 1/4 of the time in a 4 stroke, but even that load is insignificant compared to the firing stroke loads).

We'll have to agree to differ on that I'm sure the loads on crank bearings are higher when the engine is firing, but wouldn't call the the loading before this insignificant, particularly in light of the limited lubrication available. The bearings have a much lower load capacity with a limited (or nil) oil supply.

Now, cold engines rotate for a second (or more) before firing - by design in the case of electronic engines and by accident in the case of older engines. During that second or so the oil pump (generally a positive displacement pump, eg a gear pump) does deliver the oil needed to fill the void between the two surfaces of the hydrostatic bearing and so the "one off event" wear in the first second of engine running isn't anywhere near as bad as you describe.

The fact that our engines manage to survive hundreds of starts means that it's not a catastrophic event, but the mechanism for the wear is as I described.

In contrast, during the first several minutes of running an engine it isn't hot enough for the metals to have expanded to the right shape, exactly as you describe in your para beginiing "Next" in your earlier post, and it's THIS wear that I seek to avoid by running the engine for 1 minute not 30.

This is probably the key point on which we differ: I would contend that once the lubrication is established there is negligible wear of the main engine parts. I would agree that until operational clearances have been achieved (i.e. everything has warmed up) that the engine is less able to withstand high loadings (so you don't give a cold engine full revs, or full 'throttle').

BTW, I fully accept that my running the engine for 1 minute imposes wear, including the hydrodynamic bearing wear that you describe but to a much lesser extent than you describe. I just think it's a price worth paying to sloosh oil around and exercise the rubber components in both engine and g'box. But I am arguing that running the engine for a further 29 minutes imposes 30x (or whatever) more wear and adds no benefit other than removing half a teaspoon of water per 50 litres of sump oil each time

What do you reckon?

Simplistically: I reckon that the wear caused by running is at most about an order of magnitude less than the wear from a start. This is a personal opinion, so I can't cite any definitive source for it.

Aaargh!!! Now I must reconsider!
Looking forward to Andy comments on this one.

I didn't see the circumstances in which MTU recommended cranking the engine without starting - was it to circulate the oil before a start, or was it an isolated 'maintenance' procedure to be carried out without starting the engine?

If the former, then it makes sense to get the lubrication established before the heavier loads as the engine fires and picks up speed. If the latter, then they must have a reason for recommending it. (I am assuming that this refers to cranking the engine over for several seconds).

General points, not replying to anyone specifically:

I can't comment on specific engines, or other aspects as I simply don't know. The engine manufacturer will be aware of everything I've posted, as well as considerations pertaining to all the other bits that make up the engine, and will have made recommendations based on balancing all these things. If you want to do something else, why not ring up their technical department and run your plan past them? If you're engines are worth what, £15k? £20k? each then one would have to be pretty confident not to do so.

Incidentally, I'm not on a mission to change anyone's mind, just to present some information about bearing wear that I am party to thanks to hours and hours of tribology lectures, and which has helped me decide what *I* do with my engine (cast iron, normally aspirated dumper truck technology). Jfm said that he'd never seen the argument for running engines up to operational temperature laid out. Hopefully I've done that:

1) The vast majority of wear occurs on start-up, so the duration of the run is largely irrelevant from that point of view.
2) Unless the engine gets hot enough for long enough to drive them off, contaminants will accumulate in the oil.

I'm confident that both the above statements are correct. The discussion then becomes one of degree which will be impossible to conclude in a general case.

Thanks for the favourable comments on the posts - As with everything you read on the internet, take it with a pinch of salt, and look for corroboration.

Andy