too big a diesel(never at full power) and "glazing"?

[Squidly-Diddly]

I heard it mentioned that a too big a diesel that is never operated at full power will cause "glazing" problems(I assume this is on cylinder walls).

But I've also heard of diesels in just about every application being routinely allowed to idle for days if not weeks.

Now that I think of it, those are all cases of someone not quite caring about an engine as a private boat owner might, for various reasons.

Is it OK to let them idle a lot, as long as you can give them some real exercise on fairly rare occasions?


I'm sort of familiar with carbon build up on gas engines that aren't ever used at near their rated power.

The Diesel at idle will build up soot, but not glaze the cyl. liners. The underloaded engine will do both. Though electronically managed engines have less of a issue with both cases, underloading can still cause glazing.

If the engine ran only at idle for say 12hrs. opening the throttle for a while blows the soot out.
When the engine runs underloaded the soot may become carbon buildup, that will not be blown out.

Regards
Richard

[forkliftking]

I have worked on Detriots and the exhaust ports were only 1" in dia. We would close the valves and chisel and vacumn the crap out. These engine were never over 850 rpm. The bad problem is a chunk will break off and hold a valve open when they were running. Major problems.

[BTPost]

Diesel Engines like being Loaded, that is NOT in dispute. Glazing of Cyl. Walls is only an issue IF, and WHEN, the engine is operated at significantly less than Operation Temperatures, for long periods of time. Soot Buildup is usually an accompanying issue. If the Engine is at Operating Temp, then Soot and Glazing are NOT an issue.

[FAST FRED]

The commercial fish catchers face this all the time , 600+ hp to drag , 50 or so to run the Hydraulics while recovering the gear.

The solution seems to be a Non turbo engine , and square cut piston rings (not traoizoidal) that seat better at low loads.

FF

[jonr]

So how does one keep an engine running at low power output hot? Insulation, preheating of intake air with exhaust heat?

My understanding of glazing is that it has more to do with cylinder pressures and forcing the rings to scrape the oil off. This can be addressed by running at high loads/low rpms (not the same thing as high output/HP).

Originally Posted by jonr

So how does one keep an engine running at low power output hot? Insulation, preheating of intake air with exhaust heat?

A Controllable Pitch Propeller is the professional solution.

The more common is a so named "trolling valve" in the gearbox. But the latter just produces some sort of "slip" to let the engine revv. higher at slow speed. Therefore that is not the way to go.

Regards
Richard

[DennisRB]

Can someone please explain under what conditions an engine would be classed as underloaded, and how does this compare with idling to charge batteries?

[pistnbroke]

if your crankshaft to alternator speed is correct then at idle you would not get much charge current ..maybe 1/10 of max alternator output. Gearing ratio should have the alternator at 12000 rpm with engine at max rpm....even if you never run it flat out

[FAST FRED]

"Can someone please explain under what conditions an engine would be classed as underloaded, and how does this compare with idling to charge batteries?"

YES engine idiling stinks as a way to charge the batteries , as the engine will run way cool , regardless of the thermostat.

LOAD is what the engine needs at any RPM.
This is why "cruising " props are installed , or CPP props work so well.

The only way to get a LOAD while charging the batts is a Huge 200-300A Alternator and a batt set that has the ability to be rapidly recharged.

Mechanical refrigeration helps as it will take a couple of HP to run that too.

The chance of damage exists.if the the engine is at 10% to 20% of rated power (some claim 40%)

This is the rated power at the operating RPM , not the seldom seen peak power.

A 100 hp diesel may create 100hp at 2600rpm bit run at 1800 will only crank out 60hp max , use at least 30hp and there will not be problems.

Oil samples , looking for blowby and unburned fuel will help.

FF

[Pierre R]

I think the old sooting and glazing thing came into wide acceptance when nearly all higher powered boats had 2 cycle Detroits. Detroits are the engines of choice for this reputation.

4 cycle engines do not have much of a problem with sooting or glazing provided the engine reaches temperature. As a general rule, probably around 1000-1100 rpm with at least some load.

The new common rail diesels are even better as they run leaner at lower power settings. This increases engine temps better at lower power settings.

If you are going to idle all day long then gas should be a strong contender over diesel.

[Easy Rider]

Steve DeAntonio of PMM says 75% load 75% of the time. Very hard to do. If you had a diesel engine that made 120hp at 2500rpm and consumed 6gph at WOT at 2500rpm and you cruise at 4.5gph you are cruising at 75% load. Only the fuel burn will tell you the load referenced to WOT at the rated max power rpm. Almost nobody w a Lehman 120 burns 4.5gph. The hardest run Lehmans that I know of run 3gph at 50% load. I think Steve D has over stated it at least a bit but I do think we should come as close as we can but most would need to repower to a smaller engine to achieve good power loading. If you over prop you don't know at what engine speed you are overloaded.

Easy Rider

[WestVanHan]

My Dodge 3500 Cummins (350 hp,650tq) loafing down the freeway at 1400 rpm I get about (quick calc for American gallon..) 19 mpg,or 23 Imperial gallon

So at 60 mph I'm using 3 gph,which may translate to maybe 60? hp to cruise.

The 350hp is peak,and you can't convince me that it has only 110 hp continuous in order to be properly loaded at 60 hp/70%.

I ran my other one to 340,000 km (210k. miles-maybe 5000 hours)before trading it in- lightly loaded as such with no change in oil consumption from new.

[Guest62110524]

once your engine is run in the problem is small
Whilst new then yes, the drill is do not run idle, that means hi or lo idle
when you are alongside and need to run for power or freezer, , run the boat on a springline, load it up it does not take long to bed in a ringset, but even when run in , if doing the above I always load the boat,
For those who do not know, high idle is full governed max revs no load
It also does not apply to large engines, but engines of any size it is amazing, these days a machine can go onto full load new from the box, as in earhtmoving, trucking, such are the advances in mondern materials and precision engineering

[powerabout]

It is a problem to diesel engines, the last company I worked for had a dozen anchor handlers with Warsila medium (constant with cpp) speed engines and if on standby, i.e. no load floating around the engines start to smoke, increase blowby by lots and consume lots of oil.
Not to mention the combustion blowby going into the oil as the ring sealing is not as good as it should be.
You need the combustion pressure to make the rings work.
Turbo engines are worse as in the case above as they usually have lower static compression ratios and bigger ring eng gaps
A hour at full load just about cleans it up but the engine will not last as long as one that runs with load all the time.
(PS None of our engineers have anything nice to say about Warsila these days, how they ever get classification approval is beyond me)