Is it safe to run a diesel on low load?

[Anonymous]

Time and time again I have heard the argument that one shouldn't let diesels run on low load or the bores will become glazed and/or the rings stuck. At one extreme, there is running the engine on tickover, in neutral, for hours on end to generate electricity and heat water. But there are times when we want to motorsail with only a very low power. Looking at my engine and speed curves, I would get excellent economy at about 25% of max rated power and I can also see that I might like to motorsail at 10% of rated output - but is it safe to do that? At what output level can one feel completely comfortable about letting the engine run 24/24 - 10%?, 20%?,.....80%?

 

[Stevie_T]

I was always taught that it was the other way round, that you should run it under load to avoid the bores becoming glazed. /forums/images/graemlins/confused.gif

 

[reeac]

I'm surprised at the low percentages of max. power that you are considering, particularly the 10% for motor sailing as that would typically be only 2 or 3 bhp for many cruisers. I find with our Sadler 29 that 2000rpm which is about 67% of max. revs. and [because of the so-called prop. law] is 30% of max. power ie. 6 bhp. is a nice relaxed way to travel [under power alone] and is OK unless the tide is strong or the water rough but I think that 2-3 bhp. would only be useful for creeping around a marina or up to a mooring. On the other hand I feel that there's lots of nonsense talked about the need to run diesel engines hard and that 30% max. power should be fine although I would have qualms about 10% max. power. But then, one sees dumper trucks idling, it seems, all day long on building sites.

 

[Anonymous]

I have 110hp, so 10% is quite a meaningful amount of power - though on the very low side, I grant you. I am really trying to get a feel for what's safe. Thanks for your input.

 

[Ric]

Bore glazing occurs at low loads because there is not enough friction between piston-rings and the liner which enables a deposit to build up on the liner. It leads to loss of oil control, as the oil-ring starts to lose its efficiency. It is not the problem it once was, as modern engine oils are more effective at preventing deposit build ups. However, long periods of no load running could still cause a problem.

If you want to make sure your engine is running under load while motorsailing, you should get a Brunton variable pitch prop. The Brunton is the only prop that adjusts its pitch automatically to best match the torque of the motor and the waterspeed. Very good piece of kit for motor-sailing.

If you motor sail with a fixed pitch prop (or a feathering prop with a fixed pitch when unfolded like the Maxprop and most other feathering props) then motor-sailing is always a very inefficient exercise. You can have the engine turning and the effective angle of incidence of the prop-blades can be zero (no propulsive effect - and no load on the motor) or even negative (a braking effect - where the prop is trying to turn the motor). With a fixed prop, you generally have to have the engine turning a fairly high rpm before the angle of incidence of the prop-blades becomes positive and starts to contribute to propulsion.

With the Brunton prop, the blades come to an equilibrium angle of incidence that is positive at even low engine torque, so you have no risk of motor-sailing with no engine load.

 

[duncan]

I suspect that the real answer lies deeper in the cycle of use for modern engines / oils.
There is a huge difference between 50hrs of running at tickover (but in gear) then 1 hour at 80% load say before back to light loads and constant running off load with no opportunity to run up under load at all.
Also important is going to be the engine cooling because the other problem of prolonged running off load it that some engine installations won't get up to temperature.

 

[boatmike]

I am bound to get slated for this but the problem, while it exists, is overrated. Assuming that the engine has been run in properly and has good clean oil of the right grade, the problem of bore glazing is not serious in a modern engine. It is true that a diesel loves work. If it is run at very low power for long periods the problem will, as someone else said, primarily be a matter of the engine not reaching operating temperature.
This will cause several problems as it will with any engine but the most serious of these will be that any condensation in the crankcase will not be driven off as steam and will tend to pollute the oil. Also the incomplete combustion and low temperature will cause carbon build up. Many is the time I have been asked to comment on a rough running or smoking engine and have advised a good long belt at near full throttle followed by a full oil change. Usually this has resulted in the engine coughing up "coke" for a while and thereafter running sweetly and cleanly. If you run your engine a lot using low throttle, for battery charging or motor sailing, I would periodically give it some work to do just to keep it in good general health. Don't let anyone tell you that you have to belt around at full throttle all the time to stop your bores glazing up though. Just give it welly from time to time to ensure all is well and that it reaches full operating temperature and when motor sailing try to use upwards of 1/4 throttle. By the way I believe the advice that Bruntons Autoprop is good if you do a lot of motor sailing is quite correct as the pitch adjusts to maintain load on the engine even at lower revs. They are expensive though (I just bought one....Ouch!!)

 

[Heckler]

YES

 

[Ships_Cat]

I go along with what Mike and others say but add that, in my opinion, a diesel engine should not be run lightly loaded at high revs as happens when a poor selection of alternator and drive ratio is made (low load at high speed may also be why some pleasure users find their diesel driven generators become high maintenance items?).

John

 

[oldharry]

Bore glaqzing is by no means the only problem caused by running diesels off or low load, and is not all that common with modern materials and lubricants. A light loaded engine may not warm up properly causing heat stress loading specially in light ally cylinder heads often used nowadays. This leads to gasket failures and in severe cases cracked heads/ blocks. Valves may not be properly warmed up leading to condensation on the springs and premature spring failure. Meanwhile in the cylinders insufficient heat is generated to burn off carbon depositis so that the engine and exhaust become choked with carbon. Lubricating oil may not reach working temprature, sludge forms and is not shifted, clogging the oilways and feeds. Carbon combustion residue escaping past the rings is not properly cleansed by the sump oil, resulting in accelerated wear, and increasing sludging.

Sea Angling or Divinbg boats often spend a large part of the trip at trolling speeds, so when you see them roaring home flat out at the end of the days fishing, they are not just in a hurry to get back to the pub, but they are actually working their engines out to clear the carbon, and rebed the rings aftre the days light load running.

These guys depend on their engines for their livelihood and safety, and this is a known and well established way of ensuring reliable longevity for a marine diesel.

So if you MUST run your diesel light for hours, give it say 1 hour in every 6 at or near full chat to keep it in good trim. Negelect this and you will find it running rough, smokey and unresponsive, and difficult to start as it becomes clogged with carbon etc.

 

[Das_Boot]

Wont a thermostat allow the engine to come up to correct operating temp whatever the load and revs. I am putting in a new engine and would also like to look after it properly. I am also planning on using it as a diesel generator when wind and sun arent available. Is this wise?

 

[oldharry]

[ QUOTE ]
Wont a thermostat allow the engine to come up to correct operating temp whatever the load and revs. quote]

The simple answer is surprisingly - no. Diesels run much cooler than petrol engines, but the cooling systems need to be able to cope with full load conditions. At tickover, coolant must still circulate to remove heat from 'hot spots' - notably the exhaust valves. Other parts of the engine are prevented from reaching working temprature by the coolant, so the engine may be running over cool. but the hot bits still get hot, and cause localised stress points in the weakest area - the head gasket.

the engine really does need to be loaded so that it warms up efficiently, or to be run up on load fairly frequently to undo the harmful effects of off load running. /forums/images/graemlins/smirk.gif

In answer to using it as a generator, presumably you mean using the alternator? The soultion is to fit something that will boost the Alternator output via a Charge controller., Adverc make a well respected range for example.

These allow the engine to bring your batteries up to charge much more quickly, so reducing the 'off load' charging time. I believe most people find that fitting one actually allows the engine to keep the batteries fully topped up during the normal work cycle, and they rarely have to run the engine just to boost the batteries. Unless you are a sailing purist and never use it! /forums/images/graemlins/smirk.gif

 

[colin_jones]

QUOTE
"The solution is to fit something that will boost the Alternator output via a Charge controller., Adverc make a well respected range for example."

Whilst I agree that Adverc make some super gear, I would not call it an alternator booster. It gets the battery to 100% +/- charge by sensing its needs at the battery, rather than at the alternator and then feeds sufficient to charge in correct proportion. This is well explained on the company's web site <<www.adverc.co.uk>>
where there is also some good info on batteries and charging in general.

When we fitted a new Beta, I raised the question of running at moderate revs and no real load with the engineers. They pointed out that their engine, like some others , was designed specifically for tractors and generators, which are expected to 'idle' for hours on end - in cold climates for days on end, non-stop. They added, for good measure, that they have sold well over 11,000 engines and not had any reports of glazing.

I believe them.

 

[Das_Boot]

I am busy buying a beta but wanted to upgrade the alternator to 100 amp or so so as to charge the batteries quicker when neccessary. Sorry about saying generator. The engineer says my Sterling (smart charger) might not cope. I dont have the spec but think it is top of the range is this an issue. and as Beta say their engines are built to run at constant low revs. I wont be doing most of my sailing in cold waters so overheating would be more of an issue than lack of temp i would imagine.

 

[oldharry]

< I would not call it an alternator booster >

neither would I.

Its a 'charge controller' which allows the alternator to increase its output to a level that brings the battery quickly and safely up to full capacity.

thats why I suggested it.

If Beta say their engines can be left idling for long periods I hope they would back that with a full warranty. so no probs.

Unlike many other diesels which do not cope with it.

 

[vlazyjack]

My understanding is that a diesel engine has a fix swept volume of air at a given speed and the 'throttle' lever is really an engine speed control lever. When the engine is put under load i.e. put into gear and driving the boat, the governer causes more fuel to be injected to provide the power to maintain the engine revs. This means that when the engine is idling under no load there is a high ratio of air to fuel (burning lean) and like a bunsen burner with the air port fully open, it burns very hot in the cylinder. It is this action which causes the engine lubricating oil to be burnt into the scored grooves in the cylinder wall (glazing). This gives the problem. A loss of oil retention between the cylinder walls and the piston rings causes a drop in cylinder head pressure, loss of power, starting problems etc.