Underloading a Diesel

Just to throw some additional thoughts in - I am a keen game fisherman and run my twin 330hp (370hp derated) Ivecos in my 43ft launch at 1200 - 1300 (trolling at 8knts) for over 80% of the time. They get a short burst up to 2400rpm (cruise) at the end of most days, and usually a 15 - 30 minute run at 2400 rpm once up to temp in the morning. That still leaves me with 10 - 12 hours each day at 1200 - 1300rpm, which is about 20% of the cruise loading. The reason for mentioning this is not that my engines are now at 3500hrs and still run like brand new engines, but because the charter boats I know that run this same pattern regularly get 10,000 - 15,000+ hrs out of what are the same engines that recreational boaties often kill at 5,000hrs or less. There are of course a few other variables in the equation, but I don't see any evidence amongst these boats that running at a ~20% load for 1000's of hours is doing them any harm either.

 

Always best to ask the engine manufacturer for loading advice. .

Ive been led to believe that for a piston to stay centered in its cyclider it must be operated at its designed working temp.

Exhaust temperature is a good indicator of engine load.

Im presently having an MTU serviced. 13,000 hrs, the engine has been operated at 65 to 75 output and adhered to service schedules. Compression test showed even wear on all cyclinders

 

Aliboy,

The average car uses about 10 - 15 Hp to maintain highway cruising speed. That's about 5 - 10% loading for extended periods and generally believed to be the "best" mileage one can accumulate. Light loading gas engines is not the question on this thread, I believe it's diesels that are in question.

-Tom

 

Check out the original post.

-Tom

 

so put an automotive spec engine in a boat if you run at light loads
seems to be the answer?

 

These two statements are not the same:

• Some marine Diesel engines which have been run at light loads most of the time have had problems.
  
  
• Some marine Diesel engines have had problems because they were run at light loads most of the time.
  
  
• All marine Diesel engines will have problems if run at light loads most of the time.

The first statement is undoubtedly true.

The second statement is widely believed.

The third statement does not appear to be true, though it is sometimes confused with the previous statements.

 

"All marine Diesel engines"

Marine diesel is a useless generic term , like Hi FI.

A tiny Kubota lawn implement motor , a recycled BMW or Toyota .and what is in a ship or tug boat or sport fish are as different as mice and elephants.

Their care and feeding will be different as well.

FF

 

As far as I know all diesel engines are damaged by underloading.

From a diesel generator technical manual


"Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines - ideally diesel engines should run at least around 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings. This causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze, which smooth’s the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface. which are there to hold oil and return it to the crankcase via the scraper ring.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures. Un-burnt fuel leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.

The problem is increased further the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required. Under loaded running inevitably causes not only white smoke from unburnt fuel but over time is joined by the blue smoke of burnt lubricating oil leaking past the damaged piston rings, and the black smoke caused by the damaged injectors. This pollution is unacceptable to the authorities and any neighbours.

Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures, allows the piston rings to scrape glaze off the bores and allow carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves this will not have any effect.

The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines.

For emergency only sets, which are islanded, the emergency load is often only about 1/4 of the sets standby rating, this apparent over size being necessitated to be able to meet starting loads and minimizing starting voltage drop. Hence, the available load is not usually enough for load testing and again engine damage will result if this is used as the weekly or monthly load test. This situation can be dealt with by hiring in a load bank for regular testing, or installing a permanent load bank. Both these options cost money in terms of engine wear and fuel use but are better than the alternative of under loading the engine.

 

Looks like this thread is in an endless loop.

Someone says diesel engines are damaged by low loads.

Someone says automotive and truck diesels are just fine with low loads.

Someone says diesel engines are damaged by low loads.

Someone says automotive and truck diesels are just fine with low loads.

... and it continues.

 

lets have a vote then
1. have you seen a truck engine damaged by low load - no
2. have you seen gen set damaged by low load - yes
3. have you seen marine engines damaged by low load - yes

 

Does this mean all diesel engines in gen sets would be damaged by low load - no

Does this mean all marine diesel engines would be damaged by low load - no

 

Perhaps an automobile engine is designed specifically for its intended use cycle

 

There is no definative answer ALL engines need a good blast now and again.

 

Yes Frosty, and sensible use for most of the time before and after the " blast ", with regular oil changes etc. should be a kind way to use an engine such as mine, just to remind you all, a 2.8 Isuzu automotive engine, in my boat !!

I'm sorry if I have opened a can of worms with my question. Obviously there is a BIG difference between dedicated marine engines, big HP and continuous duty 24/7. I believe that most smaller recreational marine diesel engines are derived from automotive units, with the necessary marine conversion, therefor most of these engines have serious limitations when pushed hard for many hours.

Perhaps my Isuzu, with 40hp / litre has more ability to " last " than some of the newer units producing 80-100 hp / litre.

Thankyou all for your comments and advice, as I did say earlier, I am new to diesels, but I have now learnt a lot

 

Dicussion last night at the bar. Guy wants to get to the fishing ground fast with a 175 Outboard then troll for 6-8 hours .

Question 2 stroke or 4 stroke.