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Old 30-04-2011, 06:52   #1
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Full Throttle Use & Diesel Engines

Is there a reason to run diesel engines at full throttle for a period of time? I have a 2001 Volvo MD22P, so a relatively recent engine if that makes any difference. Why is this required with a diesel boat engine vs. a diesel car engine?

If running the engine at full throttle is the recommended practice - for how long & how often?

What would Nigel Calder say?

Thanks,
michael
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Old 30-04-2011, 07:16   #2
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Re: full throttle use & diesel engines?

Michael it's not running the engine at full throttle that is important, but avoiding running the engine at tick over. Diesel engines run at tickover don't allow the rings to reach a good seal against the bores which leads to the engine oil to form a smooth glaze. This hard surface then stops the engine reaching full compression.

The previous owner of our yacht used the engine very gently and sparingly which started to cause this problem. One of our first trips out and back was a 14 motor across the English channel at speed. That cleared the engine and probably the exhaust out and she started much better for the rest of the season.

There is probably a sweet spot about 3/4 revs which will give a good speed at sensible fuel economy and without the engine shaking the boat apart. That would be a good speed to cruise at.

If you can find it, a power graph will show fuel used against power generated for your engine.

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Old 30-04-2011, 07:50   #3
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Re: full throttle use & diesel engines?

I can attest to what Pete is saying. Had to re-ring my Yanmar because it was smoking and using oil. Apparently, with <100 hours, the engine had been run at low rpms and had glazed the cylinders. When I popped the pistons out, it was apparent that the cylinder walls were slick as ice. There was little friction between the rings and walls. Talked to the Yanmar tech people who confirmed that these need to be run at around 2800 rpm. If you look at the torque/fuel consumption curves in the manual, it gives you a good idea of what rpm to maintain.
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Old 30-04-2011, 08:02   #4
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Re: full throttle use & diesel engines?

Thanks Pete & smurphny - that makes sense to me. WOT seems like a hell of a lot of racket for something where I struggle to see the benefit.

The peak torque of this engine (Volvo MD22P) is at 2500rpm. So maybe occasional use to 2600 rpm should do the job? My battery charging rpm has been, 1800 and I motor at 2100 rpm.
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Old 30-04-2011, 08:11   #5
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Re: full throttle use & diesel engines?

Hmm, just run that by me again, what is the difference between battery charging and your motoring regime?

In other words do you run the engine at 1800 revs at the dock to charge the batteries?

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Old 30-04-2011, 08:21   #6
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Re: full throttle use & diesel engines?

yes 1800 rpm to charge the batteries at the dock - or when I am sailing and have enough wind to move the boat along, and the primary reason for running the engine is battery charging, I will run the engine in gear at 1800 rpm. This rpm was selected based on the alternator spec, and the pulley ratios (i.e. alternator rpm).

If I am out and have to motor (my reason for running the engine is propulsion and not charging) as there is no wind, I generally do so at 2100 rpm.
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Old 30-04-2011, 08:29   #7
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Re: full throttle use & diesel engines?

Quote:
Originally Posted by Pete7 View Post
The previous owner of our yacht used the engine very gently and sparingly which started to cause this problem.
You must have caught it in time. I ran my engine for two seasons and never was able to break the glaze. Finally, last fall, disassembled the engine right in the boat and cross-hatched and re-ringed. Was a costly ($400+ for gaskets and rings) and needless adventure caused by a simple but common lack of knowledge about these marine diesels which are designed to be run hard.
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Old 30-04-2011, 08:51   #8
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Re: full throttle use & diesel engines?

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Originally Posted by svfinnishline View Post
yes 1800 rpm to charge the batteries at the dock - or when I am sailing and have enough wind to move the boat along, and the primary reason for running the engine is battery charging, I will run the engine in gear at 1800 rpm. This rpm was selected based on the alternator spec, and the pulley ratios (i.e. alternator rpm).

If I am out and have to motor (my reason for running the engine is propulsion and not charging) as there is no wind, I generally do so at 2100 rpm.
Hmm, we are now into personal opinions, but at 1800 revs with no load on the engine those pistons are just floating up and down and could again glaze the bores. Would be better at say 1500 revs and in gear with an extra line to the dock to hold the boat against the thrust. This will load up the engine nicely all be it using more fuel. the amount of fuel needed to maintain 1800 revs in neutral is tiny but a lot more in gear having to do some work therefore pressing the rings against the bores.

Are you using a smart regulator because battery charging via the engine is very time consuming and one of the reasons we added solar. It's only a single 45w panel and even with the shadow of the boom across the panel it was giving 1ah yesterday and had been all week so batteries full.

Some thoughts away.

Pete
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Old 30-04-2011, 09:53   #9
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Re: full throttle use & diesel engines?

The reason for running a diesel periodically at full horsepower is to eliminate carbon buildup in the combustion chamber.

My Cummins diesel manual requires that I run my Cummins turbo diesels at 100% horsepower for a half-hour for 10% of the total engine hours. They specifically state that carbon buildup is the reason.

Seemingly babying a diesel by running it at low RPM's all the time or having an over-sized propeller where you cannot reach full RPM is a good way to shorten the life of your diesel. Diesels prefer to be run hard. I know it is counter-intuitive but they actually last longer. The other huge factor with diesels longevity are to maintain frequent oil changes. These are the two greatest factors if you want your diesel to last. Exercising your engine frequently also helps.

It's just too bad that running your diesel fast with a high load is not the most fuel efficient way of operating a boat....because as you double the speed of a boat you must roughly square its horsepower to do so.
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Old 30-04-2011, 11:33   #10
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Re: full throttle use & diesel engines?

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Originally Posted by David M View Post
The reason for running a diesel periodically at full horsepower is to eliminate carbon buildup in the combustion chamber.

My Cummins diesel manual requires that I run my Cummins turbo diesels at 100% horsepower for a half-hour for 10% of the total engine hours. They specifically state that carbon buildup is the reason.

Seemingly babying a diesel by running it at low RPM's all the time or having an over-sized propeller where you cannot reach full RPM is a good way to shorten the life of your diesel. Diesels prefer to be run hard. I know it is counter-intuitive but they actually last longer. The other huge factor with diesels longevity are to maintain frequent oil changes. These are the two greatest factors if you want your diesel to last. Exercising your engine frequently also helps.

It's just too bad that running your diesel fast with a high load is not the most fuel efficient way of operating a boat....because as you double the speed of a boat you must roughly square its horsepower to do so.
I think that most engine manufactures state that after the engine is run in correctly at various revs for various lengths of time and full throttle also that the maximum revs should not be more than 85% of the top revs of the engine unless in an emergency.

Diesel engines definitely don't like tick over.

So if you are on tick over which is not a good idea when you are going to stop the engine rev it up high a few times before switching off it helps to stop/remove the glaze from your tick over and reduces the thermal shock from the cooling system.

Don't forget to be in neutral out of gear and throttle open slightly when you start the engine.

I have a new Yanmar 3YM20 which has completed 197 hours including the 50 hours run in time (that was a bit boring)

I purposely bought the gearbox with a lower ratio so that the engine would be working harder, top revs are 3600 85% of that is 3060 I usually run at 2800 2900 where ever I am going but its quite comfortable at 3060. If you have an oversize propeller reduce the pitch or drop the gearbox ratio you can only get top speed according to the LWL but it will give you more power.

I use a bit more fuel. but as Onassis replied when a reporter asked him how much it cost to run his yacht. "If you have to ask how much it costs then you cannot afford it"
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Old 30-04-2011, 11:35   #11
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Re: full throttle use & diesel engines?

Is the real issue the load on the engine or ensuring that the diesel engines are at operating temperature? I thought the latter was the important factor. Cheers,
Bill
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Old 30-04-2011, 11:39   #12
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Re: full throttle use & diesel engines?

There should be an ideal rpm for the engine. It is where the hp and torque curves cross I think. Industrial engines found in dozers, excavators etc. are run at constant rpm for best power and efficiency. A marine diesel should be no different.
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Old 30-04-2011, 11:46   #13
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Re: full throttle use & diesel engines?

Operating temperature is a function of the thermostat. I would like to know the physics behind this glazing issue which seems so common. Is the reason related to compression, speed of piston, chemical reaction of oil on cylinder wall ?? A The term "glazing" suggests some sort of chemical film. This could have implications regarding the type oil. I have always used Rotella but would like to know a little more about just what creates the problem. It also suggests that maybe some additive could chemically break the glaze but I know of no such additive.
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Old 30-04-2011, 13:32   #14
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Re: Full Throttle Use & Diesel Engines

What is tick over? I have never heard this term. I do know what the Cummins manual says.
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Old 30-04-2011, 13:35   #15
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Re: full throttle use & diesel engines?

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Originally Posted by smurphny View Post
Operating temperature is a function of the thermostat. I would like to know the physics behind this glazing issue which seems so common. Is the reason related to compression, speed of piston, chemical reaction of oil on cylinder wall ?? A The term "glazing" suggests some sort of chemical film. This could have implications regarding the type oil. I have always used Rotella but would like to know a little more about just what creates the problem. It also suggests that maybe some additive could chemically break the glaze but I know of no such additive.
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