Motorsailing & occasionally revving engine

[ghostlymoron]

Can you explain this, please. I thought boat and car diesels were basically the same, except for necessary marinisation.
Thanks

Boat and car diesels may be basically the same but the operating regime is vastly different. In a boat on a long trip, you may be doing the same revs for 24hrs, you'll never do that in a car. The so-called 'Italian tune up' works wonders on both boats and cars - vastly reducing the smoking from my Merc, the difficulty is finding a suitable stretch of road. Not a problem with a boat.

 

[alahol2]

Thanks for the answers so far but I'm still confused.

Been away trying to research how (ordinary) diesel engines work. What I think I've gleaned...
Combustion air is not restricted, the cylinder always draws in a full charge.
When the 'accelerator' is opened the governor allows more fuel to the injectors and into the cylinder just before top dead centre.
The piston increase the pressure and hence temperature of the air/fuel mix until it starts to burn.
The fuel/air mix keeps burning and expanding during the down stroke of the piston thus creating the power.

Some questions I haven't found the answer to...
Does the governor try to keep the engine at a set speed for the amount of accelerator or a set volume of fuel for the amount of accelerator? (or what?)
Presumably, because there's always a set volume of air, the engine is running 'lean' for anything other than a wide open governor.
Why, for anything other than an overloaded engine would all the diesel in the cylinder not burn (it's got enough air and the compression will always be the same)?
Can anyone point to a source that describes the above in better detail?

 

[ianj99]

Not true. Diesel engines running at cruising revs are not at ideal stoichimetric conditions. Some of the fuel is not burnt fully and is deposited as carbon. In a car, with variable revs, acceleration, etc., this situation does not occur. In a boat, that may run at the same revs for many hours, it can become a problem. Yanmar recommend running at maximum revs for five minutes every two hours to burn off carbon deposits on exhaust valves and ports.

Not true.

Diesel engines tend to run with an excess of air when lightly loaded as would be the case at cruising rpm.
If the engine is heavily loaded then without a turbo it would be tending to produce more soot so there might be an argument for running it out of gear at high rpm (ie with a 'weak' mixture) to clear out the soot.

Increasing the rpm under load will only increase soot production since the load will increase so will never burn off soot on the valves and ports.

Also, are you seriously suggesting that commercial fishing vessle operators who may be cruising for hours to fishing grounds and then letting the engine idle for hours before hauling their nets, actually do this?

Where is the scientific evidence to show that if you do not follow Yanmar or any other 'experts' recommendation, you will damage or shorten the life of your engine...?

Ian

 

[ianj99]

Thanks for the answers so far but I'm still confused.

Been away trying to research how (ordinary) diesel engines work. What I think I've gleaned...
Combustion air is not restricted, the cylinder always draws in a full charge.
When the 'accelerator' is opened the governor allows more fuel to the injectors and into the cylinder just before top dead centre.
The piston increase the pressure and hence temperature of the air/fuel mix until it starts to burn.
The fuel/air mix keeps burning and expanding during the down stroke of the piston thus creating the power.

Some questions I haven't found the answer to...
Does the governor try to keep the engine at a set speed for the amount of accelerator or a set volume of fuel for the amount of accelerator? (or what?)
Presumably, because there's always a set volume of air, the engine is running 'lean' for anything other than a wide open governor.
Why, for anything other than an overloaded engine would all the diesel in the cylinder not burn (it's got enough air and the compression will always be the same)?
Can anyone point to a source that describes the above in better detail?

You've got it, and corrected those who claim the fuel is not being burned fully on part throttle.

In a diesel, on part throttle, the fuel burns in pockets where the fuel air mixture is correct for combustion to take place. Therefore, the fact the intake is open does not mean the 'mixture' is weak everywhere in the cylinder, just that there is a surplus or air available. The mixture in the region of the injectors will be rich enough to ignite on the compression stroke and the expanding flame will consume the other fuel elsewhere in the cylinder with the excess air ensuring its all burned. (so no need to keep revving it to clean the valves)

On full throttle, full load (or overload) the expanding flame cannot burn all of the fuel because there is insufficient air, hence exhaust smoke occurs to a greater or lesser extent. This is why turbos and diesel engines were made for each other - there is always plenty of air, and what was smoke is now burned and turned into useful power for no increase in consumption.

On most marine engines, the governor is not directly controlling the fuel, the throttle lever is. The governor is there to maintain the idle speed and prevent over revving. I.e its acting as a limiter.

You can easily tell if this is the case (ie that the governor is not controlling the rpm - that it does is a common misconception) if, when you turn down wind, the engine rpm increases a bit due to the reduced load, or conversely, when the prop or hull is fouled, more throttle is needed to reach the same rpm.

If the rpm was set by the governor, the engine would do the same rpm regardless of load (or try to). (This type of governor is found on generators where you do want the rpm to remain constant regardless of load: there maybe some marine engines like this but I've not come across one)

I use Millers Eclipse in my vehicles and boat fuel because it improves the combustion, reduces fuel consumption and smoke and produces a quieter, smoother running engine. It also keeps the diesel bug at bay. The one-shot version kills the bugsand the multi use version is used thereafter. It not available to consumers for coshh reasons so you'd need to find a stockist who might get you some, but is widely used by the professional fleet operators.

 

[KAM]

I have always been very sceptical about the advice to run diesel engines hard. As far as I can see its only an issue with bores glazing on some engines. High revs now and again as per Yanmar instructions should sort any carbon issues. What about all the other components which are being thrashed to death at high RPM. Pumps,valves,bearings, gears, belts, alternators, mountings, couplings let alone the higher noise, vibration, acceleration and fuel consumption. The cooling system is also working at near capacity and the oil system is working harder. Someone tell me all these components also enjoy a hard life. The only advantage is plenty of hot water. I will continue to run mine where it sounds happy!

 

[lw395]

The governor uses centrifugal force to keep the engine revs the same by supplying the right amount of fuel, if the engine starts to drop revs due to load, the governor slows down and the weights come in towards the centre, this increases the fuel supply to the engine and it speeds back up to the desired revs. If it goes faster than what the governor is set at, the weights fly further out which in turn reduces the amount of fuel to the engine by adjusting the rack. So when you are setting the throttle on a diesel you are telling it to stay at a certain amount of revs. When you press the throttle on a petrol engine you are directly controlling the amount of fuel that's going into the cylinder.

If you read the manual for the Yanmar GM engines, it explains how the governor should keep the RPM 'fairly' constant for a fixed 'throttle lever' setting under varying load conditions. IIRC there are limits on the time the governor should take to bring the engine back to within x% of the set rpm after a change in load. But there is always (or should always be ) a factor where a higher load causes a lower RPM, by a small amount. This is more or less a condition for the speed to be stable.

With modern car engines, it's all software and 'stuff' to mimic a petrol engine.

 

[vyv_cox]

I suggest Vyv you look at a few torque/power curves and revise that comment - the 3YM Yanmar produces maximum torque @ 2400 rpm and maximum power @ 3600.
You'll find most modern marine diesels have a similar distribution.
I'd agree that, unlike old-fashioned long-stroke diesels, the torque and power curves are closer together with more modern diesels.

In fact it's a matter of opinion as to whether one should run modern diesels flat out on a regular basis - if you talk to the Yanmar designers you'll find that they suggest running the engine @ as close to maximum torque is the optimum - with an CI engine more welly = more fuel = more carbon.

The stricture about opening up occasionally from low revs, has nothing to do with carbon deposits but everything to do with oil distribution and bore wear.

I'm surprised to see you promulgating this urban myth about carbon deposits!!!

Yes, power curve would have been better!

Don't know about the urban myth. I have the instructionnfrom the Yanmar workshop manual at home, not with me now, and it is quite clear about the benefits that operating at maximum revs will bring. I guess that the treatment may benefit the manifold more than it does the engine itself but there is no doubt that the Italian Tuneup works very well in many cases. When I do it there is significant black smoke for a minute or so, which then disappears. If it is simply incomplete combustion, why does it disappear?

 

[charles_reed]

Yes, power curve would have been better!

Don't know about the urban myth. I have the instructionnfrom the Yanmar workshop manual at home, not with me now, and it is quite clear about the benefits that operating at maximum revs will bring. I guess that the treatment may benefit the manifold more than it does the engine itself but there is no doubt that the Italian Tuneup works very well in many cases. When I do it there is significant black smoke for a minute or so, which then disappears. If it is simply incomplete combustion, why does it disappear?

You're quite correct about the Yanmar standing instruction about revving the engine, occasionally, when operating at low revs. This has no bearing on carbon/soot production but is to correct the lack of oil on the bores when the engine is not running @ any speed. Most diesel engine designers will recommend the same action - low revs being considered as anything under 25% of peak.
As already pointed out - all diesels increase power by an increase in the amount of metered fuel being injected into the combustion chamber, this inevitably leads to an over rich mixture and black smoke from the exhaust; only corrected when the volume of air being drawn in matches the correct air/fuel mixture.
Hence the inevitable black smoke followed by a clearer exhaust.
This is most marked in indirect injection engines - with common-rail head engines and direct electronic injection into the firing chamber it is likely to be ephemeral. Additionally nearly all common-rail diesels rely upon a forced charge from a turbo-charger which makes the imbalance between air/fuel less marked.
However, I know of no marine diesels with common-rail heads and we're usually dealing with archaic technology, even when (in an attempt to boost power) the manufacturers fit a turbocharger.
It's possible that the "Italian tune", to which you refer, clears the injector nozzles and pre-combustion chambers, though this is only an opinion and no research has been done on it. The effect on IC engines is more marked, mainly because it clears deposits off spark-plugs and produces a much fatter, hotter spark.
Recent attempts to produce ever lower CO2 from diesels involve re-breathing exhaust gases at over-run BMEPs. Fortunately, this solution has only made it into production car engines in a couple of cases, as it's accident-prone, well proven when it was used on long-distance flights during World War II, such as the Ploesti raid. It is, however, a certain method of reducing carbon deposits in either IC or CI engines.
IMHO by far the greatest danger, for water-injection marine exhausts, is the build-up of carbon-based tar deposits in the water-injection elbow. This will be be accentuated by:-
1. Allowing an engine to produce black smoke - either due to constant acceleration/deceleration, over-propping, worn injectors or a blocked air-filter.
2. The use of high bio-diesel fuel (anyone who has tried to clean Castrol R off an engine will know what I mean).
3. The use of high-sulphur fuels (such as red diesel and that €0.18/litre Tunisian diesel).

So, in answer to the original OP, if you're running your engine near to max torque rpm, not only will you get the best bang for your buck of hydrocarbon, occasionally revving the engine is also unnecessary.
If you motorsail (as I frequently do) @ 1600/1800 rpm, it is probably a good idea to gradually increase revs from time to time before reverting to the plod.
As to the "expert" who suggests 3000rpm out of gear, I'd not accept his opinion on wine, women or song either.

 

[vyv_cox]

You're quite correct about the Yanmar standing instruction about revving the engine, occasionally, when operating at low revs. This has no bearing on carbon/soot production but is to correct the lack of oil on the bores when the engine is not running @ any speed. Most diesel engine designers will recommend the same action - low revs being considered as anything under 25% of peak.

I cannot quote the wording for reasons given but I don't think that is what Yanmar say. AFAIR they are talking about revs used for cruising, of the order of 2000 rpm i would suggest.

As soon as the oil pressure is up to a decent figure, little more than tickover with an engine in good condition and probably no more than 1000 rpm with a worn one, there is plenty of oil on the bores. Oil on the bores has no relationship to engine speed. My colleague and one-time boss did pioneering work sampling oil in the rings of running engines, for which he received a prestigious award. http://www.york.ac.uk/res/gkg/ms/Stark_et_al_tribology_letters_4_3_05.doc Much of that work was done at 1500 rpm but some was at tickover speeds, when there was never a lubrication problem.

 

[macd]

if you're running your engine near to max torque rpm...you get the best bang for your buck of hydrocarbon

+1
All IC engines produce the biggest bang-per-buck (in other words, the lowest specific fuel consumption) at peak torque rpm. In practice this won't necessarily equate to the very best mpg, since other factors such as prop efficiency, hull form etc may intervene to some degree. But it'll usually be close.

 

[Tranona]

at peak torque rpm.

Many small diesels have very flat torque curves with max torque often over a range of 1800-2600 revs (out of a max of 3400-3600) as you can see from the Beta power curves. So the "efficient" range is quite wide.

 

[Tony Cross]

What an interesting thread this has been. I've learned quite a bit. Thank you all!

 

[dslittle]

Hence the Italian Tuneup, running the boat at maximum revs for 15 minutes or so, is a well-known way of improving performance where it has deteriorated due to carbon accumulations. It also works for gasoline powered vehicles that are rarely revved anywhere near their maximum.

In a diesel car the same thing happens as with the boat, but every time you take your foot off the throttle the combustion chamber is flooded with oxygen, burning the carbon away.

Some time ago, a well known dealer for a predominantly red motor car used to take their service vehicles for a 'run' along the M40 at high speeds. This was part of the 'service' to clean out the engines which had mostly been used in Central London cruising around at about 10mph...

 

[vyv_cox]

Some time ago, a well known dealer for a predominantly red motor car used to take their service vehicles for a 'run' along the M40 at high speeds. This was part of the 'service' to clean out the engines which had mostly been used in Central London cruising around at about 10mph...

I have witnessed with my own eyes a boat owner being charged £100 for 'tuning' the engine, when the only treatment it received was a flat out run to the end of Holyhead breakwater and back. Oops, given it away now!

 

[Topcat47]

Italian tune up:-

Modern cars:- Running the engine at high RPM raises the temperature of the exhaust gases. This is desirable to purge the soot filters and helps extend the life of the catalytic converter.

MOBDs (Manky Old Boat Diesels):- At the end of the injection pump stroke, the last few drops of fuel do not form the ideal droplet size. This is significant at moderate powers. MOBDs run "cool" anyway. Running for a short time at max power will ensure the exhaust temperature is as high as possible which will encourage soot in the combustion chamber, exhaust port and manifold to burn off, rather than harden and "coke up". Once the exhaust gas reaches the the water injector, it gets cooled anyway.

 

[contessaman]

well just been reading this with interest/concern . my boat has a 50hp perkins which is a lot for an auxillary on a 32 foot yacht. I tend to cruise at 1100 rpm or less = hull speed in the flat sea. pushing this up to 1600 into a large sea. not sure what the max rpm of my perkins is? but it must be at least 3000rpm. so am I doing it harm? the only time I rev it more than that is a quick burst of astern etc when manouevring which sure enough usually gets the same black cloud of smoke I get from my diesel car when I shove it from 5th into 3rd to negotiate those b*ggers who have taken to dual file cycling in my area. (they deserve it)
I always start the motor and go -I never wait for it to warm up. should I hoof the boat engine from time to time then once its up to operating temparature? its a 36 year old engine never been rebuilt with 3500 hours on it. It starts and runs perfectly with little or no smoke. I cant help but think that maybe the extreme revving would do more harm to the timing gear, bearings, oil and water pumps than good to the insides??

 

[srah1953]

Boat and car diesels may be basically the same but the operating regime is vastly different. In a boat on a long trip, you may be doing the same revs for 24hrs, you'll never do that in a car. The so-called 'Italian tune up' works wonders on both boats and cars - vastly reducing the smoking from my Merc, the difficulty is finding a suitable stretch of road. Not a problem with a boat.

Thanks for this but it was the flooded with oxygen part I didn't understand and why this is different between boats and cars

 

[vyv_cox]

Thanks for this but it was the flooded with oxygen part I didn't understand and why this is different between boats and cars

Because cars spend a lot of their operating time on the overrun with foot off throttle. No real equivalent for boats.

 

[Tintin]

Gosh. If i revved my elderly volvo md5b to max every two hours i would be overheating every two hours.

I think this "advice" needs a caution for older engines that may have s few "issues" due to age.

 

[PeterBoater]

My Yanmar 4JH2-UTE manual says that for every 2 hours continuous running at below 1000 rpm, run at 2500 rpm for 1 minute. Maximum rpm on my boat is 3700 rpm.