Yanmar sweet-spot

[NZL50505]

4JH4-TE (75hp) low pressure turbo version.

We’re told to run them hard enough to maximise longevity of engine and turbo and I’ve seen quotes of anywhere between 75-85%.

My question is % of what? Power or rpm?

See below power / rpm curve which highlights the big difference.

Let’s say we are aiming for 80%:

- 80% of max rated power (75hp) is 60hp. This is developed at only about 2000rpm.

- 80% of max rated rpm (3200) is ~2600rpm.

Which is big difference.

Anyone which one Yanmar is referring to when they say “80%”?

[Don Reaves]

Since I have a tachometer, but no power meter, I’ve always assumed that they were talking about RPM. But since you bring it up, I now realize that that really was just an assumption. It would be interesting to hear from one of the mechanical engineers on the forum.

[rene460]

My experience in other fields lead me to believe that it is 85% of power that is intended.  The requirement is to have significant pressure in the cylinder to properly bed rings, load conrod bearings etc. and that power should be utilised at least 80% of the time.  Running in neutral at that speed, so with minimal power, does not really achieve that, and is generally described as contributing to glazed cylinders and premature wear.  This is very different from the general recommendations for automotive engines running on gasoline, and is due to a very different design philosophy between industrial diesel and automotive gasoline engines.  I must admit to not knowing which philosophy is recommended for automotive diesels but I expect the automotive load cycle is the governing factor, compared with a marine or industrial engine where continuous full load is more normal.

However, Yanmar also talk about “racing the engine” in neutral if circumstances prevent the recommended loading, which implies that it’s worth running at speed, even briefly, when you can’t satisfy the full power requirement.  

My normal outings involved about a mile of channel with a five knot limit, which required only 2400 rpm on my motor, which was rated at 3600 rpm.  Here there is a further ambiguity, as to whether they mean rated speed, or maximum continuous speed which were different in the manual.

However the comparison is not so simple as following that power curve.  The curve is basically showing the power output at full throttle.  When loaded by a reciprocating compressor or a heavy conveyor belt, the load torque is relatively independent of rpm and that curve applies.  But that is all industrial experience and is not exactly comparable with a marine engine application, and specifically for how we use our yacht engines.

When loaded by a propellor, the torque and power drop rapidly as rpm reduces.  When running at reduced speed, loaded by a propellor, the throttle is reduced so as to produce only the power absorbed by the propellor.  Running at 2000 rpm, your engine will not be producing the power indicated by the red curve, but will have its power output and cylinder loading limited by the throttle, so it actually follows the blue curve of the propellor.  Motor sailing will reduce the engine load even more.

I was never a maintenance engineer, so can’t testify to how much typical boat operation actually affects the engine condition in the long term.  I do remember one prominent forum member reporting that he liked to run the engine lightly, as generally recommended for an automotive gasoline engine for less wear and tear, (and many other forum members indicated at various times that they also generally run at moderate speeds for the same reason).  But eventually there was a post following a major service, where he reported that there was considerable evidence of cylinder glazing.  I can’t remember much more detail, but I assume he modified his operation in some way after that.  I hope he will join in with some more details of his experience.

I assume that Yanmar are well aware of the propellor load characteristic and make allowance for this in their marine engine design and operating recommendations, thus making the speed the essential criteria.  In view of this, I made a practice of running for a short while at full throttle after leaving the channel, then throttling back to about 2900 rpm for cruising, which is around the 80% of the maximum continuous speed specification, though obviously a lower percentage of the rated power.  The odd occassion where I did not leave the channel for long enough to do this, I generally “raced the engine” as Yanmar describe when I returned to the berth.  I hope and expect that this will have kept the engine in good condition for the new owners.

I hope that clears the air a little, but it probably raises new questions, so please feel free to continue the discussions.

rene460

[Charlie-Bravo]

I would also think that RPM is the intended guide, given no other instrument than the tachometer to be guided by, and 2600 rpm sounds reasonable …… as long as it’s not shaking the boat.
Bimbling along for hours at low revs is unlikely to achieve the optimum engine temperature, and living at the red line is going to wear things out quicker .
More importantly for long engine life would be the start up and warming of the engine components before setting off,  and frequent oil and filter changes.
Followed by, at the other end of a long motoring trip at steady revs, a 30-100 second burst at near max rpm to clear combustion deposits, (best done before entering a marina or peaceful anchorage)

In reality, your post heading of ‘sweet spot’ sums it up, you can hear and feel when the engine and drive gear is in its happy place, and should be a better guide than the tachometer reading.

CB

[zaphod]

I think for practical purposes 80% of WOT is a good rule of thumb.

I am told another good practice is to vary the speed if you are motoring for a long time, but I am not sure what the rationale is for that practice. I do periodically throttle up for a few minutes before dropping back to cruising speed.

On our boat 80% of max rpm is 2500 which gives us around 7.5kts in flat water, which is great, but if you look at fuel consumption 2000 rpm is the sweet spot, and burns quite a bit less than at 2500, so it is a bit of a conundrum. Perhaps varying the speed is the compromise.

We typically run between 2000 and 2500.

[MartyB]

No tack on my boat. Hull sp is 6.6 knots

I noticed about 1.5 gal per hour at max speed.
At 6 about .8 gph
5.4 about .5 gph

I don't mind the increase in fuel between the .5 - .8 gph, I generally shoot for 5.8- 6 knots water speed.

Some motors like the 16 hp 3 cyl turbo yanmar in my mini excavator, or the Kabota in a Bobcat I had, pedal to the metal ALL day! Generator use is similar. With this said, those applications are probably tuned back to the perverbial 80% as is noted by OP. That extra is for going against steep waves, current, wind etc for short blasts where needed.

IF you had an HP over torque curve chart. Generally speaking, where the two lines cross, is where you get the best power to fuel use. That's probably the 2600. Best fuel is initial peak torque, whis is probably the 2000 rpm range. Anything over 2000 is good, as long as the motor is not lugging!

My 02 from a slightly different angle.

Marty

[NZL50505]

Lots of interesting insights, experiences and practices.

But does anyone actually know for real?

If it’s so important to operate within a narrow range especially for prolonged periods, then why doesn’t Yanmar print in big bold red letters on page 1 of the manual:

“Your engine loves to work at around x,000 rpm and is happy to do that for hour after hour.”

“If you need to run at lower speeds then it’s important to run it at or above x,000 rpm for at least y minutes each hour, to protect engine / turbo.”

Why is it so difficult to find the true answer? I’ve searched my manual and anything else I can find online and it’s just not stated clearly anywhere!

[rene460]

Fair questions NZL.  Always hard these days to get any definitive recommendations in plain language.

We are left with the need to put together the best of our relevant experiences and hope for the best.

The big advantage of forums like this is that we can compare ideas and experience, draw on the common elements and have a good chance the consensus will at least help us avoid the worst disasters, and fill the worst gaps in our individual knowledge.

It would be nice to hear from Yanmar.  

rene460

[sunrise4]

Looking at the engine manual for the 3JH40, 4JH45, 57, 80, 110 I find the following on page 11:

The 3/4JH common rail series engines are designed to be operated at maximum throttle*1 for less than 5 % of total engine time (30 minutes out of every 10 hours) and cruising speed*2. 

The *1 and *2 have a footnote which adds:

1* maximum throttle: fuel stop power engine speed

2* cruising speed: fuel stop power engine speed -200 min1 or less. 

The maximum RPM for my engine is 3000 RPM, so from this I conclude (if I understand the '-200 min1' poorly typed expression correctly...) that I can run the engine at 2800 RPM all day long if I wanted to.  The manual does not mention 'power-%' to limit RPM, but maybe other Yanmar versions do.

Cheers!

[sunrise4]

One more... regarding a comment in another post above, same manual as in previous post on page 110:

When operating the engine at low speed for long periods of time, race the engine once every 2 hours. Racing the engine with the clutch in NEUTRAL, accelerate from the low-speed position to the high-speed position and repeat this process about five times. This is done to clean out carbon from the cylinders and the fuel injection valves

NOTICE: Neglecting to race the engine will result in poor smoke color and reduce engine performance.

Periodically operate the engine near maximum speed while underway. This will generate higher exhaust temperatures, which will help clean out hard carbon deposits, maintain engine performance and prolong the life of the engine.

Again, not sure if this applies to non-common rail engines as well, but may be worth considering

Cheers!

[zaphod]

Feb 3, 2023 14:14:31 GMT sunrise4 said:

One more... regarding a comment in another post above, same manual as in previous post on page 110:

When operating the engine at low speed for long periods of time, race the engine once every 2 hours. Racing the engine with the clutch in NEUTRAL, accelerate from the low-speed position to the high-speed position and repeat this process about five times. This is done to clean out carbon from the cylinders and the fuel injection valves

NOTICE: Neglecting to race the engine will result in poor smoke color and reduce engine performance.

Periodically operate the engine near maximum speed while underway. This will generate higher exhaust temperatures, which will help clean out hard carbon deposits, maintain engine performance and prolong the life of the engine.

Again, not sure if this applies to non-common rail engines as well, but may be worth considering

Cheers!

I think that holds true for all marine diesels not just the common rail engines. That is pretty much what I have been told for many years.

[barryl]

Hello,

Good discussion. One point I'd like to make is that diesel tachometers are NOT know for accuracy. Unlike a gasoline engine, which uses the spark plug timing for accuracy, the diesel engine tach is usually just an estimate. 

On my current 409 the tach reads very high. On the trip home, with a clean bottom, little current, and no large seas, running at 80% of power (3000rpm X .8 - 2400) yielded a speed of under 6 kts. What! This boat should be a lot faster than that. I thought about it for a bit and then pushed the throttle wide open (WOT). To my surprise the engine revved up to around 3400 RPM. So I throttled down to 80% of that ~2700 and was then moving at 7 kts. 

Now, when I'm on a delivery and the wind doesn't cooperate I run the engine at an indicated 2700 if I have time and an indicated 3000 if I'm in a hurry. At the higher speed I can make 7.5+ kts 

Last August I made a long trip from my home port to Portland Maine. I used the engine when I needed to. I got around 1 gallon / hour and moved at ~7 kts under engine power most of the time. I am very happy with the engine motoring performance. 

The engine uses no oil and the oil looked very clean when I changed it at the end of the season (after 50 hours or so).

Barry

[Don Reaves]

You don't actually need a tachometer to get a good measurement of RPM. I calibrated my tach using an app on my iPhone that's intended for musicians. I have it listen to the engine and give me the frequency. Then a simple calculation based on the number of cylinders and the fact that there are two revolutions for per power stoke will give you an accurate value. I found that my tach reads 9-10% high over most of the range.

[NZL50505]

Feb 4, 2023 0:02:35 GMT Don Reaves said:

You don't actually need a tachometer to get a good measurement of RPM. I calibrated my tach using an app on my iPhone that's intended for musicians. I have it listen to the engine and give me the frequency. Then a simple calculation based on the number of cylinders and the fact that there are two revolutions for per power stoke will give you an accurate value. I found that my tach reads about 15% low over most of the range.

That’s the funniest post I’ve seen for a while - you managed to calibrate your diesel engine tacho with a piano tuner??!! That’s genius. Love it! 😀

[NZL50505]

Agree it would be nice to hear from someone closer to Yanmar for a more official response / confirmation of the wisdom shared here. All of which sounds close but it would be nice to hear it ratified.

I would try asking one of local NZ Yanmar agents but sadly they are not the most switched on bunch…

[manaia]

Feb 4, 2023 9:13:11 GMT NZL50505 said:

Feb 4, 2023 0:02:35 GMT Don Reaves said:

You don't actually need a tachometer to get a good measurement of RPM. I calibrated my tach using an app on my iPhone that's intended for musicians. I have it listen to the engine and give me the frequency. Then a simple calculation based on the number of cylinders and the fact that there are two revolutions for per power stoke will give you an accurate value. I found that my tach reads about 15% low over most of the range.

That’s the funniest post I’ve seen for a while - you managed to calibrate your diesel engine tacho with a piano tuner??!! That’s genius. Love it! 😀

The Yamaha motorcycle icon consists of three piano tuning forks, I was told that they were used in the early days to “tune” the exhaust note to identify top performance of the two stroke motors…

[manaia]

Lol , and that’s how urban myths are born.

A quick search shows that Yamaha started off making pianos (hence the logo), but moved into motorcycles later, retaining the same logo…

[MartyB]

Yamaha is a General Motors/Group Beneteau equal in Japan. THey make sailboats too. I believe a car model or three in that area of the world too.

I know of a number of late 70's sailboats here in the Puget SOund region of Washington state.

Marty

[MalcolmP]

Feb 12, 2023 4:37:52 GMT MartyB said:

Yamaha is a General Motors/Group Beneteau equal in Japan. THey make sailboats too. I believe a car model or three in that area of the world too.

I know of a number of late 70's sailboats here in the Puget SOund region of Washington state.

Marty

As always Marty is correct 🤠

I didn't know this, here is the Sailboatdata entry;

sailboatdata.com/builder/yamaha

[MartyB]

Found a Yamaha boat site. They appear to have switched over to power boats in the 16-25 ft range.
No sailboats in the market anymore.

Marty

[Trevor]

I love the way Don checks his engine revs........ I'm inspired to do a bit of experimenting myself.

[Don Reaves]

The app I use is called PanoTuner. I believe it is available for both iOS and android phones. And best of all, it’s free.

[Elcano]

Hi,

I checked the little plate on my motor, which also shows the motor number and Yanmar writes the values for continuous power and fuel stop power at certain revs on there.
Prob an answer for the initial question?
Cheers

[Trevor]

This is an interesting subject and although I cannot speak specifically about Yanmar I do have some experience in this area.

Many years ago I used to be very involved in large diesel generator sets for TV stations. I used to write controlling software for them and at one stage project managed the installation of some large sets. A caterpillar 1MVA (800kW) generator set uses turbos to achieve their top power rating.

After running one of these sets in standby mode for several months I noticed the turbos were literally leaking oil out of them. This oil would run down the side of the engine and didn’t look good. Here was a brand new engine, leaking oil out of the turbos and that had to be a warranty issue. I had to call in the experts and that is when my education on this subject was to begin.

When the turbo charged diesel engine is running under low load, the oil can seep past the oil rings on the piston and make its way to the top of the cylinder bore. Eventually in the case of the caterpillar engine, the ultimate manifestation of the oil seeping past the oil rings is the turbos leaking that oil.

The term used for this problem is called slobbering. If the diesel was slobbering it needed to be given a serious workout. This meant we would load the genset as much as we could to work it hard. That would burn the oil off, and remove deposits from the bore that would otherwise glaze the bore creating a permanent problem with oil seeping past the oil rings.

We had two gensets, one was half the rating of the larger set. The cummins generator would take a third of the station load and the caterpillar would take 2 thirds. Hence the tricky controlling software required to parallel the generators, load share them and parallel to mains power to allow a seamless transition to generator from mains and back again. Another issue with turbocharged generators is they are rated at a power that cannot be achieved in one hit. The load would have to be staged in as any load over about 55% of the rated load would stall the engine and it would fail on under-speed. Software was required to bring loads on sequentially to ensure the shock load did not stop the engine.

We would turn on all studio lights in all studios and load as hard as we could for at least an hour to “blow the cobwebs out”. If we could get the genset to over 100% load we would. You could load those generators for a short time to 110%. At one stage we brought in a huge resistive load bank on a semitrailer low loader to really workout the whole system.

The reason that load was required was to burn off oil residue from the bores so as not to glaze the bores. If the bores are glazed the engine will use excessive oil and the slobbering would not be able to be stopped. The slobbering also eventually stops the turbos from spinning causing a loss of power for the engine. Without turbos, the engine reverts back to its naturally aspirated rating, much lower than the turbo rating.

In my case the requirment was to supply power, nothing to do with RPMs. That engine had to deliver raw power and the manifolds and turbos glowed cherry red when they were delivering 100% or over rated power. In fact because the generator was producing 50Hz mains power the RPMs remained very constant.

This provides an interesting conundrum for selecting generator size for a facility like a TV station. If the generator is too large, running it lightly will damage it. If it is too small, the loads need to be staged carefully to avoid stalling as the shock load is too high. Also, for redundancy normally two would be installed. If run together the load will be too low for each, if run separately they are under-utilised. Many facilties have load banks so the load vcan be applied to reduce the possibility of glazed bores.

Anyway, I do ponder these things when running my little 54HP Yanmar. I accept that running the engine at pretty well idle for hours does it harm but what can we do? That is surely the job for an engine in a sailboat.

I try to run the engine at Wide Open Throttle occasionally just to “blow the cobwebs out” but realise that the runs at that power load do not last very long so maybe not doing much good. I think the nature of the sailboat use is such that we just need to accept that our little diesel engines at not being optimally used but they do seem to last a remarkably long time.

I hope this helps a bit.

Trevor

[sitara]

Thanks for his Trevor, I now understand a lot more about my diesel. I love the term slobbering!!