Did you know that many of us yachties are gradually, and sometimes not so gradually, wrecking our engines, as well as spending more on fuel and generating more carbon than we need to?
It’s true, so let’s look at the problem, and then the solution.
The Problem
Let’s define the problem: the single best thing we can do to keep our diesel engines happy—after regular oil changes and clean fuel—is to run them hard. What, I can hear you say?! Surely, if an engine is underloaded it will last forever (you are not alone, I used to believe this). Well no, the exact opposite is true.
In fact, what happens if you run your diesel engine at low power setting a lot is that the cylinder walls will glaze-up and then the rings won’t seat properly, power will drop, and your engine will start to suffer from the dreaded blow-by syndrome (as well as all kinds of other bad stuff), in which combustion gasses and unburnt fuel bypass the rings because they are no longer seating properly.
If you are seeing a slick on the water around your engine’s exhaust, and particularly if that slick remains after the engine is fully warmed up, you may have a glazed engine. Another tell-tale symptom is that the engine will no longer reach full throttle RPM, even though the boat hull is clean and the prop has not been changed.
If you catch this situation early enough, you may be able to correct it by running the engine hard for long periods—I have fixed a glazed engine myself this way—but if not, the only answer will be an expensive rebuild or a new engine.
Hi John,
While I agree with most of the observations and recommendations that you and Steve make, I believe that the recommendation you are making of 75% load is an oversimplification that is too conservative for most. What we are discussing at heart is how to make make engines last the longest before requiring rebuilding. Part of this is ensuring that we have good combustion and good oil life but there are also many other aspects such as ring wear. These different factors must be balanced for each specific engine design and application.
I agree completely with the recommendations to ensure proper operating temps. As stated, this isn’t simply a coolant temp measurement, oil temp and combustion temps are very important. Unfortunately, you can’t simply state a % operating load and get to safe temps. The reason for this is that there are a wide variety of engine designs with different characteristics. Even in a single base design, there can be many different configurations. For example, the Cummins 6BT came in configurations from ~120-400+hp continuous ratings and the Detroit 671 went from ~170-400+, the increase made by changing injectors, timing and turbos. These different versions need to operate at very different load ranges to get to the same oil and exhaust gas temps. I used to have a 6BT that would hit 600F EGT’s around 10-15% load, an ideal range of 750F or so around probably 30% load and by the time you got to 75% load, you were over 1100F, not a place I would want to operate most of the time. Getting into good temp ranges is important (and the ones Steve gives are very reasonable) but exceeding them is not constructive and it is easy to do that.
I think that it is important to look at the way most people use their engines and whether there are truly any issues. I suspect that it is a very small proportion of sailboat owners who actually run their engines at 75% load with any regularity. Many people run at 75% of max rpm but this is often more like 50% load depending on engine sizing and prop. There are indeed problems with light load running but they are not all that common and I suspect most people having problems are running at even lower loads than this. For example, many charter companies recommend charging at ~1500 rpm in neutral and these engines can get high hours at these low loads pretty quickly with surprisingly few problems. In the truck world, OTR trucks idle for days at a time with few problems although they do get blown out regularly. People with diesel pickups are often running at <10% load for extended periods of highway travel and while I know of some problems in these engines, they are again fairly limited.
On the flip side, there are aspects of running at higher loads that can shorten engine life even excluding damage from misuse. For example, ring wear is best approximated by a PV relationship where as cylinder pressure and piston velocity increase, the wear rate increases. In my experience, a decent rule of thumb is that an engine will burn a certain amount of fuel in its life and you can choose the compromise between power and hours provided you keep it reasonable. People occasionally try to cite fatigue as an issue but it is simply not an issue as engine parts subject to fatigue due to cylinder pressure need to be designed so that stress is always below the fatigue limit.
My recommendation would be to focus more on the transient states. Constant cold starting of an engine is hard on it, the lubrication is not great and there is a significant period of time before things warm up to normal operating temp. We should work to limit the time in this state by trying to limit cold starting through behavioral changes (I use this as an excuse to always sail on and off our mooring and the anchor) and putting an appropriate load on the engine as it is warming up. Synthetic oil is also interesting but not something I would switch to on an old engine. I think that the 75% rpm recommendation you often see is a decent starting point and that people should be on the lookout for any symptoms of issues with not being fully up to temp. Also, I agree completely with your recommendations on engine sizing and prop pitching.
Sorry for being so long winded.
Eric
Hi Eric,
Lot’s of interesting stuff, as always. I agree that the 75% recommendation is an over simplification, but in my defence I also say:
I will be significantly expanding on it in the next post, which is already written and while not as detailed as your comments, will go a long way, I think, to help people understand the issue and make better decisions about how they run their engines.
I will also cover the importance of achieving proper oil and combustion temperature.
The one place I might differ a bit with you is on how big the problem is. I’m seeing a lot of boats that are fitted with engines that are so overpowered for the hull form that many people are consistently running them at 20-30% power. And Steve D’ Antonio, who sees way more engines than I do, is seeing the same problem.
If you add consistent running at under say 40% load (not RPM) to frequent charging in neutral, as many sailboat owners do, I think the problem is actually quite common. As you say, you can get away with low loading for surprisingly long periods, but not if you never run at higher loads. A profile that fits a surprising number of recreational users.
Interestingly I had a fascinating conversation some years ago with the engineer on a super yacht with oversize generators. He was having a hell of a time with glazing because said generators ran 24×7, much of the time at 20% load or less. Finally he fixed it by installing a hydraulic take off on each and some kind of artificial adjustable hydraulic load (didn’t get the details on that). He found that using said gadget to load at 80% or so for 30 minutes every watch solved the problem.
I look forward to this next chapter, because I want to understand how PTOs and props (loads at either end of the drive train) can work in concert to get to the sweet spot where fuel efficiency crosses appropriate wear.
I also want to better understand, in light of your recent article on variable props, how prop pitching can be changed to load the engine properly.
Hi John,
I agree that designers are putting in engines which are too large which exacerbates the light load issues. As you suggest in the post, “overpropping” could really help in this situation to get better fuel economy and less light load issues without any sacrifice on usable power. Like everything, I believe in moderation in this and that going to any extreme is probably bad. When I was designing industrial air compressors, we always joked that we were really good at designing them for 50-75% speed and that we should just upsize everything. The fact was that we were trying to keep the displacement down so they were not very efficient at full speed.
On the topic of generators, this is a tricky one. Large yachts run them full time and size them for max load while they never run anywhere near there. I have seen exhaust brakes installed on them to get temps up just like adding an extra load, all of which is very wasteful. On smaller boats that don’t run the generator full time, I am a big fan of using solar or wind to get the last 20% of battery capacity which allows the generator to load up during the bulk phase and the renewable to do what it does best which is low power charging over long periods.
One thing that I should add on transients is that a turbocharged engine should not be shut down hot as the oil in the turbo will overheat.
Eric
Hi Eric,
Yes, I agree, over-propping can be great, but you need to know what you are doing and be careful not to over do it. I have 2500 words coming on just that and will really appreciate your input on it when we publish it in a couple of days.
And yes, I really like you point about using solar to top off batteries. In fact I’m looking at a installing a couple of panels on MC for just that purpose, if I can get over my total fixation on clear decks that is!
Sometimes I wonder how much damage is caused by “rules of thumb” in this field.
There’s such a huge variety of possible ways to set up and tune an engine that I have a hard time believing any general rule or guideline that looks too simple!
I agree with Eric that being aware of, and properly controlling, the transient states is critical. On engines large enough to justify the cost, I’m quite fond of oil pre-pressurization systems, for example, and I very much like to see electric block heaters on anything that’ll regularly be started below 0C. I’ve been running some interesting (although not very scientific) experiments on my cars lately, using OBD2-to-Bluetooth interfaces, and finding very dramatic changes in fuel consumption as various components warm up (notably, this process continues for several minutes after the coolant temperature stabilizes at nominal).
A major key, I think, is to get good numbers from the engine manufacturer for the specific model and rating you have. What’s the nominal-load EGT, the running-too-cold EGT, the nearly-overloaded EGT. What RPM range should it be reaching at full throttle. What are the tolerance bounds on the coolant temperature. How long should it be idled on start-up before being put under load (I’ve seen recommendations anywhere from 10 seconds to 15 minutes, depending on engine). What’s the recommended break-in sequence. Does it speak OBD2/CAN and if it does, what type of PID04 or PID43 LOAD readout is it going to give you (those are damned useful numbers if you can access them). Things like that.
The observation that “all members of a given engine family will burn about the same amount of fuel between overhauls, whether high power / low hours or low power / high hours” is one I’ve come across several times, and it does seem to be generally applicable (as long as you aren’t doing forced induction incorrectly).
Hi Matt,
I hear you on the drawbacks of rules of thumb, but on the other hand the big problem I have found is that marine engine manufacturers are very cagey with the specifications you mention. For example I had to badger the hell out of Perkins to get just two EGT readings, and I have more access there than the average boater because of my position here at AAC.
So given that the real numbers may not be readily available and that most boats don’t have an EGT I think that Steve S’Antonio’s rules of thumb, that I published this morning, are way better than nothing, particularly since he really does have very wide experience.
Having said that, you will notice that I do recommend an EGT for those planning to over-wheel by a lot.
Hi Matt (and John),
Since you brought it up….what is the practical difference between using PID04 and PID43? I understand one is calculated load and one is absolute load, but I have heard the way this is calculated varies by manufacturer.
How many of these metrics are useful for cruising operations vs diagnostics? I’ve found monitoring fuel flow to be useful as well as EGT. I also noticed John mentioned the difficulty in getting good numbers from the manufacturer.
Thanks,
Robert
Hi John,
Really enjoyed this article thanks, and Eric’s comment (Eric, I suspect you never need to apologise for being long winded on this site). By the way, I shared this article on my FB page since you have a “2 day, read for free” offer on. I will be interested to see what feedback I get from my sailing FB friends. One question is, should you make the follow-up article also free for 48 hours? My thought for what it’s worth – if sailors found value in the first article, they will sign up to read the second, at your very reasonable membership rates. Cheers,
Rob
Hi Rob,
I’m embarrassed to say that I made a mistake and it was not free until 10 minutes ago when I fixed it.
And yes, I agree, the second part will not be free. It’s only fair that if people want the whole story including specific information on what to do about the problem that they should become members, particularly since this kind of in depth how-to piece is takes a lot of time to research and write, and it’s the members that pay me to do that.
Hi.
Well, I just wanna add some thoughts on this subject.
Yes ,glazing cylinder liners is caused by Carbon build up (and will eventually end up in a scored and seized engine) mostly around piston rings and at piston top land which will polishing the liner and no oil will stuck and lubricate the polished surface. Poor quality of fuel, oil, or to cold combustion is the common reason to carbon build up in these areas as well as around the exhaust valves.
The clearance of piston and cylinder liner together with high combustion temp (full throttle) is also a factor to count on due to expansion of the piston which minimize the clearance.
Bottom line is to make sure to have high quality of fuel,oil and run on fully warm engine. The most fuel efficient is at Max torque according to the above shown “mapping-diagram”.
Regarding power for a displacement boat is a “rule of thumb” not to go less than 5hp/ton
(metric),,,,but the gear ratio have great impact of what size of an engine you should choose. It is quit a diff between, 2,15:1 than 1,66:1. On a given engine, a 2,15 can easily swing a 18″x14″ while 1,66 only can handle 17″x10″ prop. Normal operating rpm of a propeller is at 900-1300 rpm which will be the target.
Very interesting article/subject .
Thanx,,//Conny
Hi Conny,
Thanks for coming up with good engine information, as always.
I will be expanding on exactly those issues, particularly reduction ratios, in the next chapter in a few days.
One thing I would say though: I think that the old 5hp per ton rule generally yields engines that are more powerful than they should be, particularly for sailboats. In our case that rule yields 120HP, which is the size we had in the boat for nearly 20 years and in that time we established that it was without doubt more than we could use by about 50%! See this post for what we discovered: https://www.morganscloud.com/2010/04/27/selection-criteria-for-new-marine-diesel-engine/
Eric has also just done the drag calculations on the Adventure 40, and here again the 5Hp per ton rule would yield and engine that’s 30% too big.
The other problem I see with that rule is that it focuses people on top end HP, rather than usable power at cruise speed and torque, as you point out.
Hi John,
Great article and lots to ponder. I was wondering what your opinion is regarding the use of oil & fuel additives to improve oil performance and aid in reducing wear to injectors?
Love the website.
Fair Winds,
Ron Brown
Hi Ron,
Sorry, I simply don’t know anything about additives, other than that I believe there’s an awful lot of marketing BS surrounding them. The only additive I have used is Bio-Bore JF to limit microbial growth.
However Steve D’Antonio has a good piece on them here: http://stevedmarineconsulting.com/diesel-fuel-additives-part-i/
And thanks for the kind words on the site.
Thanks for a terrific article about an important topic, and thanks for long useful comments (Eric).
I think engines generally benefit great on a right running inn time. They will last longer, perform better and be in better shape. The most important hours in an engine’s life is when it is new. Many have the opinion that driving the engine neatly inn is good, because they think it will last longer. If an engine is run in under low heat the wear of the piston, cylinder and rings will be wrong in addition to the glazing problem. The day you will give full throttle then the pistons expand and the cylinders can pinch a little when the rings and cylinder wall has aligned themselves and been honed to each other, not okay to get the full potential of power.
My practice is when a new engine is warmed up and checked for any leaks I start up again and give it a lot of power, (not easy with road going vehicles). A turbo engine gets higher load on the piston rings and need fewer run-ins hours I guess. The hour amount is a little guesswork and some clear table I cannot come with. But if the engine is treated like it’s written about in on this site from its brand new it is good. I think I will lug on with a marine anchor soon from the stern, I have never tried this before since it will be my first new marine engine to run-in.
Regarding synthetic oil that is an interesting theme brought up. I do not know why the marine world do not use them much, is it the high price or the fact that we are changing oil often? But one thing is clear; I never use synthetic oil during a run-in period. That oil is so good that you have to use much more time to get the result you are after.
I’ve run inn many motorcycle engines hard with good experience. After warming up a new engine, then out on the racetrack with taut wire. Doing so my engines had less problems and was faster at the end of the season than most of the competitor´s engines that was nicely run in, -one of my secret trick at that time ☺
Hi Odd Arne,
I think that’s a really good point: Contrary to what many believe (including me up to a few years ago) a new engine will run in a great deal better for it’s entire life if it is not chronically underloaded when new. Having said that, it is important, I think, to follow the manufacturer’s guidelines on this.
Hi John.
I think most of the manufacturers are very conservative regarding load in the first hour of a engines life, and that is why most of us is in the belief that it is right to do so. Of course if a moving part has a small fault it will be broken faster under high load and I think they want to delay that to happen. Hopefully (to them) as long as possible, to the warranty time is out. The engines we get are tested and run for a little time before we get them, so they are not completely raw. Another issue is that it was necessary in the childhood of the Otto engine to run-in safe, and maybe that advice have followed into modern manufacturing.