The Offshore Voyaging Reference Site

Understanding An Engine Fuel Map

We’re going to take a look at several powertrain designs that, at least in theory, can offer performance improvements and fuel savings relative to traditional yacht diesel engine installations.

But before we can do that we need to take a closer look at how engines and propellers behave so we can start from solid, well-understood ground before we venture into the unknown.


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Eric Klem

Hi Matt,

Just curious what engine the fuel island plot is from if you don’t mind sharing? It seems to be an engine that is optimized for fairly high speed operation as the torque curve is relatively flat at the higher speeds and the area of maximum efficiency occurs at fairly high rpm. One thing that I find interesting is to compare the plots from mechanical injection and electronic injection engines. Probably the most interesting plot I have ever seen is for a 1.9L VW tdi engine.

I am looking forward to the discussion on matching propellers.

Eric

Eric Klem

Yeah, it can be hard enough to get a curve for private use. When I was doing the hybrid spreadsheet, I settled for including 2 curves that were publicly available on the internet. I have access to a few more but I know that some people would be mad if I ever posted them publicly. Two things that jump out at me in looking at these is that the relationship between the torque curve and the island plot is pretty consistent so you can get a decent idea without having the full plot which is good because the plots are otherwise remarkably different. I wish that I had one for our Westerbeke 38B four but I don’t.

Thanks.

Eric

David Popken

Good data, but in the real world, I’m not going to change out my 1994 Westerbeke 38B or the transmission with only 950 hours on them, unless I have a major failure. So I’m stuck with her for now, and realize that it is inefficient, but still workable. Certainly good to know that low rpms can be as bad or worse than too high rpms. It’s clear from my use of the hour meter to calculate gals used per hour, that 2500-2800 rpm is this engine and propeller’s sweet spot.

robert moulding

Hi Matt
Fantastic article
Without wishing to be pedantic and or drag you off topic
I was always taught, BMEP x surface area of piston x stroke = Torque
I appreciate we have just defined displacement, but it then leads to a discussion about “short” stroke and “long” stroke motors and different torque and power characteristics.

With regards to fuel burn Caterpillar, used to issue service and rebuild intervals based on fuel and burnt.

Best Regards

Bob M

John Harries

Hi Matt (and Eric),

What, if any, effect does the availability of computer controlled injection engines have on any of this? In other words do these engines have any ability to improve the matching between the load from the prop and fuel they are burning at less than WOT?

If I’m jumping the gun and that’s coming in a later chapter, just say so and I will wait to read it there.

Eric Klem

Hi John,

Hopefully I am not jumping the gun by chiming in here. As engines have gone from mechanical injection to common rail systems with a few iterations in between, there have been many other changes as well such as turbocharging, valves and designing for emissions regulations. These other changes have also had significant effects but some generalizations can be made about the specific effects of electronic injection.

The major difference in injection systems comes in the control of them. Older mechanical injection systems operated at much lower pressure, had much less precise injection events and had fixed timing. By getting much better control over the injection events, the engine can be tuned to run well over a wider range of rpms. The change is not drastic but it does help. This can be seen in the shape of the torque curve and also in the fuel island plot (or whatever you want to call it). In addition to efficiency, by using multiple injection events per combustion stroke that are carefully timed, the engine noise and emissions can be reduced.

If you want to see examples of plots for other engines, the engine efficiency tab in the hybrid spreadsheet shows 2. What is remarkable is just how different they are and how different they are from the one that Matt shows.

Eric

Rob Gill

Take a bow Matt,

You never fail to make “way-over-my-head” stuff really interesting and tantalisingly within reach! Being seated firmly in the back row of your AAC marine science classes, may I ask some dumb questions please:

You suggest choosing a propeller slightly greater in pitch to improve an engine’s fuel map, could be a good thing. Does this mean we could achieve the same thing by having a correctly pitched propeller with a slightly under powered engine, for a given displacement and hull form?

Background to my question – we own a Beneteau 473 of some 12 tonnes and 14.3 metres which has a naturally aspirated Volvo 55hp. Although it wasn’t our decision (bought 2nd hand), we have never felt under-powered, and we achieve excellent fuel economy. Our cruising speed is around 7.5 knots. But most other 473 I have seen advertised had 75hp diesels, and later “run-out” models even had 110hp. So could the old adage “a good big player will beat a good small player” be wrong for marine diesels? Has the industry been overselling us, heaven forbid?

Cheers,
Rob

John Harries

Hi Rob,

You should take the bow! You nailed it. Most sailboats are incredibility over-engined, which results in poor fuel economy and short engine life.

More here.

Rob Gill

Hi John, I hadn’t seen your earlier articles (still working my way slowly through the back catalogue). Seems we can add propeller tip and hull speed to life’s “gotchas”, right up there with death and taxes.

John Harries

Hi Matt,

I my experience with sailboats offshore you can’t even use that 100hp for the emergency mad dash to the harbour. The reason being that if that mad dash is upwind—if it’s downwind hoist some sail—most sailboats will pitch too much to get that power into the water—the prop will just cavitate.

The problem is the mast, which increases the pitching problem when motoring up wind much more than most sailors realize. (This is another reason that motor-sailors make little sense to me.)

On MC (56′ and 25 tons) we have still can’t use our full 87HP to motor upwind in any sort of a sea—close, but not quite—and we have a carbon mast, which reduces pitching hugely.

My guess is that most 40 foot boats would be better off with about 40 HP.

Eric Klem

Sorry for the late reply here, I have been off the grid for a few days. I wanted to add a few thoughts on engine sizing to what has already been said here. As has been said, many people want to have the ability to put down unusable amounts of power. In many cases, you hardly increase the boat speed at all and you can have large negative effects on efficiency.

The distinction that I want to make is between usable power and engine size. The plot that Matt shows scales reasonably well across engine sizes and if you notice the units, they are really efficiency units, not direct power or fuel consumption. If you were only to operate the engine at a single load and everything else was equal, you would choose an engine that put you in the efficiency island at this one load which in this case would probably be at 2200 rpm or so. By choosing an engine that runs at this speed and gives you your desired output, you would be choosing a much larger engine than you would if you simply looked at the maximum output. In a sailboat, I like to have some reserve so you can’t size exactly this way but going with a slightly bigger engine that is “overpropped” can yield some efficiency boost if you are really careful about specifying things. This can definitely be taken to excess and really only makes sense for people who generally use close to the maximum horsepower they want. There are many other problems as well such as size, weight and cost.

Engine fuel maps generally result in approximately constant cylinder pressure across the rpm band at WOT. If efficiency was constant (it isn’t), then the torque curve would be flat and horsepower would be linear with rpm. For engine designers, the easiest way to get more power out of an engine is to run it faster. Since you get about the same amount of usable work out of each combustion stroke, if you can do more of these events in a given amount of time, then you have more power. Therefore, there is a lot of incentive for engine manufacturers to make higher revving engines as they are smaller and less expensive. The unfortunate thing is that the efficiency falls off at higher rpms for a few reasons so it is not a great place to operate for long periods of time. By changing your prop loading, you are effectively changing the maximum rpm of the engine.

Eric

John Harries

Hi Eric,

Great comment, thank you. I thought I understood that area well, but I’m even more clear on it now.

Matt’s articles and your comments between them have got me to the point (finally) where I really get the theory behind what I have seen on our boat where going from a 120 HP 2800 RPM engine to a 87 HP 2400 engine that we have slightly over propped at 2350 WOT has yielded such a large fuel consumption and range benefit.

Interestingly the new engine has almost exactly the same max torque as the old.

Marc Dacey

Eric and John: This is precisely the reason I chose to install a Variprop, a design that allows in-water pitch changing and different pitches for reverse and forward rotation. When I bought the prop, it was some years prior to replacing our engine (I do everything out of order when it comes to boat gear). All I could discern was the rough displacement of the boat (prior to loading for passagemaking, so I added one tonne!) and my desire to have a “sweet spot” of maximum fuel economy at between 4.5 to 5.0 SOG in flat water. This is turning out to be about 1,900-2,000 RPM, indicating I might wish to alter the current pitch somewhat. But I intend to “prove” this empirically through fuel consumption recording, but the main point is that I will see the real world fuel map from the prop end, not the engine end.

That RPM, by the way, plus the four blades on the prop, gives plenty of power left to expend when I needed it. I have done some roaring emergency stops and I think we’ll be OK!

John Harries

Hi Marc,

Just to clarify, while the variprop will let you change pitch, it does not do that dynamically under load, (as far as I understand) and therefore it will not solve the basic problem that this post is about, and more than a properly specified fixed prop or a Maxprop would.

As far as I know, the only dynamically re-pitching propeller on the market is the Autoprop, although PYI inform me that Maxprop are working on one.

Marc Dacey

No, that is correct…it does not vary pitch “dynamically”, but my point was (in our application) our “motorsailing” RPM is largely fixed, or will be, at the point of greatest fuel economy for a rather arbitrary “we will get there when we get there” speed through flat seas…otherwise we would be sailing! So the pitch will be adjusted pragmatically to find that sweet spot. I have a friend with an AutoProp on a steel ketch with roughly our dimensionals, and while it’s a thing of beauty, we wanted a slightly different approach that would allow differential pitching for forward and reverse. I didn’t know about the MaxProp initiative, so thanks for that.

John Harries

Hi Marc,

While you are right that you can over pitch to get the sweet spot in flat seas at cruise in theory, you will then not be able to reach full revs at WOT. Worse still, I think you will find that if your engine manufacturer finds out, they will, as Matt says, void your warrantee because of the risk of lugging.

Further, no matter how you set the prop, you will be very inefficient while motorsailing. That’s just the way it is for all of us with props that don’t dynamically repitch, and the only way to fix that is to have a prop that can be made more course while motorsailing. If you were to set your prop to be efficient when motorsailing (very course prop setting) you would damage the engine in very short order due to lugging when not motorsailing.

None of this is to say that you should not overprop a bit—as we do on our boat (100RPM)—but I’m guessing that you will not be anywhere near the sweet spot for efficiency, as Matt explains in the post, unless you overprop a lot, and that has the issues I cover above.

Marc Dacey

John, thank you for the further explanation. As I’m not going to swap out this prop, I will merely aim for the “slightly overpropped” zone. We really do intend to sail as much as we can (which is where the feathering comes in; I have a Gori folder on my other boat and was mightily impressed by the difference between our previous fixed prop and the Gori). I will be happy if we are at 0.8 U.S. gallon/hr, given the boat, loaded, will weigh about 16 tonnes. I’m less interested in the ultimate speed…unless we need to stop quickly, and the engine can lug all it wants for the five seconds I need to hit full reverse. Thanks again for the clarifications.

John Harries

Hi Mark,

Happy to help. This stuff is right confusing and I’m only just getting my head really around it after three re-powers and endless tweaking.

One of the key things I have learned is that one must be careful about motorsailing too much because in that configuration if the sails are doing anything at all, and they are usually doing more than you think, then the engine is horribly under loaded and glazes up very quickly.

One thing you can do is to run the engine at WOT, or close too it, for 30 minutes after motorsailing.

Marc Dacey

I think I may have implied something I didn’t mean with the term “motorsailing”, John. We actually intend to motor (with perhaps only a staysail to steady us if needed, and bare poles if not) when we absolutely must. We will sail as long as we don’t wallow or slat, as we hope to be going nowhere fast, which is the case for any boat of our type, anyway. The plan is to motor at 75%-80% of WOT should we need to up anchor to get out to sea for the necessary pumping out…and then to come back, alternators spinning and batteries topped when we return. But for almost all passagemaking, I hope to merely sail. As I said, if you want speed, get on a plane!

Anyway, it’s a big topic and thank you for tabling it.