Specific Fuel Consumption- different engine types

[DavidM]

Warning, this is long and nerdy.

Well, it is cold outside and I am sitting here a little bored with myself, so I thought I would share some research I did on outboard engine specific fuel consumption. This is in line with a couple of recent threads on O/B powered trawlers. It is also applicable to I/O and inboard shaft drive gassers.

All TF members (well at least us nerdy types) know that marine diesels make about 20 hp per gph of fuel burned. That is an approximation and older diesels like the Lehman and Perkins make about 16 hp/gph at medium rpms and modern diesels like the Cummins QSB make about 19 hp/gph at medium rpms. These numbers are easily obtainable from the manufacturer's hp and fuel consumption curves.

What about outboards and gassers in general? I have never, ever seen a manufacturer supplied power and fuel consumption chart for any marine gasoline engine. So how can we know how much fuel that they burn?

One resource is the test data supplied by boattest.com. On all boats that they test they install fuel consumption instruments and publish what looks like very professionally done performance and fuel consumption data. I am impressed that they also list the specific test conditions including test weight which is often 50% higher than the manufacturer's listed dry weight.

But that is only good for figuring fuel consumption at wot values. That is because wot is the only place that you really know the horsepower that results in the reported fuel consumption- the manufacturer's rated maximum hp output.

So taking this limited data point, I have discovered that outboards produce about 10 hp per gph. This is half of what a modern diesel will make. Inboard gassers do a little better- 11 or so, probably because they have better controlled injection and air/fuel ratio which is required for their automotive origins. Also no one pays much attention to wot fuel consumption for outboards.

But the real question isn't how much fuel is burned at 5-6,000 rpm at wot; it is how much fuel is burned at cruising conditions, 3-4,000 rpm. Boattest reports fuel consumption at those rpms, but not hp. So how do you get the hp that is produced at 3-4,000 rpms.

Well, boatdiesel comes to the rescue and even though the power required calculator on that site is presumed to be used for diesel engines, it works just fine for gassers. Remember the old adage: props move boats, engines only turn props. The calculator doesn't know or care what is turning the prop.

So, I took a recent boattest for a pair of 200 hp Evinrude E-Tecs driving a 25' express cruiser and plugged the data into boatdiesel's calculator. I first used the test weight and the rated hp to see if it matched the observed wot speed. It was close but to get it to match I had to add about 10% to the boat's weight. Then I asked boatdiesel's calculator to figure the hp required to drive the boat to 33 kts, the 4,000 rpm value reported on boattest.

When I divided that hp by the gph that boattest reported, I got 11.8 hp per gph. That is about what I expected- a 20% improvement at medium rpms compared to wot.

I have done the same for I/O gassers and I don't get that much improvement, maybe 10% and about 12.5 hp per gph, probably because the wot starts out more efficient.

I considered doing the same exercise at low rpms, maybe 2,000 which would be where you would run to stay at displacement speeds, but I figured that the boatdiesel calculator would be too inaccurate that far from its wot data point.

The E-Tec two cycle engine that I based the above example on was surprisingly efficient compared to 4 cycle O/Bs, in fact it was essentially the same. Older, no longer sold in the US two cycles are pretty horrible- 6-8 hp per gph but the E-Tec with its oil and fuel injection competes very favorably with 4 strokes. It is essentially the gasoline version of the new electronic DDs.

So my conclusion is that you will use much more fuel to power your trawler with a gasoline engine- 16-18 hp per gph for the diesel vs about 12 for the gasser or roughly 30% more. That is roughly the same as for automotive engines.

In the marine world, gassers have some real benefits, particularly outboards: cheaper- roughly half of the total powertrain cost of a diesel and lighter- again roughly half. Another advantage is that repowering can be as simple as unbolting and rebolting four transom bolts.

I am sure that any of you can list the advantage of diesels which are many besides better fuel consumption, as well as some important disadvantages of gassers.

David

 

[Wxx3]

One question I have, that I may have not understood above, is that I thought gas engines were most efficient at WOT in terms of power produced?
In that they became ever more inefficient at smaller and smaller throttle settings, thus idle was extremely inefficient??

Your thoughts.

Richard

 

[FF]

"is that I thought gas engines were most efficient at WOT in terms of power produced?"

Most power is never most efficient use of fuel.

As tech gets better the modest power area keeps improving.

C&D discussed a new concept to cruise at a 30-1 fuel air mixture rather than about 15 as now done.

Back in the day diesels used pre combustion chambers.

This allowed the fuel to be injected where it would combust and "squirt" burning into the combustion chamber igniting a lean mixture of not yet burning fuel.

Izusu and others did this .

Now they have discovered a fuel injector can make a tiny squirt that is lit by a spark plug with the same effect. The ignition of the tiny fuel ball will ignite the rest of the chamber at unheard of lean mixes.

Cruise at modest power is cheap, full power , same as always, expensive.

 

[DavidM]

One question I have, that I may have not understood above, is that I thought gas engines were most efficient at WOT in terms of power produced?
In that they became ever more inefficient at smaller and smaller throttle settings, thus idle was extremely inefficient??

Your thoughts.

Richard

Well, there is efficient in terms of power produced and all engines produce their maximum power near the rated rpm (well, duh!!!).

But both diesel and gas engines are most efficient in terms of fuel burned per horsepower produced (or hp per gph, the inverse) at less than wot. Diesels for sure and I also suspect gassers are most efficient at about 80% of wot rpm which is not coincidentally the maximum torque point for most diesels.

And yes, hp per gph efficiency declines with rpm past that point and yes at idle they are very inefficient- maybe 10 hp per gph for diesels I am guessing, near 5 for gassers.

I will leave it to others to explain why. It is time for an afternoon nap!!

David

 

[Wxx3]

Yes, I mis-spoke. intending to say the max torque rpm was the most efficient.

What I am trying to say is that in my limited understanding of Diesels and gasoline engines, I've always thought that the crucial difference is that a diesel can be efficient (within 95%) at a very wide range of rpms, from idle to past max torque mostly because diesel's power at any given throttle setting is a function of fuel only (with excess air always being available at any given power setting). Therefore at idle, a diesel only needs enough fuel to make enough power to keep itself turning, whereas a gas engine, having a specific air to fuel ratio, means that for any given cylinder volume and throttle setting, the amount of gas needed at idle is 100x what a diesel needs.

 

[Donsan]

So taking this limited data point, I have discovered that outboards produce about 10 hp per gph. This is half of what a modern diesel will make. Inboard gassers do a little better- 11 or so, probably because they have better controlled injection and air/fuel ratio which is required for their automotive origins. Also no one pays much attention to wot fuel consumption for outboards.

Sorry, I don't understand how you came up with that number. The numbers I have seen and admittedly they are automotive numbers, are one gallon of unleaded gasoline will produce 15 hp for an hour and one gallon of diesel will produce 20 hp for an hour. If you look at the gasoline gallon equivalent tables (see https://en.wikipedia.org/wiki/Gasoline_gallon_equivalent), they indicate diesel fuel is only 14% more efficient than gasoline. I guess the problem I am having is I can't see a diesel inboard being twice as efficient as an outboard given the gasoline gallon equivalent tables. What am I missing?

 

[DavidM]

Nap time has passed, so here goes an answer to your last post:

Diesels are about 33% thermodynamically efficient, ie the btu heat energy of fuel required to produce a btu equivalent of rotational energy is about 3:1. Gassers are about 25-30% efficient.

Diesels gain their efficiency as you surmise by always operating on the maximum air that can be drawn into the combustion chamber on each stroke- there is no throttle plate. Diesels unlike spark ignition gassers can burn almost almost any air fuel ratio desired. So at idle as you indicate the fuel required is a very small amount injected into a full volume of air.

Burning the right amount of fuel to turn the crank using a full stroke of air is what makes a diesel's thermodynamic efficiency.

Gassers on the other hand can only ignite a combustion chamber full of air and fuel within limited ratios- something like 15-20:1 (guessing here). Otherwise it won't fire. Modern EFI engines try to keep that at near stoichiometric (the point where all of the air is used to burn the fuel and no more or no less) for both efficiency and emissions control.

That means at idle when there is no external load (there is still significant parasitic internal load) it takes very little fuel but because the fuel/air ratio must be met, it also means very little air, ie the throttle plate is closed so that the intake is under a significant vacuum. This makes for very poor thermodynamic efficiency.

Note that this throttle plate derived inefficiency goes away at full throttle. Gassers are most efficient at full throttle, but because internal friction increases with rpm, the best efficiency occurs near the peak torque point which is often half of the peak power output rpm.

David

 

[DavidM]

Donsan:

I came up with the 10 hp per gph number for the E-Tec outboards by taking the rated hp of the two engines used in the boattest test- 400 hp and dividing it by the gph of fuel reported by boattest of about 40 gph at wot to get 10. It is a real world number, no theory here.

Diesels beat gassers for two reasons: 1) a gallon of diesel fuel has about 14% more energy than a gallon of gas as you indicate and 2) diesels are more thermodynamically efficient than gassers due to the reasons discussed in my previous post to Wxx3.

Outboards are worse than automotive gassers at high power because emissions aren't as critical so they probably dump more fuel in at high power to keep the valves and pistons cooler. At medium power output I suspect that both automotive and outboard gassers are both at about 12 hp per gph.

At the same relative power output, diesels make about 18-19 hp per gph so they are about 33% better than gassers both due to more energy in a gallon of diesel but also because the diesel combustion cycle is more thermodynamically efficient than the throttled gasser.

David

 

[Donsan]

I came up with the 10 hp per gph number for the E-Tec outboards by taking the rated hp of the two engines used in the boattest test- 400 hp and dividing it by the gph of fuel reported by boattest of about 40 gph at wot to get 10. It is a real world number, no theory here.

Diesels beat gassers for two reasons: 1) a gallon of diesel fuel has about 14% more energy than a gallon of gas as you indicate and 2) diesels are more thermodynamically efficient than gassers due to the reasons discussed in my previous post to Wxx3.

Outboards are worse than automotive gassers at high power because emissions aren't as critical so they probably dump more fuel in at high power to keep the valves and pistons cooler. At medium power output I suspect that both automotive and outboard gassers are both at about 12 hp per gph.

At the same relative power output, diesels make about 18-19 hp per gph so they are about 33% better than gassers both due to more energy in a gallon of diesel but also because the diesel combustion cycle is more thermodynamically efficient than the throttled gasser.

I must be misreading something but it seems to me your initial proposition was a diesel at 20 hp per gph was 100% better than a gas outboard at 10 hp per gph. Now it looks like you are saying a diesel at 18-19 hp per gph is 33% better than a gas outboard at 12 hp per gph. In my mind that is saying a diesel is 50-60% better and not 33%. I have looked at the Yamaha fuel tables for our Yamaha 150 which indicates over 4 mpgs at 4000 rpm. On flat water, our 19 foot Hurricane will cruise at 25 mph at 4000 rpm. That seems rather efficient to me. I can see a diesel being more efficient but not somewhere between 50-100%.

 

[twistedtree]

Good stuff. Thanks for posting.

One thing to keep in mind when comparing BSFC is that the energy content of a gallon of diesel is about 15% higher than then energy content of gasoline. So for a fair comparison you really need to factor that out. But still, a diesel is more efficient even after correcting for fuel energy content.

 

[koliver]

Good stuff. Thanks for posting.

One thing to keep in mind when comparing BSFC is that the energy content of a gallon of diesel is about 15% higher than then energy content of gasoline. So for a fair comparison you really need to factor that out. But still, a diesel is more efficient even after correcting for fuel energy content.

Are you comparing the energy content of 100% gasoline to diesel? What happens to the comparison when you factor in E10 or even E15? Surely those have a much lower energy content per gallon, or I wouldn't need always to strive for E0 for my outboard.

 

[DavidM]

I must be misreading something but it seems to me your initial proposition was a diesel at 20 hp per gph was 100% better than a gas outboard at 10 hp per gph. Now it looks like you are saying a diesel at 18-19 hp per gph is 33% better than a gas outboard at 12 hp per gph. In my mind that is saying a diesel is 50-60% better and not 33%. I have looked at the Yamaha fuel tables for our Yamaha 150 which indicates over 4 mpgs at 4000 rpm. On flat water, our 19 foot Hurricane will cruise at 25 mph at 4000 rpm. That seems rather efficient to me. I can see a diesel being more efficient but not somewhere between 50-100%.

I started out this thread by posting that we use a rule of thumb of 20 hp/gph but in reality it is 16 for older diesels and 19 for modern ones at moderate rpms. Then I used the boattest data to show that the twin E-Tecs make 10 hp/ghp at wot and then went on to extrapolate that to 4,000 rpm using boatdiesel's calculator to show it to be about 12 hp/ghp at that more moderate loading.

So I based my conclusion on 18 (sort of a compromise between the Lehman and the QSB) for the diesel and 12 for the E-Tec gasser.

Do your Yamaha fuel tables give gph burned vs hp or just against rpms. If the latter they are just like the boattest data- they don't say anything about hp/gph at lower loadings.

David

 

[Donsan]

Do your Yamaha fuel tables give gph burned vs hp or just against rpms. If the latter they are just like the boattest data- they don't say anything about hp/gph at lower loadings.

Just rpms, mph, gph and mpg. Here is the performance bulletin I found when I bought the boat new. Really would have preferred the Yamaha 115 hp engine since we don't go over 25 mph. It had a lot better performance curve.

 

[psneeld]

This was passed along to me.....

"One thing to keep in mind when comparing BSFC is that the energy content of a gallon of diesel is about 15% higher than then energy content of gasoline."

That is one of the reasons why specific fuel consumption is based on weight, not volume like they are all doing. Their contortions just confuse the situation.

BSFC, more accurately Brake Specific Fuel Consumption is measured in pounds per brake horsepower hour, the weight of fuel burned to produce one horsepower for one hour.



Also..... since I was a kid..... taking the hp of a 2 stroke outboard and dividing by 10 gave you the fuel burn at WOT.

I cant believe todays outboards arent a bit better on fuel consumption.

 

[twistedtree]

This was passed along to me.....

"One thing to keep in mind when comparing BSFC is that the energy content of a gallon of diesel is about 15% higher than then energy content of gasoline."

That is one of the reasons why specific fuel consumption is based on weight, not volume like they are all doing. Their contortions just confuse the situation.

BSFC, more accurately Brake Specific Fuel Consumption is measured in pounds per brake horsepower hour, the weight of fuel burned to produce one horsepower for one hour.

Good point. I see that by weight, diesel is about 5% higher energy than gasoline.

That said, Dave's simplified comparison is based on fuel volume rather than weight, so in that context I believe the 15% number would apply.

 

[DavidM]

Just rpms, mph, gph and mpg. Here is the performance bulletin I found when I bought the boat new. Really would have preferred the Yamaha 115 hp engine since we don't go over 25 mph. It had a lot better performance curve.

All you can infer about specific fuel consumption from that chart directly is that at wot it makes 150 hp and burns 15.5 gph of fuel or 9.7 hp per gph.

Oh and I don't think that you wasted your money on the 150, presuming it has more liters of cylinder volume. More liters means more engine life at the same cruising hp.

And to psneeld:

I think that the divide hp by 10 to get gph consumed is a lot like the 20 hp per gph rule of thumb for marine diesels: a convenient round number but the reality can be different.

I believe that an old 2 stroke at half throttle got about 8 hp per gph, and a modern 4 stroke or E-Tec gets about 12, a big improvement- 50% better than the 2 stroke or 33% worse than the 4 stroke however you want to look at it.

David

 

[bayview]

How does that data apply to prop curve consumption?

 

[JLD]

There are actually Yamaha performance bulletins for hundreds (actually 1140) of different boat and engine combinations. The main link to these bulletins is https://yamahaoutboards.com/en-us/home/owner-resources/all/performance-bulletins

What was surprising to me, the first time I started looking at these, is how thirsty many outboard boats are. In hindsight, not to surprising since many boats have a couple of hundred horses and are often driven on 'plane'.

Here's a post that I made on the TugNuts (Ranger Tugs) forum last year:

"Performance numbers, when you can find them, are always an interesting read.

Some folks assume that a smaller boat is more fuel efficient than a larger boat, but that is only sometimes true. If you look at the published fuel burn on a R29, it actually is more efficient than the numbers shown in the Yamaha performance bulletin for the R23. On the other hand, you won't get a top speed on 38 mph on the R29!

Looking at the Yamaha data, at 12.7 mph (11 knots) the R23 gets 1.44 mpg. If one normally cruises at this speed, then maybe there are better boating choices that the R23. However, if one likes to cruise at 30 mph, the R23 gets a very respectable 2.58 mpg.

Boating is a very personal thing. Some folks like to go slow, some folks like to go fast. My ideal boat is one that I can cruise slow, yet have the ability (when needed) to go fast (or at least faster).

So many considerations when buying a new boat!" ​

Putting things into perspective, the R29 is a 29' boat with a 10' beam and a 260 hp inboard diesel while the R23 is a 23' boat with a 8.5' beam and a 200 hp Yamaha outboard.

Jim

 

[twistedtree]

All those Yamaha performance bulletins are great, but they don't tell what the HP requirement/production is at the operating points other than WOT. I think Dave's back-calculation of the approximate HP is exactly the way to fill in the blanks, given the information that's available to us. It's not perfect, but not bad either, and certainly passes the smell test/sanity test.

 

[twistedtree]

How does that data apply to prop curve consumption?

This is exactly what he is trying to calculate, in the absence of manufacturer's providing the data, which for gas engines, they don't.

 

[JLD]

Here are some other real world gas outboard numbers from the Yamaha performance bulletins that are in the 10 hp per gph area at wot:

• Cutwater C-302 (their new 30' boat powered by twin Yamaha 300's, 600 hp/53.6 gph = 11.2
  
• Ranger Tug R-27 (their new 27' boat powered by a single Yamaha 300, 300 hp/26.5 = 11.3

Jim

 

[DavidM]

How does that data apply to prop curve consumption?

There has been a lot of data bandied about this thread. Which are you asking about?

Prop curve consumption can be thought of perhaps in two ways: actual horsepower required for a specific boat at different rpms or a theoretical curve of horsepower required vs rpms usually based on a 2.5, 2.7 or 3.0 exponent formula. The latter is purely theoretical and is not based on any real boat data.

I used boatdiesel's power required calculator which I understand is a bit more sophisticated than the simple exponent formula to calculate the theoretical power required to move the test boat to 33 kts at 4,000 rpm. It is theoretical for sure, but because I "calibrated" the output so it gave real world wot numbers and my calibration was less than 10%, I think it is pretty reasonable, probably within 10% of real world hp.

David

 

[markpierce]

All I care about is that diesel fuel minimizes dangerous vapors and has more energy. Nevertheless, I'm in the minority since marine gasoline is much more available than diesel. Understandable, since most boaters don't have trawlers.

 

[JLD]

The Yamaha test bulletins are for specific boat/Yamaha engine combinations offered by the boat manufacturers. When manufacturers offer different Yamaha engine options, they to are sometimes listed in separate bulletins. The bulletins give data in 500 RPM intervals from 1000 RPM to wot. Data listed is RPMs, MPH, GPH, and MPG - with the data also grafted.

While the information doesn't give the HP at operating points, it does give potential buyers a lot of data.

Jim

 

[OldDan1943]

A general rule of thumb is, the more stuff you put on and in the boat, the greater the fuel consumption.

Realize, the factory/builder reported fuel consumption is 1/2 water, 1/2 fuel, sanitary tank empty.
In the real world, we need a lot more stuff.

Once you have loaded up the boat, construct your very own fuel consumption chart speed/RPM/fuel consumption. It will be as important as a compass card.
Oh wait, do you have a current compass card?

 

[Nomad Willy]

Diesel engine efficiency can be dramatically shown by trying to fry an egg on the exhast manifofd. They say you can’t do it at an idle w a diesel. No problem w a gasoline engine. Huge difference in the manifold temp. But at full throttle they are about the same.

I read that too in a book about repowering boats. They said that a gas engine burns the same as a diesel at full throttle. The book was dated and at best it’s probably basically a rule of thumb. But also probably close enough.

 

[North Baltic sea]

A general rule of thumb is, the more stuff you put on and in the boat, the greater the fuel consumption.

Realize, the factory/builder reported fuel consumption is 1/2 water, 1/2 fuel, sanitary tank empty.
In the real world, we need a lot more stuff.

Once you have loaded up the boat, construct your very own fuel consumption chart speed/RPM/fuel consumption. It will be as important as a compass card.
Oh wait, do you have a current compass card?

Just so last time my NT weighed when it was lifted in the winter warehouse, weighs +13,000kg and the manufacturer announces 10,500kg. Everything is accumulating on a boat ...

NBs

 

[sunchaser]

Good point. I see that by weight, diesel is about 5% higher energy than gasoline.

That said, Dave's simplified comparison is based on fuel volume rather than weight, so in that context I believe the 15% number would apply.

This interesting point is where it gets confusing. The end game for larger boats and mobile equipment is measured in cost per mile or per operating hour. So gallons or liters tend to rule for cost comparisons. And yes, there is fuel consumption variation within Diesel engine brands for the big engine users if one uses dyno testing as a tool.

But how about smaller diesels for lighter recreational duty? A more fuel efficient marine diesel that is expensive to purchase and maintain may well not be more cost effective than a less fuel efficient outboard. The outboard is lighter, cheaper to fix and with maintenance down time much lower if replacement required.

This exact overall cost conundrum is the inspiration for all sorts of outboard powered recreational vessels vessels between 20 to say 35 feet. So the end game, or one's intentions, really rules. In terms of pure numbers, outboards rule. We are even debating the pros and cons of an outboard trawler.

Lots to think about, thanks David.

 

[FF]

"A more fuel efficient marine diesel that is expensive to purchase and maintain may well not be more cost effective than a less fuel efficient outboard. The outboard is lighter, cheaper to fix and with maintenance down time much lower if replacement required."

A modern big outboard will have a car engine as power.

This means the newest tech , for lowest fuel burn , and no ripping the boat apart for an engine change.

As most diesel trawlers are displacement boats and only need 30-75HP , using a 150HP rated outboard makes sense .

When the OB mfg decide there is enough demand they will start making units with enough reduction gearing to spin a good sized prop at displacement speeds.

This will rival diesel in fuel burn and be perhaps half the engine costs over the engines useful life.

 

[O C Diver]

But both diesel and gas engines are most efficient in terms of fuel burned per horsepower produced (or hp per gph, the inverse) at less than wot. Diesels for sure and I also suspect gassers are most efficient at about 80% of wot rpm which is not coincidentally the maximum torque point for most diesels.

And yes, hp per gph efficiency declines with rpm past that point and yes at idle they are very inefficient- maybe 10 hp per gph for diesels I am guessing, near 5 for gassers./QUOTE]

I can attest to this. When repowering my trawler from a Cummins 6CTA 450 HP to a John Deere 4045TFM75 135 HP, the fuel consumption went from 3.7 GPH to 2.0 GPH at 7 knots. The transmission, shaft, hull and displacement essentially remained the same. What changed was the engine and prop. The Cummins peak torque is around 2,000 rpm for that motor and it was turning 1,200 rpm at 7 knots. The John Deere peek torque is around 1,800 rpm and it is turning 1,450 rpm at 7 knots. In addition, the Cummins is roughly 3 times the size of the John Deere which adds a fair amount of parasitic drag. Imo, the lower efficiency at 1,200 rpm and the larger parasitic drag account for this substantial difference in fuel consumption.

Ted