Turbo vs NA efficiency

[REBrueckner]

Anybody know the relative fuel efficiency of turbo engines vs NA engines at a modest HP output?
For example, say it takes 150 HP per engine to move a Hatteras at some modest cruise speed. Is an 8V71 (rated 350 HP) more efficient or less efficient than an 8V71TI (rated at 435 HP). Or are they about the same??

I suppose if one had the DD spec graphs for each engine we could look at the fuel consumption for each engine and get a reasonable idea.
For the 8V71TI, my DD spec graphs ("Marine model") show 150 HP (This is the "Power-propeller load curve) at 1600 RPM and about 8.75 gal/hr fuel consumption.
Anybody got the 8V71 figures at 150HP??

[REBrueckner]

Oops, just found another book "Detroit Diesel Propeller Selection Guide" and it has the specs for the 8V-71 NA with N70 injectors (I think this is the 350 HP variation) at 150 prop load HP it takes almost 1800 RPM and ABOUT 9 GPH..but this graph is tough to interpolate...so fuel consumption seems about the SAME as the TI.
Seem reasonable?
If so, then it seems to make little sense to "detune" a turbo engine (remove the turbo, install smaller injectors,etc)...it doesn't seem to accomplish much!!!

Comments welcome!!

[MikeP]

Right up front I will admit that I am nowhere near as experienced with diesels as I am with gas engines but generally engines are engines so...

Turbo/supercharged engines normally have lower static compression ratios than NA engines. Therefore, at lower power (below boost levels) one would expect the non-turbo engine to be slightly more efficient. Off hand I don't know the difference in DD compression between NA/turbomotors. I'm not even sure there is a difference but I'm betting there is. If so, the NA diesel engine should get slightly better "mileage" at low power settings.

[Nonchalant1]

Forget the "specs". They're a lab environment spec with a very wide range of fuel use. You'll never achieve the low end of the curve and the high end of the curve is stupid inefficient. That's because the rack controls fuel input to maintain RPMs depending on conditions. If 8V71TIs were running a rowboat, maybe you'd get the low end. If they were running a 70 ft Hatteras, maybe you see the high end. There's no mean or median on the charts that's reasonable either.

Your real answer is to go read the thread titled "single engine cruising". While single engine cruising is a bad idea, the thread establishes that both engines are fairly fuel efficient at hull speeds. The TIs have enough power to plane a 53MY but the Ns don't, so N's require you to live life in the slow lane. However, with good maintenance, Ns will outlast TIs maybe 2 to 1 before major overhaul is needed ($25K-$35K for a pair).

Doug Shuman

[Avenger]

The 71 series appears to use 18.7:1 compression for N/A's and 17:1 for turbos and some N/A applications. Diesels are not sensitive to boost like a gasoline engine because they're not compressing a fuel-air mixture. We can get way more technical, but that's the short form.

If the application only demands 150 HP that's all the fuel the governor is going to meter. IMHO 150 HP is 150HP and the efficiency differences are too small to be concerned about, since there are so many other variables on a boat that have a more significant impact on fuel consumption.

[SKYCHENEY]

By reading previous posts, I've noticed that we all seem to get right about 1nm/gal at hull speed. It doesn't seem to matter which engines or even what length Hatt you have, the fuel consumption at this speed is close to the same. That makes me think that the hull design has more to do with the efficiency than the Turbo/NA comparision.

[Genesis]

Horsepower is horsepower.

A turbo is "free" in terms of power consumption, as its power comes from exhaust gas which you were going to waste anyway (by ejecting it out the back of the boat)

A supercharger consumes real power, and its the reason Detroits are not as efficient as 4-stroke engines. However, all Detroits have 'em, so there you go.

Reality of it is that none of us can run at "hull speed" with our engines at anything over idle. True hull speed on a 45C (~42' LWL) is about 6.4kts! That's IDLING on my boat, roughly, and if I could run there I get close to THREE nmpg. But if I run there for any length of time, my enignes will be damaged - so I can't and don't.

At 9kts I'm well over hull speed and starting to "climb the hump", even though it seems imperceptable. Thus I get somewhere around 0.9nmpg. BIIIIIG difference.

[Bertramp]

Genesis .... running at idle for a long period of time will damage the engine ?
what is a long period of time ?
and what causes the damage ?
thanks

[Maynard Rupp]

Very interesting that everyone seems to get 1.0 nautical mpg @ about 9 knots. Our gasoline powered 36C @ 26000lbs. also gets 1.0 n.mpg @ 9 knots. Are we saying that whatever size Hat you have with any of the factory sized engines the results for fuel, (or gas), mileage are the same??? I have a friend that re-powered his 42 Chris Craft with Yanmmar engines of about 350 hp., I believe. He runs down from Detroit to south Florida in fall and back in spring every year. He claims 3.2 nautical mpg @ 8.5 knots. Are the new diesels that much more efficient??

[Genesis]

Genesis .... running at idle for a long period of time will damage the engine ?
what is a long period of time ?
and what causes the damage ?
thanks

Cylinder washdown primarily.

The problem with prolonged idling on a diesel is that the piston speed is not high enough to insure that good scavenging takes place, and temperatures are too low. The engine cools off......

If you are going to troll for instance, you want to pick up every hour or two and run for 10-15 minutes at cruise power to clean things out.

At 1000 rpm or so you don't have these concerns, but at that RPM you're WELL over hull speed.....

[Genesis]

Very interesting that everyone seems to get 1.0 nautical mpg @ about 9 knots. Our gasoline powered 36C @ 26000lbs. also gets 1.0 n.mpg @ 9 knots. Are we saying that whatever size Hat you have with any of the factory sized engines the results for fuel, (or gas), mileage are the same??? I have a friend that re-powered his 42 Chris Craft with Yanmmar engines of about 350 hp., I believe. He runs down from Detroit to south Florida in fall and back in spring every year. He claims 3.2 nautical mpg @ 8.5 knots. Are the new diesels that much more efficient??

VERY different hulls there Maynard....

I don't know if I believe 3.2nmpg at 8.5kts. At 6 or 7 kts, yes, I believe it - I get right around 3 nmpg at 6.5 kts on Gigabite - BIIIG difference for another couple of knots of speed.....

[REBrueckner]

I agree with Nonchalant1's conclusion, but I'm not so sure about the reasoning regarding "specs". While variation between hull types, for example, could well take place and potentially overload an engine, when the prop and reduction drive are properly sized to achieve about 2300 RPM at WOT shouldn't that remove MOST of the load variation? So the spec curves should be close to real is my way of thinking.

In any case, the virtually equivalent fuel consumption at my example of 150 HP tends to show that. This reinforces the subsequent comment that 150HP is 150HP, so fuel consumption should be about the same in each engine.

I would have expected that result, but I have been told in the past that making the change from TI back to NA would improve fuel efficiency. I had never checked the spec curves before; now I don't believe it.

[jim rosenthal]

I'm not sure, as you say, that it would improve fuel efficiency that much. And that's in the abstract. If you factor in what it would cost to make the change, ouch. :eek:

[Nonchalant1]

I agree with Nonchalant1's conclusion, but I'm not so sure about the reasoning regarding "specs". While variation between hull types, for example, could well take place and potentially overload an engine, when the prop and reduction drive are properly sized to achieve about 2300 RPM at WOT shouldn't that remove MOST of the load variation? So the spec curves should be close to real is my way of thinking.

In any case, the virtually equivalent fuel consumption at my example of 150 HP tends to show that. This reinforces the subsequent comment that 150HP is 150HP, so fuel consumption should be about the same in each engine.

I would have expected that result, but I have been told in the past that making the change from TI back to NA would improve fuel efficiency. I had never checked the spec curves before; now I don't believe it.

Have you studied the DD fuel curves? It's not an individual number for any RPM, just a high and a low curve and the actual can be anything inbetween.

The difference between fuel use in boats with similar engines is the weight, the wetted area, the shape, the LWL, the props, the cleanliness and the sea and wind conditions, etc., etc. Weight and LWL "outweigh" all the others. When you move that throttle, you're really just settting the RPMs. The governor will pour as much fuel as required to keep the RPMs. Change your props and you could double the fuel required to hold the same RPMs with everything else the same. There's no way that DD could make meaningful curves without specifying all the other variables, so they give the whole range of the engine.

Doug

[luckydave215]

A turbo increases the efficiency of a diesel. It recovers some of the lost energy (waste heat) that we are sending out the exaust. Large, fixed rpm deisels (generators, for example) are ALWAYS turbocharged for this reason, with the turbo size optimized for their fixed running speed.

[SKYCHENEY]

A turbo increases the efficiency of a diesel. It recovers some of the lost energy (waste heat) that we are sending out the exaust. Large, fixed rpm deisels (generators, for example) are ALWAYS turbocharged for this reason, with the turbo size optimized for their fixed running speed.

Yes, but at relatively low Rpm before any boost is achieved, is the efficiency any better? The original question was about only utilizing 150hp of the available 350-465 or so. At that Rpm I would think that the efficiency would be about equal. Now at 200 off WOT, there should be a difference, theoretically.

[luckydave215]

Skycheney, you have to realize that diesels are designed to spend their lives at 80% of full load, and are pretty ineffiecient at any other operating condition. Running a 450 hp engine at 30% load is so far out of the realm of designed operating regime that the efficiency is hit or miss. A na engine will typically have a higher mechanical compression ratio (efficiency ratio) than a turbo charged example, and may deliver better fuel economy when run at a load outside the design parameters. The load/crossover point at which the turbo engine has better specific fuel consumption will vary between different engines, even different installations of the same engine.
In general, a turbo charged engine is more efficient because of the recovered waste heat energy.

[REBrueckner]

Summary: The Prop load curve DOES accurately reflect real life fuel consumption for a pleasure craft engine application where rated RPM is achieved at WOT.

From the DD graph description:
PROPELLER LOAD-Indicates horsepower absorbed by a typical propeller and the corresponding fuel consumption throughout the speed range.


The upper fuel consumption curve shows fuel consumption at "RATED LOAD", an engine fully loaded over the RPM range. That's why the fuel consumption is so high and would NOT be achievable in practice. The lower PROPELLER LOAD fuel consumption curve shows fuel used over the range of RPM with a typical propeller load which permits maximum rated RPM at WOT. The curves come together at the maximum rated RPM because that is the only propeller load RPM were the engine is fully loaded.

So it IS the lower curve that represents typical fuel consumption for pleasure craft as long as the engine reaches rated RPM at WOT. If the prop is bent, oversized, bottom is dirty, engine is installed with the wrong reduction drive, and rated RPM cannot be achieved, then the prop load curve is not representative.

[SKYCHENEY]

So... what is the answer to the question? Will an 8v71ti get better fuel economy than an N at the lower end of the curve? before the turbo boost is really measurable? (all other things being equal).

And what about a 6v92ta in that same boat? DD originally tauted the 92 series as the "fuel squeezer" to imply that they were more fuel efficient than the 71 series.

I understand that this is not an optimal running range for a DD. In fact, I used to have a great mechanic that worked in our shop and he was always getting on the equipment operators to drop down a gear and put their foot into it. He hated to see anyone "lug" an engine and said that Detroits especially loved to be run near WOT. (and he overhauled a bunch of 'em over the years)

[Genesis]

There is a HUGE difference between a diesel in a TRUCK and one in a BOAT.

In a truck the "drop down a gear" is correct, because lugging an engine is VERY bad for it. But you can't "lug" an engine in a boat, because of the prop - it doesn't put much demand on the engine at lower RPMs ("hump" speed excepted - don't run there!)

As for Detroits in general, they are more volumetrically efficient at higher RPMs, just like most engines. But that assumes you need the power - if not, you're just wasting RPMs, and it takes a certain amount of power just to turn the enigne at a given speed, because of internal friction and other losses.

There isn't a large difference in economy at the low end of the output curve (assuming the same output) between the different engines. All things begin equal the lower RPM engine wins slightly (provided its not being lugged) for the same output at the low end. At the higher end the turbo engines win because the boosted output is "free", in that to get the same output without the turbo you'd have to spend some of the output power to cram the air in there that the turbo gives you "freely".

The "Fuel squeezer" thing came from the truck and bus line and the claim was due to injector calibration differences. Detroits aren't "fuel squeezers" no matter how you care to measure it - the blower guarantees that, as its a power-stealer that hurts efficiency all the time.

[luckydave215]

The blower isn't all of the reason dd's eat more fuel than some other engines. Being a two stroke design, the exaust ports open while the combustion (work) stroke is still in progress, venting some of the still burning fuel mixture to the exaust. That's why they're so damn loud.

[Genesis]

In theory, that should not be taking place. Combustion should be complete before the ports uncover.

Blowers are parasitic drag that is never returned - the rob power directly from the crank for their drive, and thus always hurt efficiency. They are NECESSARY in a Detroit due to the way they are designed, but make no mistake about it - they're responsible for a lot of the economy difference between them a a 4-stroke.

Not all, but some.

[REBrueckner]

I started this thread. It's been interesting.
Unless there are convincing additional posts with evidence to the contrary, my conclusion is that the efficiency of a TI and NA version of the same engine model (fuel consumptionrate) at moderate RPM are about the same.

I'm basing this primarily on the DD fuel consumption curves found in the DD Propeller Selection Guide.

[Dustoff44]

Horsepower is directly related to the amount of fuel being burned. Therefore IMHO it doesn't matter which engine turbo, natural or gas it takes a fixed amount of fuel to make a given amount of power. A turbo engine can rev a little higher to make more horsepower because of the increase in air pressure, but as always it takes more fuel to make more power....

[REBrueckner]

"...A turbo engine can rev a little higher to make more horsepower because of the increase in air pressure, but as always it takes more fuel to make more power...."

A turbo equipped and designed engine may or may not rev higher than it's NA counterpart. The increased HP occurs over much of the regular RPM range and results from additional air, from the turbo, and additional fuel, from larger injectors. An NA engine cannot develop as much HP because of limited fuel and air for combustion.

[rmaher]

I'm confused. If the formula for hull speed is 1.34 times the square root of the waterline length, hull speed for a 42 foot waterline would be about 8.68 knots. Is there something I'm not factoring here?

Sorry if I'm going off on a tangent but curiousity got the best of me ...

Rob

[Genesis]

The 1.34 is a "best guess".

Hulls differ. For a displacement hull the 1.34 number is "mostly" accurate. For planing hulls it can vary anywhere from the 1.34 number all the way down to 1.0.

The best way to know is to run the boat with flowmeters working and see where the efficiency drop starts to get REAL steep. There's your "knee."

[REBrueckner]

Hull speed: For true displacement hulls, the equation 1.34 (sq root LWL) is an approximate top speed. Likely Hatteras planing hulls are more like 1.1 or 1.2 factors. Those calculations will just give you a rough idea of an economical cruising speed. Slower is better in this regard.

The discussion in this thread regarding TI vs NA is about the most efficient way of deriving the HP to move at your desired cruising speed. My conclusion is that TI and NA use about the same amount of fuel (gal/hr) when developing the same HP. So it seems to make little, if any, difference bewteen the two when trying to run at moderate RPM to save fuel.

[Genesis]

Yep.

As I said earlier in the thread, if I could run at idle for long periods of time I could get roughly 3nmpg of fuel with BOTH engines up! But that's making roughly 6 kts.

At 9kts, more or less, I make about 1nmpg. That's a HUGE difference in terms of fuel burned for distance travelled...... I'm definitely beyond "hull speed" at the 9kt speed.

[Traveler 45C]

The beam of the boat factors into the equation somehow???

[Maynard Rupp]

Horsepower is directly related to the amount of fuel being burned. Therefore IMHO it doesn't matter which engine turbo, natural or gas it takes a fixed amount of fuel to make a given amount of power. A turbo engine can rev a little higher to make more horsepower because of the increase in air pressure, but as always it takes more fuel to make more power....

You cannot use that comparison for gas and diesel. diesel fuel has more BTUs per gallon than gas. All engines burn BTUs not gallons. Therefore a diesel engine using the same horsepower will use less gallons than its gasoline counterpart. That's why I drive a diesel pickup truck instead of a gasoline powered one. (I also think it sounds cooler). I don't understand why I have a gasoline powered Hatteras though.

[Genesis]

Well diesels burn fewer gallons for two reasons:

1. As mentioned, Diesel has more BTUs in it per gallon than gasoline does.
2. Second, at anything other than full throttle, gas engines lose quite a bit of efficiency due to pumping losses necessitated by the requirement to keep the mixture rich enough to light. This requirement is why you have a throttle plate even on a fuel-injected engine - but that throttling costs efficiency....

There are other factors as well having to do with thermal efficiency but those can be optimized for both gas and diesel engines. The above two factors though are pretty much "how it works" stuff....

[Nonchalant1]

The beam of the boat factors into the equation somehow???



The beam of the boat doesn't figure into the "easy" formula. It presumes that the boat is something less than square, and has a bow. However, there are big hairy formulas for calculating hull efficiency that include LWL, displacement, wetted area, hull design, beam, etc. Still, for any reasonably normal boat, it works, including thin sailboats vs. beamy Hatteras types. That's because the really limiting factor is the difference between the bow wave and the stern wave and therefore how much of a hill you're going to have to climb if you exceed hull speed. A thin boat can achieve hull speed with much less power, but it doesn't change the hull speed.

Doug Shuman

[Traveler 45C]

Thanks Doug.

[Mirach 49]

A very interesting discussion, but since the engines are running at 25% of their rated power, the boat is not planning, and should be considered as a displacement hull. So the best way to save fuel is to find the ideal cruising speed, rather than modifying your engines. :)
I'm sure you can save 30% fuel just reducing your speed of half a knot (when you are just over the displacement speed).
There are formulas to calculate it, but I think the best is to experiment. Draw a curve showing the kt/RPM and you'll easily find the point where the boat needs more RPM to increase the speed. On my 49' the economical speed is around 1300 RPM for 7 kts.

[Genesis]

Yep.

I can run 9kts at 1.1nmpg.

If I run at 10kts, I get 0.85! 1 knot costs me quite a bit......

[AquaNav77]

In order to establish the efficiency of any given engine is by what is called the BSFC (Brake Specific Fuel Consumption). The calculation is quite simple with the only variable being the weight of the fuel (gas or diesel). The prop demand curve for both horsepower determination and fuel consumption at any given rpm is the only curves to use to get the most accurate number.

The Formula for calculation is as follows:

BSFC = Fuel consumption (gallons) x weight (7.001 for diesel; 6.1 for gas) then divide this by the horsepower.

Example:
On the 8V71TI rated at 435 BHP/425 SHP if one follows the fuel curve, the consumption is 8.7 gallons burned and the horsepower is 150 SHP. This is at 1600 RPM. Now let's apply this:

BSFC = (8.7 x 7.001) / 150
BSFC = .406 lb/shp-hr.

On the 8V71N rated at 350 BHP/325 SHPF if one follows the fuel curve, the consumption is 9.5 gallons burned and the horsepower is the same 150 SHP but this time at 1800 RPM according to the curve. Now let's apply this:

BSFC = (9.5 x 7.001/ 150
BSFC = .443 lb/shp-hr.

The answer as one can see is .406 for the TI engine and .443 for the N engine. Therefore the turbo engine is more efficient in producing 150 SHP than the N engine. As to the answer, the lower the number the better efficient the engine is. Today the diesel makers routinely give these numbers on their respective spec sheets and can be utilized in a comparison of other makers engines. Caterpillar gives out these numbers routinely on their spec sheets and one can see which RPM setting is the most efficient. An excellent comparison is the CAT C12 ACERT vs the CUMMINS QSM 11-715 vs VOLVO D12-715. All three of these engines are of the same exact horsepower, however their fuel burns are not identical so therefore this is where this formula would come in to determine which one is the most fuel efficient vs the fuel hog.

For the gasoline applications, the manufacturers do not give out the prop demand curves their respective engines and therefore the only comparison that can be made is at rated Horsepower and RPM. One would find the numbers are in the .500 area. Very inefficient.

BTW, the new modern diesels usually range at about .330 at cruising speeds and about .365 at WOT.

Respectfully submitted,
AquaNav77

[REBrueckner]

The newer diesel rating of .330 vs our old .406 is at cruise is 330/406 or 81%.So upgrading to a new modern diesel might yield in the range of 100-81 of about 19%...that plus a likely reduction in engine weight is noticeable..but not so important for those who run only a 100 hours or so annually...

It's also interesting to note the difference between .330 and .365 at WOT...about 11%.

I'm going to guess that is not due so much to engine inefficiency at WOT as to increased bottom/water friction at higher SOG.??? Reduced prop efficiency, if any, at high RPM should not cause increased fuel consumption,right????, just a relative speed loss.

[AquaNav77]

My response was simply on the basis on the inquiry as to the difference in fuel efficiency b/w the 8V71N with the N70 injectors vs the 8V71TI engines rated at 435BHP which are equipped with the N90 injectors while each one was respectiively producing 150 HP.

In the one sentence paragraph in the above response, it is stated that there is a difference of the BSFC between what would be considered a cruising rpm vs WOT. Every engine has different values at each RPM setting, no exceptions. Actually sometimes they will rise, then fall or even remain constant. It depends on each individual engine and is especially so on the newer electronic engines. Your assessment as to the third paragraph is in error, because hull design hasnt even become a factor yet in these calculations. What has been presented is strictly of a mechanical nature and the results of mathematical calculation. One thing is certain however. When it comes down to the turbocharged/aftercooled diesels and the old carb gas engines, the top 20% amount of rated horsepower takes more fuel burn in order to achieve the power increase, along with the full boost of the turbos and full effects of the aftercoolers for the full cool air/charge effect in the diesels. One uses these calculations, considering the fact that by using the prop demand curves given, one is able to establish a typical base line for the boat's fuel burn. Once established, one can then ascertain true efficiencies as to speed/fuel burn determining gal/nmile. This is a much easier way to find out the boat's efficiency then by taking any rpm as a guess and starting off as such without really knowing what the engine is doing at any rpm. For further reference go to:
http://marinewizard.catmms.com/catwizards/marineWizard/jsp/searchResults.jsp?lang_id=eng

Once there, click on "567" on the first line. From there it will bring you to the CAT C9 ACERT engine and simply scroll down to page 3 for the values of this particular engine.

If further info is needed, please respond back to me on this site.

Regards,
AquaNav77