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Old 07-25-2011, 04:50 AM   #21
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RE: Fuel Economy Question

" .... By the way how can a supercharged DD be referred to as a "naturally aspirated" ...........

Can anyone answer this with a straight face?"

SURE , The blower on most DD is for exhaust scavengine and simply blows the cylinder clean.

Since the DD fuel injected there is no efficiency loss that happens with a carb engine and fuel and air are pushed thru,

The DD usually has under 1 pound of boost , if anything left after the valves close.
However since the inlet air is much warmer (the density is lower) so "Supercharging" for added power does not exist.

1 pound might be called "Supercharging" but for most the Turboed DD are the ones that make use of above atmospheric pressure for added HP.

OK?
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Old 07-25-2011, 07:37 AM   #22
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RE: Fuel Economy Question

Quote:
DavidM wrote:But I couldn't find any marine diesels that produce 12 hp per gph at low power loadings, much less 8.
*Why are you guys using hp per gph? It is a bogus home-made marketing expression. If you really want to compare fuel efficiency please use specific fuel consumption based on the propeller curve. That is what you will find on most engine spec sheets and are based on standard condtions and fuel density/heating value.
Brake specific fuel consumption or BSFC is the weight of fuel burned to produce one horspower for one hour. It is the industry and engineering standard and using something else is like using lily pads passed per fortnight as a way to describe speed.
If I am not mistaken, the basis of this thread is founded on the fantasy that there is such a huge difference in engine efficiency between the holy 75 percent and some lower number that that alone is reason not to run larger engines at low power. This has been debunked several times, and is again each time one of the gurus laments the lack of fuel maps as being the reason buyers are led so far astray.
Running the numbers on several old and new engines including a very modern turbocharged, high pressure common rail electronic marvel provides a glimpse at reality.
One common mechanically injected normally aspirated engine running at 75 percent power delivers 1 hp for every .39 lbs of fuel burned. At 25 percent it burns .43 lbs. This is .04 lbs per hp per hour difference between 150 hp and 50 hp for that 200 hp engine. Point zero four, is four one hundredths of a pound, 0.64 ounces per hp.**Another way to look at it is if that engine was equally inefficient at high*as it is at*low power, eveery hour at 150 hp it would burn an addtional 32 ounces*of fuel.* In real life what it means is that instead of burning 3 gph at low power, that engine would burn 2.88 gallons if the efficiency were the same as at high power. So, to*reduce fuel consumption from*8.3 gph at 75 percent, you slow down and burn 3 gph.*Does it really matter that you are theoretically leaving*0.12 gallons*per hour on the table? How long and how far do you have to go to even know there is a difference? I suggest not a single one of us could ever tell the difference since wind, currents, poor steering and just fooling around makes those numbers so small as to be statistically irrelevant.
The engine described above was the worst of the 4-stroke engines in the range normally installed in recreational trawlers. Other engines are very similar but with 0.37 or 0.38 at low power. The latest high tech HPCR engine with all the bells and whistles shows an increase in efficiency at low power with a BSFC (propeller curve) of .36 at 75 percent power and .35 at 25 percent power. These savings if extended over a weekend cruising will be invalidated by an extra few*moments idling at the dock before departure or by letting one of your children steer for 5 minutes.
Anyone with access to engine specs (if you are reading this you do) and a calculator can confirm this for themselves. Each boat is different and each owner operates it differently. The variation among operators and even location is far wider than the small*percentage points that separate the newest from the oldest and the best from the worst engines.
Those who claim that using a controllable pitch propeller will widen the gap are still ignoring the fact that a CPP wheel is less efficient to begin with, weighs more, costs more, and requires more maintenance than the savings of a few ounces of fuel over a weekend or*a few gallons*on a trip through the "loop" or "up the inside."*Those things are great for harbor tugs and icebreakers or other boats that require rapid and frequent maneuvering but are highly unlikely to save anything for a recreational trawler owner.

*
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Old 07-25-2011, 08:27 AM   #23
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RE: Fuel Economy Question

Once again, Rick's expert post has required me to take two aspirin and l lay down for a few minutes to prevent spontaneous brain combustion. :-D
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Old 07-25-2011, 08:33 AM   #24
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Fuel Economy Question

Quote:
DavidM wrote:

So it seems that the manufacturers are implicitly supporting hp per gph.

*

The marketing people massage the data to make it more palatable to the consumer. They also tailor it to fit a very narrow audience as it should be obvious that the hp/gph thing is meaningless to the*majority of the world that does not use the US gallon.

*Then again, who knows, maybe they are using imperial gallons to make the numbers look better*

I'll also note that hp per gph has become the parameter of choice in discussions over on boatdiesel. These are real world mechanics, shop owners and boaters. So are the members of this forum.

You can use either in these discussions but I would prefer to leave BSFC to the engineers (Uh oh, I are one!!).**

It may be the parameter of choice because the folks who ask the questions ask in the terms they hear and see "on the street."

Personally, I look at hp/gph as a form of technical ebonics. Call me a stodgy old traditionalist but BSFC is universal. What works on the corner should stay on the corner unless we want to limit our readership to the "hood."

You are right though, that if people are comfortable with it, there is no law saying they have to use legitimate metrics in forums like this, but I believe it is a form of slang that doesn't translate well.
*


-- Edited by RickB on Monday 25th of July 2011 08:35:02 AM
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Old 07-25-2011, 08:58 AM   #25
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RE: Fuel Economy Question

Tony B

You say* "Can anyone answer this with a straight face?"

Looks like you are trying to be funny But I'm not amused. If you want to be rude at other people's expense go to OTDE and play games w others of like minds.

*

Eric Henning
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Old 07-25-2011, 09:00 AM   #26
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RE: Fuel Economy Question

Quote:
RickB wrote:Call me a stodgy old traditionalist
*Ok. You're a stodgy old traditionalist.

(Can you tell I have a little free time today at work?)
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Old 07-25-2011, 09:16 AM   #27
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RE: Fuel Economy Question

Quote:
RickB wrote:
The marketing people massage the data to make it more palatable to the consumer. They also tailor it to fit a very narrow audience as it should be obvious that the hp/gph thing is meaningless to the*majority of the world that does not use the US gallon.

One of the reasons folks use the GPH figure is because it is a more easily understood metric that is exactly equivalent to lb/hp-hr and hardly a lilypads passed per hour measurement.* Which is why these two metrics*are laid out side by side as equivalent in performance tables for almost every engine.

But it is certainly true that the idea that fuel consumption is materially adversely affected at low speeds is a canard.

*
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Old 07-25-2011, 09:44 AM   #28
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RE: Fuel Economy Question

Quote:
Delfin wrote:One of the reasons folks use the GPH figure is because it is a more easily understood metric**
*I have absolutely no qualms about showing gph at various power settings, it is quick and useful and easily understood by all in the terms that apply at the fuel dock. It is what I look at*for small engines. I never suggested that publishing consumption in gph on the graph was inappropriate.

I do suggest that using "horspower produced per gallon of fuel burned" is engine ebonics.*That figure*is not displayed on the spec sheet you posted. It is an abstraction used online in boating and some engine forums simply to avoid using an established metric for reasons that are best left to OTDE.
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Old 07-25-2011, 11:10 AM   #29
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RE: Fuel Economy Question

The*metric for the mods and Tony Athens on boat diesel is BSFC. Of course many posters will spout off*other numbers.

In the off road non marine diesel business even though BSFC and its derivatives are very much monitored, *it gets more *complicated with hydraulic pumps/servos consuming huge HP and electric traction motors with grid dissipating braking downhill. For the past decade or more on board computers and telemetry to fixed stations keep tabs on fuel burn, temps, RPM, EGT, EG O2, NOX etc*content and turbo pressures (as many as 4 per engine). A savings of 5% fuel burn can easily be $1 million/year at a big mine. Plus, erratic fuel burn leads to early engine demise.

GPH is my boat key though, it tells me how far* we can go and what is best combination of RPM and through the water speed for the prevailing conditions. A friend of mine with a big Westport prefers 10 knots because it "feels right" and*he can "go further."
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Old 07-25-2011, 12:37 PM   #30
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RE: Fuel Economy Question

"That is what you will find on most engine spec sheets and are based on standard condtions and fuel density/heating value."

The big problem with Rick B 's post is that while the PEAK hp ,fuel flow , hp produced and rpm are always accuratly given , the prop curve and the fuel consumption figures for it are not measured , they are mere math creations.

"Prop demand data bases on .......... a fictional GUESS.

That good old (and unaviliable) fuel map DOES use actual measurements of load vs fuel flow.

Weather its GPM or cc per min is meaningless as all can be converted accuratly to what is easy to visualize.

The folk with pumps generators etc need REAL numbers , not math creations.
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Old 07-25-2011, 01:46 PM   #31
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Fuel Economy Question

Quote:
FF wrote:The big problem with Rick B 's post is that while the PEAK hp ,fuel flow , hp produced and rpm are always accuratly given , the prop curve and the fuel consumption figures for it are not measured , they are mere math creations.

"Prop demand data bases on .......... a fictional GUESS.
Geez FF, *Have you ever seen the amount of data collected during the certification of a marine (or any other diesel) by a manufacturer or shipbuilder for EPA, class, the owner, and for their own records as well as product development?

They don't just collect a couple of points and fill in the curve to make them meet. With the technology that exists today, anyone - and I literally mean anyone - can rent the test equipment to measure the power being absorbed by his very own personal propeller on his very own boat and measure the fuel consumption down to the milligram and make his very own curve.

We just did that on a boat, we very accurately measured fuel consumption through composition of the hydrocarbon emissions and cross checked it by balancing the combustion of fuel with air equations. This isn't voodoo or backwater magic FF, it is good ole technology in practice. We also knew what the rpm and torque was and one of the guys had a watch so that just about filled in all the boxes. And we are users, not manufacturers or a certification agency.

Since you don't hesitate to*swear by the hull speed formula and*sprinkle it liberally throught almost every power discussion, *it is difficult for me to understand why you find the same theory and practice so difficult to accept when it comes to the propeller curve.

FF,*your're still beating*a dead horse. Those engine fuel maps don't apply and if they did, what would you do with them, make a one off, one speed,*one trick pony boat? If it is that important that you trash every other set of figures available, install the test equipment on your present boat and let us know what you learn from it. Maybe you will turn conventional marine engineering and naval architecture on its head. Maybe there is a new*metric waiting to be discovered - the Fred Factor.


-- Edited by RickB on Monday 25th of July 2011 01:55:22 PM
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Old 07-26-2011, 04:41 AM   #32
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Fuel Economy Question

Maybe there is a new metric waiting to be discovered?

Not really Pro boat builder ran a series of articles , some on gen sets by Nigel Calder that gave the 300% number for fuel burn at low power from a bunch of small noisemakers.

And one of out members found out what happens with a new noisemaker at minor out put after tossing a 30 year old OHNO or Koler.

In a later series of articles NG went thru the power / fuel flow on a very well funded,instrumented (gov cash) on his 46 ft Malo sail boat.

The sail boat would be pretty close to a similar sized displacement power boat in terms of thrust required.

The article describes the failure of the attempt at using hybrid electric to extend the range.

Use the engine at high (efficient) HP , with some pushing the boat , the rest being stored in batts.

The effort failed as there is no batt system , yet that does work

AS in cars hybrid, is fine for stopped or stop and go city traffic , but useless at cruise , 65 mph for a distance.

You should be able to BS a free subscription to Pro boat builder , try a look.






-- Edited by FF on Tuesday 26th of July 2011 04:43:41 AM
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Old 07-26-2011, 08:05 AM   #33
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RE: Fuel Economy Question

Quote:
FF wrote:


Pro boat builder ran a series of articles , some on gen sets by Nigel Calder that gave the 300% number for fuel burn at low power from a bunch of small noisemakers.

AS in cars hybrid, is fine for stopped or stop and go city traffic , but useless at cruise , 65 mph for a distance.

You should be able to BS a free subscription to Pro boat builder , try a look.


*I bet you can't find that 300 percent figure and provide a link or even a quote.

Was someone talking about hybrids?

No BS required, they give it away to anyone in the industry.*I get it*delivered to the office*and I*use an online sub for research. *
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