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  #1  
Old 01-29-2003
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fuel consumption gas vs diesel

A couple of weeks ago there was a message string going about gas versus diesel engines and I was challenged when I made the comment that you get more hours of operation out of a gallon of diesel.

I just stumbled across the following on a web site called boatsafe.com.

How much do gasoline and diesel engines consume?

Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.

Note: An engine at cruising speed usually uses only about two-thirds of its maximum available horsepower. Most marine engines are designed to run continuously at between 60 and 75 percent of maximum speed. Diesels tend to be more toward the top of the range.

Gasoline four stroke inboard engines need about 1 gallon per hour for every 10 hp used. The number of gallons consumed per hour can be estimated by multiplying horsepower used by 0.100. (see note above)
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Old 09-09-2009
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fuel consumption

I've always used 18 HP per hr per gallon for diesel and 14.5 for unleaded gas. This assumes internal combustion engines, 4 stroke, normally aspirated. I did some research on small engine fuel consumption and it goes like this:
Diesel17-20 hp-hr/gal

Natural gas9-11 hp-hr/ccf

LP (propane)9-11 hp-hr/gal

Gasoline10-14 hp-hr/gal

The size of the engine, operating temperature, RPM, and Load all have an effect on the efficiency of energy conversion.
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Last edited by roline; 09-09-2009 at 09:11 PM. Reason: add graph and more info
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Old 09-09-2009
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Quote:
Originally Posted by jklewissf View Post
Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.
I am not surprised that diesel has a higher power density than gasoline, but have empirical reason to question the formula.

I have no reason to think there is anything at all special about my 55-Hp diesel that pushes 10-tons of fiberglass and lead around. It has routinely averaged 1 gallon per hour running at 2400 RPM of its design WOT of 3000 RPM.

Do I misread the formula?

respectfully,
Wayne
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Old 09-09-2009
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An old thread, but what the heck...

Over the course of 8 seasons, our current 3 cylinder 27hp diesel has consistently burned about 1/3 gph during normal sailboat operations. We push it hard (2700-3000 rpm whenever we're motoring, except when docking and during warm-up/cool down).

Our previous 2 cylinder 18hp diesel burned about 1/4 gph during normal sailboat operations.

So the figures cited in the initial post do not sync with our real world experience. I know my brother's gasoline A4 has a burn rate about 2-3 times our engine. It's putting out a bit more hp (30, I think), but it's pushing a smaller boat too.
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Old 09-09-2009
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Why not..

Old thread but why not.. My 44hp Westerbeke consistently burns .72-.74 GPH at cruise speed which is 80% of WOT and I measure it at every fill up. My old A4 used to burn well over a gallon per hour pushing a boat 9,000 pounds lighter..

I can't imagine anyone questioning diesels ability to sip fuel in comparison to gas engines guzzling it.
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Old 09-09-2009
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The Beta 2 cylinder gets 14 MPG pushing the C&C 35 at 6 knots

The 4 Hp almost NEW 2 cycle that pushes my J24 6 knots is not even close
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Old 09-09-2009
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I found an efficiency graph to add to the subject.
Attached Thumbnails
fuel consumption gas vs diesel-fuel-efficiency.gif  
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Old 09-11-2009
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There should be 4 lines for a Diesel.

PC or Non direst injected Diesel Engines.. PC stands for a pre cup injected
Di or direct injected
NA or naturally aspirated Diesel Engines
and finally..
DI / turbo..

The most efficient and powerful is the Direct injected/turbocharged engine
you can also add a sub line for inter-cooled..

trying to calculate the range of a Diesel with out this information is use less.
A Pc is the least effective at turning heat in to usable power.
the DI/T-I is the best at varying load and the most efficient at fuel-hours of operation. Its in the 90% range while a pc is more like 50%.
So when i see the term Diesel engine i just cringe..

Sorry, i will get off my sop box now and go back to my little corner of the world.
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Old 09-11-2009
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According to the calculations then..my 64 hp Perkins should be burning 3.56 gph...It is not as far as I can tell ...but I will have to have more opportunity to check it out over time to be sure ..so far I have only ran it about 2 hours...I cant ignore this feeling in my gut telling me it will come in closer to under 1/2 of that..I will be quite surprised if it does not...as using various other engine consumptions ratios I have around here to compare it with says that calculation is all wet.
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Old 09-11-2009
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Quote:
Originally Posted by dieselboy View Post
the DI/T-I is the best at varying load and the most efficient at fuel-hours of operation. Its in the 90% range
Gee... you just made me feel good about my 75hp yanmar
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