Fuel and Horsepower
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Fuel and Horsepower

There is a direct relationship between the horsepower produced or extracted from all engines in relation to the fuel they burn. This is not a bold statement, it’s just a fact, and I certainly didn’t write the rules. There are many different efficiency factors involved and many different types of fuel, conversion factors, etc… But it all comes down to the conversion of heat energy to mechanical energy. For this discussion, I’ll make a few very basic claims (?) that are generally accepted principles in the operation of the types of most common diesels used in marine service.

1. BSFC (brake specific fuel consumption) is an accepted and universally used measurement for gauging power output in relation to fuel consumed. Typical units used by Cummins, Caterpillar, Yanmar, Isuzu, etc. would be lbs/hp/hr or grams/kW/hr. A standard wt for #2 diesel might be 7.001 lbs per gallon at 60 degrees F. Most manufacturers publish graphs showing BSFC at various rpm/load levels, and that, along with some interpolation, should enable one to determine that generally all diesels are their most fuel efficient (lowest BSFC) at peak torque. Of course, in marine applications, you should NOT be able to load the engine at peak torque. This is because a vessel with the correct prop and reduction ratio (propped to reach or exceed rated RPM under maximum loaded conditions) will prevent the aforementioned condition from ever occurring.

2. All the diesels in marine service that I have come in contact with over the past 20+ years fall in to a BSFC ranges of .450 to .325 (lbs/bhp/hr.) With a little math, one can derive the “magic number” of 20 hp/gallon/hr (.355) as this is a BSFC that matches (a high average) the amount of hp that is produced by a modern 4-stroke, direct injected, turbo charged/after cooled high speed diesel of modern design. At the far end of this scale (lousy BSFC) you will find normally aspirated 2-stroke diesels ( whose design characteristics date back to pre-WWII w/ many being mechanically supercharged although called NA’s) and a few NA, indirect injection, 4-stroke diesels (which are not nearly as old). You will also find at the best end of the spectrum modern engines typically designed and used for heavy duty applications. By coincidence the most efficient engines today used to produce rotational energy for marine applications (and, I believe, the most efficient heat engines available for any off the shelf application, are large (5000+++ hp) diesel engines of the 2-stroke cross head design that operate at low speed levels (40 to 300 rpm), but do burn heavy fuel oil (possibly higher BTU content, maybe Bob Mitchell can research this and help us out a little).

3. For comparison purposes, consider the following and give this some thought:

A – A carbureted gasoline (60’s through 80+’s design) engine used in an automotive application, or adapted for another application (your typical 454 Chevy/Mercruiser), will deliver LESS than 12 hp/gal/hr of gasoline consumed.

B – A two-stroke outboard engine (carbureted) may give you 6-8 hp/gal/hr.

C – A two-stroke high output motocross bike (Honda CR 250, Suzuki RM 250, etc.) might give you 4-5 hp/gal/hr if you are lucky.

D – Your Cox .049 model airplane engine running on methanol/castor bean oil mix is off my scale. I think more fuel comes out the exhaust than is used to make hp.
The above relationships are for comparison and may help some readers to understand why a diesel powered vessel goes farther on a gallon of fuel than a gas powered vessel of similar size/design. A concept related to this is that the propeller moves the boat and not the engine. It takes a certain amount of hp to turn any given propeller under any given circumstance, so if you have an engine that produces the particular hp needed for that circumstance, then a diesel engine will do it with substantially less fuel burn.

4. Typically BSFC is better (7-10% is common) with engines that are “turboed” due to the capture of some wasted exhaust energy that is then mechanically used to compress air (oxygen) which is then fed to the engine. This allows an increase of air during combustion and increases the extraction of energy from the fuel allowing more fuel to be burned properly in a given engine which means more hp output can be extracted from that engine, if needed.. Cooling the compressed air between the turbo and the intake of the engine is referred to as “aftercooling” or “intercooling” (I don’t want to argue the semantics of the two words/terms).. This further allows more air to be put into the engine because as the air is cooled, more oxygen is available do to the density increase from cooling.. Again, more fuel can be burned efficiently to make more hp, if needed. There are other benefits from aftercooling besides; it reduces thermo loading/stressing in the combustion areas of the related components and promotes less nitrogen oxide (NOx) to be produced during combustion. A popular misconception regarding turbocharging is that it increases stresses in the engine. To some extent it does raise the compression ratio, but for the same hp output from the same basic engine, turbocharging in itself is not the culprit. It’s the fact that more hp is available to use that MAY add to a shorter lifetime of the engine. From my practical experience with modern engines like the B & C Series Cummins and others specifically designed for turbocharging, I don’t feel this point contributes to how long the engine lasts.. Rather, it’s the “nut behind the wheel” concept, that is much more the determining factor in the overall life of a modern diesel engine today.
Most boaters are concerned about fuel consumption and should be. Although there are some who take exception to using diesel engines in smaller boats as they feel the benefits of reduced fuel consumption don’t even come close to justifying the cost of upgrading to diesels, I’ll leave you with this thought. There are many (one, very visible in this industry) skeptics that preach that gas is the only practical choice for smaller boats (under 35 ft or so) based on many of their own perceptions and observations over years of “experience” All of these reasons when read and understood, are really just based on a skewed perception & dollar bills. I wonder if in all of the negativities that they preach about things or “basic principles”, if they have even considered the fact that to some of us, just the inherent safety of diesel fuel (besides a dozen or so other valid reasons) could mean more to some than that wonderful old dollar In closing, there is a very definite relationship between the amount of fuel burned and the amount of horsepower produced. This is an important concept to understand, especially for you ‘forum readers’ as discussions center around various competitive diesel engines, fuel burn rates, performance and range.

The internal combustion diesel engine still remains to this day the most fuel efficient engine designed, built and used in a variety applications. Quite a nice idea that Rudolf had back in 1880’s.