CONTINUOUSWAVE

I was curious about the efficiency of the modern Mercury four-stroke-power-cycle engines based on large displacement V8 or V10 or even V12 designs compared to modern two-stroke-power-cycle direct-injection very-low emission V6 engines such as an Evinrude E-TEC G2 300-HP.

CALCULATIONS

Mercury FourStroke Twin 300-HP V8 engine

600-HP consumes 48.7-GPH (24.35-GPH per engine), then the fuel consumption rate in gallons per horsepower-hour is 0.0812
Source: https://www.bostonwhaler.com/boat-model ... erformance

E-TEC G2 direct-injection two-stroke-power-cycle 300-HP V6 engine

300-HP E-TEC consumes 24.25 GPH producing 300-HP, then the fuel consumption rate in gallons per horsepower-hour is 0.0808
Source: https://boattest.com/engine-review/Evin ... -G2-300-HP

COMPARISON
Normalizing to Brake Specific Fuel Consumption using the density of gasoline at 6.2-lbs per gallon gives

Mercury V8 FourStroke = 0.5032-lbs/HP-hour
Evinrude V6 E-TEC G2 = 0.5010-lbs/HP-hour

ANALYSIS
The modern direct-injection two-stroke-power-cycle engine appears to be slightly more efficient at conversion of fuel to horsepower than the modern four-stroke-power-cycle engine, but the difference between them is very small and probably within the measurement error range. The Evinrude E-TEC larger-displacement V6 engines employed extensive computer modeling of the combustion chamber to optimize the efficiency of the burning of gasoline fuel. More on the G2 design at

Evinrude E-TEC G2 Engines
https://continuouswave.com/whaler/reference/ETECG2.html

For legacy E-TEC engines (not G2), I used some data from my own testing with my 2010 E-TEC V6 225-HP engine. That engine consumed 20.38-GPH producing 225-HP, for a consumption rate of 0.091-gallons/HP-hour. Converting to BSFC that gives 0.561-lbs/HP-hour. In another test at full throttle the BSFC was 0.57-lbs/HP.

The legacy E-TEC large displacement V6 is not as efficient as the E-TEC G2 V6 or the big displacement FourStroke V8.

Deducing the brake specific fuel consumption (BSFC) requires only knowing the horsepower produced by the engine and the rate of fuel flow needed to create that horsepower. Unfortunately, there is really only one throttle setting for which the horsepower being delivered can reasonably be inferred: the full-throttle setting is presumed to cause the engine to produce its rated horsepower. The power delivered at any other throttle setting cannot be reasonably deduced without some actual instrumentation and measurement of the power, which typically requires having the engine output shaft connected to a dynamometer.

The usual data collected in published boat engine performance testing comes from measurements of a boat speed in miles-per-hour and the fuel economy in miles-per-gallon produced. But from those two data the flow rate in gallons-per-hour can be calculated:

(gallons/miles) x (1/(miles/1-hour) = gallons/hour

And if the specific horsepower is known, then we divide the GPH by the horsepower power, to get gallons-per-horsepower-hour.

Finally, using the density of gasoline at approximately 6.2-lbs per gallon, we get lbs-per-(horsepower-hour), the unit of the Brake Specific Fuel Consumption.

Among outboard engine manufacturers, there is seldom to never any public data provided about the engine fuel consumption at a function of engine horsepower being produced. About all you might see is a curve of horsepower as a function of engine RPM, but without any corresponding data on fuel consumption rate, and often that data is really available to the public. You might see it in some promotional literature sent out to dealers.

Another problem in this realm is that the brake specific fuel consumption is probably not at its best value at full-throttle. There may be a particular throttle setting at some fractional setting of the throttle that actually results in the best combustion efficiency. The reason behind this is that at full-throttle setting the engine designers may have intentionally provided a richer fuel-air mixture in order to help cooling of the combustion chamber and avoid overheating and pre-ignition.

Gasoline outboard engines are not the pinnacle of achievement in best BSFC. Automobile gasoline engines tend to do much better. For example, a FORD ecoBoost gasoline four-stoke-power-cycle turbo-charged direct-injection engine can produce a broad operating curve with the BSFC around 0.40.