Fuel Efficiency of Two Suzuki Engines

[rbbiggs]

Moderator's note: this discussion was a diversion from the topic in another discussion in which the stength of the transom in a 34-year MONTAUK 17 boat was being discussed. The replies diverted the topic to discuss the fuel efficiency of two SUZUKI engines. That topic has been moved to PERFORMANCE for further discussion in order to not disrupt the original thread topic in REPAIRS and MODIFICATIONS.

Q6: What is the biggest four-stroke-power-cycle engine recommended?

[Recommends a] Suzuki DF90A [on the basis that it will be] quiet, light weight, [and produce] great [fuel economy].

[jimh]

[Recommends a] Suzuki DF90A [on the basis that it will…produce] great [fuel economy].

When characterizing an improvement in fuel economy for a particular engine, it is necessary to indicate what other engine is to be used as the comparison. Also, to describe an improvement in more actual terms, some sort of figure of merit should be used, such as stating the actual fuel economy to be expected in terms of a rate of fuel consumption.

For any internal combustion engine, the best metric to assess the actual efficiency of the engine at conversion of fuel to useful power is the brake specific fuel consumption (BSFC). Figures for BSFC are given in weight of fuel consumed per horsepower-hour. If you have data about the SUZUKI DF90A engine’s BSFC, please give that data, as well as the BSFC figures for the other engine to which the DF90A is being compared to.

Fuel economy figures like miles-per-gallon are not particularly specific to an engine, but depend on many other factors, such as the hull design, the total boat weight, the propeller being used, the boat speed, and sea state.

[rbbiggs]

[Recommends a] Suzuki DF90A [on the basis that it will…produce] great [fuel economy].

When characterizing an improvement in fuel economy for a particular engine, it is necessary to indicate what other engine is to be used as the comparison. Also, to describe an improvement in more actual terms, some sort of figure of merit should be used, such as stating the actual fuel economy to be expected in terms of a rate of fuel consumption.

See the Suzuki performance report pages at

https://www.suzukimarine.com/boat-tests ... boat-tests

Obviously there won't be data on a 34-year-old MONTAUK 17, but there is data on a Stingray182SC boat comparing the Suzuki DF90 and the DF115, and whatever Suzuki propellers were used. You're not going to be using the same exact propeller on two different engines with different displacement and torque. I can only trust that Suzuki Marine picked the optimum [rigging] for each engine on the same boat, such as propeller, and mounting height, and [other factors were as] similar as possible.

115hp burned 3.6gph at 3500rpm 24.1mph 6.69mpg, range 133.. Top speed 47.8
90hp burned 2.1gph at 3500rpm 17.6mph 8.38mpg, range 166.. Top speed 41.9

I clocked my 1990 Montauk with the DF90 at 43.5-MPH [using a GNSS receiver to get Speed over Ground].

Thats as close as anyone can come to [comparing] two different engines on the same boat. At least thats what I went off from. Any better real world non pedantic ideas on these old whalers, I'm all ears.

[jimh]

115hp burned 3.6gph at 3500rpm 24.1mph 6.69mpg, range 133.. Top speed 47.8
90hp burned 2.1gph at 3500rpm 17.6mph 8.38mpg, range 166.. Top speed 41.9.

That you have now provided some data to compare the 90-HP and 115-HP engine allows for an actual comparison to be made between these two Suzuki engines and their fuel economy.

To make a proper comparison of the engine fuel efficiently from the data selected for comparison there is a hurdle to be overcome: in the data provided the two engines are NOT performing the same amount of work. The 115-HP engine is working much harder as it is propelling the boat at 24.1-MPH, while the 90-HP engine is producing much less work, as it only moves the boat at 17.6-MPH.

We can made an adjustment for the difference in the amount of work being performed by using the boat speeds. It is well known that the relationship between boat speed and horsepower is to the 0.5 exponent, that is, the boat speed varies with the power ratio to the 0.5-exponent.

In this case the boat speed ratio in 24.1 / 17.6 = 1.37. This suggests that the power ratio must have been 1.875, that is, the 115-HP engine was making 1.875-times more power than the 90-HP engine. Now we can compare the fuel consumption figures.

The 90-HP engine consumes 2.1-GPH making its reduced power, and the 115-HP consumes 3.6-GPH making its 1.875-times more power. If we compare the fuel burn figures we see they are in a ratio of 3.6/2.1= 1.714, which means the 115-HP engine is actually running more efficiently as it makes 1.875-times more power while only consuming 1.714-times more fuel.

I don't think your intention by presenting this data was to demonstrate that the 115-HP was more fuel efficient, but that is what the data indicates.

Any better real world non pedantic ideas on these old whalers, I'm all ears.

I hope you were listening to my comments in this reply. Using actual data and actual physical relationships is not particularly considered to be pedantry by most people, although today the use of mathematics and physics might seem so to some.

What also has been demonstrated in the data provided is that operating the boat at lower speeds tends to reduce fuel consumption, but this relationship was already well known.

[jimh]

As mentioned earlier in this thread, the best indicator of fuel efficiency for an internal combustion engine is the brake specific fuel consumption (BSFC). The best operating point for lowest BSFC is highly dependent on the engine design, the engine speed, and the load on the engine. There is no simple method to predict exactly where in an engine's operating curve of power output, engine speed, and fuel consumption the best efficiency will occur, but generally it won't be at full-throttle, which is no surprise, but the best BSFC will also not generally be at a very light throttle setting, either.

Among outboard spark-ignition gasoline engines, there is seldom any BSFC data provided by their manufacturers. About all one can do to infer a value for BSFC is to look at the engine fuel burn at an engine speed where the engine is producing its rated power output, and assume at that point the power being produced is the power indicated by the manufacturer. For example, here is some data from a Honda outboard engine:

HONDA BF350
Source: https://cdn.powerequipment.honda.com/marine/pdf/props/24%20C%20Hawk%2026%20BF350A(17L+).pdf
Fuel burn at full-throttle = 29.98 GPH

Assuming power produced was 350-HP, and using the 29.98-gallons of fuel burned each hour, the BSFC can be computed as follows:
BSFC = 29.98-gallons x 6.25-lbs/1-gallon x 1/(350-HP x 1-hour)
BSFC = 0.535-lbs/HP-hour

This is a rather good figure for fuel burn at full throttle--at least for a gasoline outboard engine. Some carefully tuned modern automotive direct fuel-injected gasoline engines can have BSFC values in the 0.45-lbs/HP-hour region.

For many years a reasonable Rule-of-Thumb for outboard engines was to estimate the fuel burn in GPH as the HP/10, or in the case of 350-HP, the estimated fuel burn would be 35-GPH. With a modern engine like a HONDA BF350 the full-throttle fuel burn is down to about 30-GPH. A reduction from 30-GPH from 35-GPH is a decrease of about 14-percent.

ADDENDUM
The density of gasoline varies with temperature and octane. For boat fuel, using a 90-Octane fuel, a figure of 6.25-lbs/gallon seems to be a reasonable figure. For details see

How Much Does Gasoline Weigh Per Gallon?
https://www.jdpower.com/cars/shopping-g ... per-gallon

A prior discussion of BSFC reports many interesting values from other types of engines, notably from diesel engines which see much more fuel efficient operating. See

Brake Specific Fuel Consumption
https://continuouswave.com/ubb/Forum4/HTML/004149.html