Originally Posted by Horror Hotel
Making 60 hp requires the same fuel
whether NA or turbo.
Not quite true. Specific power (amount of fuel
required to produce a given amount of kwH or horsepower/hours) is improved by turbocharging, usually by about 10% but can be more. That is because energy from the exhaust
stream which would otherwise be wasted in a NA engine is recaptured by the turbine, and because efficiency of the combustion process is somewhat improved.
"Fuel consumption values of two medium-speed diesel engines of equal horsepower with (solid curves) and without(dash curves) turbocharging, showing signiﬁcant advantages for the tur-bocharged engine .aspirated engine.Downsized engines, for a desired power output, present smaller brakespeciﬁc fuel consumption values, Figure 5.7." Turbocharged Engines | Patric Figueiredo - Academia.edu
And some turbocharged engines are more efficient than others. For example, the Yanmar 4JH3-HTE with intercooler uses less than 160 grams of fuel per horsepower/hour over a wide range of speeds; and 153 at 2500 RPM
The 4JH3-TE, which is the same engine but without the intercooler, has best specific consumption of 167 grams/horsepower/hour. That is, about 9% more fuel consumption for a given power output, just because of the lack of an intercooler.
I couldn't find the specific fuel consumption curves for the Perkins
M4.236, but we can be sure that there's a pretty big difference to the 4JH3-HTE. The big naturally aspirated Perkins
has much bigger cylinders to give the same amount of power (actually a bit less). The greater area of those bigger cylinders will mean more heat energy is lost
to the cooling
system, rather than being converted to mechanical energy. Those bigger cylinders will produce more friction. None of the energy of the exhaust
stream is recaptured.
I would be bet that the 4JH3-HTE is 9% more efficient than the also turbocharged 4JH3-TE, then it must be 20% more efficient than the Perkins if not more.
A different question is whether or not we care -- maybe we sailors don't care that much about +/- 20% of fuel consumption, since long passages will always being done under sail anyway. Fair enough.
On smaller engines (Under 80 horsepower? Under 50?) it may also not be worth thinking about. Everyone will have to decide for himself, but turbocharged engines are more efficient; that's just a fact.
For me, I don't think 20% difference in fuel consumption is a big factor. For me, the main advantage of a modern turbo-diesel is weight and size. Weight makes a big difference to sailing performance. The Yanmar with reduction gear
weighs 256 kilos; the Perkins at 520 kg is more than double the weight! 264 kilos of difference or 580 pounds! Wow! That's a quarter of a ton of difference
To put it another way -- the 100 horsepower Yanmar plus
my heavy-duty, continuous-duty rated 6.5EFOZ Kohler genset together with sound enclosure together weigh about the same as a Perkins M4.236 of 85 horsepower.
For many sailors, that alone will be well worth the added complexity of a turbocharger.