Re: TWL: To turbo or not...
snip
It appears from the data supplied by John Deere, the fuel efficiency of
their turbo is more than "a few percent."
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I appreciate the reaction from someone knowledgeable -
I guess I was being a little simplistic. But my idea that turbo diesels
had only a little advantage in fuel efficiency was based, among other
things, on the article on diesels on the web site provided by Michael
Maurice in his posting on the 6th under the heading "Documentation".
See:-
http://www.uaf.edu/seagrant/boatkeeper/
I would be interested to see a comparison of fuel efficiency not of a
turbo diesel with the NA version of the same engine but a comparison of
a turbo diesel with a (different) NA engine of the same HP, both running
at maximum efficiency. The comparison given is of one engine at it's max
rated horsepower (obviously not at it's most efficient) and the turbo
version of the same engine at 60 percent of revs.
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We charge about $2000 to $4000 more compared the natural version.
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I would be interested in a comparison of price of 2 engines, one turbo
and the other NA, again of the same HP at their continuous ratings
respectively, since that is the choice under discussion, i.e. choosing a
power plant from scratch after the HP requirement is set by the choice
of boat and speed.
Thanks in advance to all for any comparisons contributed.
At 02:33 PM 9/7/01, you wrote:
I guess I was being a little simplistic. But my idea that turbo diesels
had only a little advantage in fuel efficiency was based, among other
things, on the article on diesels on the web site provided by Michael
Since this thread does not seem to be able to die a naturally aspirated
death. I thought I would throw in another few cents worth.
From a boat 2 years ago with a John Deere 6 cylinder turbocharged engine
of about 300 horsepower.
The mean time to rebuild: 2500 hours.
For those choosing an engine for long range cruising in foreign countries,
do not be fooled into thinking that a turboed engine is necessarily a good
thing just because it has become fashionable.
2500 hours at 8 knots is only about 23000 miles of cruising.
Fashionable doesn't cut it in the real world. Yea, I know there is the
clean air problem. But the fashionable set will scuttle the best technical
solution given a reasonable excuse.
The day is coming when only the rich will be able to afford the politically
correct engines to cruise around the world.
Capt. Mike Maurice
Near Portland Oregon.
<<I would be interested to see a comparison of fuel
efficiency not of a
turbo diesel with the NA version of the same engine
but a comparison of
a turbo diesel with a (different) NA engine of the
same HP, both running
at maximum efficiency. >>
This is a tall order, because most of the NA engines
are of low power, so it's hard to find a similar
design of turboed engine of similar power. Where the
engines are available, the data is not always
plentiful.
The best I could do, after a couple hours poring over
my 3" thick engine data sheet collection, is the
Lugger L439T 4 cyl turbo(120hp@2800rpm HO rating) vs.
their L668D 6 cyl NA(145 hp @2500 rpm HO rating).
Both these engines are built from the current John
Deere blocks. The data sheet I have is dated 1992,
but was current in 1996 when I was engine shopping.
I've used 7.00 lbs/gal for the density of the fuel.
Data below is from the Lugger brochure, except
Specific Fuel Consumption, which I have calculated
from their data.
L439T at 2800 rpm, 120 hp, 7.5 gph, SFC .437 lb/hp-hr
L668D at 2500 rpm, 145hp, 8.4 gph, SFC .405 lb/hp-hr
At wide open throttle, the NA engine beats the turbo
by 7% (but we don't run our engines here).
L439T at 1800 rpm, 1.7 gph, 31.9 hp, SFC .373 lb/hp-hr
L668D at 1500 rpm, 1.7 gph, 31.3 hp, SFC .380
At these typical minimum cruise speeds, the turbo wins
by 1.8%. I used the prop law exponent 3.0 for this
calculation, as used by Lugger.
At other engine speeds, I have read from the tiny
graphs in the brochure for specific fuel consumption
(not as precise as I'd like), but over the rpm range
of 1400-2600 for the turbo, and 1200-2300 for the
bigger NA engine, the SFC remains in the range between
.370 and .400 for both engines. At low RPM, the turbo
engine is very slightly better, at high rpm's, the NA
engine squeaks out a victory. At any comparison of
these two engines making similar power, the difference
is less than 5% on specific fuel consumption.
To me, the choice between turbo or NA diesels comes
down to this: would I rather have a heavy engine that
lasts more hours between rebuilds, or a lighter engine
that needs more frequent rebuilds. For typical
pleasure use (50-200 hrs/year), turboed engines last
"long enough". I run 800 hours/year, so I chose the
heavy, longer lived NA engine.
=====
Mark Richter, M.E., aboard M/V Winnie the Pooh,
custom Morgan 46 Pilothouse Efficiency Trawler.
"Mark's Mobile Marine" electrical systems repair & consulting. Homeport Stuart, FL
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