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| Tony's Tips |
ARTICLE DATE: 10/01/2000 |
| Boost and EGT |
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What are turbo boost and EGT, and what are their relationships
and importance to the operation of a diesel engine in a boat?
Following is a brief summary of my ideas and some data that I've accumulated
over the years relating to the above, along with the operation of a boat and
these operating measurements: |
| Boost |
"Boost" is the positive manifold pressure (compressed
air) created by the turbo-charger and/or super-charger. This piece of equipment
is merely a specialized type of air compressor in which the exhaust gases are
funneled through a turbine. In turn, this turbine is connected to a compressor
wheel and as it turns at very high speeds, the compressor wheel creates the
compressed air (turbo-boost.)
The compressed air is forced from the turbo into the combustion chamber and
may pass through an after-cooler/inner-cooler on its way to the chamber. The
amount of boost or manifold pressure that is created is directly related to
the amount of exhaust gases produced by the fuel's combustion.
Diesel fuel requires tremendous quantities of oxygen to produce good combustion.
Along with allowing the most efficient combustion, this large quantity of air
is used to scavenge and cool (discussed below) all the components within the
combustion part of the engine. Typically, in a modern turbo-charged diesel,
air (oxygen) flow needs to be in excess 12,000 CFH (cubic feet per hour) per
100 horsepower produced. On a rough average, it requires approximately 5 gallons
per hour to produce this 100 horsepower, which will equate to an exhaust flow
of approximately 30,000+ CFH. This boost/manifold pressure/compressed air that
is developed by the turbo is in direct relation to this exhaust flow, which
is determined by the horsepower that is being produced, i.e., an engine running
at no or very low load uses very small amounts of fuel and, therefore, has
very low or no boost. A simple example would be removing an intake air hose
from a modern Cummins engine and starting the engine. One would not notice
any change in the start-up or idle characteristics of the engine, and even
revving the engine up in neutral, there would be very little noticeable change
in this type of operation. If the engine were then put into gear, as the throttle
is advanced and more horsepower is asked of this engine (because of the prop
loading,) a lack of combustion air would start to occur at about 1,000 rpm
and black smoke would start to become quite noticeable (no boost.)
Since all boats have varying characteristics relating to the way the engine
is loaded during operation, manufacturers do not publish boost curves in relation
to horsepower and rpm. What they do publish is maximum boost at RATED HP and
RPM.. This is why installing a boost gauge early in the operation of a new
vessel will allow the individual operator to develop base line numbers for
the particular boat. A typical planing hull boat, heavily loaded and transitioning
from hull speeds to planing speeds, may indeed go through a speed range that
requires substantially higher horsepower/boost than is needed to maintain the
boat at a planing speed above this transition speed. There are many boats/engine
combinations that would dictate the continued operation during this step-off
mode be avoided as the engine is actually in an "overload" condition.
As the years have gone by, my experience with Cummins B and C marine engines
has shown that the engineers have developed the turbo-charging technology to
where boost pressures rise much earlier in the rpm/ power range without over-boosting
at rated horsepower. In some engines, a "waste-gate" is used to keep
maximum turbo-boost at acceptable levels. A waste-gate is merely a pressure-operated
valve on the exhaust side of the turbo, which bypasses exhaust flow to control
turbine speed/boost. These better designed "turbo maps" compensate
for some of the unusual operating characteristics in a boat by allowing a much
more controlled air flow through the different loading variables in different
boats. |
| Exhaust Gas Temperature
(EGT) |
Exhaust gas temperature is merely the temperature of the
exhaust gases coming out of an internal combustion engine.. They are typically
measured at the outlet port of the exhaust and temp probes can be placed before
or after the turbo. When placed before the turbo, readings will be much higher
and the reading will also show a much quicker change when a load-change occurs.
The turbo extracts much of the exhaust energy (heat) and this results in this
measurement difference. As I'm most familiar w/ the Cummins B's and C's, and
the factory only publishes EGT #'s for measurements taken AFTER the turbo,
I can only comment on factory #'s interpolated w/ field testing of over a hundred
boats over the past 15 yrs or so.
EGT is also affected by engine load like "boost" but has a few other
characteristics that make it an extremely important monitoring device for the
operator who wants to use ALL of the HP he paid for, but also wants to be sure
that his operation does not go "over the edge". Besides being affected
by engine load, it is a gage that will tell you that your turbo is putting
out enough boost to have efficient combustion and keep the operating temperatures
within the engine from exceeding a safe limit. (Please understand that coolant
temperatures do not affect combustion temperatures.) Some earlier Cummins B's
& C's engines did not generate enough boost at mid-range/high load conditions
and EGT could rise well above 1000 F in RPM ranges around 1800-2100, thus shortening
the life of some components. This is where boost and cooling come into play.
Higher boost at these lower RPMs not only allows more HP to be produced, it
keeps EGT at a safe limit. Boost (higher boost) is a GOOD thing.
Now, imagine your new boat with twin high performance 300 HP turbo charged
after-cooled diesels, and you are ready to start using the boat for its intended
purpose. You know from the sea trials that everything seemed to run perfect,
you've got the standard factory instrumentation and now you'd like to outfit
the boat with all the goodies you have dreamed about.. During sea trials the
boat hit 30 k's with your buddy and the yacht broker on board, had a ¾
full fuel tank and the engine went right to its rated RPM (3600) with a little
extra.. Now comes the "wake-up" call.
It's been 3 weeks now and during this time you've added 1000 lbs of high dollar
gingerbread, got your fuel and water tanks full, and you are loaded for bear
(you not only have your 6 yrs accumulation of fishing tackle on board, your
friends that are helping to pay the fuel bill have theirs too. This is when
you need to have those other monitoring gages (a PYRO and BOOST gage) ready
to go, as now it's time to start developing those base line numbers we've been
talking about.
During sea trial, the boat seemed quite spunky. Although she is still performing
well, instead of topping out at 30 k's, with an easy cruise at 23, she only
hits 27 now at barely 3600 RPM and cruises nicely at 20. Had you had a boost
gauge and pyro during the sea trials, you may have seen 34lbs of boost at 3650
with an EGT of 900 F. Cruising at 3,100 rpm (23 k's) the boost was only registering
14lbs (light engine load.) Now, with all the extra weight and a three-week
old bottom, your 3,100 rpm cruise is only getting you 18½ k's, so now
you need 3225 rpm to a comfortable 20 k cruise. Your boost now is 21lbs and
your EGT is still reasonable at 825 F. Now, what can we do over the next few
months with these types of numbers and how can they help us monitor the performance
of our engine and be sure that we are not exceeding acceptable operating conditions?
As engine hours increase, air cleaners become dirtier, turbo blades and turbine
housings become fouled, and hoses, tubes, and after-coolers get oily. All of
these conditions will contribute to lower boost pressures and higher EGT's
at any given rpm/load. As time passes, boats seem to become heavier for a variety
of reasons while bottoms and props definitely get dirtier. These two conditions
lead to higher loading at any given prop rpm as the boat moves slower at the
same rpm. These last two conditions lead to higher loads at the same rpm and
will translate into higher EGT's in relation to previous boost pressures that
were noted when the bottom was cleaner and the boat was lighter.
If a "turbo hose" develops a leak over time due to a defective clamp
or chafing, the resulting air leak will show up as a rise in EGT with a decrease
in boost. I've actually been on a couple of sea trials where a turbo pipe/hose
would blow off at high speeds and besides the instantaneous sound of an explosion,
black smoke would pour out of the exhaust before the operator could pull back
the throttle. No boost.
That pyrometer gage (pyro) you installed will tell you when something is wrong
probably better than any other gage you could have if you want to know when
your engine is going to self destruct from being over loaded. That boost gage
will be the guide to engine load for your boat and will tell you if you are
getting the HP you paid for, along with establishing various engine loads throughout
the rpm range.
Are these added gages for every application?? That's a personal choice, but
with the high performance diesels that drive this market and are producing
upwards of 1 HP/ cubic inch, (along with many of the operators of these engines
asking them to produce in excess of 75% of their maximum power when "just
cruising around",) I'd say one would be driving blind (without them) w/
a $50,000 plus engine room pushing his boat and operating his engines "close
to the edge".......... Tony |
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