| |
 |
 |
 |
| |
| Tony's Tips |
ARTICLE DATE: 02/01/2002 |
| Some Thoughts on Exhaust
Systems |
|
|
|
 |
|
 |
|
|
| There are no REQUIRED exhaust sizes for Cummins
marine engines; there are RECOMMENDED minimum sizes that may or may not
meet the required exhaust restriction requirements published by Cummins.
Those two statements above are also applicable to most marine engine
recommendations / requirements. The sizes that are recommended are based
on past experience and are merely a guide in the selection process for
designing a safe and non-restrictive exhaust system. |
For all of the Cummins marine engines that are in current production,
the maximum restriction at rated output is 3" Hg (about 1½
psi) and I believe this is close to most of the competitive manufactured
turbo-charged engines in this 100-1000 hp range.
In most exhaust systems that are in the type of
boats discussed in these forums (150-650 HP,) there are TWO distinct
parts of the exhaust system/ piping. The DRY part and the WET part. Even
on the factory "wet elbows" supplied by all of the manufacturers
that I've seen, these two sections exist, though many people don't realize
it.. The inner pipe of this "wet elbow" is actually a 90-degree
dry bend, or section, surrounded by raw water to keep the surface cool.
At the end of this "wet" elbow, where the exhaust hose attaches,
is where the water is introduced (hence the term "mixing elbow")
and the exhaust NOW becomes wet. Inside this elbow is a smaller diameter
(typically around 2 ½ - 5" ID) which is the dry side, and
where the hose attaches, it expands to (or is surrounded by) 4-8"
tubing/OD piping, depending on the engine size, etc..
When using custom exhaust risers, or most factory
supplied wet elbows, the fabricators of these systems employ various
techniques to design and build these parts. Some of the designs follow
good engineering practices with "thought out" design failure
scenarios should the system NOT last the life of the boat. But, MANY
do not. In practical thinking about marine exhaust systems, never consider
your exhaust riser or wet elbow to be "lifetime." But, in real
life, when most marine exhaust systems fail, (wet type), they lead to
contingency damage of various engine parts as the designer DID NOT figure
in a failure scenario that would (will) occur from internal corrosion/leaking.
This is usually due to the fact that when the "wet side" fails
(the failure is often in close proximity to where the salt water is first
introduced to the exhaust riser/elbow,) this water ends up in the engine
when the owner least suspects it. Internal failure of "wet elbows"
and custom water jacketed risers is an old and ongoing problem, regardless
of material choice, and/or other claimed construction features. |
|
| When building custom wet exhaust systems,
these are but a few are of "common sense" guidelines
to follow in their design that will help eliminate these types
of failures/contingency damages: |
| 1. Use all of the available height in the
engine room for the riser (where needed and is practical) BEFORE
turning over the top and injecting water; i.e. always inject the
water on the downhill side, or down stream of the top of the riser.
A wet exhaust system with a steep downward slope is always better
and safer. |
| 2. Always use gravity to your advantage. Water
flows downhill so, if you have a system that holds water (water
jacketed risers for instance) and this system fails internally
(it's not IF it is going to fail, it's WHEN it fails,) where will
the water go?? Into the turbo/exhaust manifold/cylinders?? Think
about YOUR riser or elbow, should it fail internally where you
can't see it., what might happen. |
| 3.Be sure that IF the option presents itself
in the design of a wet exhaust system, to allow for all of the
water to drain itself from the exhaust when not running. Although
this can't always be done, you can still build a safe system by
utilizing other simple design ideas, custom mufflers, surge tubes,
etc. |
| 4.When sizing the system to meet acceptable
restriction requirements in order to protect the engine from excessive
back pressure, keep bends to a minimum, especially "wet"
90 degree bends. You can assume that, for any given size of piping,
a smooth 90 degree "dry" elbow is equal to about 10 feet
of the same diameter pipe as far as restriction goes. In some instances,
wet 90s could easily be equal to 20 feet of piping when unnecessary
water is injected into the exhaust. That brings us to one of the
main reasons for this discussion: Exhaust sizes vs. restriction
"requirements." |
|
|
A typical high performance marine diesel of around
300 HP will pump about 50-70 GPM of salt water through the engine cooling
circuit at rated RPM (not necessarily rated HP.) This large amount of
water is needed to keep the engine cool and provide for some reserve,
but all of this water (excess water) is NOT necessary to inject into
the exhaust system to cool the exhaust gases and quiet the exhaust level..
Although most engine installations do indeed inject all of this water,
as it "came from the factory that way," this is not required
or necessarily desirable in many installations and repowers.
The mixing of salt water into the very hot exhaust gases accomplishes
many things: |
It cools the exhaust to levels that are safe (140
deg. F or less) allowing the use of hoses, fiberglass tubing and mufflers,
and adds considerably to the quieting and scrubbing process. But it also
adds considerably to the restriction of the exhaust system..
Although I've talked to "people in the know" that claim the
opposite (the exhaust are gases cooled and therefore have less volume,)
just believe me that when water is added to 900+++ degree exhaust and
steam is produced, that the volume of the mixture of hot gas and water
vapor is substantially higher, and therefore needs a much larger exhaust
diameter after this mix takes place. This, along with the addition of
the water itself and its own frictional resistance and volume within
the piping, all add to back pressure and require larger piping than the
dry side.. A simple way to see this is by looking at exhaust diameters
of typical 300 HP diesels in trucks and comparing them to the typical
sizes of a 300 HP marine diesel wet exhaust system.
Now, back to that excess water, and how can we use this water that is
needed for the engine cooling but not necessarily needed for the exhaust
cooling. First we need to understand how much "excess" there
is and then how we use it to our advantage.. Just from testing and building
wet and dry exhaust systems for about 20 years in boats, I've learned
that you need to inject between ¾ and 1½ GPM of water flow
per 10 HP into most any properly designed mixing elbow/wet exhaust system
to allow for more than adequate cooling and silencing of the system..
By putting a simple "T" in the hose after the heat exchanger
that goes between it and the mixing elbow, and then allowing the proper
amount of water to flow out the side or back of the boat freely, one
can reduce back pressure by a measurable margin, and now, in many cases,
use smaller than recommended sizes of exhaust hose and/ or components
and still meet restriction requirements.
From direct experience, I've used 5" wet on 450 C's and 4½"
wet on 330 Diamonds and still was able to be well under restriction limits
at rated HP and RPM. In quite a few of the repowers I run into, we are
going from a gas motor w/ around 3 ½ or 4" exhaust size,
and the recommended size for the new diesel may be 6".. This can
be a tough one in many instances (space and cost limitations) and with
some clever engineering, I've always been able to use 5", or even
less in some cases.. There are many factors involved in this selection
of smaller sizes, and only with experience to draw from, can all of needed
design features be put together in a way that will insure a safe, but
non-restrictive exhaust system. As to having to use 8" exhaust on
the Cummins 450C (mentioned in a post on the Volvo 70's forum last week);
only in an exhaust system with a design that gave little thought about
all of the options which could be used to lower restriction, would an
8" exhaust size be needed. Never have I had to use an 8" exhaust
on this engine to meet restriction requirements.
Please don't take the above to mean that I don't use 5", 6"
or even 8" systems. I'm only trying to help with some of the repowers
that come up where the recommended exhaust sizes DON'T fit the constraints
of the boat or maybe the pocket book. It takes very careful planning
to reduce from the recommended sizes but this alternative is there, if
the design is right. There are many other tricks to reducing restriction
in wet exhaust systems and these may include coring a hole through the
first input baffle inside a typical inline muffler, enlarging spigot
sizes on both the input and the output of an existing 4" or 5"
muffler to the next larger size, etc. etc. etc. Again, these all come
from past experience and field trials along with some good solid engineering.
And, as one added feature with bypassing some water out the side of the
boat, you will get a easy "visual" of this water flow, and
on boats w/ swim steps and stern exhaust, seeing this water flow can
add a feeling of comfort knowing that you are pumping lots of seawater. |
|
|
