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Old 15-04-2015, 08:58   #1
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Wet exhaust explained

I get the basic premise of my wet exhaust. Hot water from my heat exchanger is mixed with hot exhaust from the engine at an elbow which is above the water line. It then cools the hot exhaust in the mixer at the bottom of the photo and is then shot over board by the pressure from the exhaust.

Recently my exhaust developed a crack right at the first right angle coming out of the engine. And I didn't notice any change in engine performance before having it welded together.

Does the exhaust and the back pressure from it somehow aid the engine's combustion? Other than suffocation what are some of the risks of having a cracked exhaust before the water mixed with it? Will it affect long term engine performance?

Im just trying to get a better understanding of how an exhaust, more specifically a wet exhaust, interacts with the rest of the engine.

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Old 15-04-2015, 12:01   #2
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Re: Wet exhaust explained

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Originally Posted by unbusted67 View Post
Recently my exhaust developed a crack right at the first right angle coming out of the engine. And I didn't notice any change in engine performance before having it welded together.

Does the exhaust and the back pressure from it somehow aid the engine's combustion? Other than suffocation what are some of the risks of having a cracked exhaust before the water mixed with it? Will it affect long term engine performance?

I think most engine manufacturer's size the exhaust outlet to provide the correct back pressure. That's not much info about how/why back pressure, but it might at least speak to the size of your outlet, mixing elbow, etc.

FWIW, many of the gas engine guys in our owners club talk about replacing their elbows -- and risers, if there are any, and sometimes even exhaust manifolds -- every 5 years, in salt water. Dunno why the diesel guys aren't so vocal about it.

-Chris
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Old 15-04-2015, 12:03   #3
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Re: Wet exhaust explained

I guess I'm a little confused about the back pressure thing since I didn't notice any difference in performance with a substantial crack in the tube


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Old 15-04-2015, 12:11   #4
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Re: Wet exhaust explained

The engines in our boats are nowhere high enough performance to notice a small reduction in back pressure. If your wet system is even remotely correctly sized there isn't a whole lot of back pressure to begin with, and the back pressure is constantly fluctuating as water is ejected in slug flow. If you ran a large diameter dry stack out you may notice a small increase in power/small decrease in fuel consumption.......but very small.


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Old 15-04-2015, 12:17   #5
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Re: Wet exhaust explained

Ahhhh thanks sail monkey. So basically, in a small sailboat engine, as long as you are still pushing water out the back of the boat a crack isn't a huge deal?


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Old 15-04-2015, 13:33   #6
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Re: Wet exhaust explained

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Ahhhh thanks sail monkey. So basically, in a small sailboat engine, as long as you are still pushing water out the back of the boat a crack isn't a huge deal?


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Old 15-04-2015, 14:49   #7
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Re: Wet exhaust explained

The appearance of a small crack in the dry portion of an exhaust system like yours won't have a significant impact on the engine life or the engine output if it is not allowed to remain too long. If you consider the cross sectional area of a crack compared to the cross sectional area of the exhaust pipe, you will see that there isn't going to be much change in the exhaust back pressure.

However, that's not to say that the crack shouldn't be repaired as soon as possible. Exhaust leaking into an engine room can cause several problems, such as sooting up everything in the boat, injesting small carbon particles back into the engine which causes accelerated wear, and stinking up all your cushions.

If your engine is running anywhere close to correctly, you won't be suffocated by carbon monoxide in the exhaust, because there is almost no CO in diesel engine exhaust. But breathing minute carbon particles and CO2 doesn't do you any good...

The reason the gas engine guys are talking about elbows, risers and manifolds more than the diesel guys is that most gas engine manifolds, risers and elbows are made of cast iron and are salt water cooled, even when the engine is equipped with freshwater cooling. When they shut the engine down the saltwater remains in the manifold and riser, and over time the cast iron rusts and plugs up. Most of the marine diesels these days are equipped with freshwater cooled manifolds, so they aren't subject to rust. The higher output diesels are also equipped with stainless risers or elbows so they don't have these problems at all.
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Old 15-04-2015, 15:17   #8
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Re: Wet exhaust explained

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The appearance of a small crack in the dry portion of an exhaust system like yours won't have a significant impact on the engine life or the engine output if it is not allowed to remain too long. If you consider the cross sectional area of a crack compared to the cross sectional area of the exhaust pipe, you will see that there isn't going to be much change in the exhaust back pressure.

However, that's not to say that the crack shouldn't be repaired as soon as possible. Exhaust leaking into an engine room can cause several problems, such as sooting up everything in the boat, injesting small carbon particles back into the engine which causes accelerated wear, and stinking up all your cushions.

If your engine is running anywhere close to correctly, you won't be suffocated by carbon monoxide in the exhaust, because there is almost no CO in diesel engine exhaust. But breathing minute carbon particles and CO2 doesn't do you any good...

The reason the gas engine guys are talking about elbows, risers and manifolds more than the diesel guys is that most gas engine manifolds, risers and elbows are made of cast iron and are salt water cooled, even when the engine is equipped with freshwater cooling. When they shut the engine down the saltwater remains in the manifold and riser, and over time the cast iron rusts and plugs up. Most of the marine diesels these days are equipped with freshwater cooled manifolds, so they aren't subject to rust. The higher output diesels are also equipped with stainless risers or elbows so they don't have these problems at all.
DougR
That's good to know about diesel exhaust. Thanks for that.
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Old 15-04-2015, 15:27   #9
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Re: Wet exhaust explained

+1
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Old 15-04-2015, 15:28   #10
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Re: Wet exhaust explained

The only thing I have ever heard of problems with reducing back pressure is on race cars with essentially no exhaust system. You want to put plugs in the pipes after shutdown to keep cold air from reaching the exhaust valves and warping them from cooling too quickly. These are big open short pipes.

I think you only have to worry about exhaust gas in the cabin.
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Old 16-04-2015, 06:22   #11
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Re: Wet exhaust explained

Quote:
Originally Posted by DougR View Post
The reason the gas engine guys are talking about elbows, risers and manifolds more than the diesel guys is that most gas engine manifolds, risers and elbows are made of cast iron and are salt water cooled, even when the engine is equipped with freshwater cooling. When they shut the engine down the saltwater remains in the manifold and riser, and over time the cast iron rusts and plugs up. Most of the marine diesels these days are equipped with freshwater cooled manifolds, so they aren't subject to rust. The higher output diesels are also equipped with stainless risers or elbows so they don't have these problems at all.
DougR

Useful, thanks!

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Old 16-04-2015, 06:48   #12
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Re: Wet exhaust explained

Exhaust pressure builds to the point at which it drives accumulated exhaust gas and cooling water through the exhaust system to the atmosphere. The pressure is a function of the configuration of the exhaust system. A longer, smaller diameter system will generate higher exhaust pressure than a shorter, lather diameter system for example. The system should have adequate capacity to maintain exhaust pressure at less than the maximum specified by the engine manufacturer, typically a couple of psi. If exhaust pressure is too high, cooling water can be drawn into the exhaust manifold and exhaust valves where the water boils leaving salt which ultimately can damage the engine. Some of the engine's power is used to generate the exhaust pressure so lower exhaust pressure improves engine output. That is why high performance car engines use headers, dual exhaust, loud mufflers, etc. Your leak seems pretty small and likely had very little effect on performance.
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Old 16-04-2015, 07:26   #13
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Re: Wet exhaust explained

Back pressure does not aid combustion. Combustion is all done in the cylinders in diesel engines. Back pressure specified by manufacturers is the maximum allowed. Less is better.
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