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#1
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Wet exhaust sizing.
Can anyone provide sizing info for wet exhausts on diesel engine. My recent
purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David |
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#2
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Wet exhaust sizing.
Acording to the Yanmar JH installation manual the exhaust should be the
size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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#3
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Wet exhaust sizing.
I concur. For belt and suspenders, you could go to several more engine
manufacturers' sites -- most of them have specs, but 4" sounds about right to me. Another risk with a exhaust that's too small is that when you crank the engine you're pumping water into the waterlift. Until the engine starts, it doesn't go up to the exit. A smaller exhaust has a smaller waterlift and therefore is more likely to drown the engine if it doesn't start easily. The appearance of "steam" is not unusual -- after all, the exhaust stream is gases fully saturated with water at an elevated temperature. When it hits the cooler air outside the boat, some of the moisture condenses. On the other hand, smoke, after the engine is warm, , is not desirable. Color will usually tell you what it is. Roughly, if it's blue, you're burning oil; black is overloading; and white is antifreeze leaking into the exhaust. -- Jim Woodward www.mvFintry.com "Glenn Ashmore" wrote in message news:lcsqb.7087$62.5549@lakeread04... Acording to the Yanmar JH installation manual the exhaust should be the size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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#4
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Wet exhaust sizing.
On Thu, 6 Nov 2003 23:14:57 +1100, "David Flew"
wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David Here's something. Note the full throttle power rating. Say 80HP That represents about 30% of the power available from the fuel in a gas engine. It represents perhaps 40% (say) of the fuel in a diesel - it is a more thermally efficient device. But use that 30% value. Say that the remaining heat exits via the exhaust (it doesnt) that's 70/30 of 80HP = 187 HP = 187 X 750 watts = 140.3 kW How much water flow is needed to carry off 141 kW at moderate temp rise? If you allow a temp rise of 50 to 180 degF (that's 10degC to 82degC) 141 kW is mass rate (kg/sec) times temp rise (72degC) times calorie to watt conversion (1/4.2) So mass rate is (141 X 4.2 )/72 = 8.23 kg/sec or 2.25 gall/sec = 136 gall/min Size a pipe to carry this water flow at moderate speed while occupying 20% of the pipe cross section, and you have a rather conservative rule of thumb to size the exhaust pipe (and the cooling pump). The flow rate is dramatically reduced if you allow some boiling to occur. Does this help? Brian Whatcott Altus OK |
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#5
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Wet exhaust sizing.
"Brian Whatcott" wrote in message ... On Thu, 6 Nov 2003 23:14:57 +1100, "David Flew" wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David Here's something. Note the full throttle power rating. Say 80HP That represents about 30% of the power available from the fuel in a gas engine. It represents perhaps 40% (say) of the fuel in a diesel - it is a more thermally efficient device. But use that 30% value. Say that the remaining heat exits via the exhaust (it doesnt) that's 70/30 of 80HP = 187 HP = 187 X 750 watts = 140.3 kW How much water flow is needed to carry off 141 kW at moderate temp rise? If you allow a temp rise of 50 to 180 degF (that's 10degC to 82degC) 141 kW is mass rate (kg/sec) times temp rise (72degC) times calorie to watt conversion (1/4.2) So mass rate is (141 X 4.2 )/72 = 8.23 kg/sec or 2.25 gall/sec = 136 gall/min Size a pipe to carry this water flow at moderate speed while occupying 20% of the pipe cross section, and you have a rather conservative rule of thumb to size the exhaust pipe (and the cooling pump). The flow rate is dramatically reduced if you allow some boiling to occur. Does this help? Brian Whatcott Altus OK It helps, but what is "moderate speed? If I had a design velocity it would be easy .... DF |
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#6
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Wet exhaust sizing.
Glen
I was hoping for a response from you - always seem to give a good practical approach. Aside from not having a water lock, I think you are spot on in both the required sizing, the original sizing, and the potential for damage. I suspect that damage was avoided by the previous owner by limiting RPM to 2200 -2500. If 3.5 " is OK for 3500 RPM., then 2.5 " is OK for roughly 2200. Either way the exhaust is clearly too small. As to the "new" engine. It's "new" for this boat. In Australia it's very economical to buy a low mileage second hand Japanese diesel, which is what I have. David "Glenn Ashmore" wrote in message news:lcsqb.7087$62.5549@lakeread04... Acording to the Yanmar JH installation manual the exhaust should be the size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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#7
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Wet exhaust sizing.
Good luck. Replacing an exhaust system is a nasty job best left to a
circus contortionist. :-) David Flew wrote: Glen I was hoping for a response from you - always seem to give a good practical approach. Aside from not having a water lock, I think you are spot on in both the required sizing, the original sizing, and the potential for damage. I suspect that damage was avoided by the previous owner by limiting RPM to 2200 -2500. If 3.5 " is OK for 3500 RPM., then 2.5 " is OK for roughly 2200. Either way the exhaust is clearly too small. As to the "new" engine. It's "new" for this boat. In Australia it's very economical to buy a low mileage second hand Japanese diesel, which is what I have. David "Glenn Ashmore" wrote in message news:lcsqb.7087$62.5549@lakeread04... Acording to the Yanmar JH installation manual the exhaust should be the size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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#8
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Wet exhaust sizing.
On Fri, 7 Nov 2003 20:57:48 +1100, "David Flew"
wrote: "Brian Whatcott" wrote in message .. . On Thu, 6 Nov 2003 23:14:57 +1100, "David Flew" wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David Here's something. Note the full throttle power rating. Say 80HP That represents about 30% of the power available from the fuel in a gas engine. It represents perhaps 40% (say) of the fuel in a diesel - it is a more thermally efficient device. But use that 30% value. Say that the remaining heat exits via the exhaust (it doesnt) that's 70/30 of 80HP = 187 HP = 187 X 750 watts = 140.3 kW How much water flow is needed to carry off 141 kW at moderate temp rise? If you allow a temp rise of 50 to 180 degF (that's 10degC to 82degC) 141 kW is mass rate (kg/sec) times temp rise (72degC) times calorie to watt conversion (1/4.2) So mass rate is (141 X 4.2 )/72 = 8.23 kg/sec or 2.25 gall/sec = 136 gall/min Size a pipe to carry this water flow at moderate speed while occupying 20% of the pipe cross section, and you have a rather conservative rule of thumb to size the exhaust pipe (and the cooling pump). The flow rate is dramatically reduced if you allow some boiling to occur. Does this help? Brian Whatcott Altus OK It helps, but what is "moderate speed? If I had a design velocity it would be easy .... DF 6 ft/sec seems moderate to me at first glance For pipe of diameter D, its area is pi D^2/4 20% of that is pi D^2/20 that is a volume of 72/20 pi D^2 per second or 11.3 D^2 per second If we want 2.25 gall/sec to be carried in a volume of 11.3 D^2 cu ins/sec that's 231 X 2.25 = 11.3 D^2 so D^2 = 46 in ^2 and D = 6.8 inches or D = 5.25 in for 10 ft/sec water velocity.... As I say, this is conservative!! Brian W |
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#9
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Wet exhaust sizing.
Question. You say, "aside from not having a water lock", which concerns me.
I assume Glenn and you mean what I call a waterlift -- a pot at a low point from which the exhaust leads out near the bottom so that water in the pot is blown out after the engine starts. The primary function of these is to hold the water that is pumped through the engine before it starts. Something to perform that function is essential unless the engine is mounted high enough so that the exhaust drains by gravity without help. Without it, you run the risk of water backing up through the exhaust valves. If that happens and the engine then starts firing in another cylinder, you may break something in the cylinder with water in it -- certainly it's not good. This may be a low risk event, particularly if the engine and batteries are in good shape and it starts quickly every time, but most of us try to avoid even low risks of putting a rod through the block. -- Jim Woodward www.mvFintry.com .. "David Flew" wrote in message ... Glen I was hoping for a response from you - always seem to give a good practical approach. Aside from not having a water lock, I think you are spot on in both the required sizing, the original sizing, and the potential for damage. I suspect that damage was avoided by the previous owner by limiting RPM to 2200 -2500. If 3.5 " is OK for 3500 RPM., then 2.5 " is OK for roughly 2200. Either way the exhaust is clearly too small. As to the "new" engine. It's "new" for this boat. In Australia it's very economical to buy a low mileage second hand Japanese diesel, which is what I have. David "Glenn Ashmore" wrote in message news:lcsqb.7087$62.5549@lakeread04... Acording to the Yanmar JH installation manual the exhaust should be the size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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#10
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Wet exhaust sizing.
Jim
The installation is fairly typical of small wooden boats in my area - at 23 ft this is at the upper end of small. From the exhaust manifold piping angles down. Water injection point is in this downward sloping metal section. At some convenient point the system becomes rubber hose, which then connects to more metal components where the system penetrates the transom. The outlet is "about" the same level as the exhaust manifold, and the centrifugal cooling water pump is directly driven by the engine. A few boats have water lifts or water locks. I've not heard of any backing up problems as you describe, I'd guess that even during extended periods of cranking and failing to start the engine, there is enough air going out of the engine to keep the exhaust from filling with water - and there is little out put from the cooling pump at cranking speeds anyway. I suspect the hose acts a water lock, if it's big enough it certainly acts a crude silencer. I went from 1.75 inch to 2.5 inch hose on a 10 HP diesel in my "old" boat ( hope it will be my "former" boat rather than my "old" boat in a day or two ) and whilst I have no figures, subjectively the engine could be run to much higher revs before it became unpleasant to sit on the stern thwart. In the quieter mode, there seem to be discrete plugs of water ejected from the exhaust, along with continual spray. Once the RPM and exhaust velocity goes up, it's just a gas/water mix. So I suspect that water accumulates in the low points of the hose, and it's a water baffled silencer, if you get my drift. The water locks I've seen advertised suggest this is part of their function - but don't give much data to support the claim. I measured the hose on the "new" boat today - it's 2". I'm really looking forward to replacing it with 3.5". I think minimum bend radius goes up something like the cube of the diameter ... I disagree with Glen, it's not a job for a circus contortionist, but for an animal trainer. I'm really looking forward to taming the new hose. I can see the attraction of just piping the exhaust straight from the engine through the sides - even if you burnt some dangly bits every time you walked past the engine .... This time I'm going to take some noise readings as I go along, also performance details. Although the difference between 7.5 and 7.6 knots at full throttle is not exactly important! Regards David "Jim Woodward" jameslwoodward at attbi dot com wrote in message ... Question. You say, "aside from not having a water lock", which concerns me. I assume Glenn and you mean what I call a waterlift -- a pot at a low point from which the exhaust leads out near the bottom so that water in the pot is blown out after the engine starts. The primary function of these is to hold the water that is pumped through the engine before it starts. Something to perform that function is essential unless the engine is mounted high enough so that the exhaust drains by gravity without help. Without it, you run the risk of water backing up through the exhaust valves. If that happens and the engine then starts firing in another cylinder, you may break something in the cylinder with water in it -- certainly it's not good. This may be a low risk event, particularly if the engine and batteries are in good shape and it starts quickly every time, but most of us try to avoid even low risks of putting a rod through the block. -- Jim Woodward www.mvFintry.com . "David Flew" wrote in message ... Glen I was hoping for a response from you - always seem to give a good practical approach. Aside from not having a water lock, I think you are spot on in both the required sizing, the original sizing, and the potential for damage. I suspect that damage was avoided by the previous owner by limiting RPM to 2200 -2500. If 3.5 " is OK for 3500 RPM., then 2.5 " is OK for roughly 2200. Either way the exhaust is clearly too small. As to the "new" engine. It's "new" for this boat. In Australia it's very economical to buy a low mileage second hand Japanese diesel, which is what I have. David "Glenn Ashmore" wrote in message news:lcsqb.7087$62.5549@lakeread04... Acording to the Yanmar JH installation manual the exhaust should be the size of the mixing elbow outlet up to the water lock and then if the run is "long" (they don't say how long long is) to increase by 25%. From the Centrek site a 3.5" exhaust can handle 85 HP max so you might consider using 4" to have a little margin. The exhaust size for the original 80 HP gasoline engine was probably 2.5". If that system is still installed the back pressure is probably doing some damage to your new diesel. David Flew wrote: Can anyone provide sizing info for wet exhausts on diesel engine. My recent purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over that the exhaust has both a little smoke and what looks like steam to me. And the exhaust if absolutely rocketing out the pipe ! I've seen somewhere that diesels have twice the exhaust volume of similar sized petrol engines, which suggests the exhaust may be half the required area ( I bet it's the original diameter ...) but I'd love either rule of thumb or rigorous methods for sizing exhausts. Thanks David -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
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