giant exhaust elbow

[markxengineerin]

What would the disadvantages of a larger exhaust elbow be?

It's 1.5" ID now, and going to 2" would give 80 ish percent more area, which seems like 80% longer life, as an example. If that logic works, extrapolate from there until the failure becomes the material, not the clogging (?)

[Orion Jim]

You need to balance excessive back pressure while maintaining adequate exhaust pressure velocity. Your engine has intake and exhaust flow parameters based on horsepower. Fooling with one parameter effects all other parameters. It is less an art and more of a science.

[markxengineerin]

People use all kinds of exhaust elbows, downstream mufflers, etc. At best the OEMs probably follow a generic guidance from Yanmar like "minimize exhaust length and bends". I'm not as worried about the effect on the upstream engine performance due to local velocity decrease. The very non-optimal shape of the exhaust manifold shows that Yanmar prioritized packaging and cost, not any kind of wave tuning and performance. I'm looking for ideas about what I might be missing, maybe something specific to exhaust elbow performance, as I have minimal knowledge of marine-specific engines, but I design generic ones for my day job.

[tkeithlu]

Messing with the material used might be more productive than changing the size.

"specific to exhaust elbow performance" is going to be a pretty narrow specialty when the manufacturers aren't even trying to tune the exhaust. These aren't two-cycle motocross bikes.

Particular to marine engines is wet exhaust, which involves exhaust pressure shooting water up and out of a bath. You might consider the designer's expectation of velocity and pressure available for that job. I would exercise caution in messing with that, particularly with no substantive gain in mind.

I'm going to vote with Orion Jim on this one. He's pointing out something that you may be overlooking, which was your expressed concern.

[Manateeman]

Exhaust gas /water separators are found on a number of generator set ups.
We simply scaled up one for our 170 hp turbo diesel. We used CeraKote on the interior hot side of the turbo. Nice stainless mixer. The back pressure on the turbo is nicely reduced and we cannot see what harm this would do.
The manatee crew continues to work on their system for using excess heat to improve their “turbocharged “ beer fermentation vessel. They’ve been unsuccessful in patent applications but continue to hope “the Pizza Bazooka” will get funded by Save the Orcas.
Merry Christmas
Captain Mark

[markxengineerin]

Since the water is being injected at the high point of the elbow, I guess the bath you are referring to is downstream, in the wet muffler?

I think an ideal elbow would be large during the "turn" where it's impossible to clean, but could neck down to normal size at the inlet and outlet. Those spots are easier to clean using a die grinder or similar, but there's no access to the "turn" without cutting open.

[rslifkin]

What is the elbow clogging up with? I'm thinking salt scale and corroded metal? It seems like the issue is one of poor design more than the size.

[markxengineerin]

From what I've read, exhaust elbows are throwaway items for marine diesel engines that idle semi-frequently. Replace every 1, 2, 5 years, pick your number. Mine clogged several times when I had inadequate solar and had to do an hour of idling a day to keep batteries going. Replacing an expensive stainless steel casting that frequently is wasteful if there is a better design.

[Wotname]

Originally Posted by rslifkin

What is the elbow clogging up with? I'm thinking salt scale and corroded metal? It seems like the issue is one of poor design more than the size.

Principally calcium carbonate which precipitates out of sea water rather rapidly at temperatures over about 50C.

[rotrax]

I am going to try a Swedish made S/S elbow for our Yanmar 4JH4-HTE. The Yanmar Elbow fitted as OE is a huge cast bronze job, cost in the UK around £1500. The Swedish replacement £300.
I found a pinhole in the OE Bronze elbow during the 1000 hour service. By drilling, tapping 6mm and screwing in a 6mm bolt, with two locknuts on the other side and the end peened over I made a satisfactory - albeit temporary - repair.
A sculptor I know will get the bronze elbow to use when making a bronze statue!
I am a good recycler.

[Steve_C]

Originally Posted by rotrax

I am going to try a Swedish made S/S elbow for our Yanmar 4JH4-HTE. The Yanmar Elbow fitted as OE is a huge cast bronze job, cost in the UK around £1500. The Swedish replacement £300.
I found a pinhole in the OE Bronze elbow during the 1000 hour service. By drilling, tapping 6mm and screwing in a 6mm bolt, with two locknuts on the other side and the end peened over I made a satisfactory - albeit temporary - repair.
A sculptor I know will get the bronze elbow to use when making a bronze statue!
I am a good recycler.

Do you have a source or perhaps a link to share? Thanks!

[kenbo]

Mark,

I reviewed your teardown pdf, very nice work. The mixing elbow that PSC installed on your boat was a poor choice for your engine. Here's why I say that.

That design was a carry-over from the early GM series that topped out at 27 hp on a 3 cylinder engine. You most likely can't tell from the corrosion on the outlet side but the outlet is threaded for a 1.25" pipe nipple and that's after the water is injected! That's avoidable restriction.

The water injection should always be on the downhill side of the elbow. If corrosion wears through the cast iron, seawater cannot enter the exhaust manifold.

You would be better served if you went full size 1.5" and made the U from 1.5" street 90° and then fit something like this https://www.asap-supplies.com/produc...m-hose-2187351. That's assuming that PSC used 2" hose for the wet exhaust. N.B. Yanmar uses BSP threads.

That said, Yanmar specs a 3" wet exhaust hose for that engine after the water is injected. So to answer your question, yes going to 3" wet exhaust is ideal for that engine but it will require upsizing everything from the mixing elbow aft. I'd bet that PSC never did an exhaust back pressure test to certify that engine warranty but hey you did 8000 hrs and no issues. Yanmar are fairly tolerant of high back pressure.

[rotrax]

Originally Posted by Steve_C

Do you have a source or perhaps a link to share? Thanks!

I ordered it direct from the Swedish maker from ebay and had it delivered to a friend in Denmark who will bring it to the UK in the new year.


The supplying company was:- marineparts.eu


I got the price wrong - it was cheaper!

[markxengineerin]

A cut into the elbow made me realize an obvious requirement I was missing- cooling the exhaust gas enough for the rest of the system downstream to be made from cheap and convenient materials (plastic, rubber). I think this is why the water enters an internal passage and takes a long U turn instead of just being shot in with a simple fitting on the downhill side.

It's not obvious how to make the perfect elbow, but I'm thinking about something that starts here:
https://www.summitracing.com/search/...-degrees%3A180

[markxengineerin]

Couple ideas- first is the simplest, but 2nd gives more cooling with just 1 extra pipe extension and 2 welds.