Exhaust manifold heat importance?

[Number21]

I'm looking for an engine to use for a larger cogenerator setup. To me it seems like choosing something that was used as a marine motor, and has water cooled exhaust manifolds available is a good idea. Unfortunately most of these motors are gasoline, not diesel, and too big for my use.

I want to collect as much waste heat as possible into my water system. With the exhaust manifold being the hottest part, it seems like the best place to collect heat energy. It's also really hard to create a custom water cooled exhaust manifold for a motor that was never used in a boat.

Lets say I made a simple shell heat exchanger around the exhaust pipe, after the manifold on a 3-4 cylinder motor. Now I wrap the exhaust manifold in insulation. Maybe some thermal barrier coatings. Would I still lose a lot of heat through the exhaust manifold? Is it going to be less efficient if I start collecting waste heat AFTER the manifold?

[mike90045]

look for a hot-rod header for that motor, and send send it out for inner and outer ceramic coating.   Then bolt that outlet onto the inlet of your heat ex. the header will take the hottest, ceramic coating will insulate it a bit and your your heat ex is easy to swap for testing.   You will loose some heat thru the header pipe, but it will be minimized.
http://www.jet-hot.com/  and a bunch of other vendors do it. 
Been thinking about maybe doing the head and piston top on the listeroid for better thermal and maybe easier carbon shedding.

[mobile_bob]

you can also get cast iron manifolds ceramic coated inside and out
being a tighter package they out to lose less heat than a tube header?

just a thought.

and another

i am thinking the exhaust temperatures are going to be fairly low, relative to full rated
load at rated rpm.

at 15-20kw load and 1200rpm, the exhaust gas temperatures might be much lower than one would like or expect?

bob g

[Number21]

at 15-20kw load and 1200rpm, the exhaust gas temperatures might be much lower than one would like or expect?

That might be my biggest issue with water cooled manifolds. I want to burn WMO, so I probably want a little higher EGT. I also need some heat to preheat the oil, a hot manifold would be a good place to get that.

Just spent some time looking at the thermal and friction coatings for pistons and such, I didn't know they sold a DIY version. Very interesting... I've also thought about wrapping manifolds in header type insulation, maybe even the whole engine block!

[buickanddeere]

  High exhaust gas temps and clean combustion are from engines operating with coolant temperatures above 195F. And operating at the full , max fuel rack setting with the higher HP per lb of fuel per hour. Somewhere a notch or two or three below the rack setting that makes absolute max HP.   

[Jens]

A couple of random thoughts:

I found that preheating of veggy oil was not really required once the engine was at operating temperature. First of all the injector pump is quite warm and, more importantly, the injectors can easily heat the oil as it comes in. On the other hand, I found that fat crystals could easily clog the filter in my setup as the oil at that point was fairly cool.

I was struggling a lot with soot in the exhaust. I had all kinds of theories including the rings not seating well since I never cross hatched the cylinder walls. My current theory is that I was running the engine at too high a load. I was running at somewhere between 90 and 95% of maximum. There was no sign of blackness in the exhaust gases. I suspect though that there was unburned fuel which caused soot in the exhaust system even if it was not visible in the exhaust gases. I ended up having to do regular cleaning of my exhaust gas heat exchanger.

I am no longer running the engine so haven't got any recent observations.

[buickanddeere]

A couple of random thoughts:

I found that preheating of veggy oil was not really required once the engine was at operating temperature. First of all the injector pump is quite warm and, more importantly, the injectors can easily heat the oil as it comes in. On the other hand, I found that fat crystals could easily clog the filter in my setup as the oil at that point was fairly cool.

I was struggling a lot with soot in the exhaust. I had all kinds of theories including the rings not seating well since I never cross hatched the cylinder walls. My current theory is that I was running the engine at too high a load. I was running at somewhere between 90 and 95% of maximum. There was no sign of blackness in the exhaust gases. I suspect though that there was unburned fuel which caused soot in the exhaust system even if it was not visible in the exhaust gases. I ended up having to do regular cleaning of my exhaust gas heat exchanger.

I am no longer running the engine so haven't got any recent observations.

What is the temperature of the water entering the engine from the radiator? If the rad is dropping the coolant temperature too much, the cylinder walls are too cold. Ideally the coolant temp would be 210F  leaving the engine and 200F entering the engine from the radiator.