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Effect of Vacuum on Engine Exhaust

Effect of Vacuum on Engine Exhaust

Effect of Vacuum on Engine Exhaust

(OP)
I'm working on an "Evaporation Pond" project.  The goal of the project is to dispose of about 15 gpm of water produced in conjunction with natural-gas production.

The higher the temperature of the stream, the more effective the evaporation will be.

The transfer pump is powered by a natuarally-aspirated 90 Hp industrial engine.  I can install an "eductor" to let the water I'm pumping suck on the engine exhaust-stack and create a 10-15 inHg vacuum on the engine exhaust (and thereby raising the temperature of the water 10-15F).

My question (finally) is:  Will there be any adverse affects of pulling a medium vacuum on the engine?

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com

RE: Effect of Vacuum on Engine Exhaust

depends on your valve timing.  You run the risk of having a great deal of flow-thru (charge bypassing the cylinder) during overlap.  If you have the option of grinding a new cam, or if unburned fuel and efficiency aren't problems, it might be worth looking at.

RE: Effect of Vacuum on Engine Exhaust

I forgot to mention one other thing- you might start sucking a bit of oil past the exhaust valve seals.

Of course, before posting either comment, I should have asked whether the engine is 2 or 4 stroke, and whether it runs gasoline or natural gas as fuel...

RE: Effect of Vacuum on Engine Exhaust

(OP)
Four stroke, natural gas.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com

RE: Effect of Vacuum on Engine Exhaust

They may actually be some benefits, since the engine won't have to work as hard as an air pump. 15 inch Hg vacuum is about 1/2 atmosphere. Decreasing the exhaust manifold pressure one-half atmosphere will increase engine efficiency about one percent, which will translate into about a 3% improvement in fuel mileage.

With a 90 hp natural gas engine your waste coolant heat and exhaust heat will be about the same -- close to 4000 BTU/min. 15 gallons of water a minute is equal to about 120 lbs of water. 4000 BTU a minute will raise the temperature of this amount of water about twenty deg F (assuming a 100% efficient heat exchanger. You might want to consider a liquid/liquid intecooler to recover the coolant system heat, since this would essentially double your recovery to 8000 BTU/min.

I would recommend getting a custom cam that minimizes the exhaust and intake valve overlap. Otherwise the exhaust manifold suction will be sucking some of the fuel/air mixture when both the intake and exhaust valves are open.

RE: Effect of Vacuum on Engine Exhaust

how about sucking down the c-case pressure too?  That'll cut pumping&windage losses.

RE: Effect of Vacuum on Engine Exhaust

(OP)
Great ideas.  If I can get 40F temp increase on the entire stream (probably closer to 30F with real-world effeciencies), it'll almost double the evaporation rate of the system and cut the size of the evaporation pond by a bunch.  Fuel savings are just an added bonus.

David

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