Theoretical engine heat output?

[Biohazard]

I understand that an average gasoline engine is approximately 30% efficient - meaning only 30% of your fuel generates power output, the other 70% is waste heat.

Let's say theoretically you ran a gasoline powered generator and captured 100% of the waste heat from the engine to heat your house. Let's say a gallon of gas is 125,000 BTUs. If your generator is buring 1 gallon per hour, is it also giving off 125,000 BTUs of waste heat? Or would it only be 70% of that number, 87,500?

 

[RP]

It would be 70% (87,500) in heat minus the amount of energy still contained in any unburned fuel in the exhaust.

[bob g]

the typical gas engine driven generator will struggle to be 25% efficient mechanically, the generator will be no more than
perhaps 75% efficient for a total overall efficiency of maybe 18% or so

of the 75% waste heat, you can only capture approx 75% of the exhaust heat and maybe around 85% of the coolant heat
(none if it is air cooled), however

the exchangers must be sized appropriately and the engine will have to be running at a load that will keep the exchangers operating
at the designed parameter.

most diy'er efforts are not much better than 50-60% efficient, although with a lot of work you can get upwards of 85% overall efficiency
but not likely with any gas engine, here the advantage is decidedly slanted toward the diesel engine.

google  "micro cogeneration" or "combined heat and power"

btw, the technology is not new, it was routinely done as far back as edisons day, where his generation plants harvested the waste heat to
sell to local businesses for space heating.

there is a lot written on the subject, and there are many folks working on various designs

and i am one of them 

bob g

www.microcogen.info

[Biohazard]

Thanks for the info. I've been looking up the idea of microgeneration for a while now, but, it just doesn't make sense with the cheap natural gas and electricity I have at my house.

On the other hand, I have an industrial shop with no natural gas and currently electric heat...and as a commercial user I pay more for electricity. Basically for $3, I can get a gallon of gas or a bit less than 30kw of electricity. I'm considering hooking up a generator inside with exhaust through the walls, I should be able to capture most of the heat if I'm actually running the engine inside the building I want to heat. Not to mention I want to install a backup generator anyway...

Definately would love to go with a diesel, especially with all the waste oil I have, but, it seems the EPA have killed the cheap low speed china diesels...am I wrong? Is there still a source for those in the US? Anything else is way too expensive...

[comp]

Thanks for the info. I've been looking up the idea of microgeneration for a while now, but, it just doesn't make sense with the cheap natural gas and electricity I have at my house.

On the other hand, I have an industrial shop with no natural gas and currently electric heat...and as a commercial user I pay more for electricity. Basically for $3, I can get a gallon of gas or a bit less than 30kw of electricity. I'm considering hooking up a generator inside with exhaust through the walls, I should be able to capture most of the heat if I'm actually running the engine inside the building I want to heat. Not to mention I want to install a backup generator anyway...

Definately would love to go with a diesel, especially with all the waste oil I have, but, it seems the EPA have killed the cheap low speed china diesels...am I wrong? Is there still a source for those in the US? Anything else is way too expensive...

did you find anything ??

[electrondady1]

a few things you might want to try.
air injection into the exhaust will generate a lot of heat and burn up any remaining fuel.
a water jacketed exaust manifold would be required.

run a hydrogen generator off the alternator and feed that into the intake
it should give you a better burn.

[rossw]

When I built here, I made my "genset" - a CHP - based on a 4-cylinder automotive engine (a ford laser 1.8 litre, 16 valve, overhead cam).
Removed all the fuel injection crap and the engine control electronics.
Added a performance mixer before the inlet manifold and modified it for operation off propane just like a commercial car conversion.
Also added a lambda controller in a closed feedback, monitors O2 in the exhaust and controls the fuel (propane feed) to ensure optimum AFR regardless of speed and load.
Built a micro controller to do auto-start, auto-stop, sequencing, cranking, monitor temps/oil/etc, run times and service intervals.
Added an electronic governor with a pickup on the 112-tooth ring gear that operates a linear actuator that drives the butterfly valve (throttle).
Stripped off all the extra junk - water pump, alternator etc.
I got a surplus/scrap tube-in-shell type industrial heat exchanger, milled the end caps out to make plenums either end, and adapted the exhaust to run through it.
My "cooling" water now has a 60-litre-per-minute grunfos circulating pump that pushes water through the exhaust heat exchanger, then through the engine block, then out to my large heat storage tanks (heat exchangers in the bottom), and back to the circ pump.

I have a vortex flowmeter in the water line so I can see how much water is flowing, and temperature measurement of "cold" return water and "hot" exit water from the thermostat body. This lets me fairly easily calculate how much heat I'm getting from the engine.

If the conversion of fuel to electricity is 20% end-to-end, for 8kW electrical output, I should be consuming 40kW of gas, which leaves about 32kW as heat. That much heat will raise the cooling water by just over 15 deg C at 30 litres/minute flow, and that's about what I see in real life.

If anyone wants pics, holler and I'll post them.

[willib]

 YEAH
Ross i would like to see them.

[rossw]

YEAH
Ross i would like to see them.

Rather than hijacking this thread, I collected a number of images here for you and anyone else interested.

[willib]

Ross that is spectacular.
man you have been busy.

which Atmel programmer do you have?

[rossw]

Ross that is spectacular.
man you have been busy.

which Atmel programmer do you have?

Thanks. Its one of the countless jobs around here that needed (still need) doing!

The Atmel was just programmed via a dumb cable with a couple of resistors off the parallel port.

[SparWeb]

When I built here, I made my "genset" - a CHP - based on a 4-cylinder automotive engine (a ford laser 1.8 litre, 16 valve, overhead cam).
...
If the conversion of fuel to electricity is 20% end-to-end, for 8kW electrical output, I should be consuming 40kW of gas, which leaves about 32kW as heat. That much heat will raise the cooling water by just over 15 deg C at 30 litres/minute flow, and that's about what I see in real life.

Ross, that's an impressive build, there. 
The amount of heat you are able to collect sounds like it's enough to heat your house.  Is that what you do with it?
Thanks

[rossw]

Ross, that's an impressive build, there. 
The amount of heat you are able to collect sounds like it's enough to heat your house.  Is that what you do with it?

Primary aim was to make enough for domestic hot water, which it does fine.
Secondary aim was to use it to heat the house, which it does to some extent. As we've increased the amount of PV and generator run times have reduced substantially, it's making less and less of our heating. (Example, we didn't run the generator at all for the last week)

[SparWeb]

... As we've increased the amount of PV and generator run times have reduced substantially, it's making less and less of our heating. (Example, we didn't run the generator at all for the last week)

Well if you ever put up enough solar that you don't need it any more, crate it up and ship it over here!