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Old 04-03-2014, 11:14 AM   #1
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making heat: oil furnace vs diesel engine


Anyone that has any knowledge of how fuels burn may know that the mechanical force they produce is incidental to the amount of heat they produce. The mechanical force is a product of thermal expansion when the hot gasses inside the engine burn an heat up rapidly. An internal combustion engine typically is only about 30% efficient at converting the energy in the fuel into mechanical force. The rest of the energy is dissipated as heat out of the radiator, the engine itself, or the exhaust.

Now, Let's look at a a hypothetical oil boiler. Most have an optimal heat-exchange efficiency of around 80%-85% under optimal conditions. The combustion, is perhaps in the 75% to 80% range...fuel oil burns in a furnace with a white-yellow flame. Yellow flames are from the luminescence of the hot carbon in the flame. Let's take this typical oil boiler, and run it say, with 1.0 gph nozzle in it. This will produce about 115,000 BTU's of heat energy...plenty of heat energy to warm a large home around 2500 sq/ft or more

Now, let's replace this oil furnace with a diesel generator! A generator that is capable of burning 1 GPH, same as the boiler. Huh? Yep! Now, a typical diesel generator that only consumes that much fuel is only going to be able to produce about 20KW of electricity, which would make about 60,000 BTU's of electric heat...a little less than half the energy the oil furnace was making. *BUT* what about if we connected the cooling system of the generator up to the home's baseboard radiator's too....I am thinking another 90,000 BTU's of heat energy could be harnessed from the cooling system.

*BUT* we are banging into the laws of physics here. It is generally stated that there's only 140,000 BTU's per gallon of energy in a gallon of #2 fuel oil. The idea above though makes you think we are making 180,000 BTU's of energy (of any type). Now, here is where I'm scratching my head...First, I know there's some thermal expansion occurring in a furnace / boiler burning the fuel for the purposes of heating, but it does not seem like it's particularly forceful, so an engine must be converting some of the heat of combustion into mechanical force instead of merely providing incidental thermal expansion, and thus there's less heat is being produced the cooling system is having to deal with than the full 140,000 OR the mechanical force is not included in the 140,000 BTU's of energy in a gallon of fuel oil calculation, thus we are now harnessing wasted mechanical energy that the furnace discards.

What are your thoughts on this? At the minimum, it seems like a diesel generator would still be more efficient than a furnace at heating a home if both the mechanical and heat energy can be harnessed at the same time. At the very minimum, a diesel engine burns the fuel more efficiently and completely than a furnace/ boiler would.

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Old 04-03-2014, 11:25 AM   #2
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making heat: oil furnace vs diesel engine


1) Diesels are NOISY. where would you keep it and not have a noise problem. Unless you live in the bush far away from anyone that ain't going to be feasible.

2) How would you get the heat from the cooling system back into the house. The logistics of that would be difficult and complex and expensive. I worked in a hospital with diesel generator elec backup and all kinds of heat recovery systems and steam to water converters and automation and that is way beyond feasible for any resi use IMO. Commercial/industrial/institutional yeah.

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Old 04-03-2014, 11:35 AM   #3
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making heat: oil furnace vs diesel engine


Your thoughts may already be in practice with turbine set ups.
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Old 04-03-2014, 12:03 PM   #4
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making heat: oil furnace vs diesel engine


The type of setup you are discussing is known as a combined cycle combustion engine, and is commonly used for steam turbines where the waste heat is used to generate hot water or low pressure steam. I can assure you that there is no way you can get more energy out of the system than the total heat content of the fuel, so not even God can get more than 140,000 BTU's per gallon of diesel fuel. And in fact, you will not get close to that, since every real fuel cycle wastes a certain amount of heat as rejected heat, since heat exchangers are not 100% efficient, and a certain amount of heat goes up the stack.

The best combined cycle natural gas fueled generators are about 60% overall efficient.
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Old 04-03-2014, 12:18 PM   #5
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making heat: oil furnace vs diesel engine


Yuri, You obviously haven't heard my old oil furnace LOL! (or the new VW TDI diesels...they are very quiet)

Anyways, the cooling system output of the diesel generator could be, in theory, fed directly into the radiator system of the house. The optimal cooling system temp of most engines is around 180-200 degrees...right at the same temp that most home boilers run at. I wouldn't think any complex heat exchangers would be needed.....That's what sort of started this wild and crazy idea!

Daniel, that was sort of my thinking of this...some of the heat goes out the exhaust, as does with any piece of equipment. Even the the fancy new gas furnace I just recently installed that runs about 95% efficient has some pretty warm exhaust. I think in the realm of combined-cycle generators however, the goal of recovering the heat is mechanical energy to drive a steam generator...I'm merely trying to use the heat in it's original form.

Just a thought guys! Thanks for the interesting discussion
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Old 04-03-2014, 01:02 PM   #6
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making heat: oil furnace vs diesel engine


Yeah, but then you would be heating your house with glycol not water as your cooling system uses glycol. Then your boiler and rads would be running on glycol and apparently have to be bigger if using glycol.

The new diesels are apparently quite cool and my brother-in-law and cousins hubby are looking into buying them. Apparently the VW one gets more miles for the cost per $ for fuel. They start better and produce heat for the interior quicker and properly unlike the old ones. Heck I rode in a VW Bug with a gas heater under the seat. No window defrosting when that sucker failed or inside heat. Now that was primitive.
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Old 04-03-2014, 07:41 PM   #7
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You are throwing around a lot of numbers, but if you are suggesting that a gallon of oil will produce more energy in a diesel than in a boiler, you are mistaken in some of those numbers.
I would expect the combustion in a boiler is very close to 100% efficient in converting chemical energy. The losses are primarily the heat that goes up the chimney. Same with the diesel engine --- some of the energy is lost to friction, some of the heat goes into the radiator, and some goes out the tailpipe. But if your 140,000 BTU/gal number is right, both the boiler and the diesel will get that much energy from each gallon.

Possibly in the 1960s you had measurable amounts of unburnt fuel going through an engine, but I doubt that's the case any more.

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