Small engine gas consumption?

[Number21]

I'm trying to research the running cost of a small natural gas engine. Can anybody help me find some rough average numbers for a single or twin cylinder engine? I am not able to locate a published fuel consumption on natural gas or propane for any small generator. I found a few giant ones, but that probably won't be the same. I found a very rough estimate of 10,000 BTU per horsepower, but that seems way high and a little too round.

[buickanddeere]

  The ideal method is a diesel engine using pilot injection burning natural gas. That is run the diesel at minimal idle speed on diesel and add natural gas to the intake manifold to reach the desired power. A gasoline NG carb does nicely. Would probably require a 6 to 7HP diesel engine. The advantage is robust construction and dual fuel capability.
   To make 5HP with 33% thermal efficiency would use approx 38cubic feet per hour.

[Number21]

  The ideal method is a diesel engine using pilot injection burning natural gas.

Would that use less gas than a spark engine making the same horsepower?

To make 5HP with 33% thermal efficiency would use approx 38cubic feet per hour.

So, if gas was 1020 BTU per cubic foot (according to google) then that would be 38,760 BTU or 7752 BTU per horsepower. Is that a good rough number to use?

To add to the question...
If that 5hp 33% engine burns 38,760 BTU, and you were successful at catching 100% of the waste heat, how much would that be? Would that be the other 66.6%?

[Thob]

Here's a page from Briggs that lists fuel consumption:

http://www.bsapowersolutions.com/Briggs-Stratton/8-10kgenerator.php

Nat Gas, 2 cyl, 10KW, 100% load, 180 cu ft / hr  --- 50% load, 115 cu ft / hr

Nat Gas, 1 cyl, 8KW, 100% load, 121 cu ft / hr  --- 50% load, 94 cu ft / hr

YMMV, but this should give you ball park figures of what can be done in a small system.

How does that work out for $/KWH based on what you pay for natural gas?

[Number21]

Nat Gas, 1 cyl, 8KW, 100% load, 121 cu ft / hr  --- 50% load, 94 cu ft / hr

Perfect...if 8KW = 16hp, then that is about 7713 BTU per HP.

How does that work out for $/KWH based on what you pay for natural gas?

I'm looking into the idea of a natural gas powered heat pump. I'm making a lot of assumptions but I think I can make heat for about half the cost of burning straight gas.

IF 2 horsepower can spin a 2 ton compressor, for ~15,500 input BTU:
I can get 24,000 BTU heat from the compressor
I can get up to 10,230 BTU in engine heat (in a perfect world) or 8184 at 80% efficiency
Total of 32,000+ BTU from .155 therms

One therm cost $1.01, so that is $0.15655 per hour.

If I used an electric heat pump, 32,000 BTU heat would cost about 2133 watts. At 11 cents/KWH, that is $0.2346 per hour.

Just thinking out loud. Do I sound crazy?   In the summer, it would still be cheaper to run than electric A/C, and I'd get free heat for my hot water.

[buickanddeere]

  The ideal method is a diesel engine using pilot injection burning natural gas.

Would that use less gas than a spark engine making the same horsepower?

To make 5HP with 33% thermal efficiency would use approx 38cubic feet per hour.

So, if gas was 1020 BTU per cubic foot (according to google) then that would be 38,760 BTU or 7752 BTU per horsepower. Is that a good rough number to use?

To add to the question...
If that 5hp 33% engine burns 38,760 BTU, and you were successful at catching 100% of the waste heat, how much would that be? Would that be the other 66.6%?

Higher compression ratio also offers a higher expansion ratio on the power stroke. A 16 to 1 diesel beats a 8 to 1 gasser in that department.

[buickanddeere]

Nat Gas, 1 cyl, 8KW, 100% load, 121 cu ft / hr  --- 50% load, 94 cu ft / hr

Perfect...if 8KW = 16hp, then that is about 7713 BTU per HP.

How does that work out for $/KWH based on what you pay for natural gas?

I'm looking into the idea of a natural gas powered heat pump. I'm making a lot of assumptions but I think I can make heat for about half the cost of burning straight gas.

IF 2 horsepower can spin a 2 ton compressor, for ~15,500 input BTU:
I can get 24,000 BTU heat from the compressor
I can get up to 10,230 BTU in engine heat (in a perfect world) or 8184 at 80% efficiency
Total of 32,000+ BTU from .155 therms

One therm cost $1.01, so that is $0.15655 per hour.

If I used an electric heat pump, 32,000 BTU heat would cost about 2133 watts. At 11 cents/KWH, that is $0.2346 per hour.

Just thinking out loud. Do I sound crazy?   In the summer, it would still be cheaper to run than electric A/C, and I'd get free heat for my hot water.

746 watts in one HP.
Engine efficiency is usually best at or near 100% of max  output at or just above the rpms where torque is peak.
payback depends on your home's energy requirements, the initial cost of your co-generation which depends on complexity. Service and maintenance costs increase with complexity.
  $2000 in sealing air leaks around doors, windows, curtains, insulation and a programmable thermostat. may save more $$$ than a co-generation heating setup.

[Number21]

Higher compression ratio also offers a higher expansion ratio on the power stroke. A 16 to 1 diesel beats a 8 to 1 gasser in that department.

But how does it compare after you account for the diesel you have to burn also?

[buickanddeere]

  The amount of diesel is minimal. Probably 1/4 to 1/2 a gallon a day. Most of us could afford that considering the duel fuel advantage.

[buickanddeere]

Higher compression ratio also offers a higher expansion ratio on the power stroke. A 16 to 1 diesel beats a 8 to 1 gasser in that department.

But how does it compare after you account for the diesel you have to burn also?

The amount of diesel burned to obtain pilot ignition is probably about 5-7% of the fuel flow required for max power at max rpms.

[BruceM]

Spark conversion of the diesel engine is certainly possible- in the last year did a conversion of a DES 8/1 Lister CS clone to propane.  A long reach spark plug fit perfectly in the injector hole, and with a hot spark system added it now works fine.  It was a lot of fussing to get the right carb and regulator combo due to the low speed, gulping intake of the CS single cylinder engine.  Because of using propane, we did have to reduce compression to 11:1.  Pilot ignition with diesel is a lot less appealing for propane as you can only run about 25% propane.

Natural Gas is another situation entirely, since full diesel compression levels are possible, and over 90% NG can be used.  A Diesel pilot would save a fair amount of effort and fiddling and would give you nearly dual fuel capability.  A small amount of diesel would go a long way.  Spark could still be done if pure NG alone is a requirement...just remember that at those higher compression levels, a very hot spark system will be needed. 

BruceM