How many MPG's am I losing for extra 20amps? - Fuelly Forums

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Old 07-18-2008, 09:35 AM   #1
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How many MPG's am I losing for extra 20amps?

I understand pulling extra power puts an extra load on my alternator. I am pulling 20amps total (about 240 watts in all) and I'm trying to figure out how much FE this is costing me. I am running HHO and am looking for an alternative power solution. First, I am trying to determine if it will be worth it.
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Old 07-18-2008, 10:14 AM   #2
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240 watts = 0.322 HP, assuming 100% conversion efficiency. How many HP it actually draws off your engine depends on your alternator's efficiency. How that HP draw affects your fuel economy will depend on your engine's overall efficiency and where it is at that moment on it's BSFC maps.
If you can get ahold of all that info, feel free to calculate it out. If not, just hook up your 240 watt load and find out.
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Old 07-18-2008, 10:25 AM   #3
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I figure it as about 2/3 of an HP at around 60% alternator efficiency, so how much extra gas it sucks depends on the Brake Specific Fuel Consumption map of your motor. A "typical average" is normally taken as .45 lb/hp/hr thus if it takes 30hp to maintain a cruising speed of 60mph it would take 30x .45= 13.5lb of fuel with 6.5lb to the gallon, thus using 2.08 gallons over an hour for 60 miles travelled for an FE of 28.9 mpg... whereas 30.666HP would mean about 28.3 mpg, so you theoretically "lose" .6mpg

However.... this is where it gets fancy, the true situation, is that up to around the torque peak an engines BSFC improves with load, i.e. it's more efficient the more heavily loaded it is in it's most efficient RPM range. This means that while at a 30HP load you might need .45lb/hp/hr, increasing the load fractionally may move you up the BSFC curve fractionally, such that you only need .44/lb/HP an hour. (This is all dependant on the BSFC map for your particular engine, values given here for illustration purposes) So then, instead of .45 x 30.666, we do .44 x 30.666= 13.49 lb/hr.... say what? yep, by increasing load at certain points in the efficiency map, you might move to a lower BSFC region and end up using less fuel, or not as much extra fuel as you anticipate. So it could be the case that at highway speeds the 20A load costs little to no mpg loss....

However, at city speeds, idling etc, you may be using as much as .65/lb/HP/hr and only using 5HP for average speeds of 20mph. This might work out as about 40mpg if you keep moving steady (and have suitable gearing) but due to low BSFC, even with an incremental increase to .64 lb/hp/hr because the load is a higher proportion of the total load, you lose mpg to the tune of about 4 or 5 mpg.

This is partly why A/C use in city driving is so inefficient but may give barely noticeable economy differences on the highway, though in that case it's about a 3HP drain, moves you higher up the BSFC map maybe but due to it being a high proportion of total load in the city and a lower proportion on the highway, you still see a huge ding in the city.

Anyway, as surprising as it may seem, at modest highway cruise speeds just under the torque peak of the motor your might not see any mpg loss from "throwing away" 20A worth of electricity.
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Old 07-19-2008, 06:39 AM   #4
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I couldn't ask for a better reply!!! You really know your stuff. I guess I won't be running my hho off any deep cycle batteries for a while (mostly highway driving).

Thanks again!
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Old 07-19-2008, 11:15 AM   #5
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Unless you want to milk another 10A into a deep cycle batt at highway speed to feed the HHO in the city.
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Old 07-19-2008, 11:39 AM   #6
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thats a good possibility. I'll have to put some thought into this one
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Old 07-19-2008, 03:15 PM   #7
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Hmm, so my radio can load up 2/3rds hp from the engine? Gross.
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Old 07-19-2008, 04:00 PM   #8
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It would be tough to pull a steady 20 amps through a stereo. I think huge stereo systems can hit that in a very peaky way on each bass thump. If you're going to rattle the whole neighborhood, expect to burn a little gas to do it.
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Old 07-19-2008, 04:25 PM   #9
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Get rid of the HHO, it is snake-oil technology and is costing you more by reducing your MPG. Think about it, are cars made to run on hydrogen?

The 240 watts that the HHO uses is generated by the gasoline that is burned in your engine, not hydrogen. Do yourself a favor and remove the HHO from your car that is causing you more harm than good.
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Old 07-19-2008, 05:45 PM   #10
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HHO again? Why doesnt someone just use a separate battery to generate the HHO, then recharge that battery at home overnight?

Electricity is much cheaper from the power company.

See how much your mileage improves with no additional load on the engine, generate fuel using you local power company. Power companies sell that kind of HP for pennies.

regards
gary
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