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Old 04-19-2014, 01:07 PM   #43
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No, it will be the same, assuming that a diesel powered coach is carrying a diesel-powered generator and a gasoline-powered coach is carrying a gasoline-powered generator, as is almost always the case. For a given BTU cooling capacity the energy is coming from either the RV engine or the generator engine, and since these use the same fuels then the amount of additional fuel used for A/C will be about the same (or perhaps slightly more for the separate generator as explained above.)
Well, I don't think you understand what the data tells you. One diesel generator in the group uses 0.32 gph to produce 3500 watts.

The gasoline generator uses 0.70 gph to produce the same output.

Surely, you would agree that the gasoline generator will use more than twice as much fuel as the diesel generator for the same load.

Good Luck!
Wil
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Old 04-19-2014, 01:56 PM   #44
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I will give that a try…dog…….

Thanks, r0n…..
Worked for me when my dash air quit in the high 90s.
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Old 04-19-2014, 03:55 PM   #45
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The missing concept in this efficiency idea is that a generator running a house a/c unit is running 100% of the time, burning gas all the while, albeit at a reduced rate when the a/c is not cycling. The chassis a/c on the other hand is zero load on the engine when the compressor is not cycling, therefore no lost fuel consumption.

If you have a C the chassis a/c is far higher capacity than a typical A coach is. If you boost the circulation by opening a rear roof vent or window a little the vacuum does far more to circulate the cool air than any 3 fans could.
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Old 04-19-2014, 04:46 PM   #46
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Surely, you would agree that the gasoline generator will use more than twice as much fuel as the diesel generator for the same load.
The thread wasn't discussing which generator type uses less fuel, it was discussing whether using dash air vs. generator-powered A/C was more efficient. In that case you should be comparing your onboard generator to your vehicle engine, not various unrelated generator fuel types. So yes, I wouldn't dispute the figures you quoted, I guess I just missed how they were related to the thread topic(?)
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Old 04-19-2014, 04:59 PM   #47
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The missing concept in this efficiency idea is that a generator running a house a/c unit is running 100% of the time, burning gas all the while, albeit at a reduced rate when the a/c is not cycling. The chassis a/c on the other hand is zero load on the engine when the compressor is not cycling, therefore no lost fuel consumption.
Yes, you got it. There just isn't any way an engine-driven compressor (that as you note is engaged and drawing power only when needed) is somehow going to be less efficient than running a separate engine (and incurring separate friction, volumetric, and heat losses), converting that energy to electricity (incurring additional losses), then turning the electricity back into mechanical power to run a compressor (incurring additional losses.) Logically, the engine-driven A/C has to win every time (again, assuming similar BTU cooling capacity in both systems.)

This isn't necessarily a reason not to run the coach A/C because if you need it, you need it... just don't kid yourself and think that it is more efficient.
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Old 04-19-2014, 08:01 PM   #48
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Yes, you got it. There just isn't any way an engine-driven compressor (that as you note is engaged and drawing power only when needed) is somehow going to be less efficient than running a separate engine (and incurring separate friction, volumetric, and heat losses), converting that energy to electricity (incurring additional losses), then turning the electricity back into mechanical power to run a compressor (incurring additional losses.) Logically, the engine-driven A/C has to win every time (again, assuming similar BTU cooling capacity in both systems.)

This isn't necessarily a reason not to run the coach A/C because if you need it, you need it... just don't kid yourself and think that it is more efficient.
I totally agree regarding the "run it if you need it" (whatever is necessary) logic.

In a perfect world, I might also agree regarding engine driven compressor efficiency. Knowing full well perfect worlds don't exist, I'll take the counterpoint regarding efficiency. Let's add some more info regarding that engine driven compressor, from a diesel pusher perspective? Lets move that compressor 40' or more away from the evaporator (minimum) and run those hoses to the front along a hot frame rail, with nice hot air circulating around their entire length. Lets cool the condenser with air pulled from the surface of some pretty warm pavement (assuming we're talking about a 90 degree day). Now, let's assume the chassis AC is not running on max, so potentially, it's pulling HOT outside air up off the pavement and running that through the evaporator, and trying to cool that. From the evaporator, it's run through a labyrinth consisting of many feet of flex hose prior to reaching the air outlets and your face? And that's assuming whoever installed the dash didn't pinch or smash any of the duct work.

With this in mind, it doesn't take much imagination to figure that a roof AC, circulating pre-cooled inside air through an evaporator located within inches of the compressor, might be operating a hair more efficiently?

2 points really. First, I wouldn't assume the cab air is all that efficient when compared to a diesel genny running a modern roof AC. Second, if you're going to run the chassis AC in warm weather, for pete's sake, make sure it's set to "recirculate" or "max" to avoid the losses incurred trying to cool off hot muggy outside air. Help it out by letting it pull from the pre cooled air from the coach's interior. That move alone can make a very noticeable difference in AC performance. -Al
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Old 04-19-2014, 08:15 PM   #49
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... I wouldn't assume the cab air is all that efficient when compared to a diesel genny running a modern roof AC.
If you're talking about any A, let alone a DP, I'd have to agree. However in my C for example, the rooftop unit is only 15k BTU so it's no match for the chassis / dash unit.
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Old 04-20-2014, 07:41 AM   #50
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No argument, as long as the chassis AC is run on max/recirculate.
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Old 04-20-2014, 07:55 AM   #51
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Lots of discussion and technical information. I think the difference as far as the economy is negligible. If you are traveling in an RV just enjoy some comfort.

My concern was I personally could not get the House AC to cool the rig when underway and judging by all the feedback I must be doing something wrong? My House AC does work perfectly when I am parked.

r0n…..
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Old 04-21-2014, 04:50 PM   #52
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Another $0.02, since it looks like this thread needed it!

The hydraulic fan needed on a side rad to cool the chassis AC can consume up to 70HP worth of power. Given how potentially inefficient our systems are (40 feet distances and such) I bet the duty cycle of the fan is biased to on more then off when the chassis AC is running.

Wish I had a way to measure this. I have no way of knowing (AFAIK) when my fan is running whilst on the road. Anything I can see on scangauge?

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Old 04-21-2014, 05:33 PM   #53
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I've seen the 'up to 70 hp' figure quoted in a few places but most systems are capable of variable output and normally are consuming much less power than the maximum figure. How much additional power might be drawn as a result of A/C efficiency is just about impossible to say without instrumentation and testing.

No doubt there is some loss in the refrigerant lines and maybe some loss in the fan, etc., but you've still got a high hurdle in that when running a generator you're running an entirely separate engine with a whole new set of losses. It would be interesting to make an apples-to-apples comparison, however in a situation where dash air is adequate and you are choosing whether to simply run the dash air or fire up the generator and run a separate A/C, it would take more than 'maybes' to make me think the latter would be more efficient.

But bottom line... if the dash air won't cut it and you need the extra cooling capacity then it doesn't matter which is more efficient, ya gotta do what ya gotta do.
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Old 04-21-2014, 07:22 PM   #54
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Just a thought re: the 70hp draw on side rad. coaches? I don't know for sure, but my GUESS is that the AC condensor is still mounted up by the front axle, much as it would be on a rear rad. setup?

The 70hp is used cooling the engine, which is using just a little more power than normal when turning the compressor. I doubt there's a heck of a lot of difference in the demands place on that hyd. motor AC compressor running vs. not.
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Old 05-07-2014, 11:07 AM   #55
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One pint to one quart per hour, nope, not likely for an onboard genset. My 3000W Boliy portable genset uses a gallon in 4 hours. I've never seen an onboard genset that small.

Somewhere on irv2.com in a thread, someone posted a chart of fuel consumption for almost every brand built into an RV. A search should discover the post.
Not for a gas genset no. Gas gensets are simply thirstier than diesels.

1 Quart per hour running a single AC unit- pretty standard with a diesel genset.

Both my Power tech 8000 and my Onan RVQD 8000 get about 4 hours running a single AC unit on gallon of diesel.

Running both cost me slightly less than 1/2 gallon an hour.

My 7000 by comparison is a thirsty beast gulping about .9 GPH screaming along at 3600RPM running both and about half a gallon an hour running one.
Onan and Powertech publish the consumption rate on each models sheet.

Fuel burn is just one aspect to "cost"
From a pure financial perspective the generator is always more expensive to run than the coach engine as a source of power or cooling. It's rebuild cost and maintenance schedule are far less than the expected lifetime of the coach engine and their oil filter change schedules almost always more frequent forcing both parts and labor component (even if your own labor) every 50 hours with a gas genset and 150-250 in a diesel.

A few days cross country driving running a gas genny and you have an oil & filter change waiting.


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Old 05-24-2014, 04:17 PM   #56
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The Myth Busters were correct for a car running A/C with the windows closed vs no A/C with the windows open. The open windows create an air dam which catches the air and creates a lot of drag, thus making the motor work harder and burn more fuel. We go for the comfort trade-off of running the generator and house A/C. I don't believe our mileage difference is enough to consider otherwise.
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