A/C and Fuel Economy

[LouisB]

What is better on fuel economy?? when driving on the road do you run the generator and the roof A/C or only the engine A/C ???

[LouisB]

What is better on fuel economy?? when driving on the road do you run the generator and the roof A/C or only the engine A/C ???

[Retired and Happy]

I would also like to see a definitive answer on this. I have always heard that the generator was more efficient than the added load on the vehicle engine for running A/C units. But I recently saw an article in a newsletter that said to run the dash air first since the engine was already running. I personally run the roof air all the time since we also run various pieces of medical equipment while enroute. And the dash air will not cool a large motorhome by itself.

So cooling depends on what you want to cool. If there are just two of you sitting up front, the dash air should be sufficient. If you carry a load of people throughout the unit, the roof air will be necessary for comfort throughout. (In hot climates, of course.)

You may have to make a tradeoff between cost efficiency and cooling efficiency.

[Bob (WA0MQE)]

I don't know about running the generator but Onan says on a 5.5 KW gen it uses somewhere around .7 or so gallons per hour. This makes it fairly easy to calculate the additional gas being used when determining mpg.

As far as the dash air, I've driven our moho with and without it for long periods of time and I can definitely say in my case it makes very little difference in gas mileage. In my neck of the woods, Colorado/New Mexico/Texas, etc. I average 6.5 mpg everywhere I go across this country. A recent trip going east to Minnesota I averaged 7.0 coming back home it was back to 6.5 mpg, and that's with the dash air running constantly.

[TXiceman]

It is easy to figure the numbers.

At a speed of 60 mph and you cover 60 miles), and say you get 7.5 mpg, you will use 8.0 gallons of fuel.

Now just assume that with the A/C on, you drop to 7.2 mpg at 60 mph, you will use 8.33 gallons of fuel.

So in this case, the dash air is costing you 0.33 gallons of fuel per hour. If the generator will run at less than 0.33 gallons per hour, you are better off with the generator. If it uses more than 0.33 gallons per hour, you are beter of with the dash air.

Ken

[blacktie.shooter]

Louis, my son also has a Dolphin on a Workhorse chassis, and my impression, riding shotgun, is that when he kicks on the dash air, that 8.1 doesn't even feel it. I doubt that you could even measure the difference in fuel consumption without serious technical instruments and procedures.

On the other hand, a generator does need to be run under load, fairly frequently, and for at least a couple hours at a time. You're not doing it any favor by using the dash air instead, plus you get a lot more cold air from the gen-driven A/C, from what is probably close to the same amount of fuel that would go through the engine when using dash air. Bottom line, IMHO: roof or chassis central air is more efficient - better for the gen, better bang for the buck.

On my older Pace, I only use roof air, 1) to exercise the gen, 2) get more cool air for the buck, and c) not the least, because the previous owner removed all dash air components to save weight, and remove the temptation to use it! (I don't disagree; when I'm towing, wet, and loaded, I really don't need any more load on that Ford 7.5L.)

[LouisB]

Ken, you are correct the 8.1 doesn't feel it when you turn the A/C on, the only problem I noticed that when it is hot outside and the engine A/C is on the clutch fan comes on more often and that sucker is LOUD.

I guess I should keep the generator running since it will also keep the fridge cool, and turn on the roof A/C.

Thank you all.

[AFChap]

As mentioned, fuel economy is only part of the picture. We probably don't run the dash air enough to keep it in good condition. I might use it a bit in moderate weather.

Like others here, we tend to run the generator if we need ac while traveling because the house air does a much better job, and to exercise the generator. I make sure we run it at least an hour a month under at least 50% load.

As for keeping the fridge cool, if we aren't running the generator I turn the inverter on and the engine alternator puts out enough power to keep the batteries up. When we stop, I switch the refrigerator onto LP, or OFF if it is a fueling stop.

[wa7hra]

Here is a bit of factual data on the amount of fuel used by an Onan, Marquis Gold, 5.5 KW, gasoline powered genset.

From the specifications in the Onan owner's manual: fuel consumption is 1.8 lbs/hr at no load, 3.3 lbs/hr at half load, and 4.6 lbs/hr at full load.

From the Santa Cruz Public Library (Google search): gasoline weighs 5.8 to 6.5 lbs/gallon depending upon temperature.

Now, just to make the math simple, using a nominal 6 lbs/gal, we see .3 gal/hr at no load, .55 gal/hr at half load, and .77 gal/hr at full load.

I assume that a single roof top AC would be about a half load and two units would be a full load.


Bob, wa7hra