Mileage

Good Morning,

I am having trouble with my dash AC and am having it looked at Monday. A friend of mine says he was told that using the generator and running the coach AC's will provide better mileage that running the dash air. I have a Cumins ISL 6.9 (I believe). Anyone ever heard that or seen studies that support that?

Thanks

That subject has been discussed here many times without coming to any real conclusion. Everyone has their own opinion but no one has any real scientific facts to back up their claims.

The dash A/C compressor is basically the same as what you find in a large car or p/u truck.  It doesn't add very much of a load to the engine.  Like wise running the generator doesn't take that much fuel for each hour it is run to power the house A/C.

One probably uses less fuel than the other, but not enough to be concerned about. 

As others already said, it's largely an academic debate.  30 years ago an engine-driven a/c compressor sucked 25-30 hp when on, but modern systems are much more efficient and are managed better, so the overall load on the engine is modest. Estimates vary, but the guesses I see are in the range of 10-15 hp.  If your engine is using 150-200 hp to move the coach at cruising speed, the dash a/c compressor is probably no more than an additional 10%?  Somewhere in that ballpark.


The genset fuel consumption varies with the watts (amps ) being used and so varies with the amount of time the a/c cycles on and the number of a/c units powered.  Predicting fuel consumption can get complex, but even if you have two a/c's running 100% of the time, the genset probably has a small advantage.  A typical RV generator uses about 0.5 gal/hr at 50% load (50% of its max rated wattage). If you have a 6000 watt generator, 50% = 3000 watts and that's plenty to run two a/c units.  In most Class motorhomes, 0.5 gallons of fuel is only enough to go about 4 miles (@ 8 mpg), so the fuel consumption for the genset is trivial compared to engine consumption anyway.  Why bother to worry about it?

Many people end up running one (front) roof a/c to cool the area near the cab and often keep the dash air on as well to provide direct cooling to the seating area via the dash fan & outlets.  That's a very light load on the genset and maybe uses 0.3 gal/hr, plus the dash a/c doesn't cycle as much because the interior air is already somewhat cooled.

8 hours on the road, 0.5 gallons per hour to run the gen, 4 gallons used at $3 a gallon, $12 per day to stay cool.

If you are the type that only drives 5 hours a day, it's $7.50.

Personally I'd say the comfort of cool air in all of the RV is worth it.

I use the dash air until it won't keep the cabin cool, then use the genset.

I think some items that are being left out of the "which more economical" equation are the associated costs of running another engine, if it isn't needed - engine hours, wear items, oil & filter, air filter, etc. I would think these would cancel any slight fuel savings by using the genset. But if it is needed to stay comfortable, use it.

Living in Nevada we've learned to hit the road in the predawn hours when we are traveling up through the State on US 95 or US 93. Going North from Las Vegas the dash A/C is sufficient. Traveling South from Fernley, NV we usually add the house A/C after about 1300-1400 hrs.

I've also added the clear ceramic coating to the windshield and tinted the side windows, and I am grateful for the significant reduction in heat gain through the windows. 

The route from Northern Nevada on US 93 is 454 miles and the route on US 95 to US 395 (California) is 435 miles. It is virtually the only time we drive for more than 300 miles in a day. It's a hot summer drive going North and even hotter going South.

 

Dash will use less fuel for the same amount of cool. This can be easily demonstrated because.
Running the compressor off the main engine you do indeed "Sap" a bit of power from the engine but 100% of that power "Sapped" goes to the compressor NO LOSS. (There may be losses in the compressor but these will be consistant)

Running the generator there is loss, First the Generator burns fuel just to run the engine (has nothign to do with feeding the genrator) then I think Edison maximized the generator at something like 90%  And there has not been much in the way of improvement since.

Now we feed that power down wires (more loss but insignificant) to the Air Conditioners once again we are at 90% efficiency (Thank you T.Edison)

So we are at 81% efficiency.  Now the internal losses in the compressor (Same as engine) come into play.

HOWEVER.  I run all 3 and managed to keep cool last weekend .

I start the generator, turn on the front ac, and then put the rig in gear. Often the dash ac will also be running. These apply at least 90% of the time.

Note this also powers the hot spots and the refrigerator runs on ac. Full time we used 6.5 gallons of propane (filled monday) in the last 14 months.

Ernie

I have no choice. I travel with Collies. One of them has enough room to lay on the doghouse and suck up the a/c coming out of the cab vents, but the other will get hot laying on the floor in back. So I run the generator and coach a/c and turn off the cab air. It keeps the driver's area comfortably cool. I close off the vents in the bedroom and back of the coach but leave open the vents in the living room area where the dogs will be. In successive trips taking roughly the same roads and having similar weather conditions, I experienced better fuel mileage running the coach a/c than I did running the cab a/c. But it was only about .5 gallon difference. This also saves propane because the fridge is running on electric.

Most people don't run their  generator enough,  should be run at least 1/2 hr with at least 1/2 full load monthly  so I'd say running it going down the road with AC on will do the generator a load of good.

We always use the roof air. Anything that reduces the load and HEAT on the chassis engine is a good thing IMO. Also, reduces the electrical load on chassis. Those blowers and the ac clutch pull a lot of amps.

If I am travelling on the road my generator is running. Then we can use whatever AC we want and not have to worry about it. Better to use it than let it sit. Engines are meant to be run. There is not enough difference to loose sleep over.

My old coach's dash air needed a new compressor and lines were dry rotting so I just abandoned the whole set up, brackets and all. I did save everything though incase I changed my mind, some on here said there might be a resale issue, not concerned about that at all on a 29 year old unit. The front rooftop unit works good going down the road with the genset. My 1989 was a R-12 unit so I would be robbing a fair amount of the 240HP with the dash air, if I remember correctly and a new compressor, hoses and orifice was going to cost more than I wanted to spend at the time. Someday it might go back in if there is nothing else to do on that thing.............nothing else???

Deano, investigate a company called Vintage Air.  They do modern R134a a/c systems for classic cars and hot rods.  They have some universal units that might be adapted to your coach. 
There's also a company that's called Classic Auto Air.  IIRC, they can restore your old components and convert the system to 134.
They both make bracket kits that can adapt a new Sanyo style compressor to your engine.
If there's nothing else to do, that is...  ;D

Gary,
At what RV age would you say things got more efficient? 2000? 2010?
I'm still at the purchasing stage and this is yet another thing for me to consider.

Infact, maybe that's a completely different thread. If older equals higher quality build, but older also equals outdated systems (like suspensions, AC compressors, mileage efficiency, etc), then there should be a magic used RV age where you get the best of both worlds.

This is an awesome forum BTW. So refreshing to see people actually giving helpful information to each other instead of putting each other down.

Thanks.

Gary,
At what RV age would you say things got more efficient? 2000? 2010?
I'm still at the purchasing stage and this is yet another thing for me to consider.

Click to expand...

In my earlier post I said "30 years ago" and that is probably conservative.  Anything from 1990 onward ought to be pretty good and certainly anything from the late 90's, when fuel economy got really important.