Running Generator for Cooling Coach while on the Road

[Tom]

Smoky

I don't have any data to answer that question.

 

[Bob Maxwell]

Tom:

I think we would all agree that the genset cools the entire coach more efficiently than the dash air.

My interest, however, is which is more economical when going down the road.  Since I travel alone or with one other human, I don';t need the entire coach cooled when in transit.  I just want to be comfortable at the helm, and the dash air does that.

Smoky,  The first time you spend running with just the dash in 100+ ambient temps, let alone the higheway at a lot higher, for 8 hours and heat up that coach and stop expecting to be cool and relaxed, forgedaboudit.  Some older BB's have 3 roof airs to provide that kind of comfort. ;D The heartland and the west heat up RV's a lot more than 90 degrees back east, especially with a bit of altitude --and even more so when you get below sea level in the summer! Ive traveled in a mh at 115 + ambient in AZ and the S CA desert, and no way did I run without all of them going.

 

[Ron]

When we had the 88 gas powered Bounder it was more economical to run the Genset and roof air than to run the dash AC.  However I haven't really noticed much difference on the diesel powered coaches so we normally just use the dash AC now.

 

[blueblood]

Leo
I don't know why it wouldn't be even more cost effective ??? The price of fuel has no impact on the amount used, so if the generator saves fuel compared to dash air, the $ amount saved would be even higher.

Yes, I was rushing because of a slow connection and realized after I sent it that I had made a mistake.

 

[blueblood]

In a car the AC condenser coil (it's radiator) is located in front of the engine's radiator, thus if you are running the AC the air reaching the engine's radiator is hotter and thus not as able to cool the engine

In an RV,,, I'm very simply not sure,? I would have to crawl around under and over mine with a flashlight and take a closer look, I do know I have a pair of fans that are supposed to be for additional cooling for the AC per something I read but I"m not sure of the air path for engine cooling.? Does it make a difference? I don't know.? ?I've had no overheat issues using dash air

But on my rig, sometimes the rooftops are just not enough and you need dash air as well

The cooling fans are under front of chassis on an FCCC XC chassis. Thus this air is part of air being sucked back towards engine. However, the compressor is creating a lot of heat itself right in fron tof radiator.

 

[blueblood]

Tom:

I think we would all agree that the genset cools the entire coach more efficiently than the dash air.

My interest, however, is which is more economical when going down the road.? Since I travel alone or with one other human, I don't;t need the entire coach cooled when in transit.? I just want to be comfortable at the helm, and the dash air does that.? So, in that scenario, is the dash air more economical?? Hard for me to believe it isn't, but I am open to more data on this interesting subject.

I regret I'm not able to access my old study. However, the main reason is that the compressor uses a significant amount of HP (fuel) and the genset can virtually idle with a very low fuel consumption. My genset produces 60+ amps at full power so an ac unit at 13 amps when compressor is loaded requires very little fuel and when cycling to fan only obviously uses zilch. The dash ac compressor is pumped from rear engine compartment under pressure to 30-40 feet in front and return and at least two cooling fans must run to cool it before return not including the dash fan. I run one roof top ac with deflector vents over seats so air drops right down on us.  I do not try to cool entire coach.

 

[Karl]

All,

There are simply too many variables to say that one is more efficient than the other. Rotary compressors are inherently more efficient than reciprocating ones; a 7kW genset uses more gas at idle than a 3kW; cooling a driver's face to 75 degrees uses less power than cooling a big box to 75 degrees; a dash air conditioner is still moving a belt and pulley even when the clutch is disengaged, whereas a roof a/c simply shuts the motor down; etc. Add to that the possible differences in refrigerant types, evaporator and condensor sizes and efficiencies, air flow restrictions (both cold air and evaporator cooling) and other factors, and you can see that you've got a real basket full of snakes. What's good for the goose is not necessarily good for the gander; each method must be evaluated on an individual basis, and even that may vary from day to day as driving conditions change.   

 

[John From Detroit]

You need to consider the air temperture on the roof of the rig v/s the engine compartment too.

Generally the roof top unit is running "Cooler"

Another factor is the roof top units do not blow on the driver (At least not very well) though I can configure to favor the front of the coach in the case of my Damon

 

[Smoky]

Bob:

"Smoky,? The first time you spend running with just the dash in 100+ ambient temps, let alone the higheway at a lot higher, for 8 hours and heat up that coach and stop expecting to be cool and relaxed, forgedaboudit. "

Yes sir, I am well aware of that of course. ?However don't underestimate the cooling requirements of a VERY humid 98 degree WASH DC area. ? ;D ?We place some of the strongest demands in the nation on air. ?My new KS3910 does a terrific job at the dash. ?Plus we easterners are used to a hot humid environment to begin with so our demand for "coolness" may not be as great as the dehumidified Westerner. ? ;D

I still would like to separate personal preference from actual running economy in these comparisons. ?I would like to know what the fuel cost would be say on a 300 mile trip running the genset and house air in 90 degree temp, vs. ?the same trip under the same conditions using chassis air, regardless of a person's personal preference. ?Not sure if a test like this is even possible, but I am sure everyone gets the idea. ?A fascinating and difficult problem to resolve scientifically!

 

[Lorna]

Smoky, when you are in the mountains and the temps. are hot you are not going to run your dash air because the engine will overheat.  In fact, on most upgrades there are signs that tell you to turn off the dash AC.

 

[Gary RV_Wizard]

Your  large gasser is normally equipped with  a main engine in exes of 300 HP, your gen.set is probably 10 HP.

340 HP for the chassis, 8 HP for the 5500 watt genset.

Is your cooling system in good condition Gary?

Yes. It is now only 3 years old and does a great job in all conditions. Has both an engine driven fan and two electric fans, all controlled by t-stat and coming on in stages as needed. My observations have been over the past three years.

The heat generated from the AC, has nothing to do with the main radiator, well, I think in US just like on the old continent, dash AC  has his own. radiator.

As someone has already mentioned, the a/c's heat load is dumped in front of the engine radiator and transfers a lot of heat to it.  Perhaps a poor design, but pretty much standard in both cars and motorhomes. 

The a/c compressor takes a lot of HP from the engine. I have seen estimates (in published articles) of up to 30 HP (when clutch is engaged) , but don't know how acurate that number is.  If it is even remotely correct, my little genset is producing a lot more cooling from much less horsepower.  Maybe the rooftop a/c's are much more efficient?  I don't really care, as I am merely reporting my observed data.  In my case that means less impact on mpg and a rig that is cooler both inside and in the radiator.

As Karl says, there are far too many variables to evaluate. Makes a good topic for a campfire discussion [with a suitable libation in hand] though.     

 

[Karl]

Gary,

The a/c compressor takes a lot of HP from the engine. I have seen estimates (in published articles) of up to 30 HP (when clutch is engaged) , but don't know how acurate that number is.

Your figures are probably correct. I've always used 25 hp. as the engine load, but have no doubt that 30 is accurate; especially in view of the new type refrigerant being used - lower efficiency refrigerant = greater drain for same btu cooling.  Bummer!

 

[Smoky]

Lorna:

When that happens, I will most certainly be happy to use the roof air!!

 

[thenosyone]

The a/c compressor takes a lot of HP from the engine. I have seen estimates (in published articles) of up to 30 HP (when clutch is engaged) , but don't know how acurate that number is.? If it is even remotely correct, my little genset is producing a lot more cooling from much less horsepower.?

Thanks Gary
When those estimates up to 30 HP  are even remotely correct,  roof AC wins.... 
Poor design?
thenosyone Antwerp

 

[Karl]

Thenoseyone,

Poor design?

Not necessarily. Auto a/c's cycle on and often frequently and do so when the outlet pressure is still in the 200 psi. range. A rooftop or refrigerator compressor would not be able to handle those extreme cycling pressures without a huge motor, so they delay the restart time (around 2 minutes) until the input and output pressures have almost equalized. Also, the auto a/c must be able to operate over a wide range of rpm's and loads, whereas rooftop's run more or less continuously at the same speed for long periods, thus smaller motors are needed. Different requirements - different solutions. 

 

[thenosyone]

Thenoseyone,

Not necessarily. Auto a/c's cycle on and often frequently and do so when the outlet pressure is still in the 200 psi. range. A rooftop or refrigerator compressor would not be able to handle those extreme cycling pressures without a huge motor, so they delay the restart time (around 2 minutes) until the input and output pressures have almost equalized. Also, the auto a/c must be able to operate over a wide range of rpm's and loads, whereas rooftop's run more or less continuously at the same speed for long periods, thus smaller motors are needed. Different requirements - different solutions.? ?

Thanks for the info Karl
BTW what type of clutch is used?
If you need 30 HP to let the dash AC do the same work as a gen.set with 8 HP and roof AC how can we not talk about "poor design?"
thenosyone

 

[Karl]

Thenoseyone,

It's a magnetically operated friction clutch. The continuously running pulley has a spring steel mounted center section with a steel core. The compressor has an electromagnet surrounded by friction material similar to brake lining. When energized, it pulls the center of the driven pully into contact with the electromagnet/friction core which turns the compressor. The spring steel maintains a gap between the two when the electromagnet is not energized.

If you need 30 HP to let the dash AC do the same work as a gen.set with 8 HP and roof AC how can we not talk about "poor design?"

It's not poor design; it's using the right tool for the job and using the technology and space available. An auto a/c was not designed to cool an entire coach - the rooftop unit was. Internal compressors (sealed units) are more efficient than external units and don't have a belt and pulley running continuously and using energy. Also an auto a/c doesn't use 30 hp. continuously and usually has a cut-out to cut power usage when you're operating at full throttle - like when climbing long hills. Size is another consideration. You just don't have room under the dash for a large evaporator and blower assembly.

 

[Smoky]

Karl:

"An auto a/c was not designed to cool an entire coach - the rooftop unit was."

But what if you are not trying to cool the entire coach?

When we are underway, it is just the two of us.  Well 3 of us including the dog.  I don;t include the cat because the cat stays at the rear of the coach and would rather suffer hell and noise than come out from under his cubby hole when we are underway.

So I have my air vents in front of me, the dog is on the floor between us, and the Admiral has her vents in front of her.  We are comfortable traveling this way and do not mind the short hot trips to the head or the fridge.

Do we really need to turn on the coach rooftop air?

 

[Tom]

Smoky

I've not had to turn on the roof air in our current coach while travelling, although I had to on the old coach. I suspect you'll be fine with dash air unless/until you're driving while it's very hot outside. Depending on how much the interior of your coach heats up, you may find that convection from the rest of the coach may overwhelm the relatively small amount of cold air from the dash. But I doubt you'll be travelling in Arizona or Nevada when it's 115 degrees outside.

 

[Smoky]

Thanks Tom and yes I hope to avoid those conditions in AR and NV!