Running Generator for Cooling Coach while on the Road

[caltex]

Smokey, just don't forget to turn onthe roof airs in time to cool the coach for your landing.  Waiting for the coach to cool after parking on a 100 degree day isn't a lot of fun.

Robert

 

[Karl]

Smokey,

I have yet to use the rooftop units while travelling - even on the hottest days. If you're planning on driving in the heat for several hours at a time (which I do frequently), you could hang a bedsheet across your coach, floor to ceiling, behind the driver area to reduce those nasty convection currents Tom was talking about. May not be pretty, but certainly would help.

 

[Smoky]

Robert:

Good idea about firing up the genset and roof conditions in preparation for landing!

 

[Tom]

hang a bedsheet across your coach, floor to ceiling, behind the driver area

Is this a new class of motorhome? aka covertible class C-A?

 

[Karl]

Tom,

That was meant to be tongue-in-cheek, but I do drive with the side window shades down to keep the sun out. Suppose I could pull the front drapes around the windshield when running into the sun too ;D

 

[Tom]

Karl

Tongue-in-cheek or not, I thought it was a good suggestion. Then I started thinking of a roller blind that went the width of the coach and my imagination took over from there  ;D

 

[thenosyone]

you could hang a bedsheet across your coach, floor to ceiling, behind the driver area to reduce those nasty convection currents Tom was talking about. May not be pretty, but certainly would help.

That was a good idea, pity you ruined it with your next reply.
Maybe Smoky just want to find out how he can reduce pollution by burning Les fuel.
Our grandchildren have to live on this planet to.

BTW you still haven't convinced me about "poor design"

 

[Ned]

Our Itasca Spirit (type C) had a curtain just behind the cab for privacy but it certainly could be used to close off the cab area when driving in very hot weather to improve the cooling.

 

[Karl]

thenosyone,

I'm not trying to convince you of anything; merely stating the facts as I understand them. We could go into BTU's, enthalpy, differences in expansion valves, load efficiencies, etc. but it wouldn't serve any useful purpose. You're convinced it's a poor design, so we'll let it go at that. :-\ 

 

[Gary RV_Wizard]

Maybe "poor design" isn't quite the right description.  My view is that an a/c system borrowed from a passenger car is a "poor choice" for a Class A motorhome that has a much greater volume to cool and does not have the space/weight restrictions of most passenger cars.

Many years ago air conditioning was a high end option in motorhomes, but now even the entry level models have it and with many thousands of newmotorhomes being built every year, a system could be designed for the specifically for the needs and capabilities of a motorhome.  One of these days, some manufacturer will take the step and there will soon be a new RV industry standard solution.

 

[thenosyone]

Maybe "poor design" isn't quite the right description.? ?My view is that an a/c system borrowed from a passenger car is a "poor choice" for a Class A motorhome that has a much greater volume to cool and does not have the space/weight restrictions of most passenger cars.

Thanks Gary, thats what I meant....  My English does has some limitation sometimes

 

[taoshum]

running the a/c from the main engine runs the compressor with a mechanical belt directly, no big loss in efficiency.  running the a/c from the generator requires converting energy to electricity (35% efficient), then running a motor to run a compressor (70%...).  there's no way a genset can be more efficient than a mechanically driven compressor...??  maybe I'm missing something?

 

[Gary RV_Wizard]

I've never seen a reliable analysis of all the efficiency factors involved.  Traditional wisdom is that the small engine in a generator is more fuel efficient than the big engine on the chassis. The mechanical compressor on the engine is an incremental load of an engine that already has its work cut out for it, whereas the generator engine can run at essentially constant speed in its peal efficiency range.  On older gas engined coaches there was plenty of empirical evidence that less fuel was consumed in a day of hot weather traveling if the genset and house air was used rather than the dash air.  I used to notice a 0.5 mpg decrease when I ran the dash air on my 96 Southwind all day long.  But with my 2002 Dolphin, the difference was not as noticeable.  And on diesel powered coaches with diesel gensets it is even less so.

Probably the real point today is that you can cool most or all of the coach with the genset and roof air for a very modest cost. My 7.5kw Onan uses only 0.3 gal/hour to run one a/c unit and about 0.5 gal/hour to run both. The dash air can't even come close to the same amount of cooling and there is clearly some increase in engine fuel use to run it, so that makes the incremental cost of cooling the coach with house air very small.  It's a no-brainer in my book. YMMV  8)

 

[Mike and Sherrie in VA]

I'm just going by rough numberss based upon what I've seen with my 30ft. gas RV, but anyway:

If I run the engine and get  10 MPG without AC at 60 MPH, then turn on ac and get 8 MPG at 60 MPH, that means that I lose 1.5 gal. per hour cooling via the dash AC vs. no AC on at all.

10 MPG at 60 MPH over 1 hour is 6 gals burned.

8 MPG at 60 MPH over 1 hr is 80% as efficient.  7.5 gal. burned.

I know that my Onan genset can run both AC units for less than 1.5 gal. per hour, and keep the coach a lot cooler than just the driver's area off the dash AC.  I can run the Onan under load at about .625 gal per hour (5 gal used over the course of 8 hours).

So, assuming the above numbers (YMMV) running off dash AC is going to cost me 1.5 GPH whereas running off the genset is going to cost me .625 GPH, or a little less than half to obtain a lot more cooling.  Of course, that doesn't factor in maintenance cost over the life of the genset, but I personally would go with running the "house" AC on very hot days.  If temps are around 80 or below I just open windows and it is a non-issue.

 

[Gary RV_Wizard]

Thanks for contributing some real life data, Mike. However I am surprised that there is that large an effect. Most people who have made some attempt to measure it claim somewhere between 0.5 and 1.0 mpg impact. Maybe your compressor runs 100% of the time, though. That could be sucking as much as 12 hp continuously. Smaller engines seem to take a bigger hit too. Perhaps because they have to work harder to make up the  extra horsepower?

 

[Mike and Sherrie in VA]

Well, I am working with some not-necessarily factual numbers.  I was judging a bit more along the lines of what results I see from my car (in which case I have a lot more actual data).  It is probably more in the range of a 10% loss in the RV when running the AC, but it gives an idea of what/how to calculate the load.  The numbers I gave were showing a 20% loss with dash AC, but even then if we assume only a 10% loss the coach AC running off the genset is still much more efficient in terms of the area it cools.

 

[seilerbird]

How can anyone claim the genset is more efficient to run than the dash air? How do you measure the gas mileage in the two situations? If you are using the on board trip computer then you are wrong. When the dash air is used the effect of running it can be measured on the trip computer. When you are using the genset the gas mileage of the engine is unaffected by the gas consumption of the genset. You are comparing apples to oranges.

 

[Gary RV_Wizard]

The fuel consumption for many gensets is a well known and published number, seilerbird. Onan, for example, publishes them as part of the specs. A typical 7.5kw diesel Onan uses only about 0.5 gal/hour at 50% load, which is enough to run two roof a/c's continuously (100% duty cycle).  That can be compared to the observed fuel economy differences at the engine. It just takes some record keeping.  It's not real precise on the engine side, but its a usable yardstick.

 

[Mike and Sherrie in VA]

How can anyone claim the genset is more efficient to run than the dash air?

Based upon fuel consumption, and the inclination to believe that the genset running two roof A/C units cools the entire coach vs. the relatively small area cooled by dash air, I think it is safe to say that the genset is a bit more efficient.

 

[seilerbird]

In order to compare the two you would need to run the exact same route at the exact same speed for the exact same time at the exact same temperature. Otherwise you are comparing apples to oranges.