In the first thread the discussion was focused on driving techniques and patterns as well as effect use of the Mode capabilities for those equipped with an Allison Transmission with mode capabilities. Some Thoughts on Improving MPG..Driving Techniques In short according to both Cummins and Caterpillar websites, driving technique and patterns accounts for 30 to 40 percent of the variable irrespective of the speed one choice to drive. The speed one drives and the weight of the motorhome and dinghy combination establishes the base line where driving Techniques and Driving Patterns can play a role.
Just slowing down with not ensure that you will achieve the optimum MPG for the size, weight, power train and tires. How you operate your motorhome will and does play a major role within the band of speed where your particular configuration is the most efficient.
The purpose of this thread is to give those that would like a better understanding of the dynamics that conspire to consume fuel. Hopefully we will have a discussion that will be of benefit to some... For those that have no interest just block this thread.
As mentioned in the other thread discussing Driving Techniques to improve MPG the aerodynamic profile of your motorhome falls into the category of it is what it is...
There may be some slight aero improvement possible with after market attachments. But short of selecting a motorhome based on it aero profile we get what we get. The best way to counter the wind resistance and weight factor is to slow down and travel lighter. I do think it worthwhile to discuss in general terms the role and impact wind resistance has on fuel economy. This thread is an attempt to put a meat on the bones... The reality is when we speak of slowing down to improve fuel economy we are in reality talking about reducing the drag or resistance of air as the motorhome moves along the highway...
As speed increases the effect of resistance of the air the motorhome is moving through increases exponentially. This increase has to do with the frontal surface presented by the motorhome as well as anything that prevent smooth airflow over the top and along the sides. With that in mind it would appear as though a coach that is totally square in the front will offer the most resistance moving through the air. Those that have some slope at the top and rounded sides will offer some what less resistance. No need for complex formula's just think back whenever you were riding down the road with you hand out in the air. Held at 90 degrees offer the most resistance tilt slightly and the wind resistance decreases. While calibrating exactitudes of wind resistance is complex the principle is a simple as putting you hand out the window and experience the force as you alter you hand in relation to the wind. The bigger the area the broader the surface equates to more wind resistance the effect of which are compounded by speed...
Also to some extent that weight and aero shape of our dinghy's also play a role... Aero considerations relative to the dinghy is not something I have focused on but taking into account the weight of the dinghy might well be worth thinking about. The higher the percentage of weight the dinghy represents the more consideration it ought to be given...In our case our dinghy represents about 14 % of our rolling weight.which translates to just under .5 of our MPG average. In other words its going to cost us about $35 dollars to drag our dinghy to the Elkhart rally and back...
According to Cummins Cummins MPG Insights "Every 2% reduction in aerodynamic drag results in approximately 1% improvement in fuel economy" Plus "aerodynamics is the most important factor over 50 mph" in terms of consuming horsepower.
There are other factors that consume HP , Parasitic engine and drive train loses which can be as much a 12% of horsepower. There is also cooling fan whether belt driven or hydraulic driven consumes horsepower which increases with the size of the engine and Fan RPM. Plus the alternator and air conditioning if in operation and if a DP the air compressor Those discussions are beyond the scope of this thread...Plus they fall in to they are what they are except that the lower the engine RPM the lower the hp requirements to run engine driven components.
The chart below was created by using the information from the Caterpillar site and doing some extrapolation to assign values to other weight classes... I would not take the numbers to the bank since extrapolation excludes the dynamics of variable weighting. I also used .048 gallon per hour per horsepower as fuel consumption. I suspect that the diesel engines in our motorhomes are somewhat more efficient but .048 is in the ball park. Doing the same calculation for a Gasoline engine the constant would be .068 gallons per hour per horse power. Fuel usage per horsepower If anyone has a better constant let me know...
The thing to take note of is the horsepower it takes and the fuel consumption that horsepower uses to overcome wind and rolling resistance For discussion purposes there is no consideration to prevailing wind or anything but a flat smooth road surface. I would not expect the numbers presented below to be wholly accurate because of the method used to extrapolate but the scale and relationships shown for HP requirements and fuel consumption should be within reason. I would add that if one is operating with under inflated tires aside from the increased risk of blow outs you are adding to an already significant consumer of HP which translates to using additional fuel.
This chart simply illustrates how many gallons per hour is used at various speeds and MPG
* This post was
edited 04/02/08 06:24am by JohnnyT *
"The best way to counter the wind resistance and weight factor is to slow down and travel lighter."
There you have hit the key. We've sort of beat this topic to death in other threads about mileage in the past, but the truth remains.
The charts you've posted also lay out in exacting detail just what we've been saying in general terms. Perhaps they'll help convince some people who otherwise just don't want to believe what they've heard from some of us. They also can point out the difference in characteristics of diesel vs. gas powertrains.
Engine RPM and shift points come into play. I have been driving different speeds and watching my drivetrain computer for intantaneous mileage. What RPMs produce the best mileage? If I drive less than 55 on anything but the flattest of terrain, the Allison shifts into 5th and the RPM's go up to 1700 or so. Oddly, the mileage doesn't seem go down that much if at all. Is it more efficient to drive at 50 in fifth than 55 in sixth? Drive at peak torque RPM? I have upped my mileage almost two MPG by slowing down and reducing power soon after take-off, accelerating slower.
Unfortunately the lab test and resulting statistics do not always bear out in real life. I and some others here find very little change in mileage based on speed.
To some extent, in real life, you find that some engines just get what ever mileage they get regardless of how much weight they pull or the size of the vehicle. Much of that is related to engine size. Generally speaking engines running closer to their maximum output are more efficient. This would suggest that there needs to be a philosophical shift in the RV market toward slightly downsized RVs. No, I'm not thinking of tiny, but something inbetween the size that I have and the bemouths typical of today. The problem is that there is no intermediate RV market. Todays coaches are all raised rails and as big as they can possibly be. I would suggest that looking into something the height of the early 2000s Rexhalls, where the basement height is only 2 feet tall instead of 3. To do this right, you have to custom build the chassis, so there is not much frame under the coach. The coach needs to be full monocoque construction. Yes, it takes some engineering to make it work with slides, but it can be done. Secondly, go to some real effort to build it lightweight. Shell and frame construction needs to be of aluminum. For the interior, my cabinet doors and sides are hollow. This means that are very lightweight. The skin is Wilsonart and holds up very well. My cabinets looks as good as they did when the coach left the showroom. No real signs of wear. There are some also some new, strong wood/plastic composites that are built hollow to be lightweight. Japan is licensing this product for US production, but I don't know if anyone yet is producing it here. The product has been in used in Japan for many years now. Do to its design, it can be made to be structural which is something new for plastic/wood composites. Lightweight is worth the effort. My coach is about 25% to 30% lighter than coaches of comparable size. Current construction methods are simple and cheep, which is the driving force. Next, round off the corners, especially on the front and rear of the coach. The driver's area is a much wasted space. This area could be narrower, and much more rounded off, yet not have any impact on the interior storage or feeling of space. There rear roof line could be tapered down without having significant impact on space. Rounding off the wall to roof intersection does impact interior space, but some compromise could be made there, improving the current design, but limiting the interior impact.
Once you build the motorhome more efficient, then the engine requirement would not be so great and smaller, more efficient engines could be used. As a bonus, all this areo improvement will greatly improve handling as well.
The Flying Fortress
'83 Revcon Prince 31' FWD
502 w/Howell/GM 16197427 ECM/Edelbrock MPFI,Thorley's & Magnaflows,
4L85E 4 speed, KoniFSD,
6% grade = wanna drag? MISC photos Revconeers Forum
Many of the attributes you describe are some of the reasons we chose our coach:
monocoque construction, light weight (23,000 pounds loaded for a 36' DP coach), a more aerodynamic shape than a box, etc.
Are there trade-offs-- sure. Our coach is not as roomy as "modern" coaches. But the 11.3 MPG average is becoming more and more important.
And indeed compared with a house ALL our motorhomes are small. I know people who have circumnavigated in 24' boats and those who find 40' boats too confining. Our perception of "adequate space" is more the issue than actual space.
And if 11.3 MPG becomes an issue, I would certainly be willing to downsize rather than give up the RV lifestyle.
The air drag is nearly completely determined by the MH manufacturer and reducing it in most cases would not be cost effective. I became interested in reducing the drag produced by the "vacuum" and turbulence at the rear of the MH. I installed air tabs here to help. They advertise 2-4% decrease in drag and improve handling in crosswinds. I can't give fuel economy results yet until I get a longer history with them but they seem to make sense. Considering the cost of fuel the approximately $200 I paid could easily be recovered over time. Other things that might help the rear aero problem is a deflector to send air into the rear.
As J Walker suggests, closing the gap at the rear contributes significantly to the total picture and is worth pondering. I believe that its contribution can be equal to or greater than that of the nose depending on the shape of the nose. While punching a big hole in the air, the flow is more or less orderly out front with some circular flow just aft of the edges (simplified). Out back however, it's pure chaos. Since teardrop shaped MHs are impractical, modifying the aerodynamics of the trailing edges, either at the factory or with aftermarket vortex generators or foils is the best we can do.
For this same reason, I don't think that toad aerodynamics adversely affect the efficiency of the whole assembly. The toad could even reduce drag by helping to plug the hole, as evidenced by drafting.
FWIW my 08 Itasca has a more pronounced reverse slope nose than the 07's did. Not sure if Itasca did that to look futuristic or one of their engineers spent time in wind tunnel. Also the nose has some contours. They look aerodynamic as well. Maybe it just looks cool Other than the AC and Vents the profile is fairly smooth (rounded roof radius etc.)
I say this because my mileage in moutains, with TOAD appears to be on the higher side for Gas units. I do admit to staying in the powerband of the engine and being in traffic areas 70MPH is as fast as I ever get - mostly 62ish or less.
2008 Itasca Sunova 35J Class A
1997 TJ Sahara, hard and soft tops and AC
Held together via Roadmaster Falcon 2 tow bar and stopped by US Gear Unified Brake system.
Our 30' Beaver Monterey is somewhat light at a little over 21,000 wet. With the 3126 300 hp cat the performance and mileage is pretty good and runs from 8.5 to 10+.
We installed the same Air Tabs as J Walker two summers ago. On similar cross country trips both at 62-65 mph the ave trip mpg rose by a little under 1 mpg. Stability in cross wins also improved. The only objective verification that the Air-Tabs reduce the low pressure area behind the rear cap is that my rear view camera lens now becomes obscured by water as the rain now follows the contour of the rear cap and wets the camera. A wet camera is no advantage, however it does demonstrate the change of airflow.
I suppose the only way to quantify a reduction would be in a wind tunnel. Do the Air-Tabs work? They seem to, will they ever pay for themselves, perhaps, If the price of fuel keeps going up and I drive it enough.
The biggest payoff for me was the increase in stability.
Weight does not seem to be much of a factor for us. On our last cross country trip we took several thousand LB of our possessions to our new home. We drove our typical speed and got the same mileage we normally get.
When you consider the variations in chassis and rig weight used with the same engine and transmission combo there isn't much difference in reported mileage. For instance, consider a 30' non slide Beaver Monterey and a 38' multi slide Monterey with a big difference in weight.
* This post was
edited 04/02/08 06:35pm by The Beave *
30' 1997 Beaver Monterey
(The shortest DP we could find)
2005 Honda CR-V
Remco surge brake