Excellent Information on Motorhome HP Requirements and Fuel Consumption

[GDeen]

Found this link on another site to a Cat information page on their engines for Motorhome applications. Excellent data on HP demands vs. cruising speed, head winds, various atmospheric pressures and grades. Also shows parasitic demand from cooling fan at various engine speeds (up to 10% of available HP).

One thing that has been discussed here frequently and is intuitive through our own individual experiences, is the sharp decline in fuel economy at higher speeds. Their chart shows that increasing speed from 55 to 70 mph requires almost a doubling of HP to the drive wheels. Obviously explains the big jump in fuel burn, but I wouldn't have guessed that much. They also break down the HP requirements in the 2 main catagories of rolling resistance and wind resistance. Very interesting stuff.

Although this is for Catepillar engines, the HP data is directly applicable to our DD's.

https://ohe.cat.com/cda/files/287140/7/LEGT5364.pdf

[Kenneth Brewer]

Thanks, Gordon.

[Jon Wehrenberg]

Kind of makes 62.5 MPH seem like a good compromise doesn't it?

It also shows the penalty we pay for weight, something I know from flying. Unlike in the bus where power varies, in the plane I set the power and leave it. There are two factors that influence speed, one being altitude where I gain 1.4 knots for every 1000 feet of altitude and the second is weight, where I gain about one knot for every 100 pounds of fuel I burn.

On our buses if we can reduce the weight of the bus and tow vehicle we will increase our mileage. Also, our fuel mileage increases in warmer temperatures because the air is thinner and thus less resistance to the frontal area which is akin to flying in thinner air at higher altitudes and getting more speed.

[jack14r]

I have noticed the same about temperature,40 degrees verses 80 degrees seems to be about .5MPG

[phorner]

I have noticed the same about temperature,40 degrees verses 80 degrees seems to be about .5MPG

My fuel economy plumments at 40 degrees......... 'cause I'm trying to find warmer climates......... FAST

[JIM CHALOUPKA]

Good article Gordon, thanks for posting.





JIM

[GDeen]

Also, our fuel mileage increases in warmer temperatures because the air is thinner and thus less resistance to the frontal area which is akin to flying in thinner air at higher altitudes and getting more speed.

this is one I hadn't considered until I read the article. I figured you pilots would be all over it but I am surprised it is that big of a deal.

[Jon Wehrenberg]

Apart from being a target of the "go-fast" crowd that thinks they need to show off their high speed driving, I picked my driving speed because in a previous life I never slowed down. In the bus I'm already on vacation so I picked a speed that was relaxing to drive.

One of the benefits has been I use less fuel than at 65 or 70. I also am far less tense driving. If my speed matches or exceeds the average speed of traffic I cannot just sit back and relax on cruise control. At my speed almost every car and truck is going slightly faster so I do not have to come upon someone, turn off the cruise until I can pass and then resume it. I rarely have to turn off the cruise.

I do not know if the cruise is more efficient than driving the bus manually. In large jets the companies that I am aware of want the pilots to use the autopilots. Maybe John or Brian and address that.

I do know and I believe I have posted that our number one enemy of fuel economy is speed, and the best way to increase economy while keeping a reasonable average speed is to pick up the speed with a tailwind and drop speed back a little with a head wind. Fuel prices are creeping up and although the cost of fuel is chump change compared to other costs of ownership it is nice squeezing another 1/2 mile per gallon out of the bus.

[Charles Spera]

[QUOTE=Jon Wehrenberg;64455] "... I picked my driving speed because in a previous life I never slowed down. In the bus I'm already on vacation so I picked a speed that was relaxing to drive. "

I dunno, maybe it's just me but since I read about your 62.5 mph pace, and now do so also, my blood pressure is lower. Maybe it's just imagination.

[gmcbuffalo]

Looking at the charts I think the weight issue is even worst for us. The coaches in the charts with heavier weights also have larger engines, where we only have two engine option v92 or s60 to pull the weight. If you are a v92 and go from 40K to 50K of weight you are looking a sufficantly more fuel consumption that what appears on the charts.