Excellent Information on Motorhome HP Requirements and Fuel Consumption

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

Thanks, Gordon.

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.

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

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Originally Posted by jack14r

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

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

Good article Gordon, thanks for posting.





JIM

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Originally Posted by Jon Wehrenberg

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.

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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.

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.

[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.

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.

You only pay for the horsepower you use. With the exception of more engine weight and higher internal friction in a higher horsepower engine which has a modest impact on fuel economy it is your right foot that regulates your fuel consumption.

If you have a big engine, but choose to climb long steep hills using the performance of a smaller engine you will consume the same volume of fuel as the smaller engine. But that is not how we drive. If we have a big HP engine and we can pass every vehicle in the slow lane going up hill, that is what we do so we burn the fuel proportional to the HP we used. Hills and starts are what use fuel. Once rolling on level ground we are using horspower to overcome the wind and rolling resistance. But getting a heavy coach moving or going up a hill requires more fuel consumption than a lighter coach.

If you study the charts, a 10,000 pound heavier coach requires 8% more horsepower, but increasing the speed (headwind) 15 mph requires about 57%. Weight is a factor, but speed is the largest impact.

My car has 505 HP, far more than any of my other three cars, but it is the cheapest car to drive with a 24 MPG fuel economy around town, and over 30 MPG on the highway. Up until I stick my right foot on the pedal.

Jon what do those Corvettes weight?

Mine is 3400 and change. So as a result it uses little horsepower to get rolling, stay rolling and going up a hill, probably not unlike a Camry or Accord.

A Jeep or an SUV or a minivan, all of which have around 4000 pounds and much greater frontal area can be expected to get less MPG.

I am not sure of all the HP on my cars, but the S500 weighs 4000 lbs and is a 23 mpg car, the Ram 1500 weighs 5300 and gets 18 mpg, and the H2 Hummer weighs 6700 and gets 13 mpg. Every mileage figure is off the car's trip computer over many thousands of miles so it is reasonably accurate. The weights and frontal areas of the vehicles seems to support what Cat is saying. We do not drive any of the vehicles different than the other. FWIW my most economical vehicle is the bus which weighs 46,600 and gets 7.5 mpg.

I know the impact of speed on MPG because I had to pick up a truck in Hickory NC last month and Di followed me home in the Hummer. I drove the truck at 60 MPH and I verified the speedo was accurate since the truck was new to me. The Hummer got 16 MPG on the trip since it was all interstate and we were driving slower than the 70 MPH we drive on the interstates around our home in all our vehicles. It has never gotten that high a mileage.

This is from that Cat info: A diesel engine consumes approximately 1 gallon of fuel per hour at a fast idle (900 – 1000 RPM).

We're in really hot weather and I was wondering about running the engine and OTR air to stay cool for a couple of hours rather than running the generator to power the cruise airs. This info looks like it is more economical to run the engine and OTR air rather than run the generator. Not that running the engine is more comfortable with regard to sound and vibration but staying cool is a big deal. If you need a cool down before the sun sets, running the engine and OTR air seems like an ok option.

Over the entire week at OSH last year I ran the generator and I measured the hours and fuel consumed. I never ran fewer than 2 Cruise Airs plus all the normal stuff like the refrigerator, TV when I watched it, the grill (when I wasn't being fed by my neighbors, Mr. and Mrs. Sawdust) and I averaged less than 1/2 gallon per hour. I think in round numbers I ran over 160 hours and consumed 70 gallons.

If you don't want to use your generator can I have it?

Oh, I don't mind running my generator. I was just struck by the amount of fuel used while idling. Didn't know that number. You sure take good notes.

Jelmore I could be wrong but I think the 1 gal of diesel at idle doesn't taken to count the load of OTR at high idle.

On several trips where we have dry camped I have kept up with the fuel used in the generator and it has averaged .85 GPH,I am using 4 cruise airs and we invert at night and at least one other time during the day,usually my loads are between 40-65 amps on each leg.

Jim,

I checked the fuel consumption specifically so I would have the information. I topped off the tank about 2 miles from our OSH parking. I recorded the generator hours at the start and at the end of the show, and before I got on the highway I drove the two miles back and coincidently used the same pump at the same fuel stop and let it click off like it did the week before.

I noticed Jack was running more fuel through his, but his loads were greater. I left two AC on all day while I was working the show, and the coach was closed up the whole time. I wasn't there at the coach from 7:30 until 5:30 or 6:00 so except for the refrigerator, and the inverters to charge the batteries from quiet time I wasn't using much power so my loads may have been 15 to 25 amps per leg except when I was at the coach and using lights and other appliances. I used to remember how many watts equal a horsepower (750?????) and from a calculation of your loads you can estimate approximately what your fuel consumption might be. To use 1 gallon per hour the generator is working fairly hard, which incidently is the absolute best thing you can do for it.

For a quick rule of thumb, use the generator wattage, move the decimal three places to the left (20,000 watt generator equals 20) and double that figure and you have an approximation of the HP of the motor. The amount of fuel used can then be determined by determining the HP to be used and most generator manufacturers will provide the fuel used at certain percentages of the generator's capacity

Jim,

One more data point for you...I have checked genset consumption several times now for 24 hrs up to a 72 hr run straight through - all done in hot summer type weather and I like it cold in the bus so with the 3 cruiseairs cranked up. We also cook a lot in the bus, and my wife can run a hair dryer for half an hour straight it seems. Anyway, my consumption is very consistent at .7 to .75 gph.

The Pro Driver computer (at least mine) is very accurate on engine fuel burn. Could run a test case pretty easy and see what the big engine burns running the OTR with the Pro Driver. We stopped about an hour today for lunch on a return trip from Ok - wish I would have read this yesterday and I would've checked it out.

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Originally Posted by Jon Wehrenberg

You only pay for the horsepower you use. With the exception of more engine weight and higher internal friction in a higher horsepower engine which has a modest impact on fuel economy it is your right foot that regulates your fuel consumption.

If you have a big engine, but choose to climb long steep hills using the performance of a smaller engine you will consume the same volume of fuel as the smaller engine. <<< snip snip snip>>>

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Except that engines seem to acheive their optimal BSFC (brake specific fuel consumption) at their peak torque outputs; consequently, in the above example, the smaller engine will have better fuel economy.

A smaller engine will always have better fuel economy than a large one because of the energy required just to overcome the forces required to operate the drivetrain and overcome friction.

However, if I operated my engine at the peak torque output I would either have to drive around mostly in first gear, or be driving very fast at which wind resistance and other factors would damage my fuel economy.

I short shift as much as practical and get into 6th (or high gear) ASAP which in the case of the Corvette puts me at 60 MPH at 1300 RPM (approximately) significantly below peak torque and HP. Operation at peak torque might be more applicable to a diesel where peak torque is reached at relatively low RPM, such as in our buses, that are mated to a transmission whose shift pattern is set to upshift at relatively low RPM unless we go into performance mode.

Jon, Your 60 mph, 1300 rpm vette brought a grin to my face. In 1967 I had a 67 vette, 427, that turned 3000 rpm's at 60 mph. The auto industry has come a long way !

It would bring tears to your eyes when you had that 427 if a typical hot car of today, such as a Subaru WRX or even an Impala with the V8 ate its lunch in a drag race.

My 63 Corvette would do the 1/4 mile in 14 seconds at 98 mph, something close to what the average car does today. And that car needed points and plugs every 10,000 miles and it ate universal joints on the half shafts at the rate of one every 30,000 miles or so. Today we probably don't even have to change plugs for 100,000 miles and most mechanical stuff lasts far in excess of that. Aaaahhh the bad old days.