Page 4 - Summer 2020 Travelore 50

Page 4 - DRVC Travelore
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ment in fuel economy. Here’s how Cummins describes Aerodynamic Drag:
“Aerodynamic drag is the result of forces (pres- sure imbalances) acting on a vehicle as it passes through the air. The magnitude of the forces acting on a vehicle depends on speed, frontal area, and external shape. Aerodynamic drag
is the most significant contributor to vehicle power requirements above a speed of 50 mph. As the following graph shows, aerodynamic aids can have a major impact on vehicle fuel economy on an interstate duty cycle and very little impact on an intercity duty cycle.” While the descriptive categories relate more to truck- ing than RVing, the
laws of physics are the same.
Towing a trailer or a
vehicle may also influ-
ence aerodynamic drag as
well as rolling resistance.
The largest single power
requirement for an RV
is the power needed to
overcome air resistance.
As a general rule, each 10
percent reduction in air
resistance results in a 5
percent increase in mpg. As is noted below, climate also impacts fuel economy as cold is denser and increases drag on the RV.
Rolling Resistance
Rolling resistance is the second-largest consumer of power on an RV. While the tire guys will tell you that worn tires provide greater fuel economy than new tires (approxi- mately 7%), that may not be a big factor for RVers since
it takes 35, 000 to 50,000 miles for new tires to become sufficiently worn to reduce the amount their contribution to rolling resistance. What you can control, however, is tire pressure. Every 10 psi that a tire is underinflated reduces fuel economy by 1%. Tires make the biggest difference
in mpg below around 50 mph; aerodynamics is the most important factor over around 50 mph.
Diesel engine manufacturers say that “The most efficient
drivers get about 20 - 30% better fuel economy than the least efficient drivers.”
Other factors to be considered include route selection, climate, fuel blend, weight, and idle time. East/west routes experience almost continual crosswinds and headwinds. Less fuel can be used on north/south routes where poten- tial parts of the trip are not only warmer but see less wind resistance. Rough roads, rain, and snow increase the vehi- cle rolling resistance and contribute to lower fuel economy. On a cool rainy day, observe the steam (water vapor) sur- rounding the tires shortly after coming to a stop. Tires act as a pump to displace water from the roadbed. Pumping water requires horsepower and the energy turns to heat contributing to water evaporation and lower fuel economy.
Environmental and sea- sonal weather changes may require as much as
1 to 1.5 mpg difference
in fuel consumption depending on the season or weather conditions. Cli- mate affects performance and fuel economy because cold air is denser than warm air and increases the aerodynamic drag on the coach. Compared to 70°F ambient temperature,
the increased drag caused by denser air at 50°F ambi-
ent temperature represents a 5% (0.3 MPG) fuel mileage penalty. At 30°F, the penalty increases to 9% (0.6 MPG). Summer blend #2 diesel fuel (API 35 gravity) has a higher BTU (higher heat value) content than #1 winter blend (API 38) and contributes to better fuel economy. Depending
on the geographic location, winter blend fuel can make its appearance during the latter part of August. Winter blend is responsible for a 2.5% penalty (0.15 MPG).
Accessories
The alternator, air compressor, cooling fan, power steer- ing, and optional use of the generator and/or the dash air conditioner while driving all impact fuel economy. Categorically, although largely fixed, engine accessory and drivetrain losses can significantly contribute to total vehicle power requirements.
14 12 10
8 6 4 2 0
0 5 10 15 20 % Aerodynamic Aid
Interstate Intercity Rural Delivery Urban Delivery
continued on next page
% MPG Gain