Fuel Economy Basics From Caterpillar® Field Service Coordinator Jim Booth Jr. and Powertrain Technical Steward Brian Coe
With energy prices continuing to rise and no end in sight, fuel is without a doubt the largest operating expense for trucking companies today. It's vital that truck owners have a basic understanding of the many variables that affect large truck fuel economy.
Factors with the most impact on fuel usage include aerodynamics, gearing, tires, load, speed, climate, idle time and driving techniques, according to Brian Coe, powertrain technical steward, Caterpillar® Global On-Highway.
"Good fuel economy isn't a mystery-it's the result of a combination of variables that, if managed effectively, can deliver the best fuel mileage possible for that particular truck," Coe said.
Aerodynamics
Aerodynamics of any vehicle is a major contributor to fuel economy performance, especially when it comes to larger vehicles. Many exhaustive tests have been run over the years to confirm the effects of certain truck- and trailer-mounted aerodynamic devices. Industry studies testing the differences in fuel economy advantages or disadvantages between all types of semi-truck components have found that the most aerodynamic tractor can have as much as a 15 percent fuel efficiency advantage over the least aerodynamic vehicle.
The whole objective in improving aerodynamics is to increase the ability to move the tractor and trailer through the air while causing the least amount of air disturbance, and therefore improving fuel efficiency. When pulling high cube trailers such as dry vans and refrigerated trailers, the smooth-sided trailers cause the least amount of air turbulence. Aerodynamic tractors with smooth-flowing roof fairings and smooth-sided cabs can push the air around the nose of the trailer. The more traditionally shaped tractor, with its squared-off edges and externally mounted air cleaners and exhaust systems, causes a greater amount of air turbulence that negatively impacts the benefits of the smooth-sided trailers, in both fuel efficiency and overall engine performance.
When pulling trailers that cause a greater amount of air disturbance, such as livestock trailers or car haulers, the aerodynamic effects of the tractor are much less noticeable. In these applications, the extreme air disturbances caused by the larger trailer will have a much greater impact on the aerodynamic drag of the total package than that of the smaller tractor.
Something else to consider when pulling high cube dry vans or refrigerated trailers is the tractor/trailer gap. Decreasing a trailer gap from 65 inches to 25 inches can improve fuel economy by as much as 5 percent.
Gearing
The gearing of a large truck includes not only the rear-end axle ratio and the transmission type, but also the drive tire size as well. Choosing the proper gearing of a large truck can be a daunting task since gearing can affect both the performance and fuel efficiency of the vehicle. Selecting the proper gearing for a truck is about finding that balance between fuel efficiency and performance that works best for each individual application. There is no such thing as a "one spec fits all." It boils down to personal preference for your particular application.
A truck spec'ed for optimum fuel economy will typically be geared to run at a very low engine rpm while at cruise speed, the so-called "Gear Fast Run Slow" concept. Driven correctly, trucks with drivetrains set up for GFRS will deliver good fuel economy and good performance, as long as they are not geared too fast and running at too low of an rpm at cruise speed. A truck geared more for performance will be geared to run the engine at a higher engine rpm at cruise speed than the GFRS setup. Talk to your Caterpillar dealer representative for more information about the correct gearing solution for your particular application. Additional information can be obtained by visiting our Web site at
http://ohe.cat.com/drivingtips.
Tires
Tire manufacturers basically sell three different types of drive tire tread design. Deep lug tires deliver the best traction and long life but have high rolling resistance. Shallow lug tires have less tread depth to decrease the rolling resistance. Rib tires lack good traction but provide the least amount of rolling resistance, maximizing fuel economy.
All tires are least fuel efficient when new. As the tires wear, the rolling resistance decreases and fuel economy improves. Tests performed by leading tire manufacturers have shown that once a tire is 50 percent worn, you will see as much as a 6 percent increase in fuel economy.
Another choice to make when choosing a tire is whether to go with low profile or tall tires. Tall tires have a little more rolling resistance than the low profile tires because of the increased flex of the taller sidewall. Low profile tires also have a weight advantage over the taller tire, but a shorter life expectancy because of the increased tire revolutions per mile.
Driving Speed
Horsepower demand increases substantially as vehicle speed increases. It takes a lot of power to push a large vehicle through the air. A good rule of thumb to remember is that fuel economy will change about one-tenth of a mile per gallon for every one mile per hour over 55 mph road speed change. In other words, decreasing overall road speed from 70 mph to 65 mph can improve fuel mileage by around half a mile per gallon. Actual fuel mileage increases will depend on the aerodynamics and weight of the vehicle. As speed increases, truck/trailer combinations with poor aerodynamics will experience greater fuel economy losses than those with better aerodynamics.
Climate/Fuel
Cold weather will cost you fuel economy. The difference between 70-degree weather and 25-degree weather is a 13 percent fuel economy loss. This is caused by the thicker, denser cold air, as well as cold weather fuels that have lower energy content. Another contributor to cold weather fuel mileage penalties is thicker lubricating fluids with higher resistance to flow.
High winds are another cause of fuel mileage decrease and can decrease fuel economy by as much as 13 percent compared to a calm day. Testing has shown that you will see about a 3 percent penalty in fuel efficiency when switching from straight #2 diesel fuel to a winter blend fuel, depending on the blend.
Idle Time
An idling engine can burn somewhere between half a gallon to one and a half gallons per hour, depending on the engine idle rpm and accessories in use, like the air conditioner. Drivers can minimize warm-up times by idling only while doing the pre-trip inspection and filling out the logbook. Once the coolant temperature gauge starts to rise off the peg, it's okay to start moving the truck. Just be easy on the throttle until the engine reaches normal operating temperature. The engine will warm up much quicker and the transmission, wheel bearings and rear end will warm up at the same time.
Driving Techniques
The industry has found that the most significant variable to fuel economy is the driver. According to Jim Booth Jr., field service coordinator, Caterpillar Global On-Highway, the driver controls vehicle speed, shifting techniques, idle time, acceleration, brake usage, trailer gap settings and more. Studies have shown that it is not uncommon for fleets with identically spec'ed trucks to see as much as a 35 percent fuel economy difference between trucks-a difference that is solely due to variances in skills and attitudes of the drivers.
Truck owners can minimize the effects of inefficient driving by setting parameters within the engine ECM that limit the vehicle speed and idle time. Customer programmable parameters can also be set to help control poor shifting techniques. While engine electronics can provide some necessary tools to help improve fuel mileage, the most important tool by far is driver training. No matter how the truck is spec'ed or how the customer parameters are set, the one thing that is consistent in trucks getting good fuel economy is a driver with a good attitude and good understanding of proper driving techniques. For further information drivers can contact their local Cat dealer, call the Caterpillar Truck Engine Call Center at 1-800-447-4986, or visit
http://ohe.cat.com/drivingtips.
For more than 81 years, Caterpillar Inc. has been making progress possible and driving positive and sustainable change on every continent. With 2007 sales and revenues of $44.96 billion, Caterpillar is the world's leading manufacturer of construction and mining equipment, diesel and natural gas engines and industrial gas turbines. The company is a technology leader in construction, transportation, mining, forestry, energy, logistics, financing and electric power generation. Caterpillar is a leading manufacturer and supplier of truck and bus engines in the combined medium and heavy duty categories in North America and a global leader in powertrain products. More information is available at
http://www.cattruckengines.com.
