Over the years, a substantial amount of literature has been generated in the area of fuel economy.
Several white papers from reputable manufacturers of engines, tires and drivetrain components have populated the landscape. However, they primarily target the long-haul and highway truck markets. Bus applications have not enjoyed the level of attention from original equipment component suppliers, and this has created some misleading information.
Fuel economy is a vital factor in the overall operations of bus fleets nationwide, and it continues to be a growing concern for today’s bus operators.
As school bus operations are unique and do not exactly mirror most truck applications, this article provides information specifically tailored to bus operations.
Fuel economy testing
Fuel economy testing is more complicated than many people realize. Accurate testing is needed to obtain accurate data.
While informal testing is acceptable, it is not possible to obtain consistent, accurate fuel usage from the engine control module, and informal testing can be highly influenced by driver behavior, climatic conditions and route.
The best way to capture reliable fuel economy data is to use standardized industry test procedures, such as the Society of Automotive Engineers fuel economy test.
There are many factors beyond the engine that can increase or decrease fuel economy, and it is necessary to understand how they impact a bus operation.
Fuel economy factors
Driver Behavior — A bus driver’s behavior is the first factor to consider when determining how to impact the fuel economy of a bus. The bus driver can have as much as a 30-percent impact on the fuel economy based on his or her driving techniques.
Techniques like avoiding sudden accelerations and excessive idling can have a positive impact on each vehicle’s fuel economy. For example, since idling yields 0.0 miles per gallon (mpg), every hour of idling reduces fuel economy by 1 percent.
An easy way to decrease idling is to include a vehicle idle shutdown timer in a bus’ specifications. Proper driver training is essential and can ensure that the fleet will have consistent driver behavior regardless of the route or the vehicle.
Aerodynamics — When we talk about fuel economy, a common subject that comes to mind is aerodynamics. Although it can be a serious factor to consider for highway applications, most school bus routes have low average speeds. This means that aerodynamic performance is not a major contributing factor in fuel economy for school buses, except for in certain types of service, like activity trips.
Tires — One major item to consider is tires. Tires can contribute to fuel economy in two ways, and each one has a tradeoff.
A tire that is designed for high-mileage life (like the ones used in buses) will not yield the best fuel economy because of its deep tread.
However, a tire that is classified as a fuel-efficient tire (like the ones used in long-haul applications) will likely experience reduced tread life because of the shallow tread of the tire.
In school bus applications, these fuel-efficient tires are not the appropriate choice, since bus tires require a different specification due to the scrub loads and constant stop and go required to pick up and drop off passengers.
Bus tire selection should not be based on fuel economy, as fuel-efficient tires are designed for long-haul truck applications. The normal bus duty cycle is more severe, and it has higher scrub loads that could tear the fuel-efficient tire apart. It is wiser to select a bus tire that will provide longer life and keep tire replacement costs down.
Other tire tips:
Tire inflation is also important, as under-inflated tires can reduce fuel economy by at least 1 percent, and over-inflated tires reduce the life of the tire.
Broken-in tires provide 7-percent better fuel economy than new tires.
Ribbed drive axle tires provide 2- to 4-percent better fuel economy than lugged tires.
Tires make the biggest difference in mpg under 50 mph.
Lubricants — Another factor to improve fuel economy is the use of synthetic lubricant in the axle and transmission.
Synthetic axle lubricant will not only extend the drain interval — it will also improve fuel economy by around 1 to 1.5 percent.
Engine oils can also affect fuel consumption, an independently-documented, measurable fuel economy benefit relative to conventional mineral-based multigrade diesel engine oils. Because synthetics flow better at lower temperatures than mineral oil, synthetic engine oil is more fuel efficient at a lower ambient temperature.
The cost of synthetics is also a factor to consider, as the cost of mineral oil is considerably less than the cost of synthetic oil.
It is also appropriate to mention that exceeding the fill level of the crankcase will create churning, and that translates to a 2-percent fuel efficiency decrease.
Axle alignment — Tire manufacturers report that fuel economy is adversely affected by improper axle alignment. When the axle is not properly aligned, the vehicle drag increases and tires also experience excessive scrub loads against the road surface. This condition will affect fuel economy by at least 0.6 percent and will significantly reduce tire life.
Specs for improved fuel economy
We have shown several factors that influence fuel economy, but while having a properly maintained vehicle will prevent fuel efficiency degradation, there are other steps that bus fleets can take to improve fuel economy.
Fuel efficiency improvement is highly dependent on proper component specifications, budgeting and technology.
Bus fleets have access to expert advice from their local dealer. Dealers will review performance needs of each customer to help specify the right powertrain components.
Engine — Selecting a fuel-efficient engine is the first step. New engines have double-digit percentage improvement in fuel savings over pre-2007 emission compliant engines, and this alone is a tangible savings in fuel costs.
Automatic transmission — The use of the proper features in the automatic transmission also makes a difference. Using the fuel economy mode on the transmission can save between 3 and 5 percent. This feature is usually available with the push of a button.
Other features such as shift energy management (available in popular automatic transmissions) or transmission retarders can benefit the bus operation by increasing fuel efficiency and cutting operation cost.
Rear axle ratio — The introduction of the high pressure common rail fuel system and shift energy management transmission calibrations combined with increased horsepower and peak torque values yield significant improvements in fuel economy.
It is critical to note that the improved torque delivery of today’s advanced powertrains allows for a lower numeric rear axle ratio to be specified, which maintains vehicle response while improving fuel economy.
Customers who spec out a bus with the approach of ‘This is what we always order’ will miss an important means of saving fuel.
Hybrid electric systems — If a bus fleet wants to use technology to its advantage, a great solution is a hybrid electric vehicle (HEV).
Not only will an HEV provide substantial fuel savings over a similarly specified non-hybrid bus — it will also increase the life of other components, such as brakes and tires.
The anticipated fuel-efficiency improvement ranges from 20 percent to more than 70 percent, depending on the selected technology (non-plug in or plug in), the route and driver behavior.
Since HEV systems use regenerative braking, the brake pads are used less often, with resulting benefits for both tires and brakes.
Ramses Banda is marketing manager of bus product platforms for IC Bus.