Traditionally, we have heard that users of vocational trucks are more concerned about performance than fuel economy. But with the volatile price of diesel fuel, anything you can do to reduce fuel consumption is worth a second look.
There are several variables you can control when it comes to fuel efficiency. According to Kelly Como, on-highway segment manager, Sterling Truck Corp., the three main areas of focus include vehicle specs, driver education and maintenance.
Spec it right
Engine size, gearing and weight are just a few factors that impact fuel consumption.
Engine size ranks among the most important factors. "A good match between the engine and the payload is significant," says Steve Matsil, global vehicle chief engineer, General Motors.
If the truck engine is too large for the application, you add unnecessary weight and upfront cost, plus you can burn more fuel while under-working the engine. Drivers are also more prone to use the excess horsepower, resulting in higher fuel consumption.
If the engine is undersized, it will be overworked, leading to increased engine wear and poor fuel economy. "If you are constantly in an underpower condition - meaning you are putting out the maximum horsepower of the engine to accelerate to cruising speed to move into traffic - you are always in a two-pedal position either off or on the throttle," says Matsil. "I suspect that you will not get the optimum fuel economy."
If you have adequate horsepower but misuse it by accelerating harder than you need to, he adds, you also lose fuel economy.
For best results, match the power curve of a particular engine to your application. You need to consider the gradeability and startability.
"The gear ratio is another significant point," says Matsil. "If you can get away with a numerically lower gear ratio, rear axle ratio - which means your accelerations will not be as brisk - that will help with fuel economy." The trade-offs would be startability and gradeability.
Weight is another issue. A light truck makes it possible to haul a larger payload. But remember that many trucks, such as dump trucks, spend half of their life travelling around empty.
In addition, consider aerodynamic efficiency if your trucks spend much time travelling down the road. "As you improve the coefficient of drag - the aerodynamic efficiency - you squeeze more miles out of the same gallon of fuel," says Matsil.
Tire inflation is closely linked to fuel economy. "Don't over or under inflate," Matsil emphasizes. "Inflate to the manufacturer's recommended pressure."
The vast majority of tire pressures outside the normal range are below recommendations. This is because tires naturally lose pressure over time - air permeates through the rubber. "The air molecules escape between the bead of the tire and the mounting surface of the wheel," says Matsil.
As tires deflate, rolling resistance increases. "As you start to lose tire pressure, your sidewalls start to sag," Matsil explains. There is more rubber on the road. "You have more friction, so the rolling resistance increases. Consequently, the amount of energy required to move the vehicle down the road increases."
Como agrees, noting, "Tires that are under-inflated increase rolling resistance and make the engine work harder and burn more fuel. Tire pressure needs to be accurately gauged at least once a week."
However, when you measure tire pressure is critical. "When you inflate the tires, inflate them cold, meaning first thing before you start work in the morning or after the vehicle has been sitting for a while, because that is where you get the best, most accurate, readings," says Matsil. "The pressure changes 1 psi for every 10° F change in temperature."
The affects of proper tire inflation and wheel alignment on fuel economy can't be overstressed. "Tire pressure and axle alignment could account for 3% to 4%," says Como.
Another consideration is the use of radial vs. bias ply tires. "Radial ply tires should get you some incremental improvement vs. bias ply," says Matsil. That is because they have less rolling resistance.