By Paul Abelson
senior technical editor
This past summer, diesel peaked at more than $3 per gallon. It's now hovering around $2.50, more than $1 more than it was two years ago.
Not too long ago, 100,000 miles worth of fuel cost less than $25,000.
At today's prices, it will cost an operator of a 6-mpg truck more than $41,500.
If prices for the new ultra-low-sulfur diesel top $3, as many predict they will, expect to pay right at $50,000 or more a year.
And if the "bad guys" disrupt the supply of oil from the Persian Gulf, $65,000 to $70,000 isn't beyond reason.
Some of this huge increase can be made up with higher rates and fuel surcharges - if you negotiate them. Regardless whether you make it up on the revenue side, you can still cut costs by reducing fuel used in your operations.
Before we look at a few methods and techniques that have worked for others, let's understand some basics about fuel consumption.
Unless you plug into a source of electricity when you stop, diesel is the sole source of energy for your truck. The electricity cooling the food in your refrigerator is in your batteries until needed, but it's created when your engine burns fuel and turns your alternator.
Drivers have told me that's "free energy" because the engine is running anyway. It may be running, but it needs more fuel to handle any added load. The same is true for tire flexing, air turbulence and anything else that increases the amount of work your truck does. Reduce the load and you'll reduce fuel use.
Most experts say that the first step to lowering fuel use is lowering speed. Drivers tell me that you have to go fast to get the miles. That may be true for company drivers, but if it's your truck, and you pay for fuel and maintenance, you might want to reconsider.
The most successful owner-operators I know, several of whom net hefty incomes, plan their routes carefully and never drive any faster than necessary. Many cruise at or below posted limits, especially out West.
I'm not saying you should limit yourself to 55 mph. As Sammy Hagar put it a generation ago, "I Can't Drive 55." But the difference between driving 70 and slowing to 60 is startling.
One of the elements used to calculate power demand is velocity squared. At speeds above 55, the point when aerodynamics start to be significant, going 17 percent faster - which means you're running 60 to 70 - increases aerodynamics resistance 36 percent. That will cost 12 to 18 percent more fuel per mile, depending on other factors.
Put simply, if you're getting 6 mpg at 60 mph, you'll get only 5.2 to 4.9 mpg at 70 mph. That's considerable.
Acceleration consumes fuel. There is a complex formula to compute the power required to accelerate from any initial velocity to any final velocity. The major variable is time. The more gradually you accelerate, the less power you need, and therefore, the less fuel is required.
Once you've expended the energy to get to your cruise speed, don't waste it by having your brakes convert it to heat. Coast to your exits. Use brakes as little as possible.
If you can coast down for a half-mile before braking, you may save a tenth of a gallon each time you do. A tenth doesn't sound like much, but if you stop for breaks, fuel and meals five times a day, that's three gallons a week, more than 140 gallons a year. At $2.50 per gallon, that can put $350 or more in your pocket each year.
Fuel saving is a game of small numbers. They add up to make significant amounts. Take a tenth of a gallon here, a twentieth there, saved over and over again. That's how the pros achieve fuel economy that most of us find incredible.
Driving isn't the only way to save fuel. There are devices available that can pay for themselves in short order through fuel savings, and then put money back in your pocket with every use.
Idle reduction equipment heads the list, because the payback is comparatively large. There is quite a bit of technology to help you reduce idling while staying comfortable, ranging from wiring your cab for shore power to installing a fully optioned auxiliary power unit.
The important thing is to use what you have so you're not idling your engine for prolonged periods. I've seen, and you probably have too, trucks idling while parked in IdleAire slots. It's not only foolish; it's discourteous to fellow drivers.
Idling kills mileage. It also shortens maintenance intervals and engine life. As a trucker, you know that burning fuel going nowhere drops a 6.5-mpg truck to under 5.9.
Free up airflow
One of the first things hot rodders and automotive engineers do to improve performance is free-up the exhaust. Soon, it may not be possible, when diesel particulate filters come on engines for 2007 and beyond.
But for current engines, there are replacement mufflers with vanes that induce spiral airflow. Reportedly, they improve fuel economy. More than 7 percent has been claimed in over-the-road operations, but the claims are backed-up by dynamometer tests run at engine distributor locations.
I'd prefer seeing the tests from the Society of Automotive Engineers and/or Technology & Maintenance Council before naming companies, but dyno tests are often good indicators.
As truck speed increases, aerodynamics are increasingly important in fuel consumption.
Aerodynamic resistance is a function of frontal area, coefficient of drag and velocity squared. That's why aerodynamic resistance increases faster than speed increases. But we can affect the other two elements independently of speed.
Frontal area is normally a constant, but it can be affected by the setup of the trucks. I see trucks stretched out, with the fifth wheel as far back as it can go and a huge gap between tractor and trailer. When diesel was 25 cents per gallon and tractors had stiff springs, that would get you a smoother ride. Today, it gets you another 80 square feet of frontal area as air fills in behind the cab and has to be displaced a second time.
Keep the cab and trailer less than 36 inches apart and air will flow from tractor to trailer. You'll save fuel.
Coefficient of drag refers to a vehicle's shape. Aerodynamic shapes save fuel because they let the truck pass more easily through air, reducing the number of times it has to change direction. The more abruptly air must move around it, the more the coefficient of drag will be.
A perfect teardrop shape with wheels could have a coefficient of drag of 0.15 to 0.20. A classic three-box tractor with engine, cab and sleeper could have a coefficient of drag as high as 0.7 or more.
A trailer by itself - stretched out too far - could have its own coefficient of drag of 1.0 added to the tractor's coefficient of drag.
Together, a closely situated tractor and trailer assist each other like NASCAR racers drafting, for a net combination coefficient of drag as low as 0.6. A stretched out combination could have a 1.6 coefficient of drag.
Add-ons to trucks create turbulence that increases coefficient of drag. That's why new trucks have rounded aerodynamic mirrors, mounted on single bars instead of flat ones with multi-tube supports.
It's also why the technical societies are developing evidence about using tiny cameras to replace external mirrors on trucks. At a TMC meeting, it was projected that they could save close to 1.2 mpg at highway speeds.
Try to match tractor height to load height. High-rise sleepers or those with high air deflectors are great when pulling full-height vans and reefers, but lousy with tankers and platform loads. Mid-rise sleepers reduce frontal area.
Air flow-smoothing devices, such as NoseCone, gap fairings and belly fairings help manage airflow and improve mpg. Freight Wing ran SAE/TMC Type II tests in 2004. Results were impressive. Belly fairings alone reduced fuel use 3.87 percent for the test configuration.
With the front fairing added, savings went to 5.99 percent. Adding rear fairings that fool some of the air into flowing as if there were a long, streamlined tail instead of a blunt rear, increased savings to 6.94 percent with all devices in place.
Results will vary somewhat with various tractor-trailer configurations, but there will be savings.
Tire selection makes a major contribution to fuel economy. Variations in tread design, depth and width, and in rubber compound and casing structures all affect rolling resistance, which is another term for energy absorption.
Other considerations - traction, off-road operations, terrain, etc. - may require less fuel-efficient tires, but for on-highway operations, ask your supplier for the most fuel-efficient available.
Keep tires inflated to design pressure, not the maximum on the tire. Air is a structural part of any tire. Too little, and the tire will flex and heat up from internal friction. Heat is wasted energy released to the air.
Automatic tire inflation systems from Airgo, ArvinMeritor and Dana-Roadranger keep trailer tires properly inflated at all times. Their usual payback has proven to be less than two years, not even counting improved fuel mileage.
Check tires regularly. Caught early, uneven wear can be corrected. Misaligned tires not only wear out prematurely, they absorb energy when scrubbed across pavement.
For a low-profile tire turning 500 revolutions per mile, just 1/16 -inch misalignment could cause 2.6 feet of side scrub per mile. Travel 500 miles a day and your tires drag sideways more than 1,300 feet each day.
Tire makers calculate that misaligned steer, drive and trailer tires can cost up to 5 percent fuel economy.
Synthetic oils save money. Several years ago, SAE/TMC tests demonstrated that by using synthetics in the engine, transmission and axles, fuel consumption dropped by about 3 percent. Amsoil compared conventional lubes against all synthetics and replaced regular 15W-40 engine oil with their 5W-30. SAE/TMC tests showed an 8.2 percent improvement in mpg.
The greatest variable, the main determining factor of whether fuel economy will be good or bad, is the driver. TMC fleets operating identical conditions report as much as a 35 percent variation in mpg between drivers.
There are very few drivers who cannot improve their techniques to become more fuel-efficient.
Here are some tips gathered from the experts:
- Learn your engine's characteristics - engines differ by make and model as to their optimum cruise speed and shift points;
- Use cruise control whenever safe and practical;
- Accelerate and brake gently;
- Keep the engine brake off unless safety demands it. You can't coast down with an engine brake on;
- Keep your truck well maintained; and
- Spec your truck for maximum mpg - after spec'ing for maximum revenue.
Use the data in your engine management computer. There's an old adage that says, "If you want to change something, you have to measure it." The converse is, "If you don't know where you are, you won't get where you want to be."
Learn what performance you are getting under various operating conditions. Keep a diary. Check your progress against similar runs.
If you've fallen short, review what was different that day. If you've improved, ask yourself what you did better. Correct what needs correcting, and reinforce what's good.
The process is part of behavior modification, a useful tool to improve driving skills and fuel economy.
Paul Abelson may be reached at firstname.lastname@example.org.