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Driving To Survive

With diesel cost-per-mile now well over 60 cents, anything you can do to save fuel offers significant savings.

June 2008, TruckingInfo.com - Feature

by Jim Park, Contributing Editor, Equipment Editor - Also by this author

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Who can laugh all the way to the bank when it costs about 65 cents a mile in fuel alone to get there? Depending on how well you manage the stick and the pedals, fuel costs presently stand in the 60-70 cents-per-mile range if you're making somewhere between 6 and 7 mpg. If you're at the top of the heap and you're making 8 mpg, costs are still in the low to mid 50-cents-per-mile range. You don't want to know how much 5 mpg costs. It's enough to curl your hair.

Considering all the factors that affect fuel economy - aerodynamics, tire inflation, terrain, idle reduction strategies, etc. - driver performance remains the most significant. Experts suggest there can a difference of up to 30 percent between the best and the worst drivers in the fleet. Clearly, there's a role for driver training in fuel cost mitigation. Fleets such as Schneider report significant reductions in fuel costs when drivers are properly trained.

Using simulators and experienced driver trainers to emphasize proper driving techniques and to provide immediate feedback, Schneider's director of engineering, Dennis Damman, claims even struggling drivers can achieve a half mile per gallon improvement almost immediately.

"It's all about shifting techniques and throttle management," he says. "We show them how to manage road speed in relation to the engine's sweet spot, and we show them how to accelerate and decelerate less aggressively. That's where the greatest savings are."

Even if you don't have a simulator at your disposal, understanding how today's engines work, understanding the relationship between torque and horsepower, and working to improve speed and throttle management can pay large dividends - virtually immediately. No waiting 'til the next quarter for the results to show up. You'll notice the improvement next time you fuel up.

Throttle Management

Slow down, everybody says, and to a point, they're correct. That will save fuel, but there's a new wrinkle to consider: how the truck is geared. Engine speed in a modern diesel is more of a consideration than it was on the engines we used during the fuel crisis of the early 1970s. Actually, if you slow down too much today, you're in for trouble.

Slowing the truck to a point where the engine is operating well below its so-called sweet spot will affect drivability, performance, and ultimately fuel economy - especially if drivers choose to run a gear down to compensate for the poor performance. Today's torque and horsepower curves are such that engines spec'd to run at a certain rpm will suffer if they run significantly outside that ever-shrinking envelope. Realistically, there's a window of about 8 mph either side of that sweet spot before performance or fuel economy is adversely affected.

If slowing down dramatically isn't an option, there are still gains to be had in improving driving style.

Let's begin with progressive shifting. That's where you upshift at the lowest possible rpm. It's a learned skill that can become a habit.

There's absolutely no good reason why anyone should ever have to take a shift any higher than peak torque in the low end of the gearbox, says Dave McKenna, powertrain marketing manager with Mack Trucks.

"In the old days, we'd say time your shift so that when you pick up the next gear you're at or above peak torque so the engine can pick up the load," he says. "That today is really true in only the top two gears. In the lower gears, you've got the incredible mechanical advantage of the gearing in the transmission and the rear ends. Even if you accelerate up to 1,200 rpm, shift, and drop back to 700 - which is idle - it will still pick up. Next shift may be at 1,300 and back to 800."

How often do you hear a driver revving all the way to pin as they take off from a light? Very smooth drivers barely get the truck moving and they're upshifting.

"Shifting at anything above 1,500 or 1,600 rpm, all you're doing is turning very expensive diesel fuel into noise," McKenna chides.

Getting into the high side of the gearbox demands that you take the revs a little higher, but there's still no need to wrap the needle all the way around the tach. Peak torque on most newer engines is somewhere between 1,100 and 1,400 rpm, and that's where you'll get most of your acceleration. Taking the revs up to 1,600 or so will still let you slip it back into gear above or at peak torque.

If you're lightly loaded, you can get away with even fewer revs. If you're shifting on a downhill grade, you can easily get away with skipping a gear. Upshifting on an uphill grade requires more revs, but again, no more than you'll need to get it back into gear at or above peak torque.

"Any time you can take revs out of the process, do so," McKenna says.

At cruise speed, the idea is to use the least amount of fuel possible to maintain road speed. That means accelerating gently up to speed and flipping cruise control on as soon as you get there. But cruise control isn't perfect. You have one principle advantage over cruise control: you can see; it can't.

Its job is to maintain road speed, and while it might be able to finesse fuel flow better than you can on level ground, it can't tell the difference between a hill and a headwind. All it knows is some external force is conspiring to slow the truck down, and it's going to feed as much fuel to the engine as needs to maintain the set speed.

"Full cruise is designed to maintain road speed, and once you lose ground, cruise is going to power right up to try to make road speed back," says Chuck Blake, Detroit Diesel application engineer. "It's basically balls to the wall."

Blake notes that all engine makers offer some variation on the "fuzzy" cruise theme where there's a gray zone between on and off. That allows for softer acceleration and a bit of roll-out before the engine brake kicks in when the set speed is exceeded. Blake says it's an option worth activating. Engine makers have different names for the parameter, like soft cruise, elastic cruise, upper and lower droop, etc. Ask about it.

"A really good driver can always beat cruise control," says Blake, "but it's tough and it's tiring because you're always working."

With throttle management, says Ed Saxman, drivetrain product marketing manager with Volvo Trucks, fuel consumption can be greatly reduced by leaving a lot more space between traveling vehicles, and by managing hill climbs better.

"Think of the truck as a momentum bank," Saxman says. "The object is to keep as much in the bank as possible, and let the vehicle's momentum, or gravity when appropriate, work for you."

Drivers have a tendency to stay too close to a vehicle they're following. If you pay attention to your throttle, you'll notice that you're on and off it a lot. You get up close, then back out of it, and then throttle up to close the distance, etc. Every time you accelerate to catch up, you're wasting fuel.

"Why not back right out of it and let the other guy get way ahead, and then set your cruise and just motor along," Saxman recommends.

Hill climbing is another area where gains can be made.

"There's obviously not much to be done when climbing a long grade at a full-throttle 45 mph. The best you can do there is to ease up on the pedal as you crest the hill, letting momentum carry you over the top at minimum throttle, and then letting gravity work for you going down the other side," he says.

But in rolling hills, where you're working it up one side and coasting down the other, a bit of throttle or cruise control management will go a long way.

"Let it roll out to the extent you can going down, and coast as far up the other side as possible before getting back on the throttle. As you near the top, back off the throttle. You don't need to come over the top at full speed," Saxman suggests. "You'll gain it all back as you start descending anyway."

If you can see the top of the hill, and you can see what your engine speed is, you can estimate whether or not you'll need to drop another gear before topping the hill. Cruise control can't see the top of the hill. All it wants to do is get back to the set speed, and that acceleration at the top of a hill costs - big time.

Staying with the momentum theme, why power down an exit ramp when you're going to stop at the bottom? Back off the throttle half a mile ahead of time out on the interstate before the exit. You'll save your brakes on the exit ramps, too.

Understanding Torque and Horsepower

To get the best performance from your engine at the lowest cost, it's essential to understand torque and horsepower, and their relationship to each other in terms of how the engine operates.

In technical terms, torque and horsepower are simply ways of expressing the amount and type of work an engine does, but the full technical definitions aren't terribly useful to truck drivers. In less precise but more practical and meaningful terms, horsepower is what keeps you running at high speed, torque is what does the pulling at low speed.

All modern heavy duty diesels achieve their rated horsepower at the midrange of the rpm band (typically somewhere above 1,600 rpm), whereas they produce high torque at relatively low rpm (typically between 1,100 and 1,400 rpm). Fuel consumption is lowest at lower rpm. That alone should tell you the engine is most economical in the lower end of its rpm range. But there's more to it than that.

Over the past decade or so, engine makers have reshaped these torque and horsepower curves, and as a result, modern engines run differently than their predecessors. Previously, the torque curve had a tall peak with gentle slopes on either side of peak - steeper on the low rpm side, shallower on the high side of peak. The horsepower curves used to slope gently up to peak from the lower rpms, and then drop off sharply on the high side of peak.

Today, the curves are flatter, meaning you get higher horsepower across a broader range and lower in the rpm band. High torque is now available across a broader range too, and lower in the operating range. More significantly, the intersections of peak torque and peak horsepower are much closer together in modern engines.

When you're operating at optimum cruise speed, you have additional horsepower above cruise for passing or high-road speed operation. And you have higher torque available just below cruise speed, so as you begin to pull a hill, for example, the engine goes into "pulling" mode without drifting too low in the rpm range.

From the driver's seat, it feels like there's additional power under foot as the torque rolls on while the revs drop in the pull.

So, why do we figure all this stuff is worth knowing? Because you drive modern engines differently than the older ones. And for anyone not familiar with older engines, it's useful to know where your "power" is when deciding when to shift gears.

Here's a completely hypothetical example, but relevant to all today's engines - although the numbers may vary across the brands and models of engines.

Your optimum engine cruise speed is 1,400 rpm, and your truck is geared to run at 65 mph at that speed. You decide to pass another truck, so you put your foot into it and run it up to 70 mph. Your engine speed is now 1,600 rpm. You're on the high side of the fuel consumption curve, and your mileage might have just dropped from 7 mpg at cruise speed to 6 mpg at passing speed.

Here's another situation. You're approaching a hill at cruise speed. Do you speed up to take a run at the hill, or roll into it at cruise speed and let torque go to work for you? The proper procedure is to let the torque work for you. You'll burn more fuel speeding up, and that will deliver no real advantage in terms of getting to the top faster of with fewer gear changes.

As your revs drop from 1,400 at cruise, you get into the peak torque range of the engine where the pulling power is best. You've now got a 300-rpm band where the engine is running at peak torque, but not burning any more fuel. You'll lose a bit of road speed, but the torque will keep pulling. If it's a steep hill and you need to drop a gear, shift well before your torque bottoms out at 1,100 rpm. You'll downshift right back into the top of the peak torque range of 1,400 rpm.

If you shift too early, you'll get back into gear at maybe 1,600 or 1,700 rpm. That's the horsepower side of the power curve, and there's not much pull there. You'll waste fuel and accomplish little until the engine drifts back to 1,400 and below.

As you approach the crest of the hill, you're at 1,200 rpm but the revs are still dropping. Do you drop another gear or just let the engine pull?

It's safe to drop a little below peak torque for brief periods. There's still torque there, just not as much. As you approach the top of a hill, it flattens, so you may be able to keep pulling below peak torque on a shallower grade. Most importantly, you're still on the low side of the fuel consumption curve. Let the revs drift down, and ease over the top by backing off the pedal as the engine speed begins to increase. Upshift as soon as you can, and let gravity help you back up to road speed as you start down the other side of the hill - no throttle applied.

"All modern engines are designed to run a much lower rpm than before. On a Mack MaxiCruise engine, the best operating range is between 1,100 and 1,450. That 350-rpm range is where we want to see the engine operated," says Mack's McKenna. "Most fuel maps are essentially flat today, but once you push beyond 1,600 rpm, in Mack's case, the brake-specific fuel curve becomes nearly a vertical line. Fuel consumption increases exponentially per rpm beyond 1,600 - and that's essentially the same for all engine OEMs."

Detroit Diesel's Blake says, "Use the rpm band that the vehicle was designed for. In other words, if you're geared for 1,500, you're going to keep it in top gear all the way down to 1,150 - at least 1,200 - before you make a downshift."

If the thought of "lugging" an engine down to 1,100 rpm and lower causes your fillings to rattle, take heart. It won't do any harm.

Many drivers, particularly those who learned to drive on mechanically controlled engines, associate low rpm operation with "lugging" and high exhaust temperature, and are inclined to avoid operating the engine at low rpm out of concern for damaging the engine.

Electronic engine controls make it impossible to over-fuel an engine or to lug it down to the point where damage could occur. The electronic control module will only feed the engine as much fuel as it needs to maintain power output, regardless of the throttle position. In the old days, the harder you pressed on the pedal, the more fuel you pumped into the cylinders.

There was some risk back then of burning valves and running too hot. Not anymore.

Operational Factors

As we noted at the beginning, a difference of up to 30 percent in fuel economy can exist between the best and the worst drivers in a fleet. Much of that chasm stems from good or bad pedal and stick management, and the driver's willingness to use the technology at his disposal to his best advantage.

Examining trip reports from your ECM will open a world of improvement potential, for these reports tell a very detailed story on how drivers perform out on the road. ECM reports show items like time in top gear, time in cruise, and idle time - all significant factors in mitigating fuel consumption.

If you're trolling the ECM for data, don't stop at "average road speed" and "fuel consumed." There are factors below those levels that can reveal bad driving habits.

Detroit Diesel's Blake points out that while two drivers may have identical average road speeds, fuel economy can be radically different. A driver with higher cruise time and lower time above 65 mph has much better fuel economy.

"Cruise control has some influence. If you look at the time in cruise, you can determine that the driver using his foot may be really cramming the throttle, and he may be showing more time above 65 than the driver using cruise and spending less time above 65," Blake says.

A cursory read of such a report might not reveal the truth behind the fuel economy spread.

The driver who stays in top gear most of the time, and uses a bit more cruise control, is in general taking advantage of the engine's torque curve. Compare the percentages and you can see how different driving styles can produce a significant difference in fuel economy.

"Even on a two-lane road in fairly flat terrain, you can still keep it in the tall gear if it's running somewhere between 1,100 and 1,200 rpm," Blake says. "Whereas, the poorer driver might drop a gear and run at a higher rpm because it feels better."

Mack's McKenna agrees.

"On secondary roads, there's a tendency to run a gear down to keep the engine in what the driver believes is a more driveable range. For example, where the highway cruise speed was optimized for 70 or 72 mph, running at 55 mph on a secondary road creates some gearing and drivability issues," he notes. "That's a fuel economy killer. We don't want drivers doing 62 mpg at 1,900 rpm in 9th gear."

Idle Time

We saved the best for last. At idle, you're getting zero mpg. 'Nuff said. However, there are legitimate reasons to idle an engine, mostly for driver comfort and safety.

Warm-up and cool-down times today need be no longer than three to five minutes, and that's mostly to stabilize the temperature of the turbocharger and the cylinder heads. McKenna suggests that if you've driven into a truckstop under low power, and fiddled around getting parked, the truck can be shut off almost immediately. Most of the cool-down will have already occurred.

"You don't want to turn off the key right at the top after pulling the Grapevine," he says. "That would cause problems."

It's the same at start-up. In mild or warm weather you can start it and drive away. In the cold, idling a couple of minutes to let the coolant and oil warm and circulate is appropriate.

"Drivers can minimize warm-up times by idling only while doing the pre-trip inspection and filling out the logbook," Caterpillar advises. "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."

An idling engine can burn somewhere between half a gallon and one and a half gallons per hour, depending on the engine idle rpm and accessories in use, such as the air conditioner. Climate control can be a necessary evil in certain climates, and since most EPA '07 engines can now be California Air Resources Board certified for low NOx output at idle, it is legal to idle for comfort in some locations under certain circumstances.

But there's no need to run the engine up to "high idle" anymore, except maybe in extreme cold.

Cummins advises the fuel consumption at idle increases exponentially with engine speed. At worst, idle can have up a 10 percent impact on overall fuel economy. Cummins, along with the others, recommends the lowest possible engine speed at idle - and certainly no more than 800 rpm.

When reviewing ECM reports, what might be considered optimum or "normal" idle times for a highway tractor?

Assuming you're not idling for climate control, 10-13 percent wouldn't be hard to take, but that still means 60-70 minutes out of a 10- or 11-hour day. Obviously, the fewer hours spent idling the better.

If you're sleeping in the truck, 20 to 40 percent might be considered realistic, especially in winter or summer. The advantages of a well-rested driver cannot be overstated, but there are alternatives to using the big diesel for climate control that - today - are becoming more and more cost-effective.

"If you're not sleeping in the vehicle, 25 percent is okay, 10-15 percent is phenomenal," Blake says. "Summer or winter for comfort, maybe 40-45 percent."

The Ultimate Cure - Training

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'd trucks to see as much as a 35 percent fuel economy difference between trucks - a difference due solely 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'd or how the customer parameters are set, says Cat, 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.

WHAT DOES IT COST TO PUT THE PETAL TO THE METAL?

Thinking of fuel costs in terms of miles per gallon is one thing. It's good to know that you're getting 6.5 mpg instead of 6.0, and it's good to strive for 7.0 if you're now getting 6.5. But what's at stake in terms of your cost per mile? What does 6.5 mpg cost you at $4.15 per gallon? How much can you save by pushing your numbers up to 7.0 - in real dollars?


The chart to the right illustrates fuel cost-per-mile at various miles-per-gallon and at various dollars-per-gallon. 



The chart is useful in pegging the fuel portion of your operating costs, and it's helpful in determining if you're getting an adequate fuel surcharge. 



The chart is also useful in determining the increase in your cost of operation over, say, the past year. For example, at the end of April '07, the DOE national average price was $2.81. April '08 closed with diesel at $4.17. At 6.5 mpg, you paid about 43 cents per mile for fuel last April. This April you paid 64 cents per mile - an increase of 21 cents per mile. 



Five years ago, when diesel was $1.30, your cost-per-mile at 6.5 mpg would have been 20 cents. Your fuel costs have more than tripled in five years.



If nothing else, the chart makes the cost of speed abundantly clear.



To determine your fuel cost per mile, cross reference your current fuel economy with the price of diesel. The number at the crossing point is your cost per mile expressed in cents per mile.

THE WINNER BY A FOOT

Kenworth invited Executive Editor Steve Sturgess and me to participate in a fuel economy challenge a few years back. It was a 496-mile round trip between Seattle and The Dalles, Ore., under carefully monitored conditions. We drove the same truck on different days, but Kenworth's test engineers were able to zero-out the trip anomalies using data collected from hundreds of previous runs over exactly the same route. 


I burned 74.898 gallons for a raw mpg of 6.62. Sturgess burned 72.035 gallons for a raw mpg of 6.89. My mileage was adjusted upward to 6.78 to account for the rainy weather we encountered on the final third of the trip - which Kenworth engineers said came at a cost of 0.2 mpg.



The vast majority of the driving was done under cruise control. The only portions of the drive that made any real difference in fuel economy were starting and stopping, and any time when cruise wasn't engaged, like in slower traffic.



The numbers shown in the chart below reveal some interesting differences in the way Sturgess and I managed the trucks. You can see his average speed was slightly higher than mine, as was his maximum speed attained on the trip. His average engine speed was 10 rpm lower than mine, suggesting more time overall in top gear, and his average acceleration and turbo boost numbers were both lower than mine. Those last three factors were key to his victory in the fuel economy challenge.

"Manifold pressure (boost) represents power demand on the engine and aggressive throttle application results in correspondingly high manifold pressure," explains Gary Ziebell, now-retired Kenworth test engineer. "Using less throttle - sacrificing a little acceleration - uses less fuel overall at a given speed, so the driver who is gentler with the throttle pedal is going to get better mileage." 



The "Acceleration, true mean" value refers to the average percentage of throttle used to get the truck up to speed and keep it there. My value was 11.74 percent, compared to Sturgess's 7.29 percent, meaning simply that he was easier on the pedal - he applied less pressure with his foot to accelerate, or, he was a little less aggressive than I was on the throttle.



By comparison, Jim Booth, a former "test driver" with Caterpillar who still runs a fleet of trucks in revenue service hauling for Cat, regularly records acceleration numbers in 3.0 to 4.0 range on his data recorder. Believe it or not, Booth manages better than 9 mpg in a well-broken-in T2000.



Most of the data Kenworth collects during these fuel economy test runs is available from your ECM, by the way. It may cost you a bit to have it downloaded and printed, but it would be worth a look if you're serious about adjusting driving habits to improve fuel economy. Slowing down helps a lot, but so does going easier on the pedal.

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