Article

Spec Economy

Spec a truck right, and you'll get top fuel economy. Do it wrong, and you could be doomed.

June 2008, TruckingInfo.com - Feature

by Steve Sturgess, Executive Editor

SHARING TOOLS        | Print Subscribe

The basis spec of a truck or tractor-trailer should determine its lifetime fuel economy. Spec it right, and you'll get top fuel mileage. Get it wrong and the truck is doomed.

The good news is, it's all fairly basic. It simply means looking at the things that negatively impact overall fuel economy and then selecting components that minimize their impact.

The major contributors to fuel consumption are rolling resistance and air drag. Weight is also a factor, and that may or may not be under your control. A lesser contributor is friction within the vehicle.

ADVERTISEMENT

Gearing needs to be optimized to allow the engine to run at its most efficient speed, and that is often where things go wrong, so let's start there.

GEAR RATIO AND ENGINES

The gear ratio to look for is the overall gear ratio, compounded from transmission top ratio - i.e. direct or overdrive - and rear axle ratio. But don't forget the size of wheel and tire equipment also affects gear ratio. That is something of a moving target, as tire wear over the life of the vehicle will vary engine rpm at any given speed. The effect is small, but some fleet executives dial in tire revs per mile several times over the tire wear mileage so the fuel economy that is displayed through the engine electronics more nearly reflects the true tank mileage. That is good for fuel bonus programs, but it also prevents mistakes calling out tractors or drivers that seem to underperform.

So what sort of gearing should you look for? That depends on a number of factors, not least of which is where and how the truck will operate. This information is best shared at the time you're discussing the order, so the computer-based sales tools can reach the optimum balance between performance and fuel expense. These tools will also make simulated runs over routes you specify so you can get a good idea of your likely fuel economy.

There's no easy rule of thumb if you don't want to go this route. Different engines have different performance curves. Ideally, you want to run as slow as you can, keeping at the optimum point of the specific fuel curve. At all costs, you want to keep the engine out of the higher rpm end of the fuel curve. We engineers used to refer to the sharp kick-up of the curve as the scorpion's tail. In spec'ing it is just as advisable to keep clear of this tail as it is with a real scorpion.

Simulation is advisable to ensure you're not going too tall in the ratios. If gears are too fast, drivers will gear down to regain performance as the engine begins to bog down with reduced speed or small grades. The good news is that the new generation of engines are unbelievably strong at low rpms. The new Detroit Diesel DD15 is a case in point, but so too are the MaxxForce big-bore Internationals. In fact, the Internationals are based on the mechanicals of the MAN D20 and D26, and those engines are designed to be lugged down to 800 rpm, so they will not be damaged by drivers who may be more used to downshifting at 1,100-1,200 rpm.

So here's a cautionary tip: If you're going to taller gears and lugging the engines, make sure drivers know what's expected of them. And understand the new technology. There's a lot of coffee-shop wisdom out there that is unbelievably wide of the mark.

These new engines - and the Volvo and Mack diesels, which also have a European heritage - are designed to lug down and just hang in there grunting along. Similarly, the 12-liter Paccar MX engine, when it comes on stream late 2009, will behave much the same way if the European MX is any indication.

As far as North American engines are concerned, gearing fast to run slow is always better, too. The big Caterpillars really don't do well for fuel over 1,400 rpm and are quite demanding in keeping between 1,100 and 1,400 rpm as much as possible. The current ISX seems to be a lot more forgiving, but it has plenty of grunt down at 1,000 rpm and is best kept in the lower registers for best economy.

New fuel issues have surfaced with newer lower emissions engines: fan-on times and active regeneration of the diesel particulate filter. The huge grille opening of Freightliner's new Cascadia is to address the fan issue, since a big fan to cool the added heat rejection of the cooled-EGR engines may draw up to 50 horsepower that ultimately comes from the fuel tank and does no useful work getting the truck down the road.

The active regeneration issue that was new for 2007 is really a function of duty cycle. If your operation sees the exhaust getting plenty of heat during the driving day, diesel particulate filter regeneration may not be an issue. However, if the DPF frequently needs a goose, then the fuel used to get it up to temperature also will have to be paid for. It is worth doing some research if you have several alternative power options for the trucks you are buying. One thing is for sure, you want to make sure the engine electronics are set up to regenerate the DPF based on pressure drop across the device, not based on time. Active regens use a fair amount of fuel, and you want the absolute minimum number you can get away with.

And it's advisable to police what drivers are doing. If they can initiate an active regen, they may do so in the mistaken belief that performance will be enhanced with a clean exhaust filter.

This also applies to drivers manually switching on the cooling fan, instead of relying on the engine ECM. Unless there's something seriously wrong with sensors, the automatic operation of the fan is always preferred. It may be you can take control of both of these away from the driver by eliminating the switches in the spec.

A further emissions-related fuel issue comes with 2010. Do you go heavy EGR as advocated by Cummins and International in over-the-road applications, or do you follow the European model and opt for SCR? We have explored this discussion in Heavy Duty Trucking's pages before, but it bears repeating: the likelihood is that EGR will be the less expensive technology installed on the vehicle and the most driver-friendly. Selective catalytic reduction will require drivers to top off the diesel exhaust fluid (commonly called urea) tank on a semi-regular basis and the equipment on the chassis will cost more initially. But since the NOx reduction is an aftertreament, the engines can be optimized for performance and fuel economy. The difference is paid for over the lifetime of the truck in better fuel economy for the SCR route.

This is a tough one. It requires a very thorough understanding of the different costs involved, the annual mileages, and the willingness of drivers to follow instructions. The issue of the SCR fluid availability is the least of the problems. European experience shows availability shouldn't be a problem, but the other issues are the imponderables.

AERODYNAMICS

The difference between a bluff-fronted conventional like the traditional premiums and an aero model tractor is about 15 percent - or a mile per gallon. That is huge and simply can't be tolerated anymore by over-the-road fleets. Many that have used traditional conventionals as driver recruitment and retention tools cannot bear the ongoing fuel costs encountered with this configuration. With 1 percent equating to around $700 per year per truck, a 15 percent loss in economy is $11,000 - way more than the cost of losing a driver through turnover.

Other applications, though, don't feel the impact in the same way. Car carriers, bull haulers, machinery and heavy haulers all struggle with loads and equipment that is as aerodynamic as a brick and are impacted far less by the shape of a traditional conventional truck than by what attaches to it.

One potential newcomer claims to be a game changer, though. The International LoneStar is said by the company to combine the sort of rugged individuality that the big conventionals bring to the table while delivering the aerodynamic efficiency of the best competitive aero trucks. Recently, International compared its ProStar and LoneStar to several competitive truck models in the National Research Council wind tunnel in Ottawa, Canada. The results appear to bear out the company's contention that LoneStar is as good as competitive aero trucks. The LoneStar did, though, give away about 7 percent in aerodynamics to the company's own ProStar, or around 3 percent in fuel economy.

Freightliner has the advantage of its own wind tunnel, a smaller, more compact tunnel than NRC's but very sophisticated nevertheless. Having it on the doorstep outside the design and test center in Portland was a big help in developing the Cascadia, with much fine tuning completed in hundreds of wind-tunnel hours. Cascadia is undoubtedly a fuel-efficient truck with thoughtful design as evidenced by its big grille as mentioned earlier.

Cascadia has its unique deliverables, too, not least of which is a cab that is arguably the roomiest in the industry. Such a wide cab has to shoulder the air aside but nevertheless, Freightliner still insists it has the most fuel-efficient truck around.

There's no question all these offer today's most fuel-efficient designs, backed by each maker's wind tunnel tests. But according to fuel economy guru Gary Ziebell, recently retired from Kenworth, wind tunnels are great for squeezing the best shape out of a stylist's designs - and Kenworth refines its computational fluid dynamics models in wind tunnels. But it's real-world SAE type 4 testing that gives accurate fuel economy numbers. And guess what: Kenworth's T2000 has proven the best Ziebell says he's ever seen - running Kenworth and competitive trucks from Seattle to The Dalles, Ore., and back, or across I-40 from Kingman to Flagstaff.

Volvo's VN is acknowledged to be a fine design, developed with the resources of one of the world's biggest truck makers.

And so it goes. The important thing is to not take what the truck manufacturers have spent hundreds of hours and hundreds of thousands of dollars developing and refining and ruining it with improperly spec'ed components that undo the good work.

In fact, in today's $4-a gallon diesel environment, it's essential to take advantage of every aero option a manufacturer offers. Peterbilt has such a kit for the 389, which goes a long way to offset potential penalties of the big conventional, even one with the wind-cheating tricks applied to the new model over the older 379.

Never mind that skirts may make it more difficult to get under the truck, for instance, the savings are well worth having. When the Cascadia was launched, it was done so without a sunvisor. One is to be made available, but the engineers say the truck has better economy without it. You just have to get used to the idea.

Even with the best, most fuel-efficient truck, you can mess it up with add-ons like bug deflectors, aftermarket sunvisors, removing side skirts, or crunching side extenders. And those tripod mirrors, what can I say?

You'll find more detailed advice on trailer efficiency elsewhere in this issue, but the interaction between tractor and trailer is a major cause of poor fuel efficiency. North American truckers are doomed to a gap between the two units. It is much bigger than optimum because the kingpin setting at only 36 inches demands swing clearance for the trailer corners. In Europe - where fuel has historically been at least double the cost of North American diesel - a deeper 68-inch pin setting has been the norm, allowing for a much tighter tractor-trailer gap. So much so that the big cabovers feature cab side extenders hinged to allow the driver to get up behind the cab to attach air and electrical couplings. For the best overall performance, North American fleets should ensure that trailers are pulled up as closely as operations allow. Considerations include front axle loadings, axle group loadings and Bridge Law spreads. In California, for instance, the kingpin-to-axle maximum dimension has to be observed with 53-foot trailers.

Most of this optimization must be done at the time the tractor is spec'd. Wheelbase and fifth wheel position are critical and should be determined through the spec'ing software at the time the truck is ordered. An optimized wheelbase not only minimizes the effect of drag on the trailer headboard, it also can improve the overall maneuverability of the outfit and improve the tractor ride.

With optimization, the tractor fifth wheel can be fixed and even lowered. This lowers the trailer profile a smidgen, but it also saves weight that can be turned into revenue payload or fuel savings.

Optimization is key. For instance, speaking at the release of the latest wind-tunnel test results, International's Assistant General Manager Steve Gilligan announced the Maxx Mileage spec packages that International has generated ensure end users don't throw away potential savings through spec errors.

Each takes a hard look at overall performance, with gearing recommendation to keep engines cruising at their most efficient speed. There are also low rolling resistance tires and synthetic lubes rolled in, and to sweeten the offering, International is discounting the invoice by $1,000 for customers willing to take advantage of the packages.

These are in addition to the SmartWay specs that are offered by the truck manufacturers so customers can avail themselves of the benefits of joining the SmartWay partnership.

SmartWay is an Environmental Protection Agency program that is designed to pull improved fuel efficiency and lower emissions into the trucking industry. Tractors, trailers and components that have SmartWay certification or approval not only potentially improve the fleet average fuel consumption, but also, the SmartWay logo on carriers' equipment and letterhead permits them access to shippers who, through their partnership with EPA, use only SmartWay fleets.

Just recently, EPA announced a funding mechanism that allows fleets - and owner-operators - to access investment funds to make the SmartWay trade-up. Information is available at www.epa.gov/smartway.

PARASITIC LOSSES

There are a number of fuel-saving components that can minimize fuel consumption or in some way optimize the truck. One of the best is the automated transmission. This will never help the top fuel economy drivers in the fleet. Savvy drivers may even do a better job than the automatic, even toward the end of the day when they are not as sharp. But the automated transmission helps bring the less-polished drivers up to the same level of proficiency as all but the very best in the fleet. Some transmissions, such as the Volvo I-Shift, even have features such as Eco Roll, which kicks the truck out of gear on mild downgrades to save the horsepower consumed in windmilling the engine, even with the fuel cut off.

Other parasitics, such as the air compressor, can be clutched to take away even the unloaded drag on the accessory drive. Air conditioning compressors also impact the truck's economy, and drivers should be encouraged to think whether they really need to operate the A/C when opening a window a crack will achieve the desired cooling.

Electrical loads also have to be paid for, since the only energy source on the truck is the fuel tank. Spec'ing LED lighting should pay back in the long run with a few drops of fuel saved, along with the much-reduced chance of getting a ticket for a burned-out light. (Or an OSHA claim after a fall from a ladder.)

It goes without saying that any technology to reduce idling is desirable - no-idle is mandated now in California and other jurisdictions. That being said, most truck OEMs offer California-acceptable idling options that come with a sticker on the hood to let enforcement know it's okay to idle. Spec'ing these allow for a more versatile truck that can be used summer and winter in the Shaky State. But getting a driver to shut it down is the best course. In fact, with $4 fuel, idling a truck for 10 hours may be more expensive than a night in a Motel 6 or Super 8.

Spec'ing a battery powered climate system - a battery power unit or BPU - or an auxiliary power unit (APU) to cut idling altogether has proven to be the technology path for many truck fleets. There have been some obstacles such as those California has thrown at APUs that make the payback longer, but still they are a very real alternative to idling.

When it's combined with the top insulation or Arctic insulation package available for a truck, the BPU can provide the answer. Kenworth and Peterbilt share a thoroughly workable system as the Kenworth Clean Power and Peterbilt ComfortClass. There's also an alternative from Dometic. These are about as heavy as an APU and have the downside that batteries eventually have to be replaced, but while they are running they are zero emissions solutions. And if you feel like being adventurous, you could replace the vehicle batteries with a capacitive starting system like that marketed by KBi and save a considerable amount of weight and battery replacement cost.

Power steering brings its own parasitic loss, and a misaligned truck that causes a pull to one side or the other will experience greater drag on the accessory drive than a truck that runs straight down the road. The jury is still out whether the new rack and pinion steering systems will show any fuel improvement over conventional steering. Certainly drivers love them, and a happy driver is a better driver.

OPTIONS

Another spec item should be a filter minder. Replacing air filters before they impact fuel efficiency is a wise move.

With incentive programs for fuel savings likely to make a strong comeback, it's good to give drivers the tools to do the best they can. So driver displays that show average and instantaneous fuel economy are a very worthwhile accessory. Displays in Volvo and Mack trucks also show by "$" symbols how well a driver is doing overall.

Telematics systems are vital to ensure trucks are fully utilized. Integrating telematics with real-time navigation systems can be a real winner in the battle for optimum fuel economy. For one, there is the reduction in out-of-route miles, which may be as much as 10 percent. On a truck covering 100,000 miles, that's 10,000 miles or near $7,000 in wasted fuel. And there's time wasted in traffic congestion that can be put to better use if breaks are taken until the snarl has cleared, saving fuel wasted in a traffic crawl.

A major spec'ing decision that could be on the horizon shortly is the adoption of a hybrid drive. They're already viable in P&D operations and in vocational applications with lots of PTO operation, but Eaton has done simulations that show there are gains to be wrung out of hybrid technology in more typical trucking applications. (Wal-Mart is testing a hybrid Peterbilt 386 in over-the-road operations.) They are not huge, but whereas the vocational solutions save a lot of a little fuel, mainstream trucking with hybrid power has the ability to save a little, and still recoup the significant investment that the technology still demands.

It's a development to watch.

Comment On This Story

Name:  
Email:  
Comment: (Maximum 2000 characters)  
Leave this field empty:
* Please note that every comment is moderated.

Newsletter

We offer e-newsletters that deliver targeted news and information for the entire fleet industry.



GotQuestions?

LUBRICANTS

The expert, Mark Betner from Citgo will answer your questions
Ask a question

Sponsored by


WHEEL ENDS SOLUTIONS

Wheel end expert Jeff Geist from STEMCO will answer your questions
Ask a question

Sponsored by

Magazine