by John Baxter
The first two articles of this series, Fuel Economy: Part 1 - The Driver Makes All the Difference and Fuel Economy: Part 2 - Maintaining Existing Equipment, we cover the ways you can save money on fuel by both capitalizing on your driving technique and optimizing the performance of the existing equipment on your truck. Here in Part 3, we'll cover ways you can reduce your fuel consumption by considering investing in newer equipment whenever possible.
The Benefits of New Equipment
While it may sound costly, purchasing newer equipment may save you money in the long run. Even if your operation runs on a shoestring, there are many fuel economy benefits of newer equipment, as tractors and their powertrain components are continuously improved not only for competitive reasons, but because of stringent greenhouse gas emission limits, which translates directly into fuel savings.
And, even though a high truck payment is daunting, the increased reliability of newer equipment often means so much savings in downtime and direct repair costs that the truck payment turns out to be much less of a burden than expected. Routine maintenance intervals like oil changes are often less frequent with newer equipment, too, keeping you on the road.
Keep in mind that the most cost-conscious large fleets frequently use their trucks for only 400,000 miles and three years or less to reduce costs. Newer equipment often also increases the value of your service to your customers since reliability and on-time delivery are often important to them.
Keeping up with Continuous Improvements
The design of every truck tractor is constantly being fine-tuned with small alterations in everything from the bumpers and headlights to the mirrors and fairings on the side. This work is done in wind tunnels. Although their appearance changes in a subtle way, all are completely redesigned every 5-6 years or so to make them aerodynamically smoother. So, updating your equipment, even if the tractor you purchase is 3-5 years old instead of 10, will mean better aerodynamics and fuel saved because of the more sophisticated design.
Engines
2017 changes in engines is another classic example. Cummins, whose main highway engine for Class 8 trucks is a 15-liter design that was originally introduced in 1998 has made changes so extensive that the original design is hardly recognizable today. 2017 saw such big changes that the company altered the name from the ISX to the X15.
A number of changes to the turbocharger, EGR cooler, and porting in the cylinder head improved the breathing of the X15 over its predecessor. A new piston not only reduced friction, but carried heat more effectively away from the combustion chamber, meaning it needed less oil cooling. That change allowed for a smaller oil pump, which reduces fuel consumption, as well as lengthened oil change intervals.
Since most of its competitors run smaller engines between 12.7 and 13.5 liters, and the X15 can produce up to 600 hp., the company has always found ways to adapt this big engine to economical fleet usage where less horsepower is normally specified. The X15 Efficiency Series uses something called an Atkinson Cycle. This concept alters when the intake and exhaust valves open and close to make the engine behave like a smaller one in terms of how it takes air in. It then utilizes the bigger cylinders to expand the burning gases more fully--to at least 20 times the volume of the cylinder when the piston is a top/center) on the power stroke. This extra expansion leaves less energy in the exhaust and puts more through the crankshaft to the drivetrain. The Efficiency Series is available with up to 500 hp. and 1,850 lb. ft. of torque.
Volvo and Mack improved their combustion chamber by finding a way to separate the sprays of fuel before they hit the walls of the piston. This not only reduced soot in combustion and clogging of the DPF, which is a fuel economy killer, but allowed a higher compression ratio, improving engine efficiency. They also switched to a high-pressure, common-rail fuel system that helps greatly to maintain combustion efficiency at lower RPMs while also allowing finer tailoring of the combustion to control emissions while remaining efficient. All the truck diesels sold in the US now have common-rail. Volvo introduced turbo-compounding that year and more recently made it standard equipment. In this system, a second turbine that is spun by the exhaust that exits the turbine at the rear of the turbocharger captures more energy from the exhaust and sends it to the crankshaft through gears. The system actually overcomes much of the negative effect of EGR on engine efficiency.
The other engine makers have also continued to make changes that radically improve fuel economy. Navistar completely redesigned its 12.7 Liter MaxxForce engine, naming it the A26 for 2017. This engine saw a significant increase in its compression ratio for 2,022. Detroit Diesel and Paccar are less open about the changes they have made, but the Paccar engine's changes actually reduced fuel consumption by a higher percentage than some of its competitors in 2017. Detroit Diesel sells both 13 and 15-Liter engines for Class 8 trucks, and says the bigger engine is actually slightly more fuel-efficient than the smaller one. They obviously have done something to use the bigger cylinders to save fuel, either by expanding the gases more to get more energy from them, or perhaps by allowing more rapid burning of the fuel without excessive engine stresses. Most engine manufacturers have done things like making their water pumps more efficient and making their speed variable so when you are just cruising down the highway, a situation that is easy on the cooling system, the water pump will slow and it will take much less power to drive it. Most late-model trucks have air compressors with clutches that disengage once the air system is up to pressure, saving friction in the compressor.
Drivetrain
Drivetrain components also see continuous improvement. Late model transmissions have pumped lubrication rather than splash, as using the gears to spread transmission fluid around actually takes more power than a pump, and it can produce too thick a layer of fluid on gear teeth and bearing surfaces. Using a pump to distribute the lube perfectly in an ideal quantity to all the parts actually reduces friction so much that transmission cooling requirements are much lower. Aluminum transmission housings, in combination with pumped lubrication, in some cases completely eliminate the need for transmission coolers and the extra pumping needed to make them work.
Automated Transmissions
Automated transmissions came to the market in stages over many years, with the first iterations only shifting between the top two gears, and later models requiring the driver to operate the clutch during starting. Volvo shook the field up with a fully-automated 12-speed that included full clutch automation. The strategy was adding just enough ratios (compared with the popular 10-speed) to improve both acceleration and fuel economy. 12 ratios allow skip-shifting at low speeds to speed up acceleration by cutting down on turbo-lag. Then, when getting into the higher gears, the closer ratios enable the unit to keep engine RPM in the fuel consumption sweet spot.
The key to affording a fully automated transmission and clutch was lower fuel and maintenance costs. An automated box always shifts at the right RPM for fuel economy, and it always starts in the right gear, using the clutch smoothly at the lowest possible RPM to minimize clutch wear. These characteristics save both fuel and driveline maintenance and repairs.
If you are a really good driver in these two respects, you may not get the full financial benefit from an automated gearbox. But, if you are not certain about the best ways to operate your tractor to save fuel and preserve the drivetrain, or know you often get a little careless when life on the road gets difficult, an automated gearbox may pay for itself. Only the very best drivers can equal an automated gearbox when it comes to fuel economy and drivetrain life. And many drivers now prefer them because operating a tractor is much easier when you don't have to manage a manual transmission and clutch.
Also, automated transmissions are essential when setting up the drivetrain so the engine operates at a very low RPM, called "down-speeding" because annoyingly frequent shifting will be needed in hilly terrain (more on down-speeding later.)
Gearing
With the right highway cruise speed and ever stronger drive shafts and axles that can tolerate fast ratios, many trucks now cruise in a direct-drive top gear, which saves at least 2% of the fuel bill right off the bat.
Gears in drive axles have also seen improved machining smoother surfaces. One design has a device that directs fluid away from the ring gear, lowering the effective level so unnecessary churning goes away. And, many late model three-axle tractors now have only a single drive axle, which eliminates the second axle, the inter-axle differential, and the second driveshaft. This reduces friction and saves significant amounts of fuel. Proper control of the air suspension system eliminates concerns about traction and tire wear on the driving axle.
Significantly stronger bearings and gears in drive axles, as well as much stronger driveshafts, allow super-fast axle ratios, from 2.64:1 down all the way to about 2.21:1, which is often used with direct drive. Typical axle ratios in the past were 3.55:1 or 3.30:1, as you may know.
Downspeeding
Putting all this together, most diesels today provide maximum torque at a far lower RPM than in the past. While many engines saw peak torque at 1,200 or even 1,300 RPM prior to 2,010, modern fuel-economy engines produce their peak torque down to at least 1,000 RPM these days, and some, like the Volvo and Mack engines with turbo-compounding, and others depending on rating, down to 900 RPM. One of the big contributors to this change is common-rail injection mentioned earlier. Common-rail became practical as durable high pressure piping became available for diesel injection systems. The injection system now receives fuel from a small pump or pumps driven off the crankshaft or camshaft, but providing steady pressure of whatever value is required at all RPMs, rather than pumping each shot of fuel separately at the right time. Timing is handled in a new type of injector that can send several shots of fuel in, or shape injection to optimize the balance between fuel economy and emissions. The separate pumping pulses of the unit injectors used earlier cause a loss of pressure when the engine lugs down. The steadier pumping of common-rail greatly reduces many engine vibratory stresses and noise while also making as much pressure as is necessary to properly atomize and distribute the fuel available even at low RPM. In the past, injection systems worked at their best only up near governed RPM.
These changes, along with automated transmissions that take the task of shifting away from the driver, now allow ultra-low cruise RPMs, called "down-speeding" of the engine. This is the icing on the cake when it comes to optimizing the diesel powertrain. Most diesels today run at 1,150 RPM or less at cruise speed, with engines that use turbo-compounding running at around 1,050-1,060, and sometimes slower. While in the past, engines needed to run at least at 1,350 RPM to minimize the need to downshift on the highway in hill climbing, automated shifting ensures that even though more downshifts are needed, the driver won't be inconvenienced. This way, the engine can run right near a very low-RPM torque peak most of the time. Such a low cruise RPM typically saves 2-3% of the fuel bill, and this also reduces in-cab noise levels. Down-speeding makes the engine more efficient by reducing friction and increasing turbo boost and the operating pressure in the cylinders. It also helps to keep the DPF clear for further fuel economy improvement.
Worth the Investment
So there you have it. Your operation will likely be more profitable the newer the type of equipment you run. It's an additional incentive to operate economically in the meantime so you can put money away for a down-payment and take the plunge to purchase a newer, or even a brand new tractor.
Other Relevant Articles:
Fuel Economy: Part 1 - The Driver Makes All the Difference