As co-holder of the Shell Mileage Marathon fuel economy record, I’ve gained some experience with vehicular fuel economy. This month I’ll use that experience to stimulate thoughts on Class 8 truck mileage.
Class 8 engines are responsible for 1/3 to 1/2 of total vehicle fuel economy. Engine builders struggle to maximize fuel economy, but they must meet exhaust emissions requirements which severely restrict NOx formation. When legislation is introduced to minimize global warming, we should see less emphasis on NOx emissions and more emphasis on fuel efficiency.
Until then, fleet operators have only a few things they can do to improve engine efficiency. Engines can be run hotter (we ran the Mileage Marathon engine at 300 degrees F) if the engine and vehicle manufacturers concur. Less viscous engine oils also can be used to reduce friction, but don’t expect gains over 1% to 2%.
There are a plethora of “magic devices” being advertised to improve fuel economy 5% to 15%. I’ve spent years testing “magic devices,” and the only way any of them ever work (extremely rare!) is by correcting some defect in the engine or defeating the emissions control systems.
To maximize engine efficiency, run the engine no more than necessary (engines use zero fuel when turned off) at the lowest practical speed (RPM). I’d almost advocate coasting, but I’m afraid of 80,000-lb. vehicles without synchronized transmissions. Spin the engine as slowly as possible in the tallest gear possible.
Three significant contributors to fuel economy are driveline losses, aerodynamic drag and rolling resistance. A large portion of the driveline’s estimated 25% contribution to fuel economy can’t be changed. But have you considered running a single differential as opposed to tandems? Also carefully monitor driveshaft angles. Of course, manual transmissions are more efficient than hydraulic, automatic transmissions. You probably already realize that synthetic driveline oils can produce measurable fuel economy benefits.
Aerodynamic drag is responsible for 15% to 20% of vehicle fuel economy. Let’s examine Bonneville racers for ideas. Bonneville racers get their cars as low to the ground as possible to minimize the amount of air passing the very non-aerodynamic undercarriage. Should we consider the “low rider” truck or just very low front bumpers, very tight trailer gaps and complete vehicle side skirts? Would completely enclosing the trailer wheels make sense from a maintenance point of view? I would also try to shape the air coming off the back of the trailer. New tractors are pretty good aerodynamically, but trailers need help. Backup cameras are more aerodynamic than large side mirrors. Windshield sun visors and bug deflectors create significant drag.
Don’t forget the most significant way to reduce aerodynamic drag –– minimize vehicle speed. I can show you data indicating that mileage is inversely proportional to vehicle speed. We ran the Mileage Marathon vehicle at 7 MPH.
Another significant contributor to vehicular fuel economy is rolling resistance. You already know the benefits of singles versus duals. But what about monitoring such things as wheel end adjustment and vehicle alignment? We used to spend hours making sure that our Mileage Marathon car had absolutely no side scrub. The problem is much more complicated with a Class 8 vehicle, because total vehicle alignment from the front end to the back of the trailer must be considered. Aerodynamic aids are compromised if the trailer is “dog tracking.” Measure the wheelbase on both sides of the tractor to be certain both differentials are tracking true with the front end. Small manufacturing differences can significantly affect fuel economy and tire wear.
We evaluated tire pressures up to 200 PSI on our Mileage Marathon vehicle, and fuel economy got better the higher we went! You might check with your tire manufacturer to find out how much air pressure you can safely run without creating traction and wear problems. We also found that tires with a lot of tread depth got less mileage due to tread squirm.
