Improving Efficiency of Diesel Truck and Bus Fleets
Clean energy consultancy Gladstein, Neandross and Associates, in its recent report on sustainable fleets¹, predicts another decade of diesel-powered heavy trucks leading long-haul. Diesel efficiency and emissions reductions are important areas of research as broader trends toward sustainable fleet adoption accelerate.
One new technology to the diesel market is cylinder deactivation (CDA). This technology has been commercially applied to the automotive sector as “Displacement on Demand” to improve fuel economy in V-8 engines. In the diesel market, CDA not only helps with fuel economy, but it also helps to increase exhaust temperatures and reduce engine-out nitrogen oxides (NOx).
CDA would be used when the diesel engine runs below its normal operating temperatures, such as in a cold start or when the engine is under light loads (i.e., idling or cruising at highway speeds). In these conditions, the system deactivates some of the cylinders to increase the load on the remaining cylinders. When this occurs, more fuel per injection is required and this produces more heat in the engine block. This, in turn, raises the exhaust gas temperature to an appropriate operating level to activate the diesel particulate filter (DPF) burn off cycle. Successful development and commercialization of CDA technology would help the industry meet 2024 emissions standards.
Another major advancement has been in the development of ducted fuel injectors. Developed by Sandia National Laboratory’s Combustion Research Facility, the ducted fuel injectors offer many benefits. They clean emissions like soot (another potent climate change chemical second only to carbon dioxide) from vehicle fuel, and they represent an inexpensive transition by working well with conventional diesel fuels. They can easily be retrofitted into existing engines, require no after-treatment systems, and have the potential to lengthen oil change intervals.
The ducts are small tubes that are mounted to the underside of the cylinder head near the injector nozzle. Researchers continue to search for appropriate high-temperature alloys for those tubes without substantially increasing costs.
Thermal energy recovery systems
A desire to improve energy efficiency and cut greenhouse gas emissions has led to considerable interest and research in various waste heat recovery techniques. Topping the list of options are thermoelectricity and Rankine Cycle. Rankine Cycle offers the greatest potential due to its higher cycle efficiency.
Rankine Cycle is not a new concept. It has been widely employed in large-scale power plants for years. But it has yet to be fully implemented for heavy-duty trucks and buses.
The cycle works by recovering wasted heat from the engine through an intermediate heat transfer loop that is filled with a working fluid. The fluid captures some of the energy from the waste heat source. The fluid is often water, but with organic Rankine cycles, a higher molecular mass fluid with a lower boiling point is used to reduce the amount of heat needed for energy recovery.
Despite its tremendous potential, some challenges remain, such as limitations of heat available in the heat source, heat rejection constraints, backpressures during the recovery process, and safety and environmental impacts of the chosen working fluid.
The beauty of this form is that its heat source–exhaust waste heat–exists on all current engines on the market.
Creating a more aerodynamic truck
Today there is a large array of aerodynamic aids available for heavy-duty trucks and trailers, all designed to increase efficiency by reducing drag and fuel consumption by as much as 12%.
Some of the more popular options include front and underbody deflectors, side skirts, rear diffusers and boat tails. Companies have experimented with the specific design and rigidity of such aids to further enhance aerodynamic performance. While all of these can provide some benefit, the key is to identify the right combination of aids that provides enough fuel savings to offset the added costs.
Low-rolling resistance tires also improve truck aerodynamic performance by reducing resistance caused by the tires rolling on the highway’s surface, often through tread depth and design. To date, however, while more fuel-efficient truck tires have garnered interest because of their ability to minimize energy and boost fuel economy, their lower life cycles cause concerns for bottom line-oriented owners and operators.
Another trend in commercial trucking is the replacement of dual tires with super single tires. The advantage of reducing the number of tires on a large rig is minimized friction and resistance. However, safety concerns arising from what happens when a tire blows are keeping most fleets from making the switch.
Alternative fuel sources
Electric (battery or fuel cell) is not the only alternative energy source being explored, as companies continue to research the potential of various renewable fuels, like renewable natural gas (RNG), biodiesel and renewable diesel (RD) which are efficient as fuel sources while producing inherently lower greenhouse gas emissions.
California leads the way in renewable energy research with herds of dairy cows already powering fleets, homes and factories throughout the state. An especially promising development is the recycling of dairy cow waste to produce methane–an option that creates a negative carbon footprint. Known as biomethane, it is an attractive tool for battling climate change.
Beyond dairy waste, RNG can come from other sources of manure, landfills, as well as wastewater. The advantage is that it can be easily exchanged with natural gas drilled out of the ground, minimizing the need to overhaul natural gas engine designs.
Biodiesel (B) and renewable (R) diesel, both of which can be made from similar feedstocks, recycled cooking oil (i.e., oil used to make French fries), oil from algae, soybeans, and other oilseed crops, are also attractive alternatives for existing diesel engines. Not only are they carbon neutral, but, like RNG, their use does not necessitate a diesel engine modification. However, biodiesel blends above 20 percent (B20) will require use of higher performance fluorocarbon sealing compounds. Learn more in our Seal Materials for Biodiesel blog.
One of the concerns for renewable diesel is the choice of which feedstock is used. Palm oil feedstock, for example, has been linked to significant land use impacts, including deforestation, which results from allocating land to grow and farm the palm oil.
Yet another alternative fuel is ethanol. One company recently completed tests showing that its system matched the torque and power of a commercial diesel engine using ethanol instead of diesel fuel, delivering over 500 hp and 1850 ft lb. of torque without additional aftertreatment.
There are several points of concern for fleets that operate Class 4 through Class 8 vehicles with multiple power sources, i.e., some diesel, some natural gas, some electric, etc. Among other things, they will face “right to repair” and maintenance challenges as technicians must be sufficiently trained in the repair of multiple power-source vehicles. For example, mechanics working with electric systems need to be well-versed on sensors and how to properly ground them to avoid serious injuries. In addition, a wide variety of parts may need to be inventoried to maintain an array of engine types.
Another point of concern is the training of first responders—not only the emergency responders, but also tow/salvage operators. Diesel, natural gas, and electric engines require different approaches to putting out fires in a roadway vehicle crash or in a facility where electric vehicles are charged.
When considering options for lessening environmental impact, total cost of ownership still plays a role in ultimately determining which technologies will be adopted and to what extent. The ability to make these innovations affordable, safe, reliable, and sustainable is the key to the future of a sustainable transportation model.
This article was contributed by Christopher Overmyer, Senior Field Application Engineer, Parker Engineered Materials Group