The big-rig tractor-trailer trucks that we see on the highway get only about 5.8 mpg of diesel fuel. In tests, the Cummins-Peterbilt SuperTruck—a U.S. Department of Energy-supported advanced technology demonstrator unveiled last year—achieved nearly double that number: 10.7 mpg. If all the heavy-duty trucks in the U.S. were as efficient as the SuperTruck, domestic consumption of oil would fall almost 300 million barrels, a potential $15-billion savings that would reduce the annual fuel outlay of the average Class 8 operator by perhaps $10,000.
And although semitrailer trucks comprise only 4% of the vehicles on America roads, they consume about 20% of the fuel, so improved fuel economy would also cut emissions of CO2 significantly.
The Cummins-Peterbilt prototype was developed and built as part of the SuperTruck Initiative, a four-year, $78-million government-industry collaboration to develop a next-generation diesel semi-truck with greatly improved fuel consumption. Now the DOE is back with a follow-on project that aims for even better engine system efficiency.
“We learned a significant amount in the SuperTruck program,” said Wayne Eckerle, Vice President of Corporate Research & Technology for Cummins, a self-described “combustion guy.” “It gave us the chance to demonstrate the feasibility of several advanced engine technologies that we’d been working on previously and integrate them into an operating system.”
The resulting SuperTruck powertrain achieved the DOE’s target goal of a peak diesel engine system brake thermal efficiency of 50%. “That wasn’t at all easy,” he stressed, noting that “diesels today are probably 43% efficient.”
The Energy Department recently awarded Cummins, the nation’s only independent diesel engine maker, a two-year, $4.5-million grant to boost its previous mark by 5 percentage points to 55% brake thermal efficiency, Eckerle said. “Now we’re aiming to demonstrate another substantial increase in efficiency in a real-world duty cycle, an effort that leverages and carries forward what we were doing on the SuperTruck project.”
The Heavy Duty Engine Enabling Technologies Project, a 50-50 cost-shared R&D endeavor, aims to “leverage the design, analysis and development work that has been invested through the Cummins SuperTruck program to demonstrate a peak diesel engine system efficiency of 55% Brake Thermal Efficiency (BTE) while also implementing an advanced, highly integrated combustion/after-treatment system,” states DOE documents.
“There is no magic bullet to get to 55% BTE,” warned Lyle Kocher, technical advisor for advanced system integration at Cummins and principal investigator on its Diesel 55BTE project on a team that includes 20 dedicated engineers. “Reaching new fuel efficiency levels while complying with all the emission limits means that we’ll have to use multiple strategies.”
The Cummins team plans to apply new ways to fine-tune the fuel combustion process, optimize both the fuel and air handling systems, modify the emissions system, reduce parasitic losses in the base engine and the turbocharger, as well as to add a bottoming cycle to recover waste heat.
Probably the largest contribution to any overall efficiency gains will derive from improving the combustion process, Kocher observed. Various fuel combustion tweaks can better the complex fuel-burning process, which in a diesel engine entails some 2000 simultaneous and sequential chemical reactions. To improve its combustion modeling, the diesel maker reportedly licenses commercial computerized code at a cost of more than $1 million a year.
The team, the mechanical engineer said, will implement several advanced combustion strategies that will optimize heat-release rates, but still retain burning at reduced temperatures for low nitrogen oxide (NOx) emissions. “There can be a trade-off between efficiency and NOx emissions,” Kocher acknowledged.
“We want to minimize the duration of the burn to reduce heat transfer loss,” he said. “In particular, we want to control the rate shape; that is, we want to slow the front end of the combustion process and speed up the back. We also want to minimize heat losses to the cylinder by using insulating coatings and other approaches.”
Like other diesels, the 55% BTE engine will derive its basic efficiency from its fuel-stingy combustion-ignition cycle and fewer mechanical losses due to lower-speed operation. The cycle’s efficacy in burning diesel arises from high compression ratios, high combustion rates under lean conditions, and the use of air-fuel ratios to control system loading rather than throttling to avoid part-load pumping losses.
Added into all the other considerations, Eckerle said, the engineers must accomplish this goal without “exceeding peak cylinder pressures or producing noise.”
Besides implementing the latest fuel-injection techniques—which might involve higher injection pressures, finer spray control, and multiple injection events—flexible valve control and enhanced engine breathing are also key to boosting powertrain efficiency. “We need to minimize pumping losses,” Eckerle emphasized. “Whenever we have to push gases back into the cylinder or draw them in that costs us work.”
The prototype engine will rely primarily on exhaust gas recirculation (EGR) to reduce combustion temperatures. “EGR is critical way to control NOx by keeping the temperature low,” Kocher said. It also aids in controlling the pumping losses.
Turbo boosts efficiency
“We use turbochargers to maximize efficiency,” he said. “High-pressure engines run more efficiently than low pressure.” Turbocharging raises power density and recovers some of the wasted exhaust heat.
Cummins is engineering more effective turbochargers with smaller gaps between the blades and the housing, Eckerle said. “We’re taking advantage of full CFD and reaction analysis and simulation techniques to model the turbo down to brief in-cycle transient conditions—essentially, pulsations in the flow.”
“In the old days—really, only 3 years ago—you couldn’t do that,” he said. Knowing the pressure coefficients and other fine details “allows us to do a much better job of designing the turbo architecture. We optimize the general design to take advantage of the pulses within each cycle, something that we kind of ignored before.”
They said that the engine system design should also feature strategic cooling to minimize thermal energy losses and augment overall power. Parasitic pump losses will be addressed in part with variable-flow cooling pumps. Likewise, friction losses inherent in the power transfer process will be mitigated with sliding friction-reducing coatings and other techniques.
Waste heat recovery
The Diesel 55BTE team is employing an organic Rankine cycle (ORC) to capture waste heat from the engine EGR system as well as the charge air and exhaust streams, and convert it into useful work. The system, which will include heat exchangers, a heat-carrying working fluid/refrigerant, expanders, pumps and condensers, will be coupled to the engine mechanically via the turbine-expanders.
The waste heat recovery system will serve as a bottoming cycle for the engine. “It’s been a subject of research here at Cummins for quite some time,” Eckerle said. In earlier reported tests, fuel-economy benefits greater than 7.4% were demonstrated when coupled with EPA 2010 engine system under ideal conditions, “but there’s considerable room for improvement.”
Finally, the team wants the exhaust aftertreatment system to be close-coupled to the engine to avoid undue heat losses, said Kocher, who concluded by saying that he was looking forward to the challenge.