Eaton demonstrated a range of waste heat recovery (WHR) technologies for heavy-duty diesel engines at the IAA Commercial Vehicles show in Hanover, Germany, as well as variable valve timing systems, highlighting their potential to reduce fuel consumption and help reduce emissions.
Both indirect and direct WHR systems were on display. The organic Rankine cycle (ORC) design recovers waste heat indirectly using a heat exchanger with the exhaust system. Alternatively, Eaton’s “electrified” system recovers energy directly using an exhaust-driven Roots compressor in conjunction with a motor/generator.
ORC WHR systems can yield fuel-economy improvements of around 5%, but system cost is high, involving a small external combustion piston engine. Eaton looked at what fluids were already carried on board the vehicle to avoid adding another for the ORC WHR system. The first fluid the company considered was ethylene glycol, already used in engine coolant systems.
As Larry Bennett, director of advanced engineering, Eaton Vehicle Group, observes, the fluid would already be hot from use as an engine coolant, but would it have the potential to add more exhaust heat to it and extract more energy? “Now we can create most of the ORC system with existing componentry,” said Bennett. “We now have a U.S. Department of Energy (DOE) grant. We’re working with Paccar, Shell Oil and Mississippi State University, which is where all the actual testing will take place.
“Shell is working on a fluid that has the capability to do this for us. The idea is to see if we can achieve enough waste heat and form this face transition, then utilize it and see if it’s going to work.”
One of Eaton’s research scientists suggested that diesel exhaust fluid (DEF)/AdBlue urea solution for exhaust aftertreatment of oxides of nitrogen (NOx) would be an ideal ORC fluid.
“The idea is exactly the same—you perform the Rankine cycle by boiling the DEF fluid,” explained Bennett. “One of the interesting byproducts is that after you heat it up, ammonia gas comes off and when you cool it down, it doesn’t want to readily return to the liquid state. We can store that ammonia gas.”
Once the engine has cooled down and is restarted, NOx output and treatment is an issue. “If you start it back up, you’re producing a ton of NOx because you don’t have ammonia gas to be able to treat it,” said Bennett. “You need the temperature in the exhaust system to get up to 250°C in order to take the liquid ammonia you’re injecting to get it to vaporize so that you can treat the NOx.”
The ammonia stored from the evaporated DEF should be enough to treat NOx for the first 15 minutes as the engine comes up to operating temperature. “It’s all research, all modeling simulation, but it appears feasible,” he said.
The direct WHR system involves fitting a Roots compressor system right next to the exhaust manifold and using the exhaust gas flow to drive the compressor rotors. Short-term testing shows that the energy can be recaptured.
“The initial concept is to have a motor/generator hooked up to it, then basically take the energy and put it in a battery,” said Bennett. This is not the most efficient way to recapture energy, but it offers another possibility. Eaton’s research scientists believe that this system could be used as a pump when there is a need for high rates of exhaust gas recirculation (EGR). The system could potentially deliver high rates of EGR independent of engine speed.
Eaton had previously developed an electronically assisted variable speed supercharger for use with gasoline engines. “In this application, we can now vary, independent of engine speed, the amount of boost that the engine can get,” Bennett explained. “On a diesel, that has a lot of advantages in the form of downsizing and instant torque. The big thing would be to manage airflow and exhaust flow independent of engine speed.” That capability would be highly attractive to engine manufacturers.
The direct WHR system has shown through simulation a 22% improvement in fuel economy while reducing NOx, Eaton claims.
Variable valve actuation has not been widely used so far with diesel engines, but there are a number of potential advantages. The first is a compression engine brake. Early intake valve closing and late intake valve closing could reduce combustion temperatures and NOx, or improve efficiency.
“You could have early exhaust valve closing for a transient to give faster boost in a turbocharger,” said Majo Cecur, engineering manager, advanced valvetrain, Eaton Vehicle Group. “You could de-activate cylinders in light load conditions so that you could have better fuel efficiency.”
Eaton is investigating valve operation designs that would enable a range of such functions.