Eaton is rethinking how the industry combats diesel engine emissions. Traditional methods, such as exhaust gas recirculation, more effective and larger selective catalyst systems, and more complex aftertreatment systems, have about reached the pinnacle of their effectiveness, according to Mihai Dorobantu, Eaton's director of technology planning and government affairs. As Eaton's point man on the emissions technology front, he is looking at viable alternatives to current systems that can actually improve performance and efficiency while meeting future emissions reduction challenges.
"It looks increasingly likely that in the 2024-2027 timeframe, we will face the double challenge of reducing both NOx and CO2 by significant degrees," he said. "The message from the regulators is crystal clear, but our response as an industry can no longer be to just add and add and add technology. This just increases the cost and the complexity while lowering the reliability of the engines. That cannot be the way of the future."
Dorobantu's remarks came during Eaton's press conference at the American Trucking Associations’ Technology & Maintenance Council 2019 annual meeting in Atlanta March 17.
Eaton has been researching solutions at the system level for several years. That work has revealed many possibilities for reducing cost and complexity, but they will require a fresh start in engine design not just more bolt-on components. The solutions lie in deep integration of modular and smart components and making use of advanced controls, Dorobantu said.
"All we are doing now is increasing cost, weight, and complexity, and that's bad," he said. "Current incremental technology will get us through 2021, but by 2024, we will require something completely different."
Here's some of the technology Eaton is looking at.
Variable Valve Actuation
Variable valve actuation (VVA) technology can change the opening and closing timing of intake and exhaust valves to make the engine more efficient during different operating phases. For example, forcing the intake valve to stay open slightly longer would allow the engine to draw in more air and thus improve combustion efficiency. The same actuator on an exhaust valve could be used to open it slightly earlier while the exhaust gas is at peak temperature. This would get you an additional 20-30 degrees Celsius of exhaust temperature, which would help maintain higher temperatures (and efficiency) in the aftertreatment systems.
Eaton has been experimenting with hydraulic modular VVA capsules to command different valve behavior and force the valves open or closed at times other than those described by the basic cam action.
"Camless engines or electrically driving the valves is viable, too, but that type of technology is pretty far out in terms of cost and complexity," Dorobantu said. "What we're doing is keeping the basic cam-face profile and adding modifiers that achieve perhaps 80% of what a pure camless engine can do, but with significantly lower cost and complexity."
VVA also could facilitate cylinder deactivation, for instance, not as a fuel-saving strategy but for NOx reduction. By operating two or three cylinders at higher loads rather than six cylinders at light loads, you produce higher exhaust temperatures (250 degrees C at higher loads rather than 100-150 degrees at light or medium loads). Low temperatures across the catalyst limit its effectiveness, which cause more NOx to be emitted. Keeping the catalyst hot by flowing hotter exhaust gas from two or three cylinders helps maintain the catalyst efficiency in the 98-99% efficiency range.
"Running at light loads or at idle [when coasting] is a very effective way of cooling the aftertreatment system," Dorobantu explained. "The engine effectively becomes a big cold air fan for the aftertreatment system. With CDA, rather than pushing lukewarm exhaust through the aftertreatment system and pulling down the temperature, we work fewer cylinders harder and maintain higher aftertreatment temperatures."
This is particularly helpful in powertrains with a neutral-coast feature. While coasting, the exhaust temperature drops dramatically, causing the engine to go into thermal management mode, essentially dumping a lot of fuel to heat up the aftertreatment. This subverts the benefits of running the engine at idle while coasting to save fuel.
"While you're saving fuel in neutral coasting, you're paying fuel as soon as you get the engine going again," he said. "It takes a few good minutes of running the engine at higher loads to heat up the aftertreatment. So the benefits of neutral coasting are limited today because it cools down the aftertreatment system during neutral coasting. With cylinder deactivation, we don't have that problem."
Dorobantu also noted that technology similar to what is used for VVA can also double the output of a typical engine compression brake.
"This is an example of modular and flexible architecture," he said. "We use the same capsule with different valve commands to achieve multiple functionality. This gives the OEM flexibility and choice in their design space. Once we break out of the paradigm of the fixed cam, we can enable multiple engine functions with essentially one type of device."
Exhaust Gas Recirculation Pumps
EGR is a sure-fire way of reducing NOx, but current systems aren't as efficient as they could be. With EGR, the trick is to balance the quantity of exhaust directed back into the engine to reduce the oxygen content of the intake air without unduly reducing the efficiency of the engine. EGR is the first stage of the NOx reduction process; SCR takes care of the rest in the downstream exhaust flow.
To get the exhaust back into the engine, you have to create a pressure differential where the exhaust manifold pressure is higher than the intake manifold pressure so that the exhaust will flow in the right direction. Today, we do that by adjusting the vanes of a variable geometry turbocharger to create backpressure on the exhaust side of the turbo. Generally speaking, that process creates a pumping loss penalty of about 5%, according to Dorobantu.
"To eliminate some of these pumping losses, we could turn to different types of turbochargers, or we could take a totally different approach: an exhaust gas recirculation pump," he said. "We're talking about an electrically driven boost pump very similar to an automotive supercharger. It would enable lower backpressure on the engine, which really means eliminating pumping losses, and we can not only pump as much EGR flow as needed, regardless of the engine rpm, but we [also] can measure and manage the EGR flow very precisely."
The TVS EGR pump (Twin Vortices Series) is driven by a 48-volt electric motor, making it completely independent from engine speed and significantly more controllable than pumpless EGR systems. TVS technology enables EGR flow in operating regions where it wasn’t possible to drive flow before and provides more accurate EGR flow rate control for better combustion and emissions management, according to Eaton.
Using such a pump eliminates the need for a lot of sensors and venturi tubes and other things that tend to clog. And it since it's electrically driven, you do not have to sacrifice turbocharger efficiency to drive the EGR pressure differential. According to Dorobantu, the pressure differential between the exhaust and intake manifolds is roughly equal, so little energy is required to drive the pump.
"It operates at around one kilowatt, so there's no CO2 penalty [fuel economy penalty] to over come the pressure and the pumping losses," he explained.
Dorobantu said European OEMs are moving ahead with this type of technology because of the NOx restrictions already in place there. On this side of the pond, San Diego-based engine developer Achates is using a variant of the EGR pump in some of the 10.6L opposed-piston engines it is developing for the California tractor market, where further NOx reduction requirements are planned.