Toxic tailpipe emissions and greenhouse gases (GHG) impact all nations, yet there is no global consensus on how to prioritize truck-engine emissions strategies. Emissions regulations differ from one continent to another almost as much as truck duty cycles and market requirements. This means OEMs must consider different emissions-related engine technologies for different territories.
The two principal targets of emissions regulations are nitrogen oxide (NOx), which causes poor air quality harmful to health, and carbon dioxide (CO₂), which blocks heat from escaping from the earth’s atmosphere. But regulators are not yet aligned in the relative importance they attach to each of these two gases.
To paint the current regulatory picture in broad brushstroke, North America’s CARB (California Air Resources Board) standards focus more on NOx, a menace that is difficult to ignore when it forms smog over cities, while the European Union’s Euro 6 emissions standard focuses more on CO2, motivated by the legally-binding Paris Agreement to reduce GHG emissions. Regulatory priorities in Asia, including China and India, are currently closer to Europe’s than North America’s, placing less emphasis on NOx than CO2.
The CO2-NOx conundrum
The big technical challenge is that these two targets, NOx and CO2, cannot easily be hit with the same bullet. The most effective way to reduce CO2 emissions is to reduce fuel consumption, as the two are directly related – but the most widely-used technologies for reducing NOx emissions negatively affect fuel consumption.
Engine manufacturers typically have met U.S. EPA 2010 and EU6 emissions legislation by employing cooled exhaust gas recirculation (EGR) and selective catalytic reduction (SCR). Both technologies, however, have disadvantages.
EGR reduces exhaust temperature to lower NOx, but to cool the exhaust gas before sending it back into the engine, it goes through a lot of plumbing and a heat exchanger. These parts have had issues with long-term reliability, and if the EGR level is not exactly right, they can produce excessive quantities of particulate matter, hastening the clogging of diesel particulate filters (DPF).
The SCR system injects a small amount of an ammonia (NH3) and water solution (diesel exhaust fluid) onto an SCR catalyst, but this can only be done after the catalyst has warmed up to temperatures around 200°C (392°F). To get the SCR system up to operating temperature, the engine calibration can force the engine to run inefficiently in some warm-up modes, and this is at the expense of fuel economy. The SCR system’s fuel use is greatest when engines start from cold, or when the aftertreatment cools down during low-load operation and needs to be heated up.
All these technical conflicts and limitations pose a problem for OEMs: how to accommodate a technology that meets both North American and European regulations with the same global engine platform. One answer to that is a modular system that enables different actuation technologies to be integrated into the same valvetrain.
Variable valve actuation
In the U.S., cylinder deactivation (CDA) technology can help OEMs comply with the tightening rules on emissions when engines are at low loads and idling, with a focus on NOx reduction. In Europe, where regulations place more emphasis on emissions at medium and higher loads through a focus on CO2 reduction, the optimum modular solution is late intake valve closing (LIVC). For which focus the engine is optimized is determined at the start of the engine’s development.
Variable valve actuation (VVA) is highly effective, lowers overall engine system costs and integrates with minimal impact to engine overhead designs. This technology makes real-time adjustments to valve opening and closing, maintaining accurate control of valve motion. By creating a hydraulic link between the cam and the valve, VVA precisely tunes the engine across its operating range.
2-Step VVA, for example, enables the combustion engineer to optimize valve timing at two operating points instead of the traditional fixed-cam-based single-timing option. 2-Step early or late intake valve closing reduces fuel consumption, optimizes compression ratio versus load, improves transient response and start-up, and improves emissions by keeping the aftertreatment system hot during low-load operation.
Early intake valve closing is achieved by operating on an early closing profile main event with the auxiliary valve motion (normal closing) deactivated. Late intake valve closing is achieved with a late-closing cam profile activated on the auxiliary rocker arm to hold the valve open longer.
Early exhaust valve opening enables faster warm-up of the engine and aftertreatment system, improves transient turbocharger response and emissions by keeping the aftertreatment system hot during low load operation. This is accomplished with an early-opening cam profile on the auxiliary rocker arm, or lost motion system, and actuated on-demand with engine oil. Early exhaust valve opening also can be an in-cylinder solution for helping with DPF regeneration by replacing exhaust heaters and dosers.
Another aspect of VVA is internal exhaust gas recirculation. This stabilizes cold start-up combustion, shortens engine warm-up time, improves aftertreatment performance, and lowers emissions by keeping the aftertreatment system hot during low-load operation. This system responds faster than EGR systems and enables the downsizing or elimination of external EGR systems. Versions of this technology have been in production for nearly 20 years.
Though CDA has been employed successfully in passenger car engines for decades, Jacobs is the first to develop it specifically for 7- to 15-L medium- and heavy-duty engines. CDA’s hydraulically activated mechanism is integrated into a collapsing-valve-bridge system for overhead camshaft engines or with a collapsing-pushrod system for cam-in-block engines.
Jacobs’ approach uses components identical to those of its high-power density (HPD) engine brake, meaning that their durability has been proven over many millions of miles. When these mechanisms are combined with disabled fuel injection in selected cylinders, the deactivated cylinders act as a gas spring and return the compressed energy of the air to the crank.
As many cylinders as needed can be deactivated cycle-to-cycle, improving combustion efficiency at times of low-level torque demand. To optimally manage cylinder shutdowns according to driving conditions, Jacobs has collaborated with internal combustion controls specialist Tula Technology. By combining Jacobs’ CDA with Tula’s Dynamic Skip Fire (DSF) controls technology, an advanced control strategy makes decisions for an engine’s cylinders on an individual basis to best meet torque demands while maintaining performance yet saving fuel.
CDA additionally improves emissions because of its heightened exhaust temperatures. Aftertreatment temperatures are maintained even when the engine is in low-load operation, and cooling of aftertreatment is minimized during coasting because less air is pumped through the engine. CDA is mostly able to improve fuel economy through the increased load on the operating cylinders, but incremental improvements also are gained from reducing camshaft friction, reducing pumping losses in part-load conditions, and reducing or eliminating the use of the intake throttle.
Meeting future challenges
Independent validation tests conducted by Tula and Cummins, run on a Cummins X15 HD Efficiency Series diesel engine, and combined with a well-calibrated powertrain simulation tool, revealed impressive results. The tests followed the Federal Test Procedure (FTP) and combined CDA with DSF software and engine control algorithms.
CO₂ emissions were reduced by 1.5% during the hot cycle and NOx emissions by 45%. During the low load cycle (LLC) test, CO2 was reduced by 3.7% and NOx by 66%. Moreover, at a steady engine speed of 1,000 rpm, CDA with DSF delivered a fuel consumption improvement of 25%. Another more-recent study on the LLC further improved these results to a 74% reduction in NOx, 20% savings in fuel, and 5% reduction in CO2.
A whole host of new regulations are in the pipeline: CARB HD Omnibus, EPA Phase 2 GHG standards, the EPA Clean Trucks Initiative, and the Euro 7/VII emissions standard. Though details of these standards have yet to emerge, it is known that by 2024 CARB is scheduled to tighten NOx emissions regulations for low-load drive cycles. Soon after, regulators in Europe and China also will toughen their rules.
No longer will emissions strategies be allowed to choose which – NOx or CO2 – is the lesser of two evils: emissions of both will be more stringently controlled. When this happens, there can be more focus on what matters most: Real Driving Emissions, regardless of if the truck spends a lot of time operating at low loads, or any other special application drive cycle. Either way, valvetrain technology can help the engine meet those ultra-low limits.
Robb Janak, director of new technology at Jacobs Vehicle Systems, wrote this article for SAE Mobility Media.Continue reading »