Upcoming diesel-engine emission regulations will require significant reductions in NOx and CO2. According to the EPA and industry estimates, one gallon of diesel fuel emits 22.44 pounds (10,180 grams) of CO2 when combusted, and driving one mile on average emits 404 grams of CO2.
Fortunately, variable-valve-actuation (VVA) technologies that permit management of the temperature of the exhaust gases can improve fuel consumption as well as reduce these emissions.
Eaton, for one, exhibited a portfolio of technologies designed to reduce emissions and improve fuel consumption in commercial vehicles at the Shanghai Auto Show, which took place from April 18-27 in Shanghai, China. “VVA technologies,” said Bo Yang, president, Eaton Vehicle Group and eMobility APAC, “are unique in that they reduce both NOx and CO 2, while other technologies focus on one or the other.”
Eaton has developed VVA technologies for medium- and heavy-duty engines addressing needs such as early/late intake valve closing (EIVC/LIVC) or "Miller" cycle and cylinder deactivation (CDA).
A Closer Look at LIVC and Its Benefits
The company’s LIVC delays intake valve closure so that the effective compression ratio is less than the expansion ratio. LIVC increases engine thermal efficiency, improving fuel economy and reducing CO2 emissions. In addition, LIVC allows for an increase in geometric compression ratio, further enhancing its benefits—CO2 is expected to reduce 1% to 2% when combining LIVC with a higher compression ratio, according to Eaton.
Eaton’s deactivating roller technology can be employed to modulate the intake valve to achieve either early or late valve closing. LIVC makes effective compression ratio lower than expansion ratio, increasing efficiency while increasing exhaust temperature. It helps improve the efficiency of the engine, contributes to after-treatment thermal management at partial loads (keeping it warm at +40°C at low load), and enables higher compression ratios.
In the Eaton LIVC system, a standard intake rocker is combined with a switchable rocker. While the standard rocker realizes the regular intake valve lift, the switchable LIVC rocker runs on a secondary cam with increased duration. When the LIVC rocker is active, the secondary cam controls the valve closing motion and therefore it’s delayed.
Eaton’s technology can modulate the exhaust valve to achieve an early exhaust valve opening. By opening the exhaust valve earlier, halfway through the expansion event, a larger part of the combustion energy goes into heat instead of mechanical work to increase the exhaust temperature.
Cylinder Deactivation
To meet future CO2 and NOx regulations, engines will need to rely on advanced combustion processes. Cylinder deactivation (CDA) is a technique in multi-cylinder engines whereby a combination of cylinders is systematically disabled. CDA is achieved by deactivating the intake and exhaust valves for the deactivated cylinder.
In diesel engines, cylinder deactivation is used for exhaust heating. By deactivating cylinders at low loads, the remaining active cylinders work harder and produce more heat, which gets the after-treatment system hotter quicker and reduces emissions.
Diesel CDA and EIVC/LIVC technologies can be used to reduce fuel consumption between 5% and 25%, increase the rate of after-treatment warm up, and maintain higher temperatures during low-load operation.
CDA closes intake and exhaust valves on specific cylinders and forces the remaining cylinders to perform more work. These higher loads force the throttle to be opened more, reducing pumping loss in the active cylinders and improving fuel economy. Pumping loss is the power required to perform the intake and exhaust pumping functions. In spark-ignition engines, reduced pumping losses result in increased fuel economy.
Cylinder-deactivation technology reduces both CO2 and NOx, making it more efficient than other technologies, which reduce NOx at the expense of CO2. CDA reduces NOx by more than 40% and CO2 by 5% to 8%. This suits it for utility vehicles that make frequent stops, such as sanitation trucks and delivery vehicles.
Deactivating the valves on one or more cylinders reduces the overall air/fuel ratio is reduced. The amount of injected fuel is similar while the airflow is reduced proportionally to the number of deactivated cylinders. This works to increase the exhaust temperatures and ultimately to improve emissions through improved catalyst efficiency.
Other advantages of CDA include NOx reduction up to 90% when combined with advanced after-treatment. Tests also show that it’s possible to operate a diesel engine at low loads in CDA without compromising its torque and power capabilities.
Simultaneously, a fuel-efficiency benefit is possible. Eaton says it has developed a strategy for maximizing the CDA benefit while maintaining acceptable noise, vibration, and harshness (NVH). This strategy comprises deactivating variable numbers of cylinders depending on the engine's speed and load.