Improving commercial vehicle emissions and fuel economy with engine temperature management using variable valve actuation

Abstract

Commercial vehicles require continual improvements in greenhouse gas emissions to meet upcoming emission regulations and fleet fuel economy needs. Challenges for future emission standards require technologies for engine exhaust temperature management to deal with low engine load operation for optimal aftertreatment performance. The proposed ultra-low NOx emission standards of 10% of today’s US level (0.2 g/hp-hr) is challenging and requires significant temperature management strategies including heat-up strategies during the cold part of the emission cycle. Heavy duty commercial vehicle applications requires a heat source on the order of 30 kW to achieve aftertreatment temperatures for sufficient NOx reduction. There are technologies that can provide such high heat loads in a short period of time. A diesel exhaust burner is an option for fast heat-up at the expense of fuel economy. Variable valve actuation (VVA) solutions are effective for aftertreatment temperature management including early exhaust valve opening, intake valve closing modulation and cylinder deactivation. Further steps of emission legislation focus on in-service operation, including NOx emission reduction during low load operation. Such low engine load operation may result in exhaust temperatures between 100°C and 250°C, where NOx aftertreatment systems are not effective. Thus, technologies are needed to raise the exhaust temperature under such conditions. The use of VVA to vary the air-excess ratio in the cylinder is a fuel efficient method to increase exhaust temperature under low load conditions. Methods of intake air throttling are capable measures such as cylinder deactivation and Miller cycle. Cylinder deactivation during low load engine operation shows a marked increase in exhaust temperature by approximately 100°C which moves aftertreatment systems to a more optimal region, typically significantly above 250°C while also offering fuel economy benefits. The addition of a high efficiency boosting system enables Miller cycle operation to improve fuel economy. Thus, the use of VVA is a leading technology combining the future requirements to simultaneously reduce NOx and fuel consumption. This paper will show the benefits of variable exhaust valve opening, intake valve closing modulation with and without boosting, and cylinder deactivation for meeting future emission regulations and fuel economy needs. Finally, solutions combining VVA and engine braking are provided.