Analysis of the impact of early exhaust valve opening and cylinder deactivation on aftertreatment thermal management and efficiency for compression ignition engines

Leighton E Roberts, Purdue University

Abstract

In order to meet strict emissions regulations, engine manufacturers have implemented aftertreatment technologies which reduce the tailpipe emissions from diesel engines. The effectiveness of most of these systems is limited when exhaust temperatures are low (usually below 200°C to 250°C). This is a problem for extended low load operation, such as idling and during cold start. Use of variable valve actuation, including early exhaust valve opening (EEVO) and cylinder deactivation (CDA), has been proposed as a means to elevate exhaust temperatures. This thesis discusses a research effort focused on EEVO and CDA as potential enablers of exhaust gas temperature increase for aftertreatment thermal management. EEVO results in hotter exhaust gas, however, more fueling is needed to maintain brake power output. The first study outlines an analysis of the impact of EEVO on exhaust temperature (measured at the turbine outlet) and required fueling. An experimentally validated model is developed which relates fueling increase with EVO timing. This model is used to generate expressions for brake thermal efficiency and turbine out temperature as a function of EVO. Using these expressions the impact of EEVO is evaluated over the entire low-load operating space of the engine. Considering the earliest EVO studied, the model predicts an approximate 30°C to 100°C increase in turbine out temperature, which is sufficient to raise many low-load operating conditions to exhaust temperatures above 250°C. However, the analysis also predicts penalties in brake thermal efficiency as large as 5%. The second study focuses on the impact of 3-cylinder CDA on exhaust temperature and efficiency at both "loaded" and "unloaded" idle conditions. CDA at idle results in a reduction in air-to-fuel ratio, and heat transfer surface area. This enables an increase in exhaust temperature for aftertreatment thermal management, and an increase in efficiency via reduced pumping and heat transfer losses. At the loaded idle condition, deactivating 3 cylinders provides an increase in exhaust temperature from about 200°C (6-cylinders) to approximately 300°C (3-cylinders), with no fuel economy penalty. Additionally, at the unloaded condition, CDA provides an increase in exhaust temperature of about 20°C, from about 117°C to about 135°C, with a fuel consumption reduction of 15%-26%. The third study includes additional research motivating CDA as a thermal management strategy. Results of an experimental load sweep with CDA show an increase of about 5% to 7% BTE at low load (1.3 bar) with an increase in exhaust temperature from 166°C to about 245°C. By about 2.5 bar, there is no significant change in BTE, yet an exhaust temperature increase is observed from 215°C to about 340°C. At 6.4 bar, a reduction of about 10% to 15% BTE is observed with a temperature increase from 354°C to about 512°C. As noted above, these are desirable benefits during steady-state; however, when an engine transitions from low to higher load, more air is needed to accompany the additional fuel. During transient operation, the reduced air-fuel ratio as a result of CDA limits the rate at which the load can be increased, as well as the maximum load that can be achieved. In addition to demonstrating the benefits of CDA during steady state operation, this paper identifies challenges with respect to transient operation of CDA for engines incorporating "conventional" air handling systems - high pressure EGR and variable geometry turbocharging. The transient Federal Test Procedure (FTP) cycle requires a load transition from near zero load to about 6 bar BMEP within approximately one second. This study shows that at low speed (800 rpm), the test engine operating in CDA mode cannot meet the load transition required by the FTP without mode transitioning to conventional 6 cylinder operation. At a moderate speed consistent with highway cruise conditions (1200 rpm), the transient FTP heavy-duty cycle can be met only by increasing the higher load air-fuel ratio target from ∼18 to ∼21, which reduces the temperature benefit seen from CDA by ∼60°C (from 512°C to 450°C) and increases the NOx from 3.2 to 10.3 g/hp-hr. The load response required for the mid-range cycle cannot be met with CDA due to low air-fuel ratios causing large soot emissions, even when air-fuel ratio is increased to ∼23. The work presented here provides insight into the thermal management capabilities of EEVO and CDA. EEVO can significantly raise exhaust temperatures; however, this comes at a large efficiency penalty. CDA provides large exhaust temperature increase accompanied by fuel consumption benefits at low load. This thesis demonstrates the benefit of CDA, but illustrates that remaining challenges exist with enabling transient operation.

Degree

M.S.M.E.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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