Effects of Intake Valve Lift Form Modulation on Exhaust Temperature and Fuel Economy of a Low-loaded Automotive Diesel Engine

Abstract

Exhaust after-treatment (EAT) systems on automotive vehicles cannot perform effectively at low loads due to low exhaust temperatures (Texhaust < 250oC). Con-ventional late intake valve closure (LIVC) technique - a proven method to im-prove diesel exhaust temperatures - generally requires the modulation of the whole valve lift profile. However, an alternative method - boot-shaped LIVC - only needs partial lift form modulation and can rise exhaust temperatures signif-icantly. Therefore, this study attempts to demonstrate that boot-shaped LIVC can be an alternative solution to improve exhaust temperatures above 250oC at low-loaded operations of automotive vehicles.A 1-D engine simulation program is used to model the diesel engine system operating at 1200 RPM engine speed and at 2.5 bar brake mean effective pres-sure (BMEP) engine load. Boot-shaped LIVC is achieved via keeping the valve lift constant (at 4.0 mm) for a while during closure and then closing it at different closure angles. The method results in up to 55oC exhaust temperature rise through reduced in-cylinder airflow and thus, is adequate to keep EAT system above 250oC at low loads. The longer the boot is kept during closure, the lower the air-to-fuel ratio is reduced and the higher the exhaust temperature flows at turbine exit. Similar to conventional LIVC, boot-shaped LIVC improves fuel con-sumption as pumping losses are decreased in the system. Despite aforementioned improvements, EAT warm-up is affected negatively due to the significant drop-off on exhaust mass flow rates. The need to modify only some parts of the lift profile is a technical advantage and can reduce production costs.