Full-time GDCI combustion over the operating map is
achievable by using exhaust rebreathing (RB) at low loads and
cooled EGR at medium to higher loads. Rebreathing (RB) is
accomplished using the exhaust valvetrain system, which
provides a secondary exhaust lift event during the intake
stroke. It is an effective method to recuperate heat from hot
exhaust gases in order to raise mixture temperatures. This
heat promotes autoignition at low loads when boost pressure is
zero or low. Rebreathing also collapses the pumping loop
(intake and exhaust valves are open at same time) and raises
Sellnau et al / SAE Int. J. Engines / Volume 7, Issue 2 (July 2014)
exhaust temperatures significantly. In this way, a close-coupled
oxidation catalyst will remain warm over a wide range of low
loads without special maintenance heating.
The valvetrain system is a Type II, continuously-variable
mechanical system that provides the rebreath functionality. The
valvetrain has good packaging characteristics with low friction.
A Delphi electric camshaft phaser is used to actuate the
exhaust valvetrain and control the secondary valve lift with very
fast response. Figure 7shows a photograph of the front of the
engine with electric camshaft phasers