Among the existing concepts that help to improve the efficiency
of spark-ignition engines at part load, Controlled Auto-Ignition™
(CAI™) is an effective way to lower both fuel consumption and
pollutant emissions. This combustion concept is based on the
auto-ignition of an air-fuel-mixture highly diluted with hot burnt
gases to achieve high indicated efficiency and low pollutant
emissions through low temperature combustion. To minimize the costs
of conversion of a standard spark-ignition engine into a CAI
engine, the present study is restricted to a Port Fuel Injection
engine with a cam-profile switching system and a cam phaser on both
intake and exhaust sides.
In a 4-stroke engine, a large amount of burnt gases can be
trapped in the cylinder via early closure of the exhaust valves.
This so-called Negative Valve Overlap (NVO) strategy has a key
parameter to control the amount of trapped burnt gases and
consequently the combustion: the exhaust valve-lift profile. The
present study shows that intake valve-lift profile, air-fuel ratio
and EGR are also relevant parameters to control CAI combustion
mode.
First, a reference CAI operating area with constant valve-lift
timings and stoichiometric mixture is presented. Specific intake
and exhaust valve-lift profiles with reduced maximum valve-lift and
opening duration are implemented. The gains associated with dual
cam phasers and lean burn mixture on both the CAI operating area
and fuel consumption are then detailed. Lastly, the positive impact
of EGR on the extension of the CAI operating area towards higher
loads and on the improvement of efficiency is quantified.
To complete the test bench data, the operating points are
analyzed with system simulation to investigate the mean in-cylinder
conditions (species and temperature) just before auto-ignition and
to understand the consequences of the different strategies. An
energy balance on a standard spark-ignition operating point and on
various CAI operating points is also presented to point out the
origins of the fuel economy.