contd...

• Overall mixture being lean very low CO is produced as the CO produced early in combustion can be oxidized within the cylinder itself by the excess oxygen available.
• The  mixture  in piston ring crevice region being very lean the contribution of crevices to HC emissions would also  be very  low.
• The direct injection of fuel in the cylinder can decrease HC emissions during warm-up after cold start as liquid fuel film is not formed in the intake  manifold and port. Also, a smaller fuel quantity needs to be injected during cold start compared to PFI engines.
•  With direct injection of gasoline a faster dynamic response is possible hence a flatter air-fuel ratio curve during acceleration can be used that provides lower HC emissions.
• The DISC engines can tolerate higher EGR rates than the homogeneous charge SI engines and hence larger reductions in NO_x

DISC Engine Combustion Requirements

For stratified charge engine operation the following main requirements are to be met to obtain good combustion:

• Combustible mixture must form quickly.
• The liquid fuel and fuel over-rich zones  should be minimum at the time of ignition
• Suitable air motion is to be provided during compression stroke and at the time of fuel injection to accomplish charge stratification and transport mixture to the spark plug in a reproducible manner cycle after cycle. Air motion may be aided by a suitably designed cavity on the piston crown.
• Wetting of piston crown and spark plug by excessive liquid fuel deposition is to be prevented.
• Over mixed zones having excessively lean mixtures beyond flammability limits as well as under mixed over-rich zones are to be avoided.