contd...

The DISC engine gives much lower HC emissions than the PFI engine during engine cold start, engine warm-up phase and transient conditions. In the PFI engines during cold start and transient conditions, the amount of fuel entering the engine cylinder is not the same as being injected at the port as some fuel gets deposited at the port forming liquid
fuel film. Hence, in the PFI engine very rich mixture is to be supplied to have quick cold start. In DISC engines, as the fuel is directly injected in the cylinder there is no delay of fuel being inducted into the cylinder. The DISC engines on the contrary can be started on stoichiometric or even slightly overall lean mixtures.  After engine is switched on, the DISC engine achieves stable combustion in the very first or second cycle while the PFI engine requires about 10 cycles to attain stable combustion. The cold start HC emissions from a DISC engine are nearly 1/4th of that of PFI engine. CO emissions of DISC engines are very low due to overall very lean engine operation.
For the success of DISC engines, catalytic reduction of NO_x under lean engine operation is required. Although with EGR large reductions in engine out NO_x emissions are obtained, but lean operating limit is narrowed with high EGR and during lean homogeneous engine operation high EGR cannot be used. Therefore, lean de-NO_x catalyst technology is essential to meet stringent emission standards in future while maintaining fuel economy benefits of the DISC engines.  Mitsubishi and Toyota DISC engines employ a lean de-NO_x catalyst in addition to EGR to reduce nitrogen oxide emissions to the level of Euro 4 and beyond  emission standards.