Compression Ratio

The effect of compression ratio on engine emissions is shown on Fig. 3.1. The typical effect observed when the engine CR was reduced from 10:1 (CR used on high performance engines during pre-emission control period) to 8.5 and 7.0:1 are given on this figure.
Use of high CR results in

(i) Higher burned gas temperature
(ii) Lower residual gas content

These lead to higher  NO emissions on volume basis. However, as engine efficiency increases with increase in compression ratio, brake specific NO emissions decrease. High CR combustion chambers result in

(i) High surface to volume ratio and
(ii) A proportionately higher crevice volume.
(iii) Lower exhaust gas temperatures

Thus the volume of flame quenching regions increases resulting in higher HC  emissions. The problem is further enhanced as due to lower exhaust gas temperatures oxidation of  the  unburned HC is reduced  during exhaust process. These factors result in an increase in HC emissions with increase in engine CR.  At lower CR% fuel efficiency is also reduced thus increasing  specific CO emissions.