Nitrogen oxide (NOx) concentration and oxygen (O2) percentage in the exhaust gases were determined by MEXA-720 NOx gas analyzer produced by Horiba. It is a direct-installation gas analyzer using a zirconia ceramic sensor and it can measure NOx emission and fuel air ratio in the exhaust gases. The main specifications of this MEXA-720 NOx gas analyzer are given in Table 2. Smoke index K, which is a measure of the black smoke, is measured by Sun model MGA-1500 detector