Experimental Study on the Effects of EGR and Octane Number of PRF Fuel on Combustion and Emission Characteristics of HCCI Engines

The effects of Exhaust Gas Recirculation (EGR) and octane number of PRF fuel on combustion and emission characteristics in HCCI operation were investigated. The results show that EGR could delay the ignition timing, slow down the combustion reaction rate, reduce the pressure and average temperature in cylinder and extend the operation region into large load mode. With the increase of the fuel/air equivalence ratio or the fuel octane number (ON), the effect of EGR on combustion efficiency improves. With the increase of EGR rate, the combustion efficiency decreases. The optimum indicated thermal efficiency of different octane number fuels appears in the region of high EGR rate and large fuel/air equivalence ratio, which is next to the boundary of knocking. In the region of high EGR rate, HC emissions rise up sharply as the EGR rate increases. With the increase of octane number, this tendency becomes more obvious. CO emissions have strong relationship with combustion temperature in cylinder. With the increase of EGR rate, CO emissions increase. In the ordinary operation region, NOx emissions are little and the influence of EGR on it is insensitive. NO_X emissions increase abruptly as the knocking combustion occurs. The result also indicates that the operation region of the fuel with octane number 60 is largest in HCCI operation at the test condition.