This work investigates the effect of fuel reactivity, mixture preparation (premixed or diffusive) and equivalence ratio (phi=0.6, 1.0) on the combustion and emission characteristics of ethanol, diesel-gasoline, n-heptane-isooctane, n-heptane-ethanol and decane-ethanol in a constant volume bomb. It provides the direction to improve the combustion efficiency and reduce the pollutant emissions for the internal combustion engines. The fuel selection criterion is to establish the fuel reactivity gradient by blending the high and low octane number fuels. The main conclusions are summarized below: First, the premixed fuel-lean combustion, also known as homogeneous charge autoignition (HCAI), can improve the combustion efficiency by elevating the peak pressure and reducing the combustion duration. The ethanol combustion efficiency increases from 41.0% at 1.0-D condition to 68.7% at 1.0-P condition by breaking the limit of the fuel-air mixing rate, and finally reaches 72.8% at 0.6-P condition through increasing the oxygen concentration and facilitating the hydrocarbon oxidation. Second, the combination of the high reactivity (low RON) and high volatility fuels can further improve the combustion efficiency that the former acts as the distributed ignition source and the latter forms the homogeneous charge. The combustion efficiency of n-heptane-ethanol at 0.6-P condition reaches 97.2%. Third, the HCAI can reduce the NOx emission by enhancing the mixture uniformity, constructing high dilution to lower the peak combustion temperature. Fourth, the premixed fuel-lean combustion produces higher CO and HC emission compared to diffusive combustion due to the relatively low reaction temperature. Fifth, the premixed fuel-lean combustion can significantly reduce the particle mass and number concentration by decreasing the production of the accumulation mode particles. Sixth, the high volatility/RON fuel (such as ethanol) alone is not the best candidate for HCAI combustion because their high autoignition resistance makes it difficult to ignite spontaneously and produce a large number of unburned hydrocarbons. Then they nucleate to form new particles or condense on solid particles.