Why efficiency of heat engine is more in winter as compare to summer?

Because a thermal engine efficiency is directly linked to the difference of the temperature of the gases involved .As the gasoline burns at a set temperature , the lower winter temp helps to widen the temp differential between the ignited gasoline and exaust. Unfortunatley the colder air is more dense and generate more drag than in summer , more than negating the increased engine efficiency.

To put it in rough numbers , you may get 0.5% more engine efficiency but you lose some 4% more to extra drag.