Diesel engines are more efficient than petrol engines coz they have higher compression ratio which heats the inlet air and obtain higher efficiency from air fuel mixture to same quantity of air fuel mixture in petrol engine. 

* first in simple words ( efficiency is the ratio of useful output to total input ) .

In case of engines there are various components which contributes to overall engine efficiency :

1/ thermal efficiency

2/ mechanical efficiency

overall efficiency =(thermal efficiency)*(mechanical efficiency).

* first the diesel engines have higher thermal efficiency than petrol engines , because they run on higher compression ratios .

compression ratio of diesel 12 to 25

compression ratio of petrol 6 to 10 .