If you have traveled in Boeing 737 airplane, you might have noticed that its engines have a strange and unusual shape which is round at the top and nearly flat at the bottom. Whereas, most of the other aircrafts have engines that are round in shape when viewed from front. Why is it so ?
Well the answer to this question is that Boeing 737 was originally designed with turbojet engines (Almost all of the engines are now ‘turbofan’ engines) which were much compact in size compared to turbofan engines now in use today. The turbojets, for the same size of turbofans, are much powerful. However, the turbojet engines ate fuel more than turbofan engines and were noisy as well.
When the turbofan engines became popular, Boeing faced the challenge of retrofitting the Boeing 737s with turbofan engines. The 737 was too low for the turbofan engines to be fitted as there was very less clearance between ground and the engine. The turbojets had no problem as they were small in size. So a modified design, as shown in the picture below was conceived, which would flatten the bottom to allow for more space between engine and ground. That is the motivation behind its unusual design.