Engines on the 737 Classic series (300, 400, 500) and Next Generation series (600, 700, 800, 900) are not shaped like a circle like most planes. The 737 Classics had CFM56 turbofan engines. These burned much less fuel and made much less noise than the Pratt & Whitney JT8D engines which were used on the -100 and -200. However, because the 737's wings and fuselage are not very high above the ground, it was impossible to get the engines to fit on the wings. Boeing and CFM fixed this problem by putting the engines slightly ahead of the wings instead of putting them right underneath. They also moved some parts of the engines which would normally go at the bottom to the sides. This is what makes the engines non-circular.