Diesel Engine is the main power source for the locomotive. It comprises a large cylinder block, with the cylinders arranged in a straight line or in a V. The engine rotates the drive shaft and this drives the various items needed to power the locomotive. As the transmission is normally electric, the engine is used as the power source for the alternator that produces the electrical energy to drive the locomotive. Diesel engine can be classified into two-stroke and four-stroke groups. In general, two stroke engines are smaller in size and have less parts compared to four stroke ones which means that they need less maintenance. On the other hand four stroke engines have less emission and less fuel consumption.