A functional machine needs a running engine, and if the engine doesn’t crank, it doesn’t start. A properly operating and reliable diesel engine starting systems are a must for keeping a machine productive.
For many years, diesel engines have mostly used electric motors to crank them over to start the combustion process. For some applications, an air or hydraulic motor will create the torque needed to turn the engine over.

Many years ago, diesel engines were sometimes started with a smaller gas engine called a pup engine. See the above Figure for a pup engine on an older diesel engine. Another way to get a diesel engine started was to start it on gasoline and then switch it over to run on diesel fuel. This was a complex solution to a simple task because the engine had to have a way to vary its compression ratio, and it needed a spark ignition system and a carburetor. As 12V electrical systems became more popular and electric motor design improved, electric starters were able to get the job done. Many large diesel engines will use a 24V starting system for even greater cranking power. See the below Figure for a typical arrangement of a heavy-duty electric starter on a diesel engine.

A diesel engine needs to rotate between 150 and 250 rpm to start. The purpose of the starting system is to provide the torque needed to achieve the necessary minimum cranking speed. As the starter motor starts to rotate the flywheel, the crankshaft is turned, which then starts piston movement. For a small four-cylinder engine, there doesn’t need to be a great deal of torque generated by a starter. But as engines get more cylinders and bigger pistons, a huge amount of torque will be needed to get the required cranking speed. Some heavy-duty 24V starters will create over 200 ft-lb of torque. This torque then gets multiplied by the gear reduction factor between the starter motor pinion gear and ring gear on the engine’s flywheel. This is usually around 20:1. See the below Figure for how a starter assembly pinion engages with the flywheel ring gear

Some larger engines will need two or more starters to do this. Some starters for large diesel engines will create over 15 kW or 20 hp! See the below Figure for a double starter arrangement.

When a starter motor starts to turn the engine over, its pistons start to travel up in the cylinders on compression stroke. There needs to be between 350 and 600 psi of pressure created on top of the piston. This is the main resistance that the starter has to overcome. This pressure is what is needed to create the necessary heat in the cylinder so that when fuel is injected it will ignite. If the starting system can’t crank the engine fast enough, then the compression pressure and heat won’t be high enough to ignite the fuel. If the pistons are moving too slowly, there will be time for the compression to leak by the piston rings. Also the rings won’t get pushed against the cylinder, which again allows compression pressure to leak into the crankcase. When this happens, the engine won’t start or it starts with incomplete combustion. Incomplete combustion equals excessive emissions. This is another reason to have a properly operating starting system.

The faster a diesel engine starting systems can crank a diesel engine, the faster it starts and the quicker it runs clean.