Diesel Engine Starting Systems | Part 1

The Importance of Starting Systems: A functional machine needs a running engine, and if the engine doesn’t crank, it doesn’t start. A properly operating and reliable starting system is 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 Figure 1 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 Figure 2 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 Figure 3 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 Figure 4 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 starter can crank a diesel engine, the faster it starts and the quicker it runs clean. This engine cranking task is much more difficult in colder temperatures especially if the engine is directly driving other machine components such as hydraulic pumps, a torque converter, or a PTO (power take-off) drive shaft. Cold engine oil adds to the load on the starter, and this load may increase by three to four times what it would normally be in warmer weather. Engine oil that is the wrong viscosity (too thick) for the temperature will greatly increase the engine’s rolling resistance. Adding to this problem is the fact that a battery is less efficient in cold temperatures. When engineers design a cranking system, they must take into account cold weather cranking conditions and will quite often offer a cold weather starting option. This would likely include one or more of the following: bigger or more batteries, higher output starter, larger battery cables, battery blankets, oil heaters, diesel fired coolant heater, electric immersion coolant heater (block heater), and one or more starting aids like an ether injection system or an inlet heater. One more recent difficulty added to starting systems is a result of electronic controls on some engines. Some ECMs may need to see a minimum number of engine revolutions at a minimum speed before it will energize the fuel system. This equates to longer cranking times and more strain on the cranking system. Some electronic engines will crank for five seconds or longer even when the engine is warm before the ECM starts to inject fuel and the engine starts. It’s important that a machine’s starting system works properly and you should be aware of how the main components of a system work. This will give you the knowledge needed to make a proper diagnosis when you get a complaint of an engine cranking slowly or not at all. If an engine doesn’t start, then a machine isn’t working, and instead of making money, it’s costing money. The better you know how to diagnose and repair a starting system problem, the more valuable you will be as an HDET. There are lots of technicians who are good at changing starters whether the starter is faulty or not. Many times the cause of a starting complaint is something other than the starter. If a starter is used properly, it will last for well over 10,000 starts. The biggest factor in reducing the life of an electric starter is overheating from over-cranking. Never run the starter for more than a 30-second stretch, and if it does run that long, then wait at least two minutes between cranks to allow the starter to cool. For engines up to 500 hp, electric starting systems will be used for 99% of the applications. For any size engine, air and hydraulic starting systems are an option; however, they will likely only be used for special applications and usually for engines over 500 hp. (For More Parts Visit Our Blog)