How Does a Diesel Engine Start?

So how does a diesel engine turn fuel into power? Diesel engines operate using the same four strokes as gas engines:

1. Intake stroke: In this phase, the piston travels from top dead center (TDC) to bottom dead center (BDC). This results in low pressure inside the cylinder. All diesel engines are turbocharged, so air isn’t drawn in but is waiting behind the intake valve under pressure. As soon as the valve opens, the air rushes in.

2. Compression stroke: The piston travels from BDC back to TDC and prepares for combustion. The compression creates heat, which ignites atomized diesel fuel. There is no spark necessary — diesel fuel will self-ignite if it is atomized, or sprayed into the cylinder in a fine mist.

3. Power stroke: In the power stroke, the piston goes from TDC to BDC. The combustion creates pressure to move the piston downward, and this shifts the power to the crankshaft.

4. Exhaust stroke: In the last stroke, the piston travels from BDC to TDC. As it rises, exhaust gases are pushed from the cylinder and through the exhaust valve. They go into the emissions system and are then pushed from the vehicle.

Diesel engines do not use spark plugs to ignite the fuel mixture. The higher compression ratio in the engines produces enough heat to ignite the mixture. Additionally, diesel fuel itself allows for combustion without the use of a spark plug.

Diesel Engine Diagram

A key difference between diesel and gas engines is fuel management. Gas engines control the flow of air into the cylinder. By increasing airflow, gasoline increases by default. Diesel engines directly control fuel flow into the cylinder, and more fuel equals more engine speed.

To overcome the existing air pressure in the combustion chamber, diesel engines need to generate a significant amount of fuel pressure. The pictured diesel engine relies on a pressurizing fuel pump and a high-pressure common rail system to create, maintain and deliver fuel at precisely the correct time.

The high-pressure common rail sits under the valve cover. It distributes fuel to the individual cylinders via the fuel injectors. Unused fuel cools down in the fuel cooler before the return line transports it back to the fuel tank.

How This Applies to a Diesel Technician Career

It’s important for diesel technicians to know diesel engines inside and out. Understanding how each of the different components work together is what allows technicians to diagnose, repair and maintain these engines.

Because of their many benefits, diesel engines are here to stay — and as with anything in the transportation industry, they will likely continue to evolve and improve. Today’s diesel techs must be able to keep up with these changes to stay relevant.

Being able to work on diesel engines has the potential to provide a variety of career opportunities, whether it be in a dealership, repair shop, truck stop or even the railroad industry. Skilled diesel techs can take their skills to many different industries, including:

• Trucking and transportation

• Railroad

• Marine

• Agriculture

• Power generation

• Service writing

In some cases, diesel techs are able to learn skills on the job, but many employers prefer candidates who have completed a postsecondary training program, like the Diesel Technology program at Universal Technical Institute (UTI).

Students who train at UTI can learn everything from truck brakes and chassis to fuel systems, preventive maintenance, and transportation refrigeration. They also dive into diesel engines — learning the principles of operation of four-stroke engines, diesel combustion fundamentals and other characteristics unique to these powerful engines.