DIESEL ENGINES

Introduction

A diesel engine is an internal combustion engine that uses compression ignition to ignite the fuel as it injected into the engine.

Diesel versus Gasoline
It is helpful to an understanding of how diesel engines work to compare the differences between a diesel engine and a gasoline engine. The main differences between the gasoline engine and the diesel engine are:

• A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine intakes just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air ignites the fuel spontaneously. A diesel engine does not contain a spark plug.
• A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.
• Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder. A diesel engine compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power generated.

The injector on a diesel engine must be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. To ensure that the mist is evenly distributed throughout the cylinder, some diesel engines are equipped with special induction valves or pre-combustion chambers.
In addition, some diesel engines contain a glow plug. When a diesel engine is cold, the compression process may not raise the air temperature high enough to ignite the fuel. The glow plug is an electrically heated wire that facilitates fuel ignition when the engine is cold. Glow plugs are typically found on small diesel engines.

Combustion Cycle - Step-By-Step
Step 1: Intake (Down) Stroke 1: The piston moves down drawing air into the cylinder.
Step 2: Compression (Up) Stroke 1: The piston moves up compressing the newly drawn air in the cylinder. Before the piston reaches top-dead-center (TDC), diesel fuel is injected directly into the cylinder. This results in combustion of the diesel fuel.
Step 3: Intake Stroke (Down) 2: The piston draws down but the intake and exhaust valves are not open.
Step 4: Compression Stroke (Up) 2: The piston moves up forcing the combusted diesel fuel out of the cylinder in the form of exhaust.
Step 5: Process repeats.