A diesel engine converts the chemical energy in fuel into motion by burning fuel inside a cylinder. Its key feature is ignition by compression, which means the air gets so hot when squeezed that diesel fuel ignites without a spark plug. This makes diesel engines efficient and strong at low speeds, which is why they are common in trucks, buses, tractors, ships, and generators.
Understanding the four-stroke cycle helps explain how pressure, heat, and mechanical motion work together in a vehicle engine.
During the intake stroke, the engine draws in air only, then the piston compresses that air to a much smaller volume. Near the top of the compression stroke, an injector sprays diesel fuel into the hot compressed air, creating rapid combustion and a high-pressure push on the piston. The piston drives the connecting rod and crankshaft, turning straight-line motion into rotation.
Finally, the exhaust valve opens so burned gases leave the cylinder and the cycle can repeat.
Key Facts
- A diesel engine uses compression ignition, not spark ignition.
- Four strokes are intake, compression, power, and exhaust.
- Compression ratio = maximum cylinder volume / minimum cylinder volume.
- Diesel engines often use compression ratios of about 14:1 to 22:1.
- Work from one power stroke can be described by W = Fd, where force on the piston acts through a distance.
- Engine power can be estimated by P = W/t, where work per cycle is delivered over time.
Vocabulary
- Compression ignition
- Compression ignition is the process in which fuel ignites because highly compressed air becomes hot enough to start combustion.
- Injector
- An injector is a precision nozzle that sprays fuel into the cylinder as a fine mist at the correct time.
- Piston
- A piston is a moving metal part inside the cylinder that is pushed by expanding gases during combustion.
- Crankshaft
- A crankshaft is a rotating shaft that converts the piston and connecting rod motion into useful rotary motion.
- Compression ratio
- Compression ratio is the ratio of the cylinder volume when the piston is at the bottom to the volume when the piston is at the top.
Common Mistakes to Avoid
- Thinking a diesel engine uses spark plugs for normal ignition. Diesel engines normally ignite fuel by injecting it into hot compressed air, not by making an electric spark.
- Assuming fuel and air enter together during the intake stroke. In most diesel engines, only air enters first, and fuel is injected near the end of compression.
- Confusing high compression with high fuel amount. Compression heats the air, while the injected fuel amount controls much of the engine load and power output.
- Ignoring the timing of injection. If fuel is injected too early or too late, combustion pressure happens at the wrong piston position and the engine loses efficiency or can be damaged.
Practice Questions
- 1 A diesel cylinder has a maximum volume of 900 cm3 and a minimum volume of 50 cm3. What is the compression ratio?
- 2 During a power stroke, expanding gases exert an average force of 6000 N on a piston over a distance of 0.09 m. How much work is done on the piston?
- 3 Explain why a diesel engine can ignite fuel without a spark plug, and why the fuel is injected near the end of the compression stroke rather than at the beginning.