Here’s an explanation of each diesel engine term:
(a) Overlap:
Overlap refers to the period in a four-stroke diesel engine cycle where both the intake and exhaust valves are open slightly at the same time. This timing is crucial for efficient operation:
- Exhaust Valve Closing: The exhaust valve starts closing near the end of the exhaust stroke.
- Intake Valve Opening: A little before the piston reaches the bottom of its stroke (end of exhaust), the intake valve opens. This allows some fresh air to enter the cylinder to help expel remaining exhaust gases.
- Benefits: The overlap period helps scavenge exhaust gases more effectively and promotes a cooler intake charge due to the incoming fresh air.
(b) Scavenging:
Scavenging is the process of removing exhaust gases from the engine cylinder and replacing them with fresh air during the engine cycle. This is essential for several reasons:
- Combustion Efficiency: Proper scavenging ensures a mostly fresh air charge is present in the cylinder for efficient fuel combustion during the power stroke.
- Reduced Emissions: By removing exhaust gases, scavenging helps minimize pollutants like unburned hydrocarbons and oxides of nitrogen (NOX) in the exhaust.
- Cooler Operation: Fresh air entering the cylinder helps lower the overall engine temperature.
Overlap between the exhaust and intake valve cycles plays a critical role in the scavenging process.
(c) Afterburning:
Afterburning, also known as exhaust gas treatment, is not a typical part of the four-stroke diesel engine cycle itself. It refers to additional technologies used to reduce harmful emissions in the exhaust after it leaves the engine cylinder.
Common afterburning techniques include:
- Catalytic Converters: These devices promote chemical reactions that convert pollutants like NOX into less harmful substances.
- Diesel Particulate Filters (DPF): DPFs trap soot particles in the exhaust, reducing their release into the atmosphere.
(d) Compression:
Compression is a crucial stage in the four-stroke diesel engine cycle:
- Intake Stroke: The piston moves down, drawing in a mixture of air and fuel (air only in direct injection engines) through the open intake valve.
- Compression Stroke: With the intake valve closed, the piston moves up, significantly reducing the volume of the air-fuel mixture within the cylinder. This compression dramatically increases the temperature and pressure of the mixture.
The high temperature and pressure created during compression are essential for efficient fuel ignition in a diesel engine without the need for spark plugs.
(e) Expansion:
Expansion is the power stroke of the four-stroke diesel engine cycle:
- Combustion: Under the high pressure and temperature from compression, the injected fuel ignites spontaneously. This burning fuel rapidly expands, pushing the piston down with great force.
- Expansion Stroke: The expanding hot gases from combustion force the piston down, rotating the crankshaft and generating power output from the engine.
The force exerted by the expanding gases during this stroke is what ultimately propels the engine.