These terms all relate to the movement of the piston within the cylinder of a diesel engine and the resulting volumes created:
(a) Top Dead Center (TDC):
This refers to the uppermost point in the cylinder that the piston reaches during its movement. At TDC, the piston is as close as it gets to the cylinder head.
(b) Bottom Dead Center (BDC):
This is the opposite of TDC and represents the lowest point in the cylinder that the piston reaches during its movement. At BDC, the piston is as far away from the cylinder head as it gets.
(c) Piston Stroke:
The piston stroke is the total distance the piston travels between TDC and BDC. It’s essentially the length of the piston’s movement within the cylinder.
(d) Swept Volume:
The swept volume is the theoretical volume displaced by the piston during one complete stroke (up and down) within the cylinder. It’s calculated by multiplying the cylinder’s bore area (π * bore radius squared) by the piston stroke.
(e) Clearance Volume:
The clearance volume is the volume remaining in the cylinder at TDC. With the piston at its highest position, there’s still some space between the piston head and the cylinder head. This clearance volume is crucial for several reasons:
- Combustion Space: It provides the necessary space for air (or air-fuel mixture) to be compressed before ignition.
- Piston Expansion: The clearance volume allows room for the piston to expand slightly without contacting the cylinder head.
- Engine Starting: Adequate clearance volume is necessary for the engine to crank freely without the piston hitting the cylinder head.
(f) Compression Ratio:
The compression ratio is a critical parameter in a diesel engine and is calculated by dividing the total cylinder volume (swept volume + clearance volume) by the clearance volume. Here’s the formula:
Compression Ratio = (Total Cylinder Volume) / (Clearance Volume)
A higher compression ratio signifies a greater compression of the air (or air-fuel mixture) in the cylinder, leading to:
- Higher Efficiency: Increased compression creates higher temperatures and pressures during combustion, improving fuel efficiency.
- More Power: The higher pressure generated during combustion translates to more power output from the engine.
However, there are limits to compression ratio due to factors like:
- Fuel Ignition: Very high compression ratios can cause the air to ignite spontaneously before the intended injection of fuel, leading to engine knocking.
- Engine Design: The engine needs to be designed to withstand the increased forces associated with higher compression.