Engine Performance: Theoretical vs. Practical Diagrams & Timing
Engine Performance: Theoretical vs. Practical Diagrams
Key Losses in Actual Motor Operation:
- Cargo pumping and drainage loss
- Gas cylinder pumping work
Theoretical vs. Practical Diagram Differences
In a practical diagram, the admission pressure remains below atmospheric pressure, and the pump exhaust exceeds the theoretical. Negative work load occurs in the cylinder.
Losses in the Cylinder
- Incomplete cylinder filling results in compression pressure lower than theoretical.
- Heat loss to the cooling circuit.
- Pressure losses compared to the theoretical cycle, where no heat loss is considered.
Combustion Time Losses
In theory, combustion occurs at constant volume, but in reality, combustion time is required to reach maximum pressure. During this time, the piston moves, causing a volume variation.
Exhaust Losses
Premature exhaust causes a rapid pressure drop before the piston reaches Bottom Dead Center (BDC), resulting in energy loss.
Ignition Timing
Gas combustion is not instantaneous and requires time to achieve maximum pressure. Ignition should be initiated slightly before the piston reaches Top Dead Center (TDC) to maximize boost immediately after passing TDC.
Ignition advance is approximately between 5 and 40 degrees before TDC, depending on rotation speed and engine load. Faster piston speeds require earlier ignition.
Valve Timing (Distribution Chart)
EPA (Exhaust Port Opening): The piston descends during expansion. The exhaust valve opens before BDC, when the cylinder pressure is 3 to 4 bar, causing rapid gas expulsion and reducing pressure to 0.2 bar, facilitating the piston’s upstroke.
AAA (Admission Advance Angle): During the exhaust stroke, the intake valve opens a few degrees before TDC, while the exhaust valve is still open. This allows fresh gas to be drawn into the cylinder as the piston starts its downward stroke, ensuring almost complete admission.
RCE (Retard Closing Exhaust): The exhaust valve closes after the piston passes TDC. The intake valve is already open, and the piston has begun its downward stroke. Gases continue to exit due to inertia, achieving good waste gas scavenging. The angle at which both valves remain open is called valve overlap.
RCA (Retard Closing Admission): During the intake stroke, the valve closes after the piston passes BDC. The high-speed fluid entering due to intake inertia allows continuous gas entry, improving cylinder filling even as the piston begins its upward stroke.
Valve Overlap
Valve overlap occurs between AAA and CER. At the end of the exhaust stroke, gases continue to exit due to inertia. As the piston approaches TDC, this speed difference creates a slight vacuum inside the cylinder, causing fresh gases to enter and improve cylinder filling.