The Effects Of Valve Motion Upon The Early IVC Cycle
Although the early IVC strategy has been demonstrated to improve part load efficiency in the published data, the theoretical benefits of the ideal cycle as shown in figure 4 (see part2) cannot be achieved across the whole operating range of the engine.
The requirement of drawing air into the cylinder close to atmospheric pressure becomes progressively more difficult to achieve as the trapped air mass is reduced because of the limitations in the valve motion that can be achieved.
Ideal Valve Motion for Early IVC
In order to draw air into the cylinder without creating a significant pressure drop across the intake valve it is necessary to maximise the available flow area.
The ideal valve motion for an early IVC strategy would therefore resemble a square wave starting around TDC with its duration being variable to control the mass of air trapped in the cylinder.
Figure 6: - Ideal Intake Valve Motion for Early Intake Valve Closing
Figure 6 illustrates the ideal valve motion for minimising pumping losses during the intake stroke. Clearly it is not possible to produce a valve actuation system that achieves instantaneous opening and closing, and so any practical system will tend to restrict the airflow into the cylinder to some extent while the intake valve is open.
In addition, the design of most modern engines is such that valve to piston contact would occur if the valve were to achieve its maximum lift close to TDC.
The provision of sufficient valve to piston clearance to allow maximum valve lift to occur around TDC would severely compromise the combustion chamber design, and so it is accepted that valve lift must be restricted around TDC.
Practical Valve Motion for Early IVC
Figure 7: - Typical Characteristics of Camshaft Operated Early IVC System
Figure 7 shows a typical valve motion for a mechanical early IVC system, and it can be seen that the airflow into the cylinder will tend to be restricted by the intake valve, at the valve opening and closing points.
This means that in practice the trapped air mass will be controlled by a combination of valve head throttling and early IVC.
Valve head throttling reduces the benefits of an early IVC strategy by increasing the size of the pumping loop.
In some cases, valve head throttling may only cause a significant restriction close to the valve opening and closing points, effectively delaying the valve opening.
This will result in a PV diagram as shown in figure 8 where there is an initial pressure drop until the intake valve opens sufficiently and pressure returns to the more ideal early intake valve closing characteristic illustrated in figure 4 (see part 2).
If a lower valve lift characteristic is used, the airflow in to the cylinder may be restricted by valve head throttling throughout the whole opening duration, and the cylinder pressure will be reduced significantly below atmospheric pressure throughout the intake stroke as shown in figure 9.
Figure 8 - Effect of Delayed IVO
Figure 9 - Effect of Valve Head Throttling
Thus in practice the part load pumping work that can be achieved through early intake valve closing will fall somewhere between that of the throttled cycle (figure 3) and the ideal early IVC cycle (figure 4).
Conclusion
This concludes our look at the practical valve motion for an intake valve closing strategy and how this may differ from the ideal. Now continue to part5, where we look at the performance benefits associated with throttleless operation.
Part 5 - The performance benefits