This section discusses the two intake valve closing strategies that can be used to reduce pumping losses during part load as part of a throttleless load control strategy. These are termed early intake valve closing (EIVC) and late intake valve closing (LIVC). Both of these strategies keep the intake valve opening point in a fixed position, whilst moving the closing point to reduce pumping losses and hence improve fuel economy.
Early intake valve closing strategy
An ideal early IVC strategy allows air to be drawn into the cylinder as close as possible to atmospheric pressure at the beginning of the induction stroke. The intake valve closes part of the way through the induction stroke to prevent any further air from entering the cylinder, thus restricting the trapped air mass.
Once the intake valve has closed, the pressure inside the cylinder will fall as the piston moves towards BDC, but the work done by the piston to expand the air will mainly be recovered at the beginning of the compression stroke (once the intake valve is closed, the air within the cylinder will act like a gas spring, allowing this work recovery).
Figure 3 illustrates the principle of the ideal early IVC strategy in comparison to conventional throttled operation and highlights the reduction in intake pumping work that can theoretically be achieved.
It can be seen that the early IVC cycle still has a pumping loss associated because in practice the exhaust stoke will not reduce cylinder pressure to atmospheric because of exhaust back pressure, and the induction stroke will always draw air into the cylinder slightly below atmospheric pressure.
Furthermore, the expansion of the air between the intake valve closing and BDC is not quite reversible because heat transfer takes place into the trapped air from the cylinder walls.
An early IVC strategy offers the possibility of throttleless operation throughout the engine speed and load range because there is in principle no limit to the reduction of trapped air mass that can be achieved by early closing of the intake valve.
Late Intake Valve Closing Strategy
The ideal late IVC strategy allows air to be drawn into the cylinder close to atmospheric pressure for the whole of the induction stroke, thus drawing in the maximum possible air mass.
The intake valve remains open during the beginning of the compression stroke however, allowing some of the trapped air to flow back out of the cylinder into the intake manifold. Once the intake valve has closed, the air remaining in the cylinder is compressed in the conventional way.
Figure 4 illustrates the pumping loop for an ideal late IVC strategy and the reduction in pumping loss compared to conventional throttled operation.
It can be seen that as with the early IVC strategy, there is still pumping work associated with the difference in pressure between the exhaust and induction strokes, but unlike early IVC there is some additional pumping work associated with the portion of air that is drawn into the cylinder and expelled back into the intake manifold at the beginning of the compression stroke (work is done to force air past the slight restriction from the valve head).
The published research of this strategy indicates that whilst late IVC operation is able to demonstrate improvements in part load efficiency, it is not well suited to entirely throttleless operation.
This is because the trapped air mass required for light load conditions and idle can be so small that IVC needs to occur around TDC, resulting in very little charge compression.
In addition, IVC can occur significantly after the ideal ignition timing for such light loads, and so a conflict exists between the ideal IVC timing to reduce the trapped mass, and the ideal ignition timing, because ignition may only occur after IVC.
What Other Strategies Are There?
This concludes our summary of EIVC and LIVC, but what other part load strategies can be used in an SI engine to improve fuel economy ?
The next page looks at three other methods that are popular. These are gasoline direct injection, exhaust gas recirculation and cylinder deactivation.
Part 3 – Other part load strategies