Intake Systems. Ram Tube Charging

Both in the intake system and in the exhaust system, gas- dynamic processes occur that are based on the periodic excitation of the piston and natural frequency of the system. These can be used to improve the charge cycle process. These gas-dynamic effects in the intake system are divided into ram tube and resonance effects. A schematic illustration of both intake systems is shown in Fig. 10-30.

Fig. 10-30. Diagram of the ram tube and resonance

Ram Tube Charging. The ram tube effect is based on the vacuum wave triggered by the descending piston that travels in the induction pipe opposite the direction of flow to the common plenum chamber and is reflected there at the open tube end. The overpressure wave that arises in this manner increases the cylinder charge by increasing the pressure gradient via the intake valve. This effect is particularly useful briefly before the intake valves are closed while the piston is ascending. The pressure wave prevents the expulsion of the fresh charge from the combustion chamber into the induction pipe and generates a charging effect.

Corresponding to the acoustic design, the pressure wave requires the following time at speed a to leave and return in the ram tube:

The inlet time (from IO to IC) should average one-third of the time required for an engine revolution at a given speed:

This allows the optimum length of the induction pipe to be determined at a given speed n:

Hence, the induction pipe length is the quantity that determines the ram tube effect. Corresponding to the acoustic design, there is a preferred speed for each induction pipe length at which there is maximum air expenditure. This has been demonstrated in engine tests in which only the induction pipe length was varied. Figure 10-31 shows the influence of the induction pipe length on the maximum mean effective pressure. A shorter induction pipe shifts the torque peak in the direction of higher speeds and vice versa.

Fig. 10-31. Influence of the induction pipe length L₁ on the maximum mean effective pressure over the speed

In real engine operation, however, the influence of the induction pipe length is more complex and partially overlaps with the influence of other intake-side parameters. For example, in addition to the pressure characteristic before the closing intake valve, the charge cycle is strongly influenced by the formation of a free vibration in the induction pipe in the period between IC and 10 in correlation with the intake vibration that forms in the period between IO and IC.

A fixed induction pipe length is therefore advantageous only within a specific range of speed. At higher speeds, a short induction pipe length is desirable, and, at slow speeds, a long pipe is desirable. Engines are therefore designed with a multistage manifold; i.e., the induction pipe length is adapted to the engine speed (Fig. 10-32).

Fig. 10-32. Intake system with two-stage manifold; diagram (Audi V6)

When the throttle valve is open, the intake wave coming from the cylinder is reflected at this point (high speeds from 4000 min^-1). At speeds up to 4000 min^-1, the throttle valve is closed (long induction pipe). Figure 10-33 shows a further developed three-stage intake manifold. Recently, stageless variable induction pipes have also been used.

Fig. 10-33. Intake system with three-stage manifold

While the time of the waves depends on the induction pipe length, the amplitude of the wave is influenced by the induction pipe cross section. The flow speed in the induction pipe rises with the rpm so that the amplitude correspondingly rises as well (Fig. 10-34). Sufficiently high amplitudes to yield a corresponding recharging effect at low speeds can be created with a small induction pipe cross section. At high speeds, however, the cylinder charge falls with a small flow cross section. A good cylinder charge at high speeds, therefore, requires a large induction pipe cross section.

Fig. 10-34. Air expenditure as a function of the pipe diameter

When there are several intake valves such as those used in four-valve engines, the induction pipe cross section can be adapted as a function of the load and speed by closing a port (Fig. 10-35). At low speeds and a low load, only the primary port is used. As the speed and load increase, the secondary port is added.

Fig. 10-35. Intake systems with channel closing

Fig. 10-36. Influence of closing a port on air expenditure

At lower speeds, the cylinder charge is better when the shutoff valve is closed (Fig. 10-36). In addition, a specific charge motion (swirling) can be generated with the inflow to improve the mixture. This increases the efficiency during partial-load operation, especially when the engine is operated with a lean mixture (lean engine).