Design of Gas Exchange Devices in Four-Stroke Engines

Charge Cycle Energy Loss. The loss of charge cycle energy reduces the indicated work and, hence, the indicated output; this causes the specific consumption to rise. It occurs either as expansion energy loss at the beginning of the charge cycle [between EO (exhaust opens) and BDC] or as increased pumping work during the charge cycle. Pumping work represents the work required by the piston to draw a fresh charge during the intake cycle into the combustion chamber, and to expel the exhaust from the combustion chamber during the exhaust cycle. Correspondingly, the pumping work can be divided into intake work and exhaust work. In calculations, the pumping work is represented with the aid of the average effective pump pressure.

During intake, pressure losses occur at several locations, and this raises the intake work: Flow loss when the medium enters and leaves the intake system, pressure drop in the lines because of bends and rough surfaces, pressure loss in the air filter, at the airflow sensor, at the throttle valve, and loss at the valves. Assuming quasistationary flow, the overall pressure loss in the intake system toward atmospheric pressure can be described by the sum of the individual losses of the various components

where equals the loss coefficient, v_i the local flow speed, and A_i the smallest flow cross section of the respective component. This makes it clear that to achieve less pump work in the charge cycle, greater flow cross sections are desirable, and that the pressure loss depends on the average piston speed v_K or the rpm; i.e., it increases with engine speed. The cross-sectional flow can be increased by increasing the size of the geometric opening cross section (valve stroke, valve seat ring diameter, number of valves).

Increased intake work primarily arises from throttle control while operating under a partial load. In spark- ignition engines, the amount of charge in the partial-load range required for the desired load is attained by adjusting the throttle valve, i.e., by changing the flow cross section. The piston must aspirate corresponding to the pressure loss at this point against a lower pressure than the atmosphere (the absolute pressure of the intake pipe drops). In the idling range, the increased intake work can be up to 30% of the work accomplished by the engine (Figs. 10-26 and 10-27).

Fig. 10-26. Intake energy loss without throttling under a full load

Fig. 10-27. Intake energy losses with throttling under a partial load

The charge cycle energy losses are represented by the hatched area (without throttling) and by the hatched and dotted areas (with throttling). From the EO to the BDC, there is a loss of expansion work. The expulsion of the exhaust produces loss from exhaust work. Upon induction of the fresh charge at a vacuum, intake work is expended. Throttling loss occurs during throttling, in addition to the expansion and flow losses from the actuators. In a best- case scenario, the losses (hatched area) to the left of the compression line in Fig. 10-27 are avoided by controlled intake without throttling, such as by infinitely variable valve gears. In this case, the supplied amount of charge is controlled by adjusting the valve timing (IC is very important in this instance) or—depending on the variability of the system—by the variable stroke of the intake valve.

Increased pump work results not only from the intake of fresh air at a vacuum, but also during the expulsion of exhaust. Although the combustion gases are at a higher pressure than atmospheric pressure, they cannot leave the cylinder at the right time through the outlet and the exhaust system without work being done by the piston (i.e., before the end of the expulsion cycle). The exhaust counterpressure has a decisive influence on this process (Fig. 10-28).

Fig. 10-28. Exhaust work

Fig. 10-29. Expansion energy loss

For the charge cycle, the time of the EO is very important. This time always represents a compromise. When the EO is late, more expansion work is gained, and consumption is lowered. At higher speeds, however, greater exhaust work is required for the exhaust to leave the cylinder within the shorter period, which increases consumption. With an early EO, less exhaust work is necessary since the cylinders can be purged more easily and quickly. However, expansion work is lost, and the thermal load on the exhaust valve increases (Fig. 10-29).