Scavenging and supercharging

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In the intake systems of all 2-stroke cycle diesel engines and some 4-stroke cycle diesel engines, a device, known as a blower, is installed to increase the flow of air into the cylinders. The blower compresses the air and forces it into an air box or manifold, which surrounds or is attached to the cylinders of an engine. Thus, more air under constant pressure is available as required during the cycle of operation.

The increased amount of air, a result of blower action, fills the cylinder with a fresh charge of air. During the process, the increased amount of air helps to clear the cylinder of the gases of combustion. The process is called SCAVENGING. Therefore, the intake system of some engines, especially those operating on the 2-stroke cycle, is sometimes called the scavenging system. The air forced into the cylinder is called scavenge air, and the ports through which it enters are called scavenge ports.

Scavenging must take place in a relatively short portion of the operating cycle; the duration of the process differs in 2- and 4-stroke cycle engines. In a 2-stroke cycle engine, the process takes place during the latter part of the downstroke (expansion) and the early part of the upstroke (compression). In a 4-stroke cycle engine, scavenging takes place when the piston is nearing and passing TDC during the latter part of an upstroke (exhaust) and the early part of a downstroke (intake). The intake and exhaust openings are both open during this interval of time. The overlap of intake and exhaust permits the air from the blower to pass through the cylinder into the exhaust manifold, cleaning out the exhaust gases from the cylinder and, at the same time, cooling the hot engine parts.

When scavenging air enters the cylinder of an engine, it must be so directed that the waste gases are removed from the remote parts of the cylinder. The two principal methods by which this is accomplished are referred to as PORT UNIFLOW SCAVENGING and VALVE UNI-FLOW SCAVENGING. In the uniflow method of scavenging, both the air and the burned gases flow in the same direction. This action causes a minimum of turbulence and improves the effectiveness of the scavenging action. An example of a port uniflow system is shown in figure 6-1. An example of a valve uniflow system is shown in figure 6-2.

Scavenging and supercharging are not common to all diesel engines. For instance, in some 4-stroke cycle engines, the air enters the cylinder as a result of a pressure difference created by the piston as it moves away from the combustion space during the intake event. This type of intake is sometimes referred to as the suction-type, or naturally aspirated, intake; however, the air is actually forced into the cylinder because of the greater pressure outside the cylinder.

An increase in airflow into the cylinders of an engine can serve to increase power output, in addition to being used for scavenging. Since the power of an engine comes from the burning of fuel, an increase in power requires more fuel; the increased fuel, in turn, requires more air since each pound of fuel requires a certain amount of air for combustion. The supplying of more air to the combustion spaces than can be supplied through the action of atmospheric pressure and piston action (in 4-stroke cycle engines) or scavenging air (in 2-stroke cycle engines) is called SUPERCHARGING.

In some 2-stroke cycle diesel engines, the cylinders are supercharged during the air intake simply by an increase in the pressure of scavenging air. The same blower is used for super-charging and scavenging. Scavenging is done when air is admitted under low pressure into the cylinder while the exhaust valves or ports are open. Super-charging is done with the exhaust ports or valves closed, a condition that enables the blower to force air under pressure into the cylinder and

Figure 6-1.-Port uniflow system in a Fairbanks-Morse engine.

Figure 6-2.-Valve uniflow system in a General Motors 2-stroke cycle diesel engine.

thereby increase the amount of air available for combustion. An engine is referred to as super-charged when the manifold pressure exceeds the atmospheric pressure. The increase in pressure, resulting from the compression action of the blower, will depend on the type of installation. With the increase in pressure and amount of air available for combustion, there is a corresponding increase in combustion efficiency within the cylinder. In other words, an engine of a given size that is supercharged can develop more power than an engine of the same size that is not supercharged.

For a 4-stroke diesel engine to be super-charged, a blower must be added to the intake system since exhaust and intake in an unsupercharged engine are performed by the action of the piston. The timing of the valves in a super-charged 4-stroke cycle engine is also different from that in a similar engine that is not super-charged. In the supercharged engine, the closing of the intake valve is slowed down so that the in-take valves or ports are open for a longer time after the exhaust valves close. The increased time that the intake valves are open (after the exhaust valves close) allows more air to be forced into the cylinder before the start of the compression event. The amount of additional air that is forced into the cylinder and the resulting increase in horse-power depends on the pressure in the air box or intake manifold. The increased overlap of the valve openings also permits the air pressure created by the blower to remove gases from the cylinder during the exhaust event. Study figure 6-3 (at the end of this chapter) so that you will understand how the opening and closing of the intake and exhaust valves, or ports, affect both scavenging and supercharging. Also, note the differences in these processes as they occur in supercharged 2- and 4-stroke cycle engines.

In figure 6-3, the circular pattern represents crankshaft rotation. Some of the events occurring in the cycles are shown in degrees of shaft rotation for purposes of illustration and easier comparison only. (When dealing with the timing of a specific engine, check the appropriate instructions.)

In studying figure 6-3, keep in mind that the crankshaft of a 4-stroke cycle engine makes two complete revolutions in one cycle of operation, while the shaft in a 2-stroke cycle engine makes only one revolution per cycle. Also, keep in mind that the exhaust and intake events in a 2-stroke engine do not involve complete piston strokes as they do in a 4-stroke engine.


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