Scavenging Air Supply

Whereas the pressure gradient for the charge cycle arises from the expulsion and intake process of the engine- transmission unit itself in four-stroke engines, the required scavenging pressure gradient for the charge cycle in two- stroke engines is generated by a separate scavenging blower (compressor). The cylinder can be scavenged only when the intake and exhaust organs are open simultaneously.

The flow through the intake and exhaust organs can be described in simplified terms as a flow through two series-connected throttles that can be replaced by an equivalent cross section. Since, apart from influences such as pressure pulsation, gas temperature, and exhaust counterpressure, it does not matter whether the ports or valves open and close a few times slowly or many times quickly within a given period of time, the air flow rate through a two-cycle engine to produce a respective scavenging pressure gradient is independent of the engine speed.

In contrast, there is a quadratic relationship in the first approximation between the scavenging pressure gradient and the scavenging air quantity. At higher engine speeds, a much higher scavenging pressure is needed to attain the same result. The amount of scavenging air can be varied over wide ranges for a corresponding mapping point depending on the required engine temperature, exhaust temperature, emissions, consumption, and engine performance (supercharging), assuming that the scavenging blower is correspondingly flexible. A displacement-type compressor (reciprocating piston compressor and rotary piston compressor) as well as flow compressors can be used for scavenging or possibly supercharging two-stroke engines. Figure 10-60 shows an overview of different blowers and supercharging designs.

Fig. 10-60. Overview of the different designs of blowers and superchargers: (a) Vane-type superchargers, (b) Roots superchargers, (c) Rotary piston superchargers, (d) Screw compressors, (e) Spiral superchargers (G-superchargers), (f) Turbochargers

Reciprocating piston compressor: The simplest type of reciprocating piston compressor for two-stroke engines uses the crank housing and the bottom of the piston to enclose the working volume. With this design that is particularly widespread among small two-stroke spark ignition engines (the advantages are a compact design, low additional costs, steep compression curve, low additional drive power), the working gas generally flows through holes in the cylinder wall or piston shaft into the crank housing when the piston moves upward. When the piston subsequently executes a downward movement, the fresh gas is compressed and flows via overflow ports and from scavenging ports exposed by the piston head into the crank housing.

In the following upward movement of the piston, the fresh gas is compressed and flows via overflow channels and scavenging ports exposed by the piston head into the cylinder. By using reed valves or rotary-disk valves, or by changing to a crosshead charging pump, the amount of scavenging air can be increased that is limited by the stroke-to-bore ratio and the dead space. In particular, given the limited scavenging efficiency of two-stroke engines and the fact that operating at a full load generally requires a substantial amount of excess air even in modern diesel combustion systems because of the smoke limit, the low volumetric efficiency of the crank housing scavenging pump is a profound disadvantage, apart from the complicated stepped piston design.

Assuming that a highly effective, flow enhancing oil separator with low-pressure loss cannot be used for the scavenging air, the necessity of minimizing the lubrication oil in the scavenging air (problem: hydrocarbon and particle emissions, piston ring deposits, racing engine) means that the engine-transmission unit generally cannot be bom on tried-and-true, low-noise, economical, and reliable friction bearings with oil spray cooling of the pistons. Another substantial disadvantage of crank housing scavenging pumps is that the crank chambers need to be sealed from each other in multicylinder engines. Using a separate, mechanically driven reciprocating piston compressor avoids some of the cited disadvantages; however, apart from the limited flexibility in adjusting the fuel delivery, substantial additional installation space is required, and major additional costs are involved.

Rotary compressor: Under the general term of the “rotary compressor” (rotary piston compressor), we find a series of compressors whose delivery or compression is determined by the compressing effect of rotating elements or pistons. The driveshaft is mechanically coupled to the crankshaft of the engine to scavenge or supercharge internal combustion engines. Belonging to this group of superchargers are Roots superchargers, vane-type superchargers (encapsulated blowers), rotary-piston superchargers, spiral-type superchargers (g superchargers), and screw compressors.

Similar to reciprocating piston compressors, the delivered mass flow is approximately proportional to the drive speed and decreases slightly at higher pressures because of increasing leakage. In general, average compressor efficiency is attained. With an equivalent delivery rate, the dimensions of reciprocating piston compressors and radial compressors are approximately the same.

Flow-type superchargers: Of the flow-type superchargers, primarily radial compressors (turbocompressors) are used for vehicle engines. The delivered flow of radial compressors is approximately linear, and the pressure is approximately the square of the drive speed. Modem radial superchargers offer highly efficient compression. Since, in contrast to four-stroke engines, the two-stroke engine has a mass flow rate characteristic that is only more or less independent of the engine speed that can be defined as an opening (throttle) with a constant cross section, a radial blower mechanically coupled to the engine is a suitable scavenging blower. Corresponding to the goal of limiting the size of the radial supercharger, it is useful to drive the supercharger with a high-speed transmission.

To optimally adapt the air mass flow delivered by the supercharger largely independent from the crankshaft speed for each mapping point to the desired scavenging or supercharging level of a two-stroke engine, it is desirable to drive the supercharger with a variable transmission ratio like the previously discussed displacement supercharger. Such a solution was, for example, used for the “ZF-Turmat”. Apart from high construction costs, problems with vibration, and the useful life of variable drive transmissions, a general disadvantage of mechanically driven superchargers is that a substantial amount of the effective output must be sent to the crankshaft to drive the supercharger. This correspondingly increases the specific fuel consumption.

Exhaust turbochargers: The exhaust turbocharger that has been successfully used for decades in four-stroke engines can also be used in two-stroke engines for passenger cars and trucks as a scavenging and supercharging blower. The advantage of turbocharging is that the exhaust energy converted in the turbine is used, which would otherwise largely be lost. According to Schieferdecker a requirement for the use of freewheeling turbochargers in two-stroke engines is that the joint efficiency of the turbine and compressor must be at least 60%, which is more or less attained with modem turbochargers used in passenger cars and trucks.

To utilize as much exhaust energy as possible in the turbine, it is also essential that the exhaust lines from the respective cylinder to the spiral housing of the supercharger be optimized for both good flow and minimal heat loss. In addition to a short, cramped port design, the air gap insulation, and possibly even the use of port liners, needs to be considered. To ensure a positive scavenging pressure gradient over as wide a mapping range as possible, superchargers should be used with variable turbine geometry [adjustable blades, sliding supercharger, double helix supercharger (Twin skroll, Aisin)].

An advantageous side effect of turbocharging and supercharging with superchargers that have an adjustable turbine geometry is that the backup of exhaust in front of the turbines allows highly effective charging even for scavenging approaches with symmetrical timing (such as loop scavenging). Such an approach, although in an extreme form, was used for the turbocompound airplane engine, the Napier Normad.

To generate a positive scavenging pressure gradient when accelerating from a low load and low rpm and when starting an engine, you need a series-connected additional mechanically or electrically driven supercharger or a mechanical auxiliary turbocharger drive. An interesting alternative is an electrically supported turbocharger. With these types of superchargers, a part of the propulsion power for the compressor is supplied as needed by, e.g., an asynchronous electrical motor integrated in the supercharger.

For the thermodynamic conditions when coupling with two-stroke engines for the pressure wave supercharger (Comprex supercharger), the same observations apply that were made for turbochargers. A basic disadvantage is that the fresh gas is heated when it briefly and directly contacts the exhaust, and that mechanical or electrical support of the compressor output is impossible given the functional principle of the supercharger.