Gas Exchange and Supercharging
Beginning at the conclusion of the expansion stroke, the gas exchange phase basically performs two functions, namely: – replacing the utilized cylinder charge (exhaust) with fresh gas (air in a diesel engine), a basic prerequisite for an internal combusti
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Gas Exchange and Supercharging Helmut Pucher
2.1
Gas Exchange
2.1.1
General
Beginning at the conclusion of the expansion stroke, the gas exchange phase basically performs two functions, namely: – replacing the utilized cylinder charge (exhaust) with fresh gas (air in a diesel engine), a basic prerequisite for an internal combustion engine, and – dissipating the heat as required to conclude the thermodynamic cycle. Gas exchange can proceed based on the four-stroke or the two-stroke cycle. Regardless, the outcome of gas exchange can be characterized and evaluated by a series of dimensionless parameters, the following being stipulated first: mZ total mass of working gas in the cylinder at the end of gas exchange, mL total air mass that has flowed into the cylinder through the intake element, mLZ mass of fresh air in the cylinder at the end of gas exchange, mRG mass of residual exhaust gas in the cylinder at the end of gas exchange, rL density of the air before the intake elements mLtheor theoretical air mass. The theoretical air mass mLtheor ¼ rL Vh
(2-1Þ
corresponds to the mass of air with the density rL brought into the cylinder to exactly fill the cylinder displacement Vh. The air efficiency la ¼
mL mLtheor
H. Pucher (*) Technische Universita¨t Berlin, Berlin, Germany e-mail: [email protected]
(2-2Þ
is a measure of the total quantity of air relative to the theoretical air mass that flows into the cylinder during gas exchange. For a steady-state engine operating point, it corresponds to the measured air flow rate. The volumetric efficiency ll ¼
mLZ mLtheor
(2-3Þ
specifies the quantity of inflow air mass remaining in the cylinder relative to the theoretical air mass. Accordingly, the retention rate is defined as: lz ¼
mLZ ll ¼ : mL la
(2-4Þ
Boost efficiency and scavenging efficiency play a role as well, especially for two-stroke engines. Boost efficiency lt ¼
mZ mLtheor
(2-5Þ
specifies the amount of working gas mass located in the cylinder at the end of gas exchange relative to the theoretical air mass. The proportion of the working gas mass mZ that consists of fresh air mLZ is expressed by the scavenging efficiency: ls ¼
mLZ mZ
(2-6Þ
The following applies to the residual gas mRG, i.e. residual working gas from the preceding combustion cycle remaining in the cylinder: mRG ¼ mZ mLZ :
2.1.2
Four-Stroke Cycle
2.1.2.1
Control Elements
(2-7Þ
Gas exchange in four-stroke reciprocating piston engines is now almost exclusively controlled by valves. Although formerly used in vehicle gasoline engines, slide valve control [2-1] failed to even establish itself in gasoline engines because
K. Mollenhauer, H. Tschoeke (eds.), Handbook of Diesel Engines, DOI 10.1007/978-3-540-89083-6_2, Ó Springer-Verlag Berlin Heidelberg 2010
31
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H. Pucher
of sealing problems resulting from the variable cold and hot clearance as well as thermally induced warping. It is thoroughly unfeasible for diesel engines, particularly since they have higher in-cylinder pressures. Universally common today, a valve
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