Environmental Engineering Reference
In-Depth Information
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Fuel economy benefit (mpg/mpg) × 100%
Figure 1.30 Fuel economy to consumption benefit conversion
After combustion the exhaust valve opens and immediately vents approximately
50% of the spent charge under conditions of supercritical pressure. As the working
piston completes the exhaust stroke it forces most of the remaining spent gases out of
the chamber. Top dead centre (TDC) of the piston is called the gas exchange TDC or
overlap TDC because the intake and exhaust valves may overlap in their opening and
closing events, respectively. Figure 1.31 illustrates the four stroke gas exchange
process.
0-40°
5-20°
IP
IO
EC
TDC
BDC
IC
EO
40-60°
45-60°
Figure 1.31 Four stroke engine cycles and timing
The points in Figure 1.31 define the intake, and exhaust valve opening and closing
timing as well as ignition timing all relative to piston TDC and bottom dead centre
(BDC). Proceeding clockwise in Figure 1.31 the intake valve opens (IO), admitting a
