Environmental Engineering Reference
In-Depth Information
lift and profile, manifold dynamics, tuning and losses, and the heat transfer during
the induction process (minimal):
Q actual
60 n e = 2
q air ideal ¼ r air V d
h v ¼
q air actual ¼
ð kg = h Þ
ð 1 : 39 Þ
q air actual
q air ideal
Making the appropriate unit conversions leads to the definition of volumetric
efficiency as stated in (1.40):
9 : 568 Q air
n e V d ð P amb ð kPa Þ= T amb ð K ÞÞ
h v ¼
ð
1
:
40
Þ
where the air charge is corrected for temperature and pressure deviations from STP.
Engine speed is given in rpm and cylinder displacement in litres.
1.7.2 BSFC sensitivity to BMEP
Brake specific air consumption (BSAC) is defined as airflow per brake output
power. ISAC is defined similarly except that indicated power is used:
Q air
BP
BSAC ¼
ð g = kWh Þ
ð 1 : 41 Þ
Q air
IP
ISAC ¼
where brake power (BP) and indicated power (IP) are in kWand airflow in kg/h. BSAC
is defined in terms of brake and indicated power by rearranging the terms in (1.41) to
BSAC ¼ ISAC IP
BP
BSAC ¼ ISAC IMEP
BMEP
ð g = kWh Þ
ð 1 : 42 Þ
The derivation of BSFC follows the same procedure as (1.41) and (1.42) for air
consumption. Instead of airflow in kg/h the measured variable is fuel flow q f in g/h.
The relevant definitions are
q f
BP
BSFC ¼
q f
IP
BSFC ¼ ISFC IMEP
BMEP
ð g = kWh Þ
ð 1 : 43 Þ
ISFC ¼
From (1.43) the BMEP is related to fuel consumption through the non-linear
behaviour of ISFC. Figure 1.33 illustrates the relationship of ISFC, BSFC, IMEP
and BMEP.
MEP as a function of BMEP is offset by the engine friction MEP. The offset is
linear over the engine cold motoring range of BMEP (Figure 1.34).
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