Page 738 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Exterior noise: Assessment and control C HAPTER 22.1

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where Note: If V d were in m then the resulting bmep would
m f ¼ mass of fuel inducted per cycle (kg) be in pascals.
Now introducing the volumetric efficiency h v for the The bmep is commonly used to assess the torque
four-stroke engine only (where there is a distinct in- output or load on an engine. If bmep is measured to be
duction stroke) (Heywood, 1988) less than the known maximum bmep expected from an
2 _ m a engine, then the engine must be operating at part load.
h ¼ (22.1.8)
v
r V N bmep tends to zero at idle (h m tends to zero), with the
a;i
d
useful work produced during the cycle being used up in
where overcoming pumping losses (Heywood, 1988).
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v d ¼ volume displaced by piston (m ) Note that:
3
r a,i ¼ inlet air density (kg m )
imep ¼ bmep þ fmep þ pmep þ amep (22.1.15)
The following equations are obtained for four-stroke where
engines (Heywood, 1988):
imep ¼ indicated mep (bar)
h h NV Q r ðF=AÞ
f v
hv a;i
d
P I ¼ (22.1.9) bmep ¼ brake mep (bar)
2 fmep ¼ rubbing friction mep (bar)
h h V Q r ðF=AÞ pmep ¼ pumping (or gas exchange) mep (bar)
d
hv a;i
f v
T I ¼ (22.1.10)
4p amep ¼ accessory (alternator, pumps etc.) mep (bar)
imep ¼ h h Q r ðF=AÞ (22.1.11) So, to use Tim Drake’s criteria for excellent perfor-
hv a;i
f v
mance, the above equations show that the designer
The mechanical efficiency h m allows the conversion of needs:
indicated power to brake power P b (the useful power as
measured by a dynamometer) to maximise h v and r a,i in order to give good airflow;
P b P f to maximise h f and Q hv along with (F/A) whilst mini-
h m ¼ ¼ 1 (22.1.12) mising bsfc;
P ig P ig
where P ig is the gross indicated power (found from the _ m f 1
gross indicated work per cycle – the area under the PV bsfc ¼ P gJ or g=kWh ðHeywood; 1988Þ
diagram for compression and expansion strokes only) (22.1.16)
being the sum of brake power and friction power P f .
Therefore,
P ig ¼ P þ P f (22.1.13) 22.1.3.10.1 Some basic background terms
b
Friction power includes the power used to overcome Each cylinder of an engine with poppet valves (conven-
friction in the engine (the rubbing friction) and in tional four-stroke engines) will have the following
pumping the gas in and out of the engine (the pumping components:
loss). Therefore, the mechanical efficiency is a function Intake system, usually comprising a dirty-side
of throttle setting and engine speed. duct (or snorkel), supplying air to a filter (housed
Brake mean effective pressure (bmep) is a useful mea- in a filter box), which in turn supplies air to a manifold.
sure of the torque output of an engine. It will be maximum Intake manifold, distributing air (possibly air–fuel
at the engine speed corresponding to peak torque mixture) to each cylinder. Different configurations
(Heywood, 1988) and will be 10–15% less at peak power. of intake manifold exist, most are variants of end-
Typical maximum bmep are (Heywood, 1988): feed and centre-feed designs (Figs. 22.1-4 and 22.1-5
respectively).
NA gasoline engine 8–11 bar The intake manifold has two sections – a plenum
Turbo gasoline engine 12–17 bar being a volume commonly equal to the swept
NA diesels 7–9 bar volume of the engine, and primary lengths (or
Turbo-diesels 10–12 bar runners) being the ducts that connect the plenum to
the cylinder head.
The relationship between bmep and brake torque is
(Heywood, 1988):
2pn r TðNmÞ
bmep ðkPaÞ¼ (22.1.14)
V d
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where V d is in dm . Fig. 22.1-4 End-feed intake manifold.

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