Environmental Engineering Reference

In-Depth Information

Volume

Figure 8.3
Diagram of the pressure, as a function of volume, in a four-stroke-cycle SI engine cylinder.

the friction of the piston, connecting rod, and crankshaft bearings, along with the power consumed

in operating the valves, pumping the lubricant, and cooling the water, reduce the power output of

the engine below that delivered by the gas in the cylinders [equation (8.5)]. This net output

P
b
,

called the
brake power,
is determined while running the engine in a laboratory at various rotational

speeds
N
and throttle settings. The power is computed as the product of the brake torque

T
b
(which

is measured) and the shaft angular frequency 2

π

N
:

P
b
=
T
b
(

2

π

N

)

(8.6)

Sometimes the power and torque are expressed in terms of an average cylinder pressure
p
bmep
,

called the
brake mean effective pressure,
using equations (8.5) and (8.6)

n

P
b

V
e
N
=

2

π

n

T
b

p
bmep
≡

(8.7)

V
e

where
V
e
is the
engine displacement,
the product of the cylinder displacement and the number of

cylinders. The brake mean effective pressure is an indicator of the cycle work per unit displacement

volume of the engine, and is little affected by engine size. It directly reflects the amount of fuel

burned per unit of cylinder volume.
8

The size of engine installed in a vehicle is determined by the power required to propel the

vehicle at the speed and acceleration needed to perform satisfactorily. An important characteristic

of an engine is its power and torque when operated at maximum fuel input per cycle. For an SI

engine, this occurs when the throttle is wide open. Figure 8.4 is a sketch of the brake power

P
b

and brake mean effective pressure
p
bmep
[which is related to the brake torque

T
b
by equation (8.7)]

as a function of engine speed
N
. The brake power

P
b
rises monotonically with speed, reaching a

maximum value
max

P
b
at an engine speed
N
m
. On the other hand, the brake mean effective

8
Pressure has the dimensions of energy per unit volume.