Environmental Engineering Reference

In-Depth Information

max p
bmep

98 %

94 %

90 %

80 %

70 %

60 %

50 %

40 %

30 %

N
i

N
m

Engine speed

Figure 8.5
A diagram of contours of engine efficiency
η
e
as a function of engine speed
N
and brake mean

effective pressure for an SI engine. Contour values are expressed as a percent of the maximum value.

=

.

=

.

For gasoline of
SG

85,

the
LHV
is 41.18 MJ/kg or 35 MJ/L.
10
Converting to thermal efficiency by equation (8.8), the best

SI engine efficiency is about 31% while that of a CI engine is 44%.

The engine efficiency

0

72, the
LHV
is 43.55 MJ/kg or 31.6 MJ/L, while for diesel fuel of
SG

0

η
e
is a function of the engine speed
N
and brake mean effective pressure,

as demonstrated in Figure 8.5. Its peak value occurs at about 35% of the maximum engine speed

N
m
and 80% of the max
p
bmep
, where the engine power is about 35% of the maximum power. The

efficiency declines rapidly with decreasing
p
bmep
but less so with increasing speed. This decrease

of efficiency is a consequence of the relative increase of engine friction and flow losses, compared

to engine output, as
p
bmep
and engine power are reduced below the optimum value. Because a

road vehicle engine must provide the full range of its power over the speed range of the vehicle, it

cannot operate at maximum efficiency all the time. Nevertheless, by proper matching of the engine

to the vehicle, it is possible to minimize the fuel consumption needed to meet a particular vehicle

driving cycle.

8.4

VEHICLE POWER AND PERFORMANCE

Moving a vehicle along a highway requires the expenditure of mechanical power to turn the wheels.

Part of this power is needed to overcome the drag force exerted by the air on the moving vehicle.

Another part is needed to counter the resistance of the tires moving over the ground, called the

rolling resistance. If the vehicle is climbing a hill, additional power is needed to lift it vertically

10
Diesel fuel has more heating value per unit volume than gasoline, by about 11 %. Where vehicle fuel

efficiency is measured in terms of miles per gallon or kilometers per liter, a diesel-powered vehicle has an

inherent 11 % advantage, other things being equal.