Environmental Engineering Reference
In-Depth Information
boosting with a turbocharger or supercharger is required. Boosting under high loads leads to high
temperatures in the cylinder and thus fuels with high octane ratings are more desirable for boosted
applications. A related technique is to operate a larger engine with variable displacement.
SI engines are able to meet the lowest levels of regulated pollutant emissions standards (U.S.
Tier II, bin 2 and CA PZEV) with relatively inexpensive three-way catalysts coupled with advanced
sensing and fully electronic control. However, there is a tradeoff in efficiency to meet these strin-
gent emissions standards: three-way catalysts require stoichiometric operation of the fuel and air to
simultaneously reduce HC, CO, and NO
x
emissions. A limited number of SI gasoline-fueled engines
have been produced, primarily in Europe. They have lean combustion systems in which the NO
x
emissions are not as stringent and fuel sulfur levels are lower, enabling lean NO
x
aftertreatment
devices to be more effective.
SI engines are continuing to evolve, improving engine efficiency, performance, and power den-
sity. New technologies including direct injection, turbocharging with downsized engines, optimized
valve timing, lift, and duration, homogeneous-charge CI technology, variable displacement, and
VCRs (Jones 2008) will continue to enable significant improvements over the next decade.
10.2.3 ci E
nginES
(d
iESEl
E
nginES
)
To initiate combustion in CI engines, the compression process heats the gases (including air, prior
cycle residuals, and exhaust gas), and when the fuel, which is injected near the end of compression,
mixes with the hot gases, it undergoes rapid exothermic reactions (autoignition and combustion).
Conventional CI engines (often called diesel engines in recognition of their inventor, Rudolf Diesel)
operate using diesel fuel, which has a high cetane number (see Table 10.2, column I). The cetane
number is a measure of the fuel's ignition delay. The cetane number can be thought of as the oppo-
site of an octane number, and fuels with a high cetane number have a low octane rating. The higher
the cetane number, the easier (lower temperature) and faster autoignition will occur. In Table 10.2
it is seen that diesel has the lowest cetane rating, indicating that at least from an autoignition stand-
point, the alternative diesel fuels are better fuels than the base petroleum diesel fuels.
Several factors typically give CI engines a higher efficiency than SI engines. CI engine compres-
sion ratios are higher than SI engines because the compression ratio in a SI engine is limited by com-
bustion knock due to limited octane ratings. Compression ratios for CI engines range from 16 to 24,
as opposed to 9 to 13 for a typical SI engine, as shown in Figure 10.8. Higher compression ratios
lead to higher efficiency on the basis of the thermodynamics of the cycle. The two lines shown in
Figure 10.8 correspond to heat addition at constant volume (CV) and constant pressure (CP) for the
Ty pical SI CR
Ty pical CI CR
65
60
55
50
45
40
35
30
25
6
8
10
12
14
16
18
20
22
Compression ratio
FIGure 10.8
Compression ratio effect on ideal efficiency and ranges of current gasoline and diesel engine
operation.
