Environmental Engineering Reference
In-Depth Information
respective ideal thermodynamic cycles. In practice, engines use a limited pressure cycle in which
combustion occurs at a rate somewhere between these, yielding ideal efficiencies that fall between
these lines. Increasing the compression ratio (from SI to CI levels) yields improvements in effi-
ciency, with actual engine efficiencies falling below these lines because of the nonideal cycles with
various losses including heat transfer, rate-limited induction and exhaust of the working gases, and
friction. Another unique aspect of CI engines is the load control mechanism. Unlike an SI engine,
in which engine load is controlled by restricting the flow of air into the engine, a CI engine controls
load by injecting more or less fuel into the cylinder. By controlling load in this manner, the engine
does not waste energy across a restriction as it does in the SI engine during throttling. Furthermore,
at light and even moderate loads, when a relatively small amount of fuel is being injected, the overall
air-to-fuel mixture is lean of stoichiometric (excess air), which also leads to the higher efficiency of
CI engines relative to SI engines. Further options to improve CI engine efficiency and fuel economy
include advanced technologies such as turbocharging and injection pressure increases (Jones 2008).
It should be noted that the addition of exhaust gas recirculation (EGR) typical in 2004 and newer
engine systems may require the addition of an intake air restriction to draw relatively low pressure
exhaust gas into the intake manifold. This restriction acts like a throttle on an SI engine and reduces
the pumping efficiency. New technology using high-pressure EGR loops and variable geometry
turbochargers alleviate the addition of an intake throttle for EGR implementation.
10.2.3.1 diesel engine emissions: no
x
and Pm
Despite higher efficiencies, CI engines fueled with diesel exhibit a disadvantage in terms of regu-
lated toxic emissions and their reduction in aftertreatment systems. Of particular concern are NO
x
and PM composed of dry soot and soluble organic compounds (SOCs). There is an inherent tradeoff
in these two emissions in a diesel-fueled CI engine: when one is reduced the other will increase.
NO
x
is formed in the higher-temperature, near-stoichiometric combustion regions with soot forming
in the lower temperature, fuel-rich regions of the combustion zone. These emissions can be reduced
using various means internal and external to the engine. Internal reduction methods include changes
in fuel injection pressure and timing as well as fuel-air mixture dilution. External reduction meth-
ods include aftertreatment systems consisting of an oxidation catalyst for HC and CO reduction, a
particulate filter to reduce PM emissions, and a selective catalytic reduction (SCR) catalyst to reduce
NO
x
emissions.
The use of oxygenated fuels, such as biodiesel, in a CI engine reduces PM emissions with resul-
tant smaller increases in NO
x
. As a means for decreasing NO
x
emissions with biodiesel, EGR can be
added or combustion phasing can be delayed, resulting in similar NO
x
emissions but still lower PM
emissions relative to diesel fuel, as seen in Figure 10.9 (Polonowski et al. 2010).
In addition to biodiesel, DME is a promising fuel for CI engines, with low exhaust emissions
including very low PM. However, because of it is high volatility (gas at ambient temperature-pres-
sure) and low lubricity, the fuel and injection system must be modified from a conventional design.
DME can be blended with conventional diesel, but this still results in a high-volatility fuel requiring
specialty systems.
10.2.4 i
mpact
of
B
iofuElS
on
c
urrEnt
E
nginE
/v
EhiclE
o
pEration
It should be apparent that biofuels affect engines and transportation vehicles in many ways. Biofuels
affect emissions (as previously discussed) and can have a significant impact on full load power.
The impact areas that vehicle owners and operators tend to be most sensitive to are fuel economy,
maximum driving range between fueling, and initial purchase cost. Additionally, vehicle owners
are affected by the significant variation in implementation costs for alternative fuels. From the
manufacturers' point of view, enabling a vehicle originally designed to run on petroleum-based fuel
to operate on biofuel can be a significant challenge. For example, the fuel system materials may
