INTRODUCTION Exhaust after treatment like Selective Catalytic Reduction (SCR),


Internal combustion
engines (IC Engines) were invented in the 19th century, and ever
since they are used as primary power systems for both stationary and mobile
applications. As time progresses, IC engines went through remarkable changes in
terms of combustion, performance and emissions, and there is substantial development
because of electronic control in engine management system. Now a days new
development in engines are inspired by better fuel economy and meeting
stringent exhaust emission norms. Because of this, it is very crucial to make
cleaner and fuel-efficient engine without sacrificing the performance of the
engine. The diesel engines are popular because of their high fuel economy,
robustness ad mechanical durability. In addition to this, overall fuel-lean
operation and typically higher expansion ratio results in high thermal
efficiency in diesel engine. Further, at part load lack of throttling is
advantageous for fuel economy. The biggest challenge facing CI engines is
difficulty in simultaneously reducing NOx and Smoke. As emission limits get
more and more stringent, manufactures have to look for new ways to reduce

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One of the routes is to
modify the engine so that the combustion process can be improved in order to
reduce formation of pollutants within the cylinder itself. Many new
technologies like Common Rail Direct Injection (CR-DI), Pre-mixed Charge
Compression Ignition (PCCI), exhaust gas recirculation (EGR), intake air
boosting, and Variable Valve Timing (VVT) are used these days. Exhaust after
treatment like Selective Catalytic Reduction (SCR), Three Way Catalytic (TWC)
converter, and De-NOx converter are being implemented along with the above
mentioned technologies. 

Another approach is to
look for renewable alternative fuels which can emit low levels of harmful
components and enhance their performance through the use of modern fuel
injection and control strategies. Renewable alternative fuels like alcohols,
biogas, biodiesel and hydrogen have been and are being evaluated for
application in both in CI and SI engines. Alcohol is quite attractive and
promising because it can be easily stored, can be produced from renewable
sources and can be handled easily. Fuels with less carbon content and high
energy density are preferred as they emit less CO2 on combustion.

An overview of some of
the above mentioned technologies is described in upcoming sections to explain
their importance based on performance and emissions:


With improvement in
electronic control and manufacturing techniques it is possible to inject fuel
at a very high pressure (1600-1800 bar) directly inside the combustion chamber.
Common rail direct injection (CR-DI) is a fuel injection technique, in which a
high pressure pump is used to pressurize fuel and this will be supplied to a
common rail via a high pressure hose. Common rail is an accumulator which used
to store fuel at high pressure. High pressure fuel from the common rail is
supplied to these injectors via a high pressure hose. Solenoid or piezoelectric
operated injectors are used for direct injection. The opening and closing of
the injector is controlled by sophisticated electronics which operates solenoid
or piezoelectric valves. With the help of CR-DI system multiple injections (as
many as 5 injections per cycle) are possible with precise control over
injection timing and duration. High injection pressure ensures a fine spray and
leads to better atomization, mixing and combustion. Flexibility in injection
timing and number of injections gives better control over NOx and smoke
emissions, it also aids in reducing combustion noise and vibrations of the engine.


Recirculation (EGR) is a highly effective method to lower NOx emissions in a
diesel engine. A distinction is made between: Internal EGR and External EGR.
Valve timing is used to control level of internal EGR. In case of external EGR
it uses additional lines and control valves which is connected to combustion
chamber. Using EGR NOx reduction is because of following reason: reduction in
local excess-air factor, oxygen concentration and exhaust-gas mass flow. In
addition to this, combustion rates also drops and this result in lower local
peak temperature. Addition of EGR plays three different effects – Dilution
(reduction in oxygen), thermal (increase heat capacity of inlet charge) and
chemical (combustion process modification) effects.      

NOx formation requires
high partial oxygen pressure and high local temperature (> 2000 K), the
methods listed above helps in substantial reduction in NOx formation with
increase in EGR rates. However, with increase in EGR it results in reduction in
amount of oxygen and because of this smoke and HC emissions increase. Further,
this limits amount of EGR. Amount of EGR also effects ignition delay period. At
lower loads range if high amount of EGR is used, it results in longer ignition
delay and before start of combustion a large amount of air fuel has been mixed.
Charge will be partial homogenous in nature and combustion will be fully
premixed and diffusion part of combustion will be diminished. This results in
low NOx and smoke emissions at part-loads conditions.


Valve timing

As the engine operates between wide ranges of load and speed range the
valve timing is optimized for a particular operating range and at other places
it’s a compromise. This is because, without using additional features such as
the cam control system, adjustment of camshaft or multistage manifold one
cannot optimize the charge cycle at both maximum load and maximum torque
condition.  This issue can be solved by
making a valve train which is flexible also called Variable Valve Timing (VVT).
With the help of VVT it’s possible to get optimum valve timing for wide range
of operation i.e. over wide operating range we can get good volumetric
efficiency. Also NOx emissions can be reduce by trapping combustion gases
inside to combustion chamber (internal EGR) by valve overlap. Variable valve
actuation can be achieved by mechanically, hydraulically, electrically, and
pneumatically. VVT results in faster warm-up reduce fuel consumption, improved
starting and idle, increased low-speed torque, engine breaking (compression
release), Lower emissions, exhaust gas temperature control for aftertreatment
systems. In 4 stroke SI (Spark Ignition) GDI engine along with VVT can offer
throttle less operation.


In a HCCI engine the
fuel and air are mixed together either in the intake system or in the cylinder
with early direct injection. This premixed air and fuel mixture is then
compressed and at the end of compression stroke combustion is commence by
auto-ignition similar to conventional CI engine. In this at the end of
compression stroke the temperature of the charge has to reach auto-ignition,
this is done by retaining hot combustion products in the cylinder or preheating
the intake air. Because of this everywhere in the compression process higher
gas temperature are attained, this increase the rate of chemical reaction and
led to start of combustion of homogenously air and fuel mixtures. Throughout
the combustion chamber at the same time auto-ignition and combustion takes
place in an idealised HCCI engine. The maximum localise temperature zone is
removed as combustion takes place at the same time at several location, hence compression
effect on the burned gases. More importantly, presence of trapped or recycle
exhaust gas results in significant reduction in overall combustion temperature.
By this peak temperature can be kept below 1800K, which results in lower NO
emission formation. Further, combustion of lean premixed mixture results in
zero soot. However, because of high levels of dilutions it has lower power
density and combustion efficiency. Operating range of HCCI engine is limited,
as combustion takes place at the same time at several places this result in
high level of combustion noise and combustion phasing is poor because of combustion
process governed by in-cylinder charge composition, and history of pressure and


Researchers are trying
to combine the best features of Spark Ignition (SI) and Compression Ignition
(CI) engines. Premixed Charge Compression Ignition (PCCI) engine use a
homogenous mixture and this mixture use heat of compression for auto-ignition
of premixed charge. It combines some of the merits of both SI and CI engines.
In PCCI engine a lean premixed air-fuel mixture is formed and auto-ignited;
this auto-ignition will be much different than convention diesel like
auto-ignition. As in PCCI the mixture is more homogenous and it will
auto-ignites simultaneously and spontaneously. Because of homogenous in nature
this type of combustion results in lower NOx and smoke emissions. However, load
range is limited by misfire at low load and knocking at high load. Hence it can
be implemented in combination SI or CI (Compression Ignition) mode of operation.
A wide range of fuels like biogas, alcohols, LPG can be used with PCCI


Dual-fuel engine operation is another promising technology; it allows the
use of high self-ignition temperature fuels like alcohols by igniting them after
compression by a spray of high reactivity fuel like diesel. A primary fuel is
inducted through intake manifold or by port injection along with air, this
homogeneous air-fuel mixture is compressed during the compression stroke but
cannot auto-ignite on its own due to lean homogenous mixtures and high self-ignition
temperature of the primary fuel. Hence, the need for secondary fuel arises;
pilot injection of diesel like fuels leads to the ignition process. Brake
thermal efficiency (BTE) of a dual-fuel engine is higher than that of pure
diesel operation for medium and high load ranges of operation also NOx and
smoke emissions are lower. Wide range of fuels like biogas, natural gas, LPG
and alcohols can be used in conjunction with diesel in a dual fuel engine.
Further, high HC (Hydro-Carbons) emissions and poor BTE were observed at low
load operation and higher energy share (more energy supplied by primary fuel)
in duel fuel engine. At low loads and high energy share, the amount of pilot
fuel injected is low this results in poor ignition source. Primary fuel-air
mixture is inducted in suction stroke because of that some of this fuel can
reach in crevices and quenching zone hence combustion is not complete which
results in high HC and carbon monoxide levels in exhaust.


For Compression ignition (CI) engines alcohols like ethanol, propanol,
butanol and pentanol are considered as viable alternative fuels owning to their
moderate cetane number (it depends on type of alcohol), high-octane number and
other advantageous physical and chemical properties. Bio-alcohols are renewable
in nature and reduce global warming. Alcohols have been used as the sole fuel
and also in the blended form with diesel in CI engines.  Among the different alcohols, butanol is an
emerging fuel which can be used in both SI and CI engines. Like other alcohols,
butanol can be produced from biomass based feedstock by the alcoholic
fermentation process. It is a more complex alcohol as compared to ethanol and
methanol as it has a four carbon atoms in its molecule. It has oxygen which can
lead to reduction in soot.

Conventional fermentation process is used to produce acetone, n-butanol
and ethanol (ABE fermentation) in the ratio of 3:6:1. Fermentation is basically
a metabolic reaction of bacteria or yeast in the absence of oxygen. Starch and
glucose are converted into alcohols and other by-products like carbon-di-oxide,
hydrogen and acids etc. (Owen and Coley, 1990; Durre, 2011). Methanol can be
produced from synthetic gas formed when lignite or municipal solid wastes are
made to react with steam or oxygen (Wagner et al., 1979). Methanol was used earlier
in engines. However, it is not preferred currently due to its limited availability.
N-butanol is being evaluated for its suitability for use in CI engines as its properties
are close to those of diesel and gasoline.


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