INTRODUCTION the routes is to modify the engine so

INTRODUCTION

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
emissions.

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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:

1.1. COMMON RAIL DIRECT
INJECTION (CR-DI)

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.

1.2. PREMIXED CHARGE
COMPRESSION IGNITION (PCCI)  

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
operation.

1.3. EXHAUST GAS
RECIRCULATION (EGR)

Exhaust-Gas
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.

 

1.4. VARIABLE VALVE
TIMING (VVT) 

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.