EGR in spark-ignited engines
In a typical automotive spark-ignited (SI) engine, 5% to 15% of the
exhaust gas is direceted back to the intake as EGR. The higest
quantity is limited by the requirement of the mixture to sustain a
contiguous flame front during the combustion event; excessive EGR in an
SI engine can affect misfires and partial burns. Although EGR does
measurably slow combustion, this can largely be compensated for by
advancing spark timing. The impact of EGR on engine efficiency mainly
depends on the specific engine design, and sometimes leaads to a
compromise between efficiency and NOx emissions. A functionaly operating EGR can theoretically increase the efficiency of gasoline engines
through
several mechanisms:
1) Reduced throttling losses. The addition of inert exhaust gas into the
intake system means that for a given power output, the throttle plate
must be open further, resulting in increase inlet manifold pressure
and reduced throttling losses.
2) Reduced chemical dissociation. The lower peak temperatures result in
more of the released energy remaining as sensible energy near TDC,
rather than being bound up (early in the expansion stroke) in the
dissociation of combustion products. This causes the relatively smaller
compared to the first two.
3) Reduced specific heat ratio. A lean intake charge has a higher
specific heat ratio than an EGR mixture. A reduction of specific heat
ratio reduces the amount of energy that can be gained by the piston.
EGR is typically not employed at high loads because it would reduce high
power output. This is because it reduces the intake charge density. EGR
is also omitted at primary (low-speed, zero load) because it would
affect
unstable combustion, resulting in rough idle.
EGR in diesel engines
In modern diesel engines, the EGR gas is cooled through a heat
exchanger to allow the introduction of a greater mass of recirculated
gas. Unlike SI engines, diesels are not limited by the need for a
contiguous flamefront; furthermore, since diesels always operate with
excess air, they benefit from EGR rates as high as 50% (at idle, where
there is otherwise a very large amount of excess air) in controlling NOx
emissions.
Since diesel engines are unthrottled, EGR does not lower throttling
losses in the way that it does for SI engines (look above).However,
exhaust gas (largely carbon dioxide and water vapor) has a higher
specific heat than air, and so it still serves to lower peak combustion
temperatures; this aids the diesel engine's efficiency by reduced heat
rejection and dissociation. There are trade offs however. Adding EGR to
a diesel decrease the specific heat ratio of the combustion gases in the
power stroke. This reduces the amount of power that can be extracted by
the piston. EGR also tends to reduce the amount of fuel burned in the
power stroke. Particulate matter (only carbon) that is not burned in
the power stroke is wasted energy. Stricter regulations on particulate matter(PM) call for further emission controls to be introduced to
compensate for the PM emissions introduced by EGR. The most basic is
particulate filters in the exhaust system that result in reduced fuel
efficiency. Since EGR increases the amount of PM that must be dealt with
and reduces the exhaust gas temperatures and available oxygen,filters
need to function properly to burn off soot, automakers had
to consider injecting fuel and air directly into the exhaust system to
keep these filters from plugging up.
EGR implementations
Different circulation is usually achieved by piping a route from the exhaust
manifold to the inlet manifold, which is called external EGR. A control
valve (EGR Valve) within the circuit regulates and perodic the gas flow.
Some engine designs perform EGR by trapping exhaust gas within the
cylinder by not fully expelling it during the exhaust stroke, which is
called internal EGR. A form of internal EGR is used in the rotary
"Atkinson cycle engine".
EGR can also be used by using a variable geometrical turbocharger (VGT)
which uses variable inlet guide vanes to construct sufficient backpressure
in the exhaust manifold. For EGR to flow, a pressure difference is
required between intake and exhaust manifold and this is created by
the VGT.
Methods that have been experimented with are using a throttle in a
turbocharged diesel engine to decrease the intake pressure to initiate EGR flow.
EGR systems were relatively unsophisticated, utilizing manifold vacuum
as the only input to an start/end EGR valve; reduced performance and/or
drivability were same side effects. Slightly later (mid 1970s to
carbureted 1980s) systems included a coolant temperature sensor which
diable the EGR system until the engine had achieved normal
operating temperature. Many added systems like "EGR timers" to disable
EGR for a few seconds after a full-throttle acceleration. Vacuum
reservoirs and "vacuum amplifiers" were few used, adding to the
maze of vacuum hoses under the hood. All vacuum-operated systems,
especially the EGR due to vacuum lines necessarily in close proximity to
the heat exhaust manifold, were highly prone to vacuum leaks caused by
cracked hoses; a condition that plagued early 1970s EGR-equipped autos
with bizarre reliability problems. Hoses in these automobile should be
checked by passing an unlit blowtorch over them: when the engine speeds
up, the vacuum leak has been found. Modern systems utilizing electronic
engine control computers, multiple control inputs, and servo-driven EGR
valves typically improve performance/efficiency with no impact on
drivability.
Exhaust gas recirculation
Exhaust gas which is auto
parts, recirculation (EGR) is a nitrogen oxide (NOx) emissions
reduction technique used in most gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to
the engine cylinders. Intermixing the incoming air with recirculated
exhaust gas dilutes the mix with inert gas, lowering the adiabatic flame
temperature and (in diesel engines) reducing the amount of excess
oxygen. The exhaust gas also increases the specific heat capacity of the
mix, lowering the peak combustion temperature. Because NOx formation
progresses much faster at high temperatures, EGR serves to limit the
generation of NOx. NOx is primarily formed when a mix of nitrogen and
oxygen is subjected to high temperatures.
In the past, a fair number of car owners disconnected their EGR systems
in an attempt for better performance and some still do. The belief is
either EGR reduces power output, causes a build-up in the intake
manifold, or believe that the environmental impact of EGR outweighs the
NOx emission reductions. Disconnecting an EGR system is usually as
simple as unplugging an electrically operated valve or inserting a ball
bearing into the vacuum line in a vacuum-operated EGR valve. In almost
all cases, a disabled EGR system will cause the car to fail an emissions
test, and may affect the EGR flow in the cylinder head and intake
manifold to become blocked with carbon deposits, necessitating extensive
engine disassembly for cleaning.
