EGR in spark-ignited engines
In a typical automotive spark-ignited (SI) engine, 5 to 15 percent of
the exhaust gas is routed back to the intake as EGR. The maximum
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 cause 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 largely
depends on the specific engine design, and sometimes leads to a
compromise between efficiency and NOx emissions. A properly operating
EGR can theoretically increase the efficiency of gasoline engines via
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 opened further, resulting in increased 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 effect is relatively minor
compared to the first two.
It also decreases the efficiency of gasoline engines via at least one
more mechanism:
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 extracted by the piston.
4)
EGR is technically not employed at high loads because it would reduce peak
power output. This is because it reduces the intake charge density. EGR
is also omitted at idle (low-speed, zero load) because it would cause
unstable combustion, resulting in rough idle.
Exhaust gas recirculation
Exhaust gas which is car
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.
EGR implementations
Recirculation 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 times 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 geometry turbocharger (VGT)
which uses different inlet guide vanes to build sufficient reverse pressure
in the exhaust manifold. For EGR to flow, a pressure difference is
required across the intake and exhaust manifold and this is created by
the VGT.
Other 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 on/off EGR valve; reduced performance and/or
drivability were common side effects. Slightly later (mid 1970s to
carbureted 1980s) systems included a coolant temperature sensor which
didn't enable 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 sometimes 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 hot exhaust manifold, were highly prone to vacuum leaks caused by
cracked hoses; a condition that plagued early 1970s EGR-equipped cars
with bizarre reliability problems. Hoses in these vehicles 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.
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 cause the EGR passages in the cylinder head and intake
manifold to become blocked with carbon deposits, necessitating extensive
engine disassembly for cleaning.
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 provide the diesel engine's efficiency by reduced heat
rejection and dissociation. There are trade offs however. Adding EGR to
a diesel reduces 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 (mainly 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 common 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 these
filters need to function properly to burn off soot, automakers have had
to consider injecting fuel and air directly into the exhaust system to
keep these filters from plugging up.
