This article was first published in 2008.
E-G-R stands for Exhaust Gas Recirculation. It’s
been around for decades but it’s now becomingly increasingly important in both
diesel and spark ignition engines.
Computer-controlled, often externally-cooled, and
potentially of much greater use than for just decreasing emissions, EGR is
making a major resurgence. So what are the emissions and fuel economy
implications of EGR? Some, including reducing full-load exhaust gas temps in
turbo engines without the need to run rich air/fuel ratios, are not at all what
So let’s take a new look at EGR.
Exhaust Gas Recirculation
Exhaust Gas Recirculation (EGR) is a process
whereby some of the engine exhaust is fed back into the intake.
External EGR is achieved by means of a pipe
that connects the exhaust to the inlet manifold, with a control valve interposed
in this line to regulate EGR flow.
For exhaust gas to flow in this pipe, the pressure
in the exhaust must be higher than the pressure in the intake.
In conventionally throttled spark ignition
engines, this pressure differential is present in varying degrees at all loads,
and is highest in part-throttle conditions. However, in throttle-less engines
like diesels and some direct injection spark ignition engines, this pressure
differential is highly dependent on exhaust backpressure. In turbocharged
engines, exhaust pressure in front of the turbine is always higher than intake
Internal EGR occurs when the valve timing
is arranged so that there is some back-flow into the combustion chamber from the
exhaust, or all exhaust gases are not pushed out of the combustion chamber on
the exhaust stroke. Such engines normally have variable valve timing so that
internal EGR occurs only when dictated by the ECU; when internal EGR is
required, this is achieved by increasing valve overlap.
Internal EGR appears to be a better approach (at
least on engines with variable valve timing) as it avoids the need for external
pipes and valves, reducing cost and improving packaging. However, external EGR
has a significant advantage – the recycled exhaust gas can be cooled before
being fed back into the intake. This is termed cooled EGR.
In cooled EGR systems, the amount of EGR flow that
actually occurs depends not only on the pressure differential between the
exhaust and intake, but also on the pressure drop through the EGR cooler. The
actual amount of EGR occurring can be indirectly measured by the intake airflow
meter – more on this in a moment.
Traditional EGR Systems
EGR was first widely adopted in the 1970s as a
means of reducing oxides of nitrogen (NOx) emissions.
In the depicted Nissan system, the amount of EGR
depended on two factors – engine load (registered as intake manifold vacuum) and
coolant temperature (monitored by a thermal vacuum valve). The higher the
manifold vacuum (ie the lower the load), the greater the EGR. EGR was fully
activated only at coolant temperatures above 63 degrees C, was partially
activated at 40 – 63 degrees C, and was inactive at temperatures below 40
This Leyland EGR flow control valve consisted of a
diaphragm and a valve. The diaphragm was subjected to manifold vacuum and so
valve lift decreased with load.
However, many EGR valves used more complex
operation that this - for example, monitoring exhaust backpressure as well as
intake manifold pressure, so preventing EGR on the over-run and at idle.
Even in this period, some cars additionally used
solenoid control of the vacuum signal to the EGR valve, the solenoid being
controlled by a dedicated electrical unit.
Electronically-Controlled EGR Systems
Given that the optimal EGR valve opening varies
with load, coolant temperature, pressure differential between exhaust and intake
manifold (and other factors), electronically-controlled operation of the EGR
valve has obvious advantages. All external EGR systems fitted to current diesel
and spark ignition cars use electronically-controlled flow valves.
This Bosch spark ignition EGR system uses a
vacuum-controlled EGR valve that is electronically controlled by the ECU.
This Honda system uses an
electronically-controlled EGR valve that incorporates a feedback sensor to
determine the actual valve opening that is occurring.
Here is the electronically-controlled Honda valve
that is used on the Honda Insight.
This is a generic diagram of a typical diesel car
EGR system. The important point to note is the presence of the airflow meter. By
comparing the modelled airflow requirements of the engine (based on rpm, intake
manifold pressure, intake air temp and volumetric efficiency) with the actual
amount of air being breathed, the amount of EGR that is occurring can be
calculated and adjusted as required.
turbo engines, where the EGR source is in front of the turbine, exhaust energy
previously available to drive the turbo is reduced. If the amount of EGR is
sufficiently large, the turbo will need to be sized to take this into account.
EGR for Emissions Reductions
EGR reduces the emission of oxides of nitrogen
NOx emissions are problematic because their
generation is associated with lean combustion. Diesels (that most frequently run
air/fuel ratios of 50:1 – 100:1, with a richest air/fuel ratio of about 20:1)
and spark ignition engines run in stratified combustion and lean cruise modes
(eg air/fuel ratios of 20:1 – 25:1) produce large amounts of NOx. EGR is
therefore most often used on engines running lean air/fuel ratios.
EGR reduces NOx emissions in three ways.
EGR for Fuel Economy Improvements
In engines that use throttles, high pumping losses
occur in part-throttle conditions. That is, the engine needs to do work in order
to draw air past the partly closed throttle. One result of this is that as load
decreases and the throttle is closed to a greater extent, Specific Fuel
Consumption (ie the fuel consumed per power produced) becomes increasingly
This graph shows the phenomenon. At 100 percent
load (ie wide open throttle) this engine has a minimum Specific Fuel Consumption
(SPC) of 0.43 – see the bottom curve. With the throttle open only 25 per cent,
the minimum SPC has risen by 63 per cent, while at 2000 rpm, it has risen by a
massive 117 per cent!
EGR can help reduce pumping losses. This is
because the cylinders can still be filled, but without all the gas having to be
drawn past the throttle.
Up until a certain point, increasing EGR results
in lower fuel consumption. However, once that point has been passed, further
increases in EGR result in poor combustion behaviour, so increasing emissions of
hydrocarbons (HC) and Specific Fuel Consumption. About 40 per cent EGR appears
to be the current practical maximum in spark ignition engines.
In addition to reducing pumping losses, EGR can be
used to reduce the fuel enrichment otherwise needed at high loads. This is
especially relevant in current downsized turbocharged engines that spend more
time at high loads than conventionally-sized engines.
In highly stressed turbo engines, fuel enrichment
is traditionally used to cool the combustion process and avoid detonation.
Recirculation of cooled exhaust gas can be used to perform the same function,
with a consequent improvement in high load fuel economy. The reduced fuel
consumption also serves to decrease HC and CO2 emissions.
UK researchers working for the engine component
manufacturer Mahle conducted testing on a 2-litre, 4-valves-per-cylinder, direct
injected, turbocharged and intercooled petrol engine. The EGR system was of the
external type and used a cooler that reduced exhaust gas temp to just 20 degrees
C. The system added the cooled exhaust gas well ahead of the turbo compressor
(note: so apparently not after the throttle body).
When running at high power outputs, the engine
used a modest amount of fuel enrichment and a large amount of cooled EGR to
achieve Specific Fuel Consumption up to 16 per cent better than with fuel
enrichment alone. Under the same operating conditions, the reduction in NOx was
about 30 per cent, the reduction in CO was 70 per cent, and HC was 80 per cent.
The authors also state that “an additional benefit
of the cooled EGR technique may be that higher geometric compression ratios can
be tolerated in boosted downsized engines...”.
Incidentally, in other Mahle literature, it is
suggested that on a 140kW engine using 15 per cent EGR and with an exhaust
temperature of 980 degrees C, an EGR cooler rated at 24kW is required.
Another point is that other literature suggests
that either hot or cold EGR reduces exhaust gas temperatures, important in
highly stressed turbocharged engines.
Finally, implicit in any discussion of using EGR
for improving fuel economy is that leaner air/fuel ratios can be used (often in
conjunction with a NOx adsorbing cat converter) without exceeding legislated NOx
EGR has a long-proven history in reducing NOx
emissions. Lean air/fuel ratios are associated with heightened NOx outputs and
so reducing this specific emission is a current priority in engine development.
Reductions in combustion temperature and pumping
losses can be achieved using EGR (and especially cooled EGR), reducing the need
for fuel enrichment at high loads in turbocharged engines and improving
part-load fuel economy.
As a relatively simple and low cost technology,
expect both internal and cooled external EGR systems to be very widely used on
both diesel and petrol engines.
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Part and Full Load Fuel Economy in a Turbocharged Gasoline Engine, SAE
(2000) Insight Service Manual
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