Introduction:
An EGR (Exhaust Gas Recirculation), sometimes referred to as “UGR” in Dutch by some brands (Uitlaat Gas Recirculatie), is tasked with directing a portion of the exhaust gases back to the engine. The EGR valve is utilized in both gasoline and diesel engines. The EGR is mounted on the exhaust side of the engine block.
From the exhaust manifold, a thick metal pipe runs to the EGR cooler. This cooler lowers the temperature of the hot exhaust gases (effect of a heat exchanger). From the EGR cooler, the exhaust gas flows to the EGR valve, which mixes the exhaust gases with the intake air. EGR can be configured as high pressure (short route) or low pressure (long route). Both systems have their own advantages and disadvantages. This is described in the chapter High and Low Pressure EGR (further down this page).
The EGR valve and pipes can be mounted at different locations, but the exhaust gas is always tapped as close to the exhaust manifold as possible. The image below shows the locations where the components of the EGR are mounted on the engine block.
Because the EGR valve needs to be opened in multiple positions or fully closed, a stepper motor is used to operate the valve.
Purpose of the EGR System:
By adding small amounts of exhaust gas to the fresh air in the engine’s intake tract, the oxygen content for combustion is reduced. This results in a ‘less optimal’ combustion. Additionally, both the combustion speed and temperature decrease. The lower temperature of the combustion gases reduces the nitrogen oxides (NOx) levels. Thus, the exhaust gases become cleaner, which is beneficial for meeting environmental standards.
In older engines, the EGR valve remains closed when the engine is cold. In newer engines, the valve opens even when cold to help heat the engine with warm exhaust gases. However, during acceleration, deceleration, and idle running, the EGR is inactive. During partial load, its operation is fully utilized.
Operation:
The computer controls the amount of exhaust gases that can be recirculated back to the engine based on the following data:
- Engine load (EGR operates only at partial load; at full load, the valve is closed).
- Engine speed (at too low or too high RPM, the valve is closed).
- Intake air temperature.
With this data, the computer controls the solenoid valve with a specific duty cycle. If the difference is minimal, the vacuum converter is regulated such that the EGR valve is not activated. The solenoid valve, in this idle state, passes the ambient air pressure to the EGR valve. However, if the intake air quantity exceeds the required amount, the solenoid valve is controlled so that the vacuum of the vacuum pump is applied to the EGR valve. The solenoid valve thus regulates the vacuum from the vacuum pump to the EGR valve. Consequently, the EGR valve is opened just enough so that the difference between the required and intake air remains minimal.
High and Low Pressure EGR:
The exhaust gas recirculation system can be distinguished into two systems, namely high pressure EGR and low pressure EGR. The difference lies in the point where the exhaust gases are tapped from the exhaust. The functioning, advantages, and disadvantages of both systems are described below.
High Pressure EGR (short route):
The exhaust gases are redirected back to the engine before the turbo. Between the exhaust manifold and the turbo, there is an EGR line. Often, there is a cooler on this line to reduce the exhaust gas temperature. The EGR valve is located at the end of this line. When the EGR valve opens, the exhaust gases in the line mix with the intake air that is being directed from the intercooler to the engine’s intake.
Advantages of High Pressure EGR:
- EGR can be adjusted very quickly to the dynamic engine state.
- In a cold start, the EGR helps to increase the temperature in the combustion chamber as quickly as possible (standard in diesel engines of EURO III generation and later).
Disadvantages:
- Contamination in the intake tract due to soot formation.
- Exhaust gas must be cooled more than when a low pressure EGR is applied.
- Exhaust gas flow is not fully utilized to drive the turbo’s turbine; a portion is tapped off before the turbo.
Low Pressure EGR (long route):
The exhaust gases are only redirected to the engine’s intake after the soot filter. Usually, the exhaust of the EGR valve connects to the compressor’s inlet (as pictured), but sometimes it connects between the intercooler and the intake manifold. The drawback is that the recirculated exhaust gas has too low a pressure and speed. To resolve this, an additional valve is installed in the exhaust (after the EGR branching). When this extra valve closes, the pressure between the turbo and the valve increases. Due to the increased pressure, the exhaust gases are directed through the branching to the EGR valve.
Advantages of Low Pressure EGR:
- All exhaust gas is already utilized in the turbo’s turbine
- Exhaust gas temperature is low (without using an EGR cooler) compared to high pressure EGR.
- EGR gas is cleaner since the soot filter has already treated the exhaust gas, resulting in less engine contamination due to soot formation.
Disadvantages:
- Adding exhaust gas to the intake air (to suit the dynamic engine state) occurs slower because the EGR travels a longer route.
- The EGR valve is further from the intake valves.
Disadvantages and Possible Malfunctions:
The EGR valve does not benefit the engine’s performance. The use of the EGR valve can reduce performance and increase fuel consumption. Internal contamination also significantly increases (see image). Dirt accumulates everywhere, contaminating the intake manifold and intake valves of the engine. With a calm driving style, these impurities linger in the engine.
The only advantage of the EGR is that the exhaust gases are cleaner; NOx emissions decrease. In older diesel engines, EGR valves could be easily blocked with a closure plate. Manufacturers of new cars ensure through sensors and other methods that it is no longer possible to disable or deceive the EGR, due to pressure being measured at various points in the pipeline. Thus, newer cars also require a software modification in the engine management system. This is often part of new software in chip tuning.
The EGR valve may unintentionally remain open while not being controlled. In this case, too much exhaust gas is directed to the engine. With a diagnostic tool, we can identify this by:
- a (persistently) high intake air temperature due to the exhaust gases (see the page about the temperature sensors);
- a low value of the mass air flow meter: a large percentage of the incoming air consists of air from the EGR rather than the air filter.
EGR Cooler:
The temperature of the exhaust gases is about 300 degrees under partial load (at constant speed on the highway) and more than 700 degrees at full load. However, the cooler the intake air, the better the engine performs. Cool air contains more oxygen. Warm air is more expanded and thus contains less oxygen. Yet, warm exhaust gases are still recirculated into the engine.
There is often an EGR cooler between the EGR valve and the engine intake. This functions as a heat exchanger. Coolant flows through the EGR cooler (in fixed channels, so it does not directly contact the exhaust gases). The coolant absorbs the heat from the exhaust gases. The coolant warms, and the exhaust gases cool down. The EGR cooler lowers the temperature as much as possible, preferably to 100 degrees.
More information on this heat exchange technique can be found in the chapter heat exchanger.
Besides the EGR, gasoline engines also use a secondary air pump to treat exhaust gases, and for diesel engines, the SCR (Selective Catalytic Reduction) catalyst with an AdBlue injection system is applied.