EGR vs SCR: which technology is best?
Why do we favour EGR and SCR?
With the arrival of the new Euro VI European standard, which applies to models released after September 2015, vehicle manufacturers have had to find innovative technologies to reduce pollutant emissions on their diesel models. There are two technologies that can reduce levels of nitrogen oxides (NOx) and particulate matter (PM) to the levels required by the standard: Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR). Manufacturers either use both or only use SCR.
How do EGR and SCR work?
EGR works by taking some of the exhaust gas and adding it into the intake air. With a lower proportion of oxygen, the fuel does not burn completely and the cylinder temperature is lower. This reduces the amount of NOx formed, but does also lead to the formation of soot particles, which then need to be filtered out by a Diesel Particulate Filter (DPF) in order to stay within the prescribed limits. During operation, the filter must be periodically regenerated by burning off the soot. EGR alone is not effective enough to meet the Euro VI standard.
SCR works with AdBlue®, a urea- and water-based fluid, to convert NOx emissions from the engine into nitrogen and water.
There are clear and quantifiable benefits to SCR
An SCR system has many advantages:
- No power draw to cool the exhaust gases
- No fuel used for active regeneration or to boost SCR performance
- No additional complexity for the manufacturer of the body
- No engine noise during forced regeneration when at a standstill
- Lower operating temperatures inside the DPF and no risk of thermal runaway
- Longer oil change intervals (up to 90,000 miles/150,000 km)
- Longest DPF service interval (up to 360,000 miles/600,000 km)
- A much simpler engine means a lower risk of breakdowns