What it is and what it is used for
To understand why Euro 6 diesel cars must necessarily mount the SCR catalyst, previously used only on trucks, we must start from the basics of diesel engine operation and also consider some recent events in the automotive world.
How do diesel engines work?
The diesel engine is a type of internal combustion engine that uses a mixture of air (comburent) and diesel (fuel) as fuel, and that of compression. In fact, diesel engines are defined as compression ignition engines (unlike petrol which are defined as positive ignition).
The ignition of the diesel engine occurs in four stages:
- suction: the piston is lowered and the pressure difference thus generated inside the combustion chamber attracts the air molecules from the outside; the intake valve opens and atmospheric air enters through the cylinder;
- compression: according to the principle of compression, a gas, if compressed, heats up. When the intake valve closes again, the piston begins to rise towards the cylinder head, compressing the air and causing an increase in pressure and temperature in the combustion chamber;
- injection: when the piston reaches the cylinder head, an injector releases the diesel fuel into the combustion chamber in the form of small droplets and in a measured quantity; the fuel absorbs the heat of the air molecules and ignites, starting combustion. The thrust generated by combustion causes the piston to descend and the triggered movement rotates the vehicle wheels;
- exhaust: combustion generates a series of residual gases composed of unburned molecules and highly polluting substances, which are expelled from the combustion chamber through the exhaust valve and then released into the environment through the exhaust pipe;
This type of engine certainly has advantages, including lower fuel consumption and higher efficiency; but it also has its weak points, for example it is noisier, larger and produces more particulate matter (PM) and more nitrogen oxides (Nox), two highly polluting and dangerous substances for human health.
To reduce particulate emissions, the manufacturers have been adapting for some time (Euro 4 – Euro 5) by setting up an exhaust after-treatment system on diesel engines, the DPF.
For NOx, however, the legislation has become stricter after the 2015 Dieselgate scandal: in that year the United States Environmental Protection Agency (EPA) denounced Volkswagen for having manipulated the diesel engine control units of some cars in such a way that detected acceptable Nox emission values for type-approval purposes, although the real emissions on the road were significantly higher. The scandal then involved numerous other manufacturers around the world and together with various other consequences, led to greater attention on the controls and monitoring of diesel engine emissions.
From 2018/2019 (Euro 6d), the European Commission has introduced the obligation to adopt a Nox abatement system on all diesel engines.
What are Nox and what does the SCR do?
To understand what NOx are, let’s go back to the composition of the fuel + comburent mixture with which diesel engines are powered: air + diesel.
It is a poor mixture, that is, a mixture where the amount of air is higher than the ideal one to burn the fuel totally (the air-fuel ratio in diesel engines also reaches 25: 1).
78% of the atmospheric air entering the cylinder is composed of nitrogen, a gas which is inherently inert, 21% of oxygen, which is instead oxidizing, and 1% of other inert gases.
During the compression phase of the diesel cycle, due to temperature and pressure peaks, nitrogen and oxygen react together and give rise to different molecular forms: nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3) and nitrogen pentoxide (N2O5). Generally, the expression NOx is used to refer to these elements.
NOx have extremely serious consequences for both the respiratory system and the environment.
To eliminate them, the manufacturers have adopted a system of post-treatment of the exhaust gas specific only for NOx: it is called Selective Catalytic Reduction, or SCR.
The Selective Catalytic Reduction system
This system is based on the triggering of a targeted NOx reduction chemical reaction. The reducing agent used is generally aqueous ammonia (NH3) or urea (CO (NH2) 2). Since in high concentrations these substances can be toxic or irritating, they are usually diluted. The highest quality and most popular product on the market is Ad Blue, consisting of 32.5% of urea [CO (NH2) 2] and 67.5% of deionized water.
The Ad Blue is contained in a tank located upstream of the SCR catalyst. It is pumped into the waste stream by means of a doser; the quantity of Ad Blue injected is established by the engine control unit which in turn communicates the information to the control unit of the Ad Blue tank. In contact with the high temperature exhaust gases, Ad Blue transforms into ammonia (NH3) which reacts chemically with Nitrogen Oxides (NOx) “by disassembling” them and generating two completely harmless substances: nitrogen (N ) and water vapor (H2O).
The reduction takes place inside a porous ceramic element coated with Vanadium, at a temperature of about 265-425 ° C, and has an efficacy of up to 80%.