Q&A: Why do diesel cars need to add urea and gasoline cars don't?

Why do diesel cars need to add urea, while gasoline cars don't?

This is mainly because: the exhaust gas of the diesel engine contains a large number of nitrogen oxide compounds that are seriously polluting the atmosphere, and these substances must be reduced to harmless substances with urea; and the nitrogen oxide content in the exhaust gas of the gasoline engine is very small, and it can be treated cleanly by using a three-way catalytic converter.

Let's explain in detail below.

When the diesel engine is working, it will let a large amount of air enter the combustion chamber, and then compress it strongly to form a high temperature and high pressure air. Near the compression stop point, the injector begins to inject the right amount of diesel into the high-temperature and high-pressure air. Due to the high viscosity of diesel, the diesel injected into the cylinder is not completely vaporized, but exists in small particles, and the mixable gas formed after mixing with air in the combustion chamber is not uniform. In order to make the diesel fuel fully combustible, it is usually that the amount of air is much greater than the amount of diesel, and the air-fuel ratio generally exceeds 20 (the standard is about 15).

In this way, the combustible mixture composed of diesel and air forms oxygen-rich combustion when it is burned. Since the main component of air is nitrogen, nitrogen reacts with oxygen in the air in a high-temperature and high-pressure combustion chamber to form nitrogen oxides (nitric oxide, nitrogen dioxide, etc.). Nitrogen oxide is a toxic and harmful gas, in the air will be combined with water to form nitrite, the formation of acid rain, which in turn leads to soil acidification and water eutrophication, so nitrogen oxide pollution of the atmosphere is more serious.

So how to reduce the emission of nitrogen oxides from diesel engines?

On the one hand, by improving the combustion of combustible mixtures, the diesel engine generates as few nitrogen oxides as possible, such as reducing the temperature of the combustion chamber, improving the shape of the combustion chamber, improving the injection law of the injector, and adopting exhaust gas recirculation technology (EGR). These methods are collectively referred to as "in-flight purification".

However, in-machine purification has little effect on reducing the amount of nitrogen oxides generated, and requires extremely high cost and a long time. In the face of increasingly stringent emission regulations, people can only find another way to deal with nitrogen oxides in diesel exhaust gases by means of off-machine purification.

The so-called off-machine purification is to directly purify the exhaust gas of the diesel engine. This extra-machine purification is actually a comprehensive measure, not only to reduce nitrogen oxide emissions, but also to reduce particulate matter emissions, the main measures are oxidation catalytic converter (DOC), particle trap (POC), particulate filter (DPF) and selective catalytic reduction technology (SCR). The first three are mainly for particulate matter emissions, while the subsequent selective catalytic reduction (SCR) technology is mainly for nitrogen oxide emissions.

The working medium of this selective catalytic reduction system (SCR) is urea, which is generally configured with 32.5% high-purity urea and 67.5% deionized water, and the scientific name is called "diesel exhaust treatment fluid". When the diesel engine is working, a certain amount of aqueous urea solution is injected into the exhaust pipe and mixed with the high temperature and high pressure exhaust gas; then the urea undergoes hydrolysis and pyrolysis reaction at high temperature to generate ammonia (NH3); then the ammonia gas enters the SCR catalyst together with the exhaust gas, and on the high temperature surface of the catalyst (300 ~ 500 ° C), the ammonia reduces the nitrogen oxide to non-toxic and harmless nitrogen and water, and the excess ammonia is also reduced to nitrogen. In this way, the nitrogen oxide content in the exhaust gas is greatly reduced, and only nitrogen, carbon dioxide and water are discharged, so as to achieve the purpose of reducing the nitrogen oxide content in the exhaust gas.

This is why diesel engines need to add urea. If urea is not used to reduce nitrogen oxides, the exhaust gas of the diesel engine is difficult to meet the standard. As for why only urea can be used and no other substances are used, it can only be said that no substance has been found so far more efficient than urea reduction of nitrogen oxides.

So why don't gasoline engines use urea? This is mainly because the combustion process of gasoline engines is very different from that of diesel engines, and the composition of the substances in the exhaust gas is also different. Gasoline engine is standard combustion, gasoline and air mixing ratio is generally standard 1: 14.7, gasoline and air are almost equally consumed. The main components in the exhaust gas are carbon monoxide, hydrocarbons and small amounts of nitrogen oxides.

These substances are also harmful to the atmosphere, so they should also be treated. However, the main component in the exhaust gas of gasoline engines is hydrocarbons, so the disposal of hydrocarbons is the main task. In this regard, the exhaust gas treatment device used in the gasoline engine is a three-way catalytic converter, which is a honeycomb ceramic body containing three metal oxides of platinum (Pt), rhodium (Rh), and palladium (Pd). When the tail gas flow of the gasoline engine passes through here, a series of complex oxidation-reduction chemical reactions are carried out: carbon monoxide is oxidized into colorless, non-toxic carbon dioxide at high temperatures, hydrocarbons are oxidized into water and carbon dioxide at high temperatures, and nitrogen oxides are reduced to nitrogen and oxygen. In this way, the three harmful gases have become non-toxic and harmless common gases, and the exhaust gas of the car can be purified.

Since the nitrogen oxide content in the exhaust gas of the gasoline engine is very small, the three-way catalytic converter is enough to complete the purification effect, so the gasoline engine does not need to specifically remove the urea of the nitrogen oxide. However, the current National VI emission gasoline engine has high requirements for particulate matter emissions, so many engines need to install a particle trap in the exhaust pipe, which is very easy to block and will be a big trouble in the future.