Reducing toxicity of exhaust gases (Fiesta 4)

In essence, gasoline consists of carbon and oxygen molecules. When gasoline burns in the engine cylinders, carbon combines with oxygen in the air to form carbon dioxide (CO₂), and hydrogen combines with oxygen to form water (H2O). About 0.9 liters of water are produced from 1 liter of gasoline, which is usually not visible because it comes out of the exhaust system as steam, which it turns into under the influence of high temperature. Only when the engine is cold, especially in the cold season, are white clouds of exhaust gases formed by condensed water visible.

These combustion products are formed when air and fuel are mixed in an optimal proportion (14.7:1). But, unfortunately, this ratio is not always maintained, which is why harmful substances are present in the exhaust gases.

The Fiesta is equipped with a controlled three-way catalytic converter, the diesel engine is equipped with an oxidation catalytic converter

All vehicles without exception are equipped with a controlled three-way catalytic converter, vehicles with Endura-DE diesel engines are equipped with an oxidizing catalytic converter. The controlled catalytic converter reduces the content of carbon oxides by approximately 85%, hydrocarbons by 80%, and nitrogen oxides by 70%. Oxidizing catalytic converters have no effect on the concentration of nitrogen oxides. With increasing mileage, the efficiency of the catalytic converter decreases. The designation "controlled" indicates that when the engine is running, the composition of the exhaust gases is constantly monitored using an oxygen concentration sensor and the content of harmful substances in the gases is reduced to the standards prescribed by law.

Oxygen concentration sensor function (lambda probe)

Fig. 11.4. Location of the oxygen concentration sensor (1) in the exhaust pipe (2), where the exhaust gas temperature is highest

The oxygen concentration sensor (HO2S) on the Fiesta is installed before the catalytic converter in the front exhaust pipe (Fig. 11.4) and operates on the principle of a galvanic cell with a solid electrolyte in the form of a ceramic material made of zirconium dioxide and yttrium oxide. The ceramic material of the sensor is exposed to the exhaust gases from the outside, its inner surface is connected to the ambient air. To reduce the time it takes to bring the sensor to normal operating mode, it is equipped with electric heating. Due to the difference in the oxygen content in the exhaust gases and the ambient air, a potential difference arises in the sensor, which, at a certain residual oxygen content in the exhaust gases, increases significantly. This voltage jump occurs exactly at a fuel-to-air ratio of l = 1. With a lack of oxygen (l < 1), i.e. with a rich fuel-air mixture, the voltage is 0.9–1.1 V. With a lean mixture (l > 1) the voltage decreases to 0.1 V.



The signal from the oxygen concentration sensor is transmitted to the fuel injection system control unit. The unit enriches or depletes the fuel-air mixture to maintain the fuel-air ratio as close as possible to the optimum l=1.

Working area of the catalytic converter

The efficiency of the catalytic converter is a function of the operating temperature. The converter starts working at a temperature of approximately 300°C, which is reached after 25–30 seconds of driving. An operating temperature in the range of 400–800°C provides optimal conditions for maximum efficiency and a long service life of the converter.

The ceramic catalytic converter is sensitive to extremely high temperatures. If its temperature exceeds 900°C, the process of intensive aging begins, and at temperatures above 1200°C, its performance is completely disrupted.

The active layer consists of metals that are sensitive to lead content in the fuel, and when lead is deposited, the activity of the catalytic layer rapidly decreases. Therefore, engines with catalytic converters should only be operated on unleaded gasoline.



Fig. 11.5. Scheme of operation of the catalytic converter. Harmful substances NOx (nitrogen oxides), CO (carbon monoxide) and CH (hydrocarbons) come from the engine, and after the reaction in the catalytic converter, N₂ (nitrogen), CO₂ (carbon dioxide) and H2O (water) come out: 1,2 – metal meshes; 3 – body; 4 – perforated funnel



The catalytic converter has a porous ceramic base coated with precious metals - platinum and rhodium and enclosed in a stainless steel shell. The ceramic base, located on a wire mesh, is penetrated by a large number of parallel channels. An intermediate layer is applied to the walls of the channels to increase the active surface of the catalytic converter (Fig. 11.5).

The catalytic converter contains 2-3 g of precious metals, with platinum promoting oxidation and rhodium promoting the reduction of nitrogen oxides.

The catalytic converter neutralizes harmful substances such as carbon monoxide, hydrocarbons and nitrogen oxides (that's why it's called a three-way catalytic converter).



PRACTICAL ADVICE

Operation of vehicles with a catalytic converter

If your Fiesta does not start because the battery is dead, do not try to start the engine by pushing or towing the vehicle. This will cause a lot of unburned fuel to get into the catalytic converter, which will eventually cause it to fail.

If there are misfires or ignition failures, the ignition system must be checked immediately and high engine speeds must be avoided when driving further.



Before applying protective mastic to the underbody, carefully close the catalytic converter, otherwise a fire may occur.

Always check the heat shields whenever the vehicle is lifted.

Leakage in the exhaust system (burnt gasket, crack from high temperature, etc.) before the oxygen concentration sensor leads to incorrect measurement results (high oxygen content). Therefore, the electronic engine control unit will enrich the mixture, which will lead to increased fuel consumption and premature wear of the catalytic converter.



TECHNICAL DICTIONARY

Exhaust gas composition

Carbon monoxide (CO).

The richer the fuel-air mixture, the more carbon monoxide is formed. Precise control of the amount of fuel injected, correctly set ignition timing and uniform distribution of the mixture in the combustion chamber reduce the content of carbon monoxide in the exhaust gases. Never measure the content of carbon monoxide in closed rooms, as carbon monoxide is poisonous and even a small concentration of it in closed rooms can be fatal. In the air, carbon monoxide relatively quickly combines with oxygen and forms carbon dioxide. Although carbon dioxide is not poisonous, it participates in the formation of the "greenhouse" effect.



Hydrocarbons (CH).

Hydrocarbon compounds are grouped together. The HC content depends on the engine design (unchangeable value). Too rich or too lean a fuel-air mixture also increases the proportion of HC in the exhaust gases. Some of them are safe, others can cause cancer. All hydrocarbon compounds together with nitrogen oxides (NOx) form smog (hardly soluble foggy clouds of exhaust gases).

Nitrogen oxides (NOx or NO) -

are formed primarily due to the presence of nitrogen in the air entering the combustion chamber (more than 3/4). Their concentration is especially high in engine designs with low fuel consumption and low CO and CH content in the exhaust gases. These engines are characterized by high combustion temperatures and a lean fuel-air mixture. At high concentrations, nitrogen oxides can damage the respiratory system. When combined with water, acid rain is formed.

Carbon dioxide (CO₂).

Formed when fuel containing carbon is burned and combined with oxygen in the air, carbon dioxide reduces the beneficial effects of the Earth's ozone layer, which protects against harmful ultraviolet radiation from the Sun.

Toxic substances contained in exhaust gases from diesel engines.

When a diesel engine is running, it produces a small amount of CO and CH. Because of its higher compression, a diesel engine produces less nitrogen oxide. However, other harmful substances in the combustion products are typical of a diesel engine. For example, soot is a typical component of diesel exhaust gases. Soot consists of unburned carbon and ash. Soot particles can cause cancer when they enter the respiratory system. Sulfur dioxide (SO2) is also formed in the presence of sulfur, especially in diesel fuel. It contributes to the formation of sulfuric or sulfurous acid in rain (acid rain). Diesel-powered vehicles are responsible for 3% of acid rain.



Carbon dioxide is produced during combustion of diesel fuel only at higher concentrations.