Let’s go in detail about Nitrogen Oxides (NOₓ) in automobiles — their formation, types, effects, and control methods.
In this article:
1. What Are Nitrogen Oxides (NOₓ)?
Definition:
Nitrogen oxides (NOₓ) are a group of highly reactive gases composed of nitrogen (N) and oxygen (O) atoms. The most common types in vehicle exhaust are:
- Nitric oxide (NO)
- Nitrogen dioxide (NO₂)
Chemical formulas:
- NO → Nitric oxide
- NO₂ → Nitrogen dioxide
Properties:
- Colorless (NO) to reddish-brown (NO₂)
- Toxic and reactive
- Contribute to air pollution and smog
2. Formation of NOₓ in Engines
Nitrogen oxides are primarily formed due to high-temperature combustion in the presence of nitrogen and oxygen in air.
A. Chemical Reaction (Zeldovich Mechanism)
At high temperatures (>1500°C), atmospheric nitrogen reacts with oxygen:
B. Factors Increasing NOₓ Formation
- High combustion temperature → CI (diesel) engines have higher NOₓ.
- Excess oxygen (lean mixture) → more O₂ available to react with N₂.
- Long combustion duration → gives more time for NO formation.
- Turbocharging & supercharging → higher pressure and temperature favor NOₓ.
C. Difference in SI and CI Engines
| Engine Type | NOₓ Formation | Reason |
|---|---|---|
| SI (Petrol) | Moderate | Lower combustion temperature, near stoichiometric mixture |
| CI (Diesel) | High | Higher compression, higher temperature, lean mixture |
3. Sources of NOₓ Emission in Automobiles
- Compression ignition (diesel) engines → main source due to high combustion temperature and pressure.
- Spark ignition (petrol) engines → lower, but still significant at high load or high speed.
- Other factors: Poor maintenance, clogged injectors, high engine load, or incorrect timing.
4. Environmental and Health Effects of NOₓ
A. Environmental Effects
- Smog formation: NOₓ reacts with hydrocarbons (HC) under sunlight to form photochemical smog, which reduces visibility.
- Acid rain: NOₓ reacts with water to form nitric acid (HNO₃) → acid rain damages soil, vegetation, and buildings.
- Ozone depletion & greenhouse effect: Contributes indirectly to climate change and air quality degradation.
B. Health Effects
- Respiratory problems (asthma, bronchitis)
- Irritation of eyes, nose, and throat
- Chronic exposure → lung damage
- Increases susceptibility to heart disease
5. Control of NOₓ Emissions in Vehicles
A. Engine-Level Controls
- Exhaust Gas Recirculation (EGR):
- Recirculates a portion of exhaust gases back into the combustion chamber.
- Reduces combustion temperature → less NOₓ formation.
- Optimized combustion:
- Control fuel injection timing and mixture to lower peak temperatures.
B. After-Treatment Systems
Selective Catalytic Reduction (SCR):
Three-Way Catalytic Converter (TWC):
Used in petrol engines to simultaneously reduce NOₓ, CO, and HC.
Converts NOₓ → N₂ + O₂.
C. Fuel Quality
- Low-sulfur diesel → reduces catalyst poisoning and NOₓ formation.
- Biofuels → some blends can slightly reduce NOₓ emissions.
6. Typical NOₓ Levels in Vehicle Emissions
| Engine Type | Without Control | With Modern Control |
|---|---|---|
| Petrol SI | 0.5–1.0 g/km | 0.06–0.08 g/km (Euro 6 standard) |
| Diesel CI | 3–5 g/km | 0.08–0.17 g/km (Euro 6 standard with SCR) |
7. Summary Table
| Aspect | Details |
|---|---|
| Pollutant Name | Nitrogen Oxides (NOₓ) |
| Chemical Formulas | NO, NO₂ |
| Source | High-temperature combustion in engines |
| Main Engine Type | Diesel engines (high), Petrol engines (moderate) |
| Health Effects | Respiratory problems, eye irritation, chronic lung damage |
| Environmental Effects | Smog, acid rain, ozone formation |
| Control Methods | EGR, SCR, Three-Way Catalytic Converter, low-sulfur fuels |
In Short:
Nitrogen oxides (NOₓ) are harmful gases formed in engines at high combustion temperatures.
Diesel engines produce more NOₓ than petrol engines.
They are controlled by EGR, SCR systems, and catalytic converters to protect human health and the environment.
💡 Fun Fact:
Even a slight increase of 100°C in combustion temperature can double NOₓ formation — that’s why temperature control is critical in modern engines.
FAQ section
Here are 10 frequently asked questions (FAQs) about Nitrogen Oxides (NOx):
- What are Nitrogen Oxides (NOx)?
NOx is a group of gases, mainly nitric oxide (NO) and nitrogen dioxide (NO₂), produced during combustion of fuels at high temperatures. - How are NOx gases formed?
They form when nitrogen and oxygen in the air react under high temperature, such as in engines, power plants, or industrial processes. - Why are NOx gases harmful?
NOx contributes to air pollution, smog, acid rain, and respiratory problems in humans. - Do vehicles produce NOx?
Yes, diesel and petrol engines emit NOx, especially under high combustion temperatures. - What is the effect of NOx on the environment?
NOx gases contribute to ozone formation, acid rain, and eutrophication, affecting ecosystems and human health. - Can NOx be controlled in engines?
Yes, technologies like Exhaust Gas Recirculation (EGR), Selective Catalytic Reduction (SCR), and catalytic converters reduce NOx emissions. - What is the difference between NO and NO₂?
NO is colorless and less reactive, while NO₂ is reddish-brown, toxic, and more reactive, contributing to smog. - Is NOx harmful to human health?
Yes, exposure can cause respiratory problems, lung irritation, and aggravate asthma. - Are there legal limits for NOx emissions?
Yes, many countries have emission standards for vehicles and industries to limit NOx levels. - Can NOx be measured?
Yes, NOx levels are measured using gas analyzers, sensors, or air quality monitoring systems.
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