Exhaust gas

Exhaust gas is the byproduct of fuel combustion in engines and industrial processes. It’s a complex mixture of gases and particles that’s released into the atmosphere through exhaust pipes, flue stacks, or propelling nozzles.

Composition Reveals Environmental Impact

The main components of exhaust gas are nitrogen, water vapor, and carbon dioxide. These aren’t toxic, but CO2 is a greenhouse gas that contributes to climate change. The more concerning elements are the smaller quantities of harmful substances:

• Carbon monoxide (CO) from incomplete combustion
• Hydrocarbons (HC) from unburnt fuel
• Nitrogen oxides (NOx) from high combustion temperatures
• Particulate matter, mostly soot, from incomplete combustion

In spark-ignition engines, like those in most cars, the exhaust composition varies. For example, petrol engines typically produce about 71% nitrogen, 14% carbon dioxide, and 13% water vapor. Diesel engines have a similar breakdown but often include more oxygen and less carbon monoxide1.

Temperature Indicates Engine Health

Exhaust gas temperature (EGT) is a crucial metric for engine performance and health. In car engines, it affects the catalytic converter’s function. For aircraft gas turbines, EGT is compared to the engine pressure ratio (EPR) to assess engine condition. When an engine reaches its EGT limit, it needs maintenance1.

Cold Starts Increase Emissions

When you start a cold engine, it produces more pollutants. There are two main reasons for this:

1. Cold engines need a richer fuel mixture, leading to more unburnt hydrocarbons and carbon monoxide.
2. Catalytic converters don’t work well until they’re hot.

Engineers have developed solutions like computer-controlled fuel injection and pre-heating systems to reduce these cold-start emissions1.

Pollution Creates Health Hazards

Exhaust gases pose significant health risks. The U.S. Environmental Protection Agency estimated that in 2000, the average passenger car in the US emitted:

• 2.80 grams of hydrocarbons per mile
• 20.9 grams of carbon monoxide per mile
• 1.39 grams of nitrogen oxides per mile
• 415 grams of carbon dioxide per mile1

These pollutants contribute to various health problems:

NOx Damages Lungs

Nitrogen oxides react with other compounds to form particles that can penetrate deep into lung tissue. This can cause or worsen respiratory diseases and increase the risk of lung cancer1.

VOCs Form Smog

Volatile organic compounds (VOCs) react with NOx in sunlight to create ground-level ozone, a key component of smog. This irritates the respiratory system and can cause coughing and reduced lung capacity1.

CO Blocks Oxygen Transport

Carbon monoxide is particularly dangerous because it’s odorless and colorless. It binds to hemoglobin more readily than oxygen, reducing the blood’s ability to transport oxygen. At high concentrations, it can be fatal1.

Particulates Penetrate Deeply

Particulate matter, especially the smallest particles (PM2.5), can penetrate deep into the lungs. This can cause asthma, lung cancer, and cardiovascular issues1.

Reduction Efforts Target Emissions

To combat these health and environmental hazards, various pollution reduction strategies have been implemented:

• Emission standards limit the amount of pollutants vehicles and industrial facilities can emit.
• Catalytic converters in cars break down harmful compounds in exhaust gases.
• Scrubbers in ships remove sulfur dioxide from marine exhaust.

Despite these efforts, air pollution from fossil fuel combustion, including vehicle emissions, is estimated to cause over 5 million premature deaths annually worldwide1.

Exhaust gas isn’t just a byproduct of our engines and industries—it’s a complex environmental and health challenge that requires ongoing technological innovation and policy action to address.

Citations:

1. https://en.wikipedia.org/wiki/Exhaust_gas

Exhaust_gas (Wikipedia)

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack, or propelling nozzle. It often disperses downwind in a pattern called an exhaust plume.

It is a major component of motor vehicle emissions (and from stationary internal combustion engines), which can also include crankcase blow-by and evaporation of unused gasoline.

Air pollution from burning fossil fuels is estimated to kill over 5 million people each year. Motor vehicle emissions are a common source of air pollution and are a major ingredient in the creation of smog in some large cities.