Exhaust gas

In spark-ignition engines the gases resulting from combustion of the fuel and air mix are called exhaust gases. The composition varies from petrol to diesel engines, but is around these levels:

The 10% oxygen for "diesel" is likely if the engine was idling, e.g. in a test rig. It is much less if the engine is running under load, although diesel engines always operate with an excess of air over fuel. The CO content for petrol engines varies from ≈15 ppm for well tuned engine with fuel injection and a catalytic converter up to 100,000 ppm (10%) for a richly tuned carburetor engine, such as typically found on small generators and garden equipment.[8]

Exhaust gas from an internal combustion engine whose fuel includes nitromethane will contain nitric acid vapour, which is corrosive, and when inhaled causes a muscular reaction making it impossible to breathe. People who are likely to be exposed to it should wear a gas mask.[9]

See also: Diesel exhaust and Soot

• In aircraft gas turbine engines, "exhaust gas temperature" (EGT) is a primary measure of engine health. Typically the EGT is compared with a primary engine power indication called "engine pressure ratio" (EPR). For example: at full power EPR there will be a maximum permitted EGT limit. Once an engine reaches a stage in its life where it reaches this EGT limit, the engine will require specific maintenance in order to rectify the problem. The amount the EGT is below the EGT limit is called EGT margin. The EGT margin of an engine will be greatest when the engine is new, or has been overhauled. For most airlines, this information is also monitored remotely by the airline maintenance department by means of ACARS.

In jet engines and rocket engines, exhaust from propelling nozzles which in some applications shows shock diamonds.

• Flue gas

In steam engine terminology the exhaust is steam that is now so low in pressure that it can no longer do useful work.

Mono-nitrogen oxides NO and NO₂ (NOx) (whether produced this way or naturally by lightning) react with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of NO species increases the risk of lung cancer[10] and colorectal cancer.[11] and inhalation of such particles may cause or worsen respiratory diseases such as emphysema and bronchitis and heart disease.[12][13][14]

In a 2005 U.S. EPA study the largest emissions of NO_x came from on road motor vehicles, with the second largest contributor being non-road equipment which is mostly gasoline and diesel stations.[14]

The resulting nitric acid may be washed into soil, where it becomes nitrate, which is useful to growing plants.

When oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight, ground level ozone is formed, a primary ingredient in smog. A 2005 U.S. EPA report gives road vehicles as the second largest source of VOCs in the U.S. at 26% and 19% are from non road equipment which is mostly gasoline and diesel stations.[15] 27% of VOC emissions are from solvents which are used in the manufacturer of paints and paint thinners and other uses.[16]

Ozone is beneficial in the upper atmosphere,[17] but at ground level ozone irritates the respiratory system, causing coughing, choking, and reduced lung capacity.[18] It also has many negative effects throughout the ecosystem.[19]

Carbon monoxide poisoning is the most common type of fatal air poisoning in many countries.[20] Carbon monoxide is colorless, odorless and tasteless, but highly toxic. It combines with hemoglobin to produce carboxyhemoglobin, which blocks the transport of oxygen. At concentrations above 1000ppm it is considered immediately dangerous and is the most immediate health hazard from running engines in a poorly ventilated space. In 2011, 52% of carbon monoxide emissions were created by mobile vehicles in the U.S.[21]

Chronic (long-term) exposure to benzene (C₆H₆) damages bone marrow. It can also cause excessive bleeding and depress the immune system, increasing the chance of infection. Benzene causes leukemia and is associated with other blood cancers and pre-cancers of the blood.[22][23]

The health effects of inhaling airborne particulate matter have been widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, premature death.[24][25][26] Because of the size of the particles, they can penetrate the deepest part of the lungs.[27] A 2011 UK study estimates 90 deaths per year due to passenger vehicle PM.[28] In a 2006 publication, the U.S. Federal Highway Administration (FHWA) state that in 2002 about 1 per-cent of all PM₁₀ and 2 per-cent of all PM_2.5 emissions came from the exhaust of on-road motor vehicles (mostly from diesel engines).[29] In Chinese, European, and Indian markets, both diesel and gasoline vehicles are required to have a tailpipe filter installed, while the United States has mandated it for diesel only. In 2022, British testing specialist Emissions Analytics estimated that the 300 million or so gasoline vehicles in the US over the subsequent decade would emit around 1.6 septillion harmful particles.[30]

Carbon dioxide is a greenhouse gas. Motor vehicle CO₂ emissions are part of the anthropogenic contribution to the growth of CO₂ concentrations in the atmosphere which according to the vast majority of the scientific community is causing climate change.[31] Motor vehicles are calculated to generate about 20% of the European Union's man-made CO₂ emissions, with passenger cars contributing about 12%.[32] European emission standards limit the CO₂ emissions of new passenger cars and light vehicles. The European Union average new car CO₂ emissions figure dropped by 5.4% in the year to the first quarter of 2010, down to 145.6 g/km.[33]

Vehicle exhaust contains much water vapour.

There has been research into ways that troops in deserts can recover drinkable water from their vehicles' exhaust gases.[34]