Spark Ignition Engines: Emissions Guide

Emissions have become a focus of the automotive industry in recent years, in light of environmental concerns. Internal combustion engines are responsible for a large portion of the pollutants in our atmosphere; and so engine makers should bare responsibility for reducing this damage. Cars contribute over 15% of the global fossil fuel carbon dioxide emissions.

The main exhaust products are: Carbon dioxide, carbon monoxide, unburned hydrocarbons, nitrates, hydrogen gas, small quantities of water vapour and oxygen.

The differing levels of these products are dependent on a number of factors, many of them are sensitive the air fuel ratio within the engine. Also, the sources of these pollutants are known, and can be quantified as to their significance, but not eliminated.

For spark ignition (petrol) engines, unburned hydrocarbons come from a variety of phenomena. 5.2% come from crevices in the cylinder itself. The fuel can sit around the piston ring at the cylinder wall and not be exposed to the flame, leading to unburned gases being expelled to the environment. Other sources of this include: exhaust valve leakage, in-cylinder oxidization and quenching. Quenching is the effect of rapid cooling of the fuel at the cylinder wall preventing it from burning; this is also a source of aldehydes.

In SI Engines CO (Carbon Monoxide) is controlled mainly by the air fuel ratio (AFR). In lean mixtures (too much air for the quantity of fuel) dissociation occurs, whereby carbon dioxide decays into carbon monoxide and oxygen. They can also occur as unburned hydrocarbons partially oxidize during the exhaust stroke.

Nitrogen oxide (NOx) emissions are more complex, and are dependent on chemical reactions in series; such as the Zeldovich mechanism. Nitrogen and nitrogen oxides do increase as the temperature of the flame increases inside the cylinder. Flame speed also has an effect on the formation of NOx gases, and NOx will also increase with lower engine speeds.

So, in SI engines, CO and unburned hydrocarbons can be reduced by running with a lean mixture, but this will also lower the engine output.