So why is this fine old English verb working overtime lately? It features in almost everything Iíve seen written about the Volkswagen diesel emission cheat scandal. Itís the general mediaís word of choice when it comes to describing the real-world NOx emissions that the engine software of the affected cars was deviously designed to disguise.
But NOx Ė oxides of nitrogen Ė is just one very small cube in the mixture of stuff that comes out of a carís exhaust pipe. Itís true that NOx is nasty stuff. And itís true that Volkswagen deserves to be condemned and punished for its deception. But itís also worthwhile keeping things in perspective.
Choosing the word spew doesnít help here. As well as creating a misleading mental image of the quantities in question, spewing also carries strong
negative connotations. What comes up with the carrot is all disgusting, right? Yet almost all of what comes out of a carís tailpipe isnít nauseating at all.
No matter what fuel an internal combustion engine burns, the gas coming out of its exhaust is mostly nitrogen. This gas makes up 78 percent of the air that we breathe and the exhaust of an idling engine will contain only a slightly lower percentage by volume. The nitrogen is warmer than when it went into the engine, but otherwise none the worse for wear.
Then thereís oxygen, which makes up 20 percent of air. Only a little of this will be consumed while idling, although running at maximum output the oxygen content of the exhaust stream from a spark-ignition will be low, ideally zero. Making power means burning fuel, and the upper limit is determined by the amount of oxygen available for combustion.
The liquid and gaseous hydrocarbons we call petrol, diesel, LPG and methane are compounds, in varying proportion, of hydrogen and carbon.
These combine with oxygen during combustion to produce carbon dioxide and water vapour, which together account for almost everything else in the tailpipe gas mix.
A perfect internal combustion engine would emit nothing more than these, but perfection is elusive. There are always unwanted secondary reactions in the combustion chamber. These produce the stuff that typically makes up less than one percent of the exhaust gas mix, but which require after-treatment by catalytic convertors and other pollution-reduction equipment.
Some fuel isnít burned or burns incompletely.
Result? Hydrocarbon pollutants, including some considered carcinogenic.
Sometimes the fuelís carbon content oxidises to form toxic carbon monoxide, instead of carbon dioxide. And sometimes, in the heat of combustion, a small amount of the nitrogen trapped in the combustion chamber couples with oxygen to create oxides of nitrogen. NOx is linked to lung ailments and smog formation.
Itís important that emissions of these pollutants are tightly controlled, no question. But the fact remains that nitrogen, oxygen, carbon dioxide and water vapour are what engines mainly emit. These also happen to be the gasses that make up most of every lungful youíve ever exhaled.
And thereís quite a difference between breathing and spewing.
DIESELS canít help emitting higher levels of NOx than sparkignition engines. Itís an unavoidable result of their higher local peak combustion temperatures.
Measures to cleanse their exhaust gasses of NOx are complex and expensive.
Or, in Volkswagenís case, cheap and cheaty.