By P Sanchez
Last time we talked about how the muffler, perhaps the most prominent feature of the exhaust system, has been designed to address a character flaw of the internal engine: the noise. Now we’ll talk about the real dirt on driving and see how science, policy, and engineering have effected an automobile feature that’s staving-off the death of the planet (or at least making things a lot less harmful to public health.)
Hint: It doesn't go "meow".
Where There Is Smoke...
Let’s face it. In spite of all its engineering and technological advancements - like fuel injection, variable valve timing, turbocharging, and even electronic engine management - the internal combustion engine remains as an inherently inefficient and pollutive system.
The transportation is like a factory for air pollution on a global mass-production scale. According to the EPA, the sector is responsible for over 55% of nitrous oxide emissions inventory of the US. NOx in the atmosphere contributes to acid rain and destruction of the ozone, and it’s also a respiratory irritant. Other chemicals found in the exhaust are carbon monoxide (poisonous), sulfur dioxide (contributes to acid rain), benzene (carcinogenic), and particulate matter (cardiovascular and lung disease). A full list of common exhaust chemicals and their ill-effects can be read here.
So it’s bad, we get it. What gives? Liquid fuels like gasoline are made of long chains of hydrocarbons. Touching a bit on basic chemistry, burning is just a rapid oxidation process that yields heat, and with the burning of liquid fuel, hydrocarbons molecules are completely broken down to carbon dioxide and water (H2O). But that’s the ideal picture.
Far From Perfect
Unfortunately, the conditions inside an engine are not always ideal. Fuel-to-air ratios are not always correct and many times the mixture becomes too fuel rich and without enough oxygen, some considerable amount of noxious vapor leaves the engine unburnt. Pressures in the cylinder can also get high enough that normally inert nitrogen reacts with oxygen forming nitrous oxide. The fuel itself is filled with additives that when burnt, creates other harmful byproducts.
And so far we’re talking about harmful emissions from optimally running engines. Things get even worst as an engine age: seals start to fail, parts start to wear-out, mechanical and electronic timing become off. Without periodic maintenance from your trusted mechanic, your old beloved ride easily becomes a gas-guzzling, smoke belching, headache over the years.
A Little History
As early as the 1950s, studies about the adverse effect of air pollution and how the internal combustion engine is a prime contributor - have been out in the public. Around this time, French mechanical engineer Eugene Houdry has developed an oxy-catalyst which he was awarded the US patent for it. The technology was meant to clean the exhaust fumes from smokestacks and types of machinery that use low-grade fuel.
It was not until the enactment of the Clean Air Act of 1970s that made catalytic converters suitable for use on automobiles. The policy required vehicles to cut harmful emissions by 75% by 1975 and part of it was the removal of tetraethyllead as an anti-knock additive for fuel. Not only was “lead” found to be a serious health hazard but it also interfered with catalytic converters. Further development and standard application of catalytic converters on vehicles have been seen ever since.
But how does a “cat” work? A catalyst is something that hastens a chemical reaction without itself being affected by it. Cats use rare earth metals that do exactly this, break down harmful compounds in exhaust gases into simpler, more benign molecules. The metals are impregnated on the surface of a ceramic honeycomb material that not only retains heat better which aids in the reactivity of the catalysts but also provide a very large surface area to catch all the chemicals.
How good are cats? Very. They can breakdown as much as 99% of the target chemicals when brand new. However, they are not completely impervious and certain chemicals in the exhaust (like carbide, phosphorus, and even carbon ) can coat the ceramic surface or clog the honeycomb filter and render the cat less effective. After a few thousand miles, they drop to 95% efficiency in converting harmful emissions. Falling below 92% efficiency will usually trigger an electronic warning to the driver which will signal the need for clean-up and/or repairs.
At the Tail End of Things
We are still dependent on fossil fuels and as long as that’s the case, our way of living - or more specifically, our way of driving - greatly impacts the environment we live in. Modern exhaust systems have definitely reduced the strain of automotive vehicles on the environment and we have its component, the catalytical converter, to thank for that.