In Part 1, I regaled you all with the brief history behind the discovery of photochemical smog, and then proceeded to dazzle everyone with the intricacies of intake-related pollution controls. Here, in Part 2, I’ll continue down the long, winding path of emissions systems. This time around, we will focus on the biggest contributor to vehicle emissions—the exhaust.
Before we discuss the smog pump, a brief aside on names.
Names are important. They convey meaning and help our brains form a connection to the enormous world around us. And most of the time, they make sense, but sometimes we humans have a habit of assigning purposefully confusing names. Take the Mountain Chicken, a species native to the Caribbean. Most of us will see that name and think, well, that must be some sort of mountain-dwelling chicken. But it’s not a bird at all, it’s a rather large frog that just happens to taste like a chicken.
The smog pump, native to the engine bay of certain older vehicles, is not a pump for smog—it’s an air pump. It’s also an important piece of emissions equipment. The pump is one component of a larger system called Air Injection Reaction (AIR) or, in the parlance of our time, Secondary Air Injection Reaction (SAIR).
Air injection became part of the underhood surroundings in 1966 for Californians and 1968 for the rest of the United States. It was an important first step in the reduction of hydrocarbon and carbon monoxide emissions from vehicle exhaust. The idea behind AIR came around because, as it turned out, the internal combustion engine wasn’t quite as good at the whole combustion thing as people thought.
Vehicle exhaust was loaded with hydrocarbons (HC) and carbon monoxide (CO), the direct result of incomplete combustion. Unburned fuel is a type of hydrocarbon, and carbon monoxide forms when there is a lack of oxygen during combustion. So, people figured out that if you pumped fresh air into a hot exhaust manifold, the HC and CO would react with the oxygen and finish what started in the combustion chamber, and would effectively change the HC and CO to H2O and CO2. Introducing fresh air into the exhaust manifold proved to be quite effective, and it was especially important during cold starts when the air/fuel mixture was richer than Mr. Burns.
In its early years, SAIR made use of a belt-driven pump mounted to the front of the engine, and so it just kept pumping no matter what. Because of all the parts needed to get the air into the exhaust, it was a bit of a blight on the presentation of the engine bay. Unsightly metal tubes protruded from the manifolds, with large, cumbersome check valves on the ends that kept the air flowing in the right direction. Of course, there was the pump itself, which on some cars was awkwardly placed, with yet another frumpy valve mounted nearby.
The whole apparatus really cluttered up the engine bay, which might have been one of the reasons SAIR systems sometimes got chucked in the bin. Well, that, and the rumor that the pump was a massive drain on horsepower. The rumor wasn’t exactly true—the load the pump put on the engine was trivial. However, there were other factors at play to prevent any excess load.
That frumpy valve I mentioned was called a diverter valve, and it was in charge of directing the pump’s airflow. When you were just cruising around like a normal person, the valve sent air to the manifolds, and when the throttle was abruptly lifted, the sharp spike in vacuum would open the valve to vent the air to the atmosphere. The third function was to relieve the pressure when you put the pedal down and unleashed all the fury of a ’70s American sedan, the relief valve kept the pump’s load at a minimum, and also protected it. In some cases, the relief valve was built into the pump.
Air injection stuck around for ages, though it went through a few changes over the years. Once the catalytic converter became a regular player in the emissions game, SAIR became an integral part of the converter’s functionality. All those chemical reactions created between the additional oxygen and the HC and CO made the exhaust extra spicy, so the converter could reach its operating temperature a lot faster. This meant less uncatalyzed exhaust entered the atmosphere. And because a catalytic converter needs a sufficient supply of oxygen to work properly, the air injection system was also used to pump extra O2 down to the converter. Eventually, the belt-driven pumps were replaced by electric units controlled by the ECM, which was only turned on during cold starts, and no longer fed additional oxygen to the converter.
These days, between advanced engine controls and better converter placement (i.e., closer to the cylinder head), SAIR has been rendered almost entirely obsolete among modern vehicles. Unless, of course, you own a Toyota 4Runner, which utilized air injection until 2024. And that’s only because the Toyota used the same basic 4.0 V-6 that debuted during George W. Bush’s first presidential term.
Everything up until this point focused on curtailing HC and CO emissions that enter our air. But what about all those pesky NOx emissions? Well, for that particular chemical compound, engineers cooked up something special, and it strikes fear in the hearts of diesel enthusiasts everywhere. It’s called EGR.
EGR stands for exhaust gas recirculation, and it came into existence around 1971 for the ‘72 model year, with the sole purpose of cooling off the combustion chamber. Why cool the combustion chamber, you ask? Well, because harmful NOx pollutants only show up when combustion temperatures exceed 1300ºC (≈2500ºF). So, EGR recirculates exhaust into the intake to dilute the air-fuel mixture, which slows the combustion process and turns the heat down just enough to keep too much NOx from turning up in the exhaust.
At the heart of this system is the infamous EGR valve. And every manufacturer since 1972 has used one in some form to control the flow of exhaust gases into the intake—except for Chrysler, in its 1972 products. In this one-year-only case, exhaust would flow through a passage into the intake manifold, and floor jets (small fittings with a metered orifice) within this passage would admit a small amount of exhaust below the carburetor, and thus into the combustion chamber. Because there was no valve to regulate when exhaust would recycle into the intake, this arrangement produced a lot of drivability issues. Fortunately for Mopar fans, Chrysler started using EGR valves in 1973.
Before any sort of computer controls existed, engine vacuum was the go-to source for engine controls, including the EGR valve. The simplest setups used ported vacuum coupled with a thermal vacuum valve (TVV) to control when the EGR valve opened. The TVV made sure EGR wasn’t active when the engine was too cold, and the use of ported vacuum kept the valve shut at both idle and wide-open throttle (vacuum is approximately zero at WOT).
Keeping things simple is great for a lot of situations, but when it comes to controlling EGR, simple isn’t exactly better. Most folks prefer their car without weird drivability issues that may be caused by ill-timed EGR flow. So automakers explored more accurate ways of determining when the exhaust should flow into the intake.
Some harnessed the power of the exhaust backpressure transducer. This handy little device regulated when ported vacuum would be sent to the EGR valve based on the amount of positive backpressure in the exhaust. When backpressure reached a certain point, a diaphragm would overcome spring tension and close a port vented to the atmosphere, and the EGR valve would open. Sometimes the device was built into the EGR valve.
Others decided on the use of venturi vacuum, pulled from a port on the carburetor at the choke point in the venturi. This was linked to a vacuum amplifier that received manifold or ported vacuum. When the venturi vacuum reached a certain point, it operated a diaphragm that let the vacuum through to open the EGR valve. Thermal vacuum valves were still used to stop unwanted EGR flow on a cold engine.
These were just a few ways EGR valves operated in a pre-computer-controlled world. As the march of time continued, engine controls steadily evolved, but vacuum was still the go-to source for certain companies to operate the EGR valve. The big difference was that the vacuum was controlled by the powertrain control module (PCM), which used inputs from a variety of sensors around the engine for precise EGR timing. Parts like the throttle position sensor, coolant temperature sensors, MAP, MAF, and usually a duty-cycled vacuum switching valve to direct the flow. EGR valves found on today’s vehicles use small electric motors and are much more precise in their delivery of recycled exhaust. And most of them, much like the EGR on a diesel, are liquid-cooled.
(Consider this an abridged text on EGR—I could easily fill an entire two-thousand-word story with the various forms and functions of this piece of emissions equipment.)
And finally, the part you’ve all been waiting for, the part that everyone loves to hate, the part voted most likely to be hacked off with a Sawzall in the middle of the night: the catalytic converter.
In 1975, the two-way catalytic converter came into existence, also called an oxidation catalyst. It was necessary to meet the ambitious goal of a ninety percent reduction in vehicle emissions set by the Clean Air Act in 1970. This style of converter was only capable of reducing HC and CO emissions, hence the name. Oh, and it basically put an end to leaded gas at conventional pumps, because lead destroys the converter.
Standard two-way converters got a new friend in 1978, and it was called the three-way converter, sometimes referred to as a reduction catalyst. Three-way converters are responsible for handling NOx emissions and were either added as a separate unit or combined in the same shell.
A lot of manufacturers used a ceramic monolith converter, which is the style most of us are familiar with, and it kind of resembles a honeycomb. The other type, developed and used almost exclusively by General Motors, was the pellet converter. Pellet converters were packed with coated alumina pellets. As you can imagine, the pellet-filled catalysts created a major restriction in the exhaust system and are probably the reason everyone thinks their “cats” are power-robbing exhaust gremlins.
How does a catalytic converter perform all these complex chemical reactions? It uses a combination of heat, oxygen, and elements like cerium, palladium, platinum, and rhodium that act as catalysts to convert the harmful emissions into harmless emissions.
Heat is a crucial component for converter function. They have to hit a minimum temperature of around 300° Celsius (572° Fahrenheit) before they can even think about working properly (“converter light-off”). The good news was that SAIR was there to help during cold starts, so the converter could get to work faster. Heat wasn’t too much of an issue after things warmed up. Cerium is like the squirrel of the converter. It stores up all the oxygen it can get and releases it when necessary. The platinum and palladium are part of the oxidation catalyst and work by adding oxygen and hydrogen molecules to change HC and CO into H2O and CO2. The reduction catalyst uses either platinum or palladium with the addition of rhodium to remove oxygen, so NOx is reduced to harmless nitrogen and oxygen.
For proper care and feeding, three-way converters needed a more reliable air-fuel ratio than a standard carburetor could handle. The first company to take a swing at this was Volvo, in 1977. In order to meet California emissions regulations, the Swedish company installed oxygen sensors on its fuel-injected vehicles equipped with three-way converters. The oxygen sensor told the engine controller how rich or lean the engine was running, so it could make adjustments accordingly and keep the air-fuel ratio hovering around 14.7:1. Converters love a nice, stoichiometric air-fuel ratio!
The problem was that fuel injection wasn’t the industry standard in the ‘70s—carburetors were. So, some manufacturers came out with the ever-popular computer-controlled feedback carburetor while they caught up. These irritating little beasts used a mixture control solenoid that was duty-cycled by an engine controller based on oxygen sensor feedback. While this wasn’t the most elegant solution, it still did a decent job of providing a balanced air-fuel ratio. You could always tell they were working because vehicles with feedback carbs had a somewhat a shaky idle. And while it was a bit of a slog to get from feedback carbs to the industry-wide adoption of fuel injection, the changeover happened eventually. Fuel-injected engines started to trickle in during the late ‘70s and early ‘80s, and started to become more widespread as the ‘90s approached. Electronic fuel injection just took off after that, and most of us pushed feedback carbs from our memories.
Nowadays, you can’t discuss emissions without spending a little time talking about diesels. Diesel engines have been at the forefront of emissions-related news for the past few years, thanks to DieselGate and the EPA slapping massive fines on diesel tuners everywhere. So what’s the deal with diesel exhaust emissions anyway?
Well, it’s kind of similar to what you’d find in a gasoline engine, except for a few details. Diesel engines do have catalytic converters. One is the diesel oxidation catalyst, or DOC. Just like a gas engine, the DOC is responsible for oxidizing HC and CO into CO2 and H2O. It even uses the same precious metals as its gas counterpart.
The difference between the emission controls of diesel and gas vehicles comes after the DOC, when NOx is reduced. Because diesels use compression ignition, they produce a tremendous amount of heat, and excessive heat in the combustion chamber leads to excessive amounts of NOx. Diesel exhaust also doesn’t play well with a standard reduction catalyst, so diesel vehicles use a special selective catalytic reduction catalyst (SCR). The SCR is the reason behind diesel exhaust fluid (DEF). DEF is made of deionized water (67.5%) and urea (32.5%). When conditions are met, the ECM injects DEF into the exhaust, and the heat breaks down the urea into ammonia, which reacts with NOx and converts it to N2 and water vapor.
Then you have the issue of particulate matter (PM). To combat the PM, diesel-powered vehicles use a diesel particulate filter (DPF). The DPF is made of a porous material and has tiny passages throughout that dead-end, so to speak. The dead ends allow the DPF to collect the larger chunks of PM, but the porous nature of the filter material allows the exhaust to continue its journey out of the tailpipe. And when these chunky little bits of diesel soot fill up the DPF, the ECM puts the vehicle into regen mode. Regen puts extra fuel into the exhaust and brings the temperature up to 640°C (1184°F) and effectively burns off most of the built-up PM.
Exhaust after treatment on diesel engines uses a number of different sensors to report conditions to the engine control module: NOx sensors, exhaust gas temperature sensors, pressure sensors, and sometimes soot sensors, all just happily living their best lives in the exhaust and reporting back what they see.
It’s important to note that the early emissions systems weren’t directly responsible for the death of horsepower. That happened because we were pursuing lower emissions without the proper technology to achieve that goal. Naturally, compression ratios lowered and cam profiles became more conservative. (The removal of lead in the gas played a role as well.) But automakers made do with what they had at the time, and eventually came out the other side better than ever.
An honorary mention goes out to things like computer-controlled ignition systems, variable valve timing, and the variety of other advancements that led to the perfect combination of efficiency and power—some of which might not have happened if it weren’t for the noble pursuit of a clean-burning internal combustion engine.