Understanding Emissions

To a service professional, a “parts changer” is a technician who breaks the fundamental rule of automotive troubleshooting: changing things before understanding the problem. Yet the complexity of modern emission control systems and the sophistication of scan tools and engine analyzers (as well as OBD systems) allows many emission system failures to be treated by replacing parts “by the book”. There’s nothing wrong with that, if it leads to a fast and efficient repair, but sometimes the problem defies quick diagnosis. In that case you’re on your own, and with the clock ticking, it helps to understand how and what the various sub-systems are trying to control.

Power, economy and clean air are all about efficient combustion. No matter how efficient the engine design, however, the fuel, air ambient conditions and the need to operate at part throttle/part load conditions means that all tuning is a compromise. Graph 1 shows the dilemma. CO (carbon monoxide) and HC (hydrocarbon) emissions drop as torque reaches its peak near the 14.7:1 stoichiometric air-fuel ratio, but Nox (oxides of nitrogen) soars.

Excessively lean mixtures can spike HC, but this is unlikely to occur without noticeable driveability issues. CO reduction benefits from lean mixtures, but are tolerable at the magic 14.7:1 The graph is clear: precise mixture control near stoichiometric gives near peak torque and fuel consumption properties while keeping HC and CO within ranges easily mopped up by catalytic converters. The high combustion temperatures around the 14.7:1 ratio, however, mean that more than the oxygen ingested by the engine is reacted in the combustion process with gasoline. Air is 78 percent nitrogen, and some of that normally harmless gas reacts to form smog-causing oxides of nitrogen, or NOx. It’s often pronounced “knocks”, but the “x” in NOx is just an algebraic way to represent small numbers like two or three. It’s necessary because there are more than one kind of nitrogen oxide: NO, NO2 and NO3 can all be present, complicating the cleanup of these pollutants. Lowering the combustion temperature by EGR does the trick, and in it’s early, vacuum-controlled non-computer form, had the unfortunate side effect of tilting mixtures toward the lean side, increasing HC and reducing fuel economy. Computer control eliminated these shortcomings by allowing advanced ignition timing to more than make up for the power loss, while allowing the use of EGR in some systems as a detonation control strategy as well. From a service perspective, it also means that a NOx failure can involove more troubleshooting than just the valve and ECU. Some late-model engines with variable valve timing use the ultimate EGR system: none at all. The trick is to alter valve timing to leave some exhaust gas behind as the intake charge fills the cylinder. Cylinder scavenging is never perfect, and the effect is always present to a small degree in all engines, but variable valve timing allows engine designers to tailor a profile that effectively varies the amount of residual exhaust diluting the incoming air.

Volumetric efficiency is another component of low emissions, and maximizing it can also use the “self EGR” technology. “Adding” large amounts of exhaust gas can effectively pack cylinders that don’t need a full intake air charge during idle and part throttle operation, allowing the engine to be throttled through valve and injection timing rather than conventional throttle butterflies. If this technology becomes widespread, the implications for emissions service will be significant, as the EGR system disappears as a stand-alone unit, and valve timing controls both NOx and engine RPM. Diagnosis may require some means of checking valve timing vs. RPM to check the actuator, or at least a scope trace of the signal from the ECU. On the other hand, a VVT failure in a high-tech engine may make it unthrottleable, so the vehicle may arrive “on the hook” anyway. Better emissions would come automatically with the repair. Direct actuation of cam timing (possibly by stepper motors or eventually without a cam by solenoid-driven electric valve actuation) is possible, but current systems convert the ECU signal into variable oil pressure, which drives a helical spline on the cam drive gear, advancing or retarding timing. For good driveability and clean emissions, oil system cleanliness and oil of the right viscosity will be even more important in the future. Similarly, use of the wrong grade of oil or of viscosity enhancing additives may become a factor. As the new engines age, it might become worthwhile to ask owners about their top-up habits, and suggest an immediate oil change with the correct product before a smog check.

Catalysts and mixture control

Modern three-way catalysts, fed an exhaust stream using mixture checked by O2 sensors, are an excellent way to mop up both HC and CO emissions. Inside a three-way converter, platinum, palladium and rhodium reduce the three major pollutants by about 90%:

1. HC is converted to CO2 and water vapour

2. CO is converted to CO2

3. NOx is converted to nitrogen and oxygen in their natural gas forms

Add more converters and the emission levels drop even more.

A look at Graph 2 shows why the feedback from O2 sensors is necessary. Good conversion efficiency of HC and CO need stoichiometric air/fuel ratios, and O2 sensors can not only monitor mixture cheaply and efficiently, but can monitor catalyst performance too. Without sensors downstream of the converters, a poisoned catalyst would be difficult to isolate in a high-mileage engine without eliminating underhood system problems, making it often cheaper to just replace a suspect converter to get a vehicle into compliance. An important factor to keep in mind about oxygen sensors is that they’re just that: devices that respond to changes in the oxygen level of the exhaust gas stream. The ECU infers mixture from the oxygen levels reported by the O2 sensors, so if something alters oxygen content in the exhaust, the O2 sensors will send a signal that the ECU will use to adjust mixture, possibly wrongly. An example is ignition misfire, which sends unburned (or more accurately, unreacted…it’s the fuel that “burns”) oxygen and raw fuel into the exhaust, causing it to send a lean mixture signal to the ECU. That in turn can drive the mixture excessively rich, driving up HC and CO, while simultaneously reducing the converters’ ability to clean up the excess. That’s why replacing secondary ignition components like plugs and wires can have such a dramatic effect on emissions performance.

The current trend is to drive emission control systems into closed loop operation as fast as possible, and to achieve this, pre-converters are increasingly appearing under the hood to speed light-off. The 2003 Honda Accord V-6 takes this principle to the next level by eliminating the exhaust manifold altogether. Exhaust ports are fed to a common plenum in the head, and the pre-converter is bolted directly to it.

There is far more to emissions than can be covered in any one article, but the ability to explain to a client why sometimes plugs and wires will do, while other situations require expensive repairs, will ease “sticker shock” at testing time, and prepare them for reality after the repair cost exemption expires. Look for more emissions-related coverage soon in SSGM.