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OBDII for Diesels

Emission Controls Pick Up Steam for Diesel Systems
Saturday, March 1, 2008 - 01:00

Emission Controls Pick Up Steam for Diesel Systems

Onboard diagnostic (OBD) systems were pioneered and refined in gasoline engines, but now this technology has taken a firm hold in the diesel engine world as well. As of the 2007 model year, all diesel-powered vehicles rated at less than 14,000-pound gross vehicle weight rating (GVWR) must now meet OBDII requirements for monitoring the performance of their emission control systems.

Benjamin Franklin once wrote, "But in this world nothing can be said to be certain, except death and taxes." If Franklin had been able to look into the future, he might very well have added ever-tightening emission control regulations to his list. Since the California Air Resources Board (CARB) first started regulating emissions that had a negative impact on its state's air quality, standards have only gotten tougher — and technology has been steadily improving to meet these new challenges.

OBD systems are nothing new to diesel-powered vehicles. OBDII first started phasing into production vehicles in 1994 and then was required by both CARB and the U.S. Environmental Protection Agency (EPA) for light-duty diesel vehicles (less than 8,500-pound GVWR) starting in 1997. From 1997 on, California regulations also required OBDII compliance for all medium-duty vehicles (8,500- to 14,000-pound GVWR), both gasoline and diesel-powered. The EPA did not immediately follow California's lead in this area, however, and allowed an exemption from OBDII compliance for federal vehicles in the 8,500- to 14,000-pound GVWR class. The agency required that onboard diagnostic systems be used, but it did not need to be nearly as thorough as those that were required for California vehicles.

In good time, it became clear that changes would have to be made, and starting in 2004, OBDII began phasing into the EPA's "heavy-duty" class (8,500- to 14,000-pound GVWR). As of 2007, all vehicles sold in the U.S. in this weight class must be OBDII compliant.

OBDI vs. OBDII

Since 1997, there were a number of model years where federal diesel-powered pickups and vans were being sold that were OBDII exempt. These vehicles were required to have onboard diagnostics, but the systems did not need to be as comprehensive as their OBDII counterparts. How different were the federal versions from the OBDII vehicles being sold in California and other states that have adopted California emission standards?

Generally speaking, the majority of the differences were in specific Powertrain Control Module (PCM) calibrations, as the same serial data bus, data link connector and basic PCM software were used in both versions. Fewer supported monitors were used in the federal models, but the same generic and enhanced scan tool modes worked in both. For the most part, it would be difficult to tell any major differences with a simple visual inspection.

An exception in some vehicles was the glow plug system, where different hardware could be used to control and monitor operation of the glow plugs. In the federal vehicles, no glow plug monitor was required, whereas the California versions had to be able to diagnose a malfunctioning glow plug, set a Diagnostic Trouble Code (DTC) and turn on the Malfunction Indicator Light (MIL) accordingly. See the sidebar with details of the federal (OBDI) calibration for a 2003 Ford 6.0 liter turbodiesel. Overall, it is much less likely for the MIL to be illuminated in the federal vehicles than it would be in the OBDII-compliant California models.

Diesel Engine Monitors

The function of an OBDII-compliant emission control system pivots on the operation of its supported monitors. With OBDII, all components and systems that play a significant role in the vehicle's emissions output must be monitored using one or more of the following tests:

Electrical tests. Testing sensors and actuators for continuity, short circuits, signal out-of-range, etc.

Rationality tests. In the case of sensors, determining whether the data provided makes sense in light of other data input.

Functional tests. Determining whether a device is responding properly to computer commands.

Functional tests can be performed using either active or passive means. Passive testing is waiting for an actuator to receive a command from a vehicle's computer during normal operation and then looking for sensory data that would indicate proper operation. Active testing is where the computer takes control of the actuator for testing purposes only.

Diesel engines use a number of monitors that are similar to those used on gasoline engines. A short list of examples would include comprehensive component monitor and exhaust gas recirculation. There's also a misfire monitor, but it runs only at idle. In some cases, components that were once exclusive to gasoline engines now are being used to perform monitoring functions in diesel engines.

A good example of this is the Mass Air Flow (MAF) sensor, which is now being used with diesel engines to monitor the operation of the Exhaust Gas Recirculation (EGR) system. Similar to gasoline applications, total airflow into the engine is measured while the EGR valve is closed, then an associated drop in airflow is expected to occur as the EGR valve opens.

In some cases, this new measurement is compared to a speed-density calculation that is performed using a Manifold Absolute Pressure (MAP) sensor and the engine's rpm signal. The difference between the two measurements is the effective EGR gas flow, and this is compared to what was called for by the vehicle's computer to determine whether a fault exists in the system.

While there are certain monitors that diesel engines have in common with gasoline engines, there are also those that are unique to diesels. One example would be the EGR cooler monitor. The exhaust gas recirculation system plays a major role in controlling nitrogen oxide (NOx) output from today's diesel engines. Very high rates of EGR flow are required, and it is necessary to cool the EGR gases to achieve the desired effect.

For instance, the Ford 6.4 liter Powerstroke uses two liquid-cooled EGR coolers in series to perform this function. Because the EGR cooler has a major impact on vehicle emissions, it must have an associated OBDII monitor to check on its operation. In the case of the 6.4 liter Powerstroke, two temperature sensors are used to monitor the EGR cooler operation: one on the exhaust manifold as it leads into the EGR system, and one near the EGR valve itself.

When the EGR cooler monitor runs, the PCM can check the efficiency of the coolers by looking for a temperature difference between the inlet and outlet sensors with the EGR valve open. A first failure of this test would generate a pending code and freeze frame, while a failure during the next trip would lead to a DTC being logged and the MIL illuminated.

Another example of a monitor that is unique to diesel engines is the Diesel Oxidation Catalyst (DOC) efficiency monitor. The DOC is used to oxidize hydrocarbon (HC) and carbon monoxide (CO) emissions, as well as dealing with certain fractions of Particulate Matter (PM). In the past, DOCs have been used in diesel-powered pickups and vans, but their operation was not monitored, even in California OBDII applications. This has changed as of the 2007 model year (more specifically, vehicles built since Jan. 1, 2007), and now these same vehicles must incorporate a catalyst efficiency monitor in their OBD strategy.

While catalysts in gasoline engine applications use oxygen sensors, DOCs most often use exhaust gas temperature sensors to determine their efficiency. Typically, this monitor will run during active regeneration of the Diesel Particulate Filter (DPF), which is located downstream from the DOC. A tiny amount of fuel is injected when the exhaust valve is open. When this post-injection fuel makes its way into the DOC, exhaust gas temperature will rise as the catalyst oxidizes the excess HC. The temperature of the exhaust gases is measured at the DOC inlet, and this is compared to the readings at the outlet.

The minimum expected temperature increase is based on the amount of fuel injected for DPF regeneration purposes. If exhaust gas temperature does not increase to the expected minimum, a DTC is logged and the MIL illuminated.

Closed Loop Operation

In the not-too-distant future, we can expect to see closed-loop operation in diesel engines. New piezo-resistive sensors have been developed that can be built into the same package with a glow plug. Because the glow plug reaches into the engine's combustion chambers, it is now possible to measure cylinder pressures during a combustion event and make fuel control adjustments based on this feedback. This is a huge leap forward in diesel emission control, because the ability to limit peak pressures in the combustion chamber will also limit NOx formation.

Diesel engine OBD can also be enhanced with cylinder pressure data that could be used to execute new and more sophisticated monitors, making the scan tool progressively more important for diagnosing diesel engine fuel and emission control system issues.

If you're comfortable with scan tool diagnostics on gasoline engines, you'll definitely have an edge when it's time to work with these new diesel systems. But there's still a lot to learn — and since some of Ford's training material is already posted on Motorcraft.com, that might be a good place to start.

Tony Martin is an assistant professor of automotive technology at the University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 18 ASE certifications, including CMAT, CMTT, L1 and L2.

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