Don’t fear the emission test

Faraz Anwar Shah

IN THE 1980s General Motors Corporation implemented their proprietary testing protocol to test the Powertrain Control Module (PCM) through CAN BUS technology. A cable from a scan tool attaches to the ALDL (Assembly Line Diagnostic Link) port of the vehicle, which is able to read the diagnostic information from the vehicle, including any Diagnostic Trouble Codes (DTCs) if any. A DTC sets the check engine light that we see in our vehicle when there is a trouble present. It should however be noted that not all DTCs illuminate the check engine light, only powertrain related DTCs affecting emission control is more likely to set the check engine light. In the late 1980s the Society of Automotive Engineers (SAE) recommended standardizing the ALDL port and connector to read the vehicle’s information.
The advent of On board Diagnostics or OBD paved way for the government to monitor vehicle emission controls. It is mandatory for every new vehicle to comply with the clean air act and emit the mandated emissions as regulated by law. Emission control is necessary to protect our environment, safeguard the future, and ensuring we give a better cleaner world to our successors. Governments across the world are implementing laws to test vehicle emissions on a periodic basis. The vehicle owner must take their vehicle to a certified center that will check the vehicle and give recommendations based on their testing.
In light of the above concerns, the out-going US administration went into an agreement with automakers in the year 2011 to raise the fuel economy of the vehicles to approximately 55 mpg by the year 2025. This is a monumental task to achieve for the automakers. With crude oil prices falling, and an abundance of supply of gasoline in the market, the automakers are reluctant to work on this expensive idea. Besides not being very cost feasible, to achieve a fuel economy of 55 mpg on petrol vehicles is not an easy task. This requires a lot of engineering research and development. Even then the results are not guaranteed.
Many of us who must take our vehicles for periodic emission testing fear that their vehicle may fail the test, especially if it is an older model vehicle or if it is not properly maintained. Therefore it is required that we keep our cars properly maintained to increase the probability of passing the emission and periodic inspection. Later on I will discuss some scenarios that contribute to periodic inspection test failure, and how these failures can be avoided. But first let’s first give a short overview on how the emissions test is performed.
The vehicle is driven into a test center that is equipped with a dynamometer cell. Dynamometer consists of rollers for all four wheels that measures the load on the vehicle engine, and can also increase or decrease loads on the engine by varying the forces to rotate the tyres.
The dynamometer is plugged into a machine which analyses the engine performance at idle and at different velocity, and at the same time reads the diagnostic information from the vehicle’s on-board computer or PCM. It also analyzes data from a sniffing device that measures the particulates in the exhaust pipe. These particulates consist of hydro carbon, carbon monoxide, and nitrogen oxide. This is a pretty similar concept to the stress test that is performed on humans.
We will discuss three scenarios that can be taken care of in order to increase the possibility of passing the emissions test. The first aspect to properly understand is the Air-Fuel ratio (AFR). The AFR is the ratio of the volume of air required by the internal combustion engine to ignite the gasoline. The correct AFR for gasoline engines is 14.7 parts of air to 1 part of fuel. Consequently for alcohol burning engines the AFR is different due to the combustibility of alcohol ranging at 6.4 to 1 and 14.5 to 1 for compression ignition engines (diesel vehicles). Any changes in the AFR can cause the emission test to fail.
Scenario 1:
High amounts of Hydrocarbon and Unburned fuel. If there is a rich air fuel mixture present in your combustion chamber, chances are that the vehicle is getting more gasoline and less air. Probable causes could be clogged air filter, a defective oxygen sensor, or leakage in the fuel injector. If there is a lean air fuel mixture this means that there is less fuel and more air coming in. Possible causes are clogged fuel injectors or a vacuum line leak. High amounts of hydrocarbons can also be detected if the ignition coil is defective, worn out spark plug wires, or spark plugs or if the engine’s timing is incorrect
Scenario 2:
High amounts of Carbon monoxide is the result of incomplete combustion. The first suspect of an incomplete combustion is the rich condition, with above stated factors for rich condition as in scenario 1.
Scenario 3:
High amounts of nitrogen oxide means that there is combustion at very high temperature. The usual suspects are issues with the cooling system (check radiator clogging and hoses), a lean air fuel mixture or a problem with the exhaust gas recirculation (EGR) valve. The way to know if the EGR valve is defective, the vehicle will pass the emissions test at idle but fail at 2500 rpm. Some other causes are bad catalytic convertors that raise the level of nitrogen oxide. Another scenario can also be over advanced engine timing.
Before the test it should also be noted that the tyre pressure is kept at manufacturer recommended number. Low tyre pressure increases the contact surface area between the tyre and the dynomometer’s roller, hence increasing friction. With increased friction the PCM assumes that there is greater load on the vehicle (which is not the case), therefore the PCM dumps more fuel in the engine. Since there is not enough air to fully combust that extra gasoline, the car experiences a rich condition.
Revisiting the original discussion of achieving greater fuel economy number whilst not sacrificing performance, a new study is being implemented to combine super charger and a turbo charger into one calling it a super-turbo. The super charger works at low rpm rotating at engine crankshaft speed giving forced induction to the engine. While at higher speed the exhaust gases turn the turbo charger to give boost. This method greatly reduces turbo lag while increasing the performance of the engine. And since forced induction engines are inherently small, they do not consume as much fuel as a large engine with more cylinders. Force induction engines have denser air coming entering that raises the top cylinder pressure (air gets hot when compressed), increasing the possibility of engine knock.
As the air fuel mixture is lean, the temperature is high and engine knock can be greater. Therefore these engines run rich. This can be avoided by either retarding the engine timing or reducing the amount of boost to achieve maximum power with highest amounts of durability.
—The writer is an automotive industry professional with 17 years’ experience in vehicle powertrain design & development, duty cycle testing, developing safety standards, forensic investigation of failed components, accessory marketing, supply chain optimization, supplier quality assurance, and is a Toyota business practice (TBP) certified instructor.

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