Vehicle Emissions Reduction

The term denotes all tests during which no external load is exerted and the car operates with the transmission in neutral position.

a. Idle / fast idle test

The test involves carbon monoxide (CO), hydrocarbons (HC) and eventually carbon dioxide (CO₂) concentration measurements in the raw exhaust gas at idle speed and/or a higher engine speed (2000 - 3000 rpm). The test could last from less than one minute in the case of a one-speed idle test without pre-conditioning to about 10 minutes in the case of a two-speed test with "second chance" test including pre-conditioning. A garage-type non-dispersive infrared (NDIR) analyser capable of measuring CO, HC and CO₂ concentrations is sufficient.

Today idle / fast idle tests are still widely used tests in IM programs because they are the fastest, cheapest and easiest to perform with the minimum possible testing equipment.

For carburetted cars they can effectively identify malfunctioning mixture preparation systems by checking the performance of the carburetor's idle mixture orifice in the idle test and the main fuel-metering orifice in the fast idle test.

However, modern cars equipped with electronic fuel injection and ignition systems and three-way catalysts may have a defect -- such as defective sensors and degraded catalyst efficiency -- that cannot be detected through their pollutant emissions at idle.

Worse, the great bulk of emissions may be generated during transient engine operation. An additional very significant drawback is the negligible amount of NOx emissions at idle.

b. Idle / fast idle tests with lambda test

For catalyst equipped cars a lambda test may be coupled with an idle / fast idle test in order to check the performance of the mixture preparation system. Three types of tests can be performed:

1. The air/fuel ratio is indirectly determined through measurement of CO₂, CO, O₂ and HC concentrations at fast idle (2000 - 3000 rpm) in the raw exhaust.
2. The air/fuel ratio is artificially modified by adding oxygen, propane or recirculated exhaust gas to the intake air, or by tampering and then the response of the lambda control system is checked. Long response times would imply that the oxygen sensor is degraded, while no response would mean that the lambda control system is out of operation.
3. One or more of the characteristics of the electronic lambda control circuit are measured and compared with auto manufacturers' specifications.

Germany has adopted since December 1993 a test that involves both test types 1 and 2; preliminary investigations have shown that the test performs fairly well with excess emitters. A combined idle / fast idle / lambda test (involving lambda test types 1 and 2) is also in force in Austria, where it has also demonstrated satisfactory effectiveness. A similar test (but with lambda test type 1 only) is also foreseen for three-way catalyst cars in all EU countries with Directive 92/55/EC, which came into force in 1997.

As NOx emissions at no-load conditions are negligible, a loaded test is therefore necessary in order to measure NOx emission levels, which constitute a critical issue for urban air pollution.

The simplest loaded tests are the steady-state loaded tests. These involve a dynamometer with steady-state power absorption. A simulation of the car's inertia weight is not required because there is no transient phase in the emission test: the car is driven at constant speed and load, and pollutant concentrations (CO, HC, NOx and CO₂) are measured during the load phase.

Already in the seventies several loaded tests were developed in the US such as the Federal 3-Mode Test, the Clayton Key-Mode Test and the CalVIP. However their implementation was limited due to the high cost of the dynamometer and the NOx analyser.

More recently and due to the introduction of 3-way catalyst equipped cars, the Acceleration Simulation Mode (ASM) Tests were developed and evaluated. According to the ASM principle the car is driven on a chassis dynamometer at a constant speed and steady-state power absorption that is equal to the actual road load of the car during acceleration. One can achieve a realistic simulation of the car's load at a specific driving mode without the need of flywheels for inertia simulation.

However, at high speed / high acceleration combinations the required power absorption is too high to be achieved without engine overheating problems.

Pollutant concentrations (CO, HC, NOx) are in principle measured in the raw exhaust. Each steady-state test mode would require about 10 minutes for preparation, pre-conditioning, actual testing and documentation.

In transient tests cars are driven on the dynamometer according to a specific driving schedule; their main differences from type approval tests are the duration of the driving cycle and the hot start. Since exhaust gas emissions are expressed in mass units, a CVS system and laboratory-quality analysers are required in order to detect low pollutant concentrations in the diluted exhaust sample. A multiple-curve dynamometer with flywheels is also required in order to simulate the instantaneous road load and the necessary power to accelerate the inertia masses of each car.

A number of transient loaded short tests were developed in the 70s in the United States and were examined as to their correlation with FTP 75. However, the cost of the implementation of such tests for generalized IM programs was prohibitive, and thus the idea was abandoned. The fact that cars equipped with three-way catalysts were just starting to enter the US market in the late 70s and the performance of these tests with such cars had not been examined also played a role in that decision.

More recently though the interest for transient short tests was renewed. Thus, first the CDH 226 test was developed by the Colorado Department of Health (CDH) and aimed at achieving high correlation with the FTP, especially for three-way catalyst cars. Numerous studies have demonstrated correlation coefficients of 0.79 to 0.96 for all three pollutants. Excess emission identification rates were about 90% for all three pollutants at 5% errors of commission.

However, the US EPA decided to develop a more transient alternative to the CDH 226 in order to better simulate the FTP and therefore came up with the IM 240. Emissions in the diluted exhaust gas are normally derived on a mass basis with a CVS and the test takes in total about 10 minutes to perform. The IM 240 showed correlation coefficients (R2) with the FTP hot start portion of 0.89 to 0.97 for all three pollutants; another test sample had coefficients of 0.54 to 0.82 with the full FTP including cold starts.

The EPA proposed that states or regions, which will have to implement the so-called "Enhanced IM Schemes," enforce the IM240 at least for the cars of the newest model years.

Running the IM240 procedure requires a constant volume sampler and laboratory grade analysers for CO, HC, NOx and CO₂.

In order to reduce the cost, several states including New York and Massachusetts investigated an alternative test that used less expensive equipment but gave similar results as the IM240 procedure. The test that was developed used field grade analysers and a less expensive dynamometer and was capable of driving several different transient driving cycles as illustrated below.

This procedure evolved into the VMass test procedure, which has demonstrated very close correlation with the IM240 test but at much lower cost.