The two-speed idle (TSI) method is a widely adopted exhaust emission testing procedure for in-use spark-ignition (gasoline) vehicles, measuring carbon monoxide and hydrocarbon concentrations at both low idle and high idle engine speeds. This article explains the technical principles, applicable standards, sample scope, testing procedures, and typical applications of two-speed idle emission testing, serving as a practical reference for inspection stations, fleet operators, and accredited third-party laboratories.
Technical Overview
The two-speed idle (TSI) method, also known as the dual idle speed method, is a steady-state exhaust emission test procedure applied to in-use vehicles powered by spark-ignition engines. Rather than measuring emissions under simulated driving loads on a chassis dynamometer, the method evaluates pollutant concentrations while the engine operates at two unloaded, stabilized conditions: normal (low) idle and a specified high idle speed, typically 2500 ± 100 rpm.
During the test, a nondispersive infrared (NDIR) exhaust gas analyzer draws a continuous sample from the tailpipe and reports the volumetric concentrations of carbon monoxide (CO) and hydrocarbons (HC). Modern four- and five-gas analyzers additionally measure carbon dioxide (CO2) and oxygen (O2), and optionally nitric oxide (NO), which enables the calculation of the excess air coefficient (lambda, λ), a key indicator of air–fuel mixture control quality in closed-loop, catalyst-equipped vehicles.
- Simple, low-cost, and repeatable screening procedure suited to inspection lanes without chassis dynamometers
- Governing specification in China: GB 18285, which defines emission limits and measurement methods for vehicles equipped with spark-ignition engines using the two-speed idle method and simplified loaded-mode methods
- Instrumentation performance requirements align with internationally recognized references such as ISO 3930 and OIML R 99 for exhaust gas analyzers
- Serves as a complementary or alternative procedure to loaded-mode tests (e.g., acceleration simulation mode and simple transient methods) where regulations permit
Testing Samples
In-Use Light-Duty Spark-Ignition Vehicles
The primary sample population consists of registered passenger cars and light commercial vehicles of categories M1 and N1 powered by gasoline or gaseous-fuel spark-ignition engines. These vehicles undergo periodic mandatory inspection, and the applicable emission limits depend on vehicle category, fuel type, and first registration date as tabulated in the governing standard.
Heavy-Duty Spark-Ignition Vehicles
Heavy-duty vehicles of categories M2, M3, and N2 equipped with gasoline engines are also within scope. Because dynamometer-based loaded testing of heavy vehicles is operationally complex, idle-based measurement is frequently retained for these vehicles in periodic inspection programs.
Vehicles Unsuitable for Dynamometer Testing
Certain vehicles cannot safely or practically undergo loaded-mode testing, including full-time all-wheel-drive vehicles, vehicles with traction control systems that cannot be disabled, vehicles with insufficient ground clearance, or configurations explicitly excluded by the manufacturer. Where regulations allow, the two-speed idle method provides an accepted alternative for such vehicles.
Testing Items
The measured parameters and evaluation criteria include:
- CO concentration at low idle, expressed as percentage by volume (% vol)
- CO concentration at high idle, expressed as % vol
- HC concentration at both idle speeds, expressed in ppm vol and reported as n-hexane equivalent
- Excess air coefficient (lambda, λ) at high idle, applicable to closed-loop, catalyst-equipped vehicles
- Supporting measurements of CO2 and O2 for mixture computation, leak detection, and sample dilution verification; NO measurement is optional where five-gas analyzers are used
Emission limits vary according to vehicle category, fuel type, and first registration date, following the limit tables of the applicable standard. As an illustrative reference, limits for many late-model light-duty gasoline vehicles are commonly set at or below 0.5–1.0 % vol for idle CO and 100–150 ppm vol for HC, with lambda typically required to remain within 1.00 ± 0.05 at high idle. Users should always consult the current edition of the governing standard for definitive limit values.
Testing Methods
Instrumentation and Calibration
The test bench consists of an NDIR analyzer for CO, HC, and CO2, and electrochemical sensors for O2 and (where fitted) NO. Before testing, the analyzer must complete its specified warm-up period, pass a sampling-line leak check, and be zeroed and span-calibrated using certified calibration gases traceable to national metrology standards. Response time, repeatability, and accuracy must satisfy the instrument classes defined by the applicable standard and related metrological specifications.
Vehicle Preparation
- Bring the engine to normal operating temperature (oil or coolant temperature typically at or above 80 °C where specified)
- Verify that the exhaust system is intact, with no leaks upstream of the sampling point that could dilute or falsify readings
- Place the transmission in neutral or park, switch off accessories, and confirm no external load on the engine
- For vehicles with multiple tailpipes, sample according to the procedure defined by the governing standard, typically evaluating the worst-case pipe or a combined configuration
Test Procedure
- Insert the sampling probe into the tailpipe to a depth of at least 400 mm, or to at least 80 % of the pipe length for short pipes, and seal the annular gap
- Low idle phase: allow the engine to stabilize at idle, then record the average CO and HC values over a measurement interval of at least 30 seconds
- High idle phase: raise engine speed to 2500 ± 100 rpm, or to the manufacturer-specified value if different, allow stabilization, and again record the average CO and HC values over at least 30 seconds
- Capture engine speed, ambient conditions, and all gas concentrations at both speeds, and archive the results in the inspection record
Lambda Calculation and Result Evaluation
The excess air coefficient is calculated at high idle from the simultaneously measured CO, CO2, HC, and O2 concentrations using the standardized calculation formula (commonly derived from the Brettschneider equation) with fuel-specific constants. Each measured value is compared against the applicable limit for the vehicle category and registration date. Any single exceedance of a limit constitutes a failed result, triggering diagnosis, repair, and retest of the vehicle.
Applications
- Periodic statutory inspection of in-use gasoline vehicles at authorized vehicle inspection stations
- Post-repair verification following service of the three-way catalytic converter, oxygen sensors, fuel injection system, or ignition system
- Roadside and enforcement screening programs for gross emitters
- Condition assessment during used-vehicle transactions and fleet maintenance management
- Measurement campaigns and comparative studies conducted by accredited third-party laboratories
FAQ
What is the difference between the two-speed idle method and loaded-mode tests such as ASM or simple transient methods?
The two-speed idle method measures emissions at unloaded steady-state engine speeds and therefore does not replicate real driving loads. Loaded-mode tests, such as acceleration simulation mode or simple transient procedures, drive the vehicle on a dynamometer through defined speed–load cycles and better reflect on-road emission behavior. Idle testing is simpler and cheaper, which is why it is often applied to older vehicles or to fleets and facilities where dynamometer infrastructure is unavailable.
Why is lambda measured at high idle?
At high idle, a properly functioning closed-loop fuel control system should regulate the mixture near stoichiometry, which is required for efficient three-way catalyst conversion. A lambda value outside the permitted window at high idle typically indicates sensor drift, fuel system faults, or ECU control problems, even when CO and HC concentrations alone may appear marginal.
Can diesel vehicles be tested using the two-speed idle method?
No. The two-speed idle method applies only to spark-ignition engines. Diesel vehicles are assessed using free-acceleration smoke opacity testing, for example in accordance with the corresponding national standard for diesel smoke measurement, using an opacimeter rather than a gas analyzer.
What are common causes of high CO or HC readings?
Frequent causes include an excessively rich air–fuel mixture, a degraded or contaminated three-way catalyst, a faulty oxygen sensor, ignition misfires, clogged air filters, incorrect idle adjustment on older carbureted vehicles, and exhaust leaks that alter sample composition. A systematic diagnosis of fuel, ignition, and aftertreatment systems is recommended before retest.
How long does a typical test take?
The complete procedure, including vehicle check-in, preparation, probe insertion, and both measurement phases, generally takes five to ten minutes. The actual gas measurement at the two idle conditions requires only one to two minutes once the engine and analyzer are stabilized.
How are results judged?
All recorded parameters are compared with the limit tables of the applicable standard according to vehicle category, fuel type, and first registration date. A pass requires every measured value, including lambda where applicable, to remain within its respective limit; otherwise the vehicle fails and must be repaired and retested before returning to service.
