Low-Temperature Cold Start Emission Test: Technical Principles, Procedures, and Regulatory Framework

The low-temperature cold start emission test is a mandatory vehicle certification procedure that measures exhaust emissions when an engine is started under cold ambient conditions, typically at minus 7 degrees Celsius. This article explains the technical principles, sample requirements, test methods, and applications of low-temperature cold start emission testing for light-duty vehicles.

Technical Overview

The low-temperature cold start emission test, also known as the Type VI test in many regulatory frameworks, is designed to evaluate exhaust pollutant emissions from light-duty vehicles during engine startup and warm-up under cold ambient temperatures. During a cold start, the three-way catalyst or diesel oxidation catalyst has not yet reached its light-off temperature, fuel enrichment strategies are active, and engine friction is elevated. As a result, carbon monoxide (CO) and hydrocarbon (HC) emissions can be several times higher than those observed under normal ambient temperature conditions.

The test is required by major regulatory programs worldwide, including the China 6 standard (GB 18352.6-2016), the European Type VI procedure defined in EU Regulation 2017/1151, and the United States EPA cold temperature CO test specified in 40 CFR Part 86. The vehicle is operated on a chassis dynamometer inside a temperature-controlled climatic chamber, and diluted exhaust emissions are collected and analyzed over a standardized driving cycle.

• Test temperature: typically minus 7 degrees Celsius for China 6 and Euro 6, or approximately minus 6.7 degrees Celsius (20 degrees Fahrenheit) for US EPA procedures.
• Primary regulated pollutants: carbon monoxide and hydrocarbons, evaluated on a mass-per-distance basis.
• Driving cycle: the Worldwide Harmonised Light Vehicles Test Cycle (WLTC) or an equivalent regulatory cycle such as FTP-75.
• Purpose: to verify that emission control systems, particularly catalysts and cold-start calibrations, perform adequately in cold climates.

Testing Samples

Light-Duty Gasoline Vehicles

Spark-ignition gasoline vehicles represent the primary category subject to low-temperature cold start emission requirements. These vehicles are evaluated for CO and HC emissions, which are most significantly affected by cold enrichment strategies and delayed catalyst light-off. Vehicles should be properly run in, typically within a mileage range of 3,000 to 15,000 kilometers, and must be in factory configuration with all emission control systems functioning as designed.

Light-Duty Diesel Vehicles

Compression-ignition diesel vehicles are also subject to low-temperature testing under applicable standards. The test focuses on CO and HC behavior, while particulate and NOx control devices are monitored for proper function. Diesel vehicles with exhaust aftertreatment systems such as diesel oxidation catalysts and particulate filters require verification that these systems reach effective operating conditions during cold operation.

Hybrid Electric Vehicles

Hybrid electric vehicles and plug-in hybrid vehicles must undergo low-temperature cold start testing with consideration of their specific energy management strategies. Test laboratories must account for charge-sustaining and charge-depleting operating modes, and the hybrid control logic during cold start is evaluated to confirm that emissions remain within regulatory limits when the engine starts under cold conditions.

Testing Items

The low-temperature cold start emission test measures distance-specific mass emissions from a complete cold-start driving cycle. The principal measurement parameters include:

• Carbon monoxide (CO) emissions, expressed in grams per kilometer.
• Total hydrocarbons (THC) and non-methane hydrocarbons (NMHC), depending on the applicable standard.
• Carbon dioxide (CO2) concentration, recorded for fuel consumption calculation and data validation.
• Nitrogen oxides (NOx), monitored or regulated depending on the regulatory program and fuel type.
• Ambient parameters, including chamber temperature, humidity, and barometric pressure, which must be recorded throughout the soak and test phases.
• Vehicle conditioning data, such as engine oil and coolant temperatures, to confirm thermal equilibrium before the cold start.

Testing Methods

Vehicle Preparation and Conditioning

Before testing, the vehicle undergoes a preconditioning procedure consisting of a stabilization drive on the chassis dynamometer, followed by refueling with the specified reference fuel for low-temperature testing. The fuel tank is filled to a defined level to ensure consistent evaporative and combustion behavior. Tires are set to manufacturer-specified pressure, and the vehicle is inspected to confirm conformity with its type approval documentation.

Climatic Chamber Soak

The conditioned vehicle is placed in a climatic chamber maintained at the specified test temperature, typically minus 7 degrees Celsius with a tolerance of plus or minus 3 degrees Celsius. The vehicle must be soaked until engine oil, coolant, and battery temperatures stabilize within the tolerance of the chamber temperature, which generally requires a minimum period of 12 hours and a maximum of approximately 36 hours. This ensures that the subsequent engine start reproduces genuine cold-start conditions.

Dynamometer Operation and Driving Cycle

After the soak period, the vehicle is started and immediately operated over the applicable driving cycle on the chassis dynamometer. The dynamometer is configured according to the vehicle inertia class and road load coefficients determined during WLTP or equivalent coastdown characterization. The driver follows the speed trace using a driver aid display, and deviations from the tolerance bands are recorded. The complete cycle begins at engine start, capturing the cold-start and warm-up phases that dominate total emissions.

Emission Sampling and Analysis

Diluted exhaust gas is collected using a constant volume sampling (CVS) system, which meters the total volume of the dilute exhaust mixture through a positive displacement pump, critical flow venturi, or sonic nozzle. Proportional samples are collected into sampling bags over the full cycle. Analysis is performed with regulatory-grade instrumentation:

• Non-dispersive infrared (NDIR) analyzers for CO and CO2 measurement.
• Flame ionization detectors (FID) for total hydrocarbon measurement, with gas chromatography used where methane separation for NMHC is required.
• Chemiluminescence detectors (CLD) for NOx where monitoring is applicable.

Mass emissions are calculated from the bag concentrations, diluted exhaust volume, and pollutant gas densities, then divided by the cycle distance to yield grams per kilometer. The results are compared against the low-temperature emission limits specified in the applicable regulation, and calibration gases traceable to national standards are used to verify analyzer accuracy before and after each test.

Applications

Low-temperature cold start emission testing serves a wide range of regulatory and engineering purposes across the automotive industry:

• Type approval and certification of new light-duty vehicles under China 6, Euro 6, and US EPA Tier 3 programs.
• In-service conformity verification and market surveillance testing of production vehicles.
• Research and development of cold-start engine calibrations, including fuel enrichment strategies and idle speed control.
• Development and validation of exhaust aftertreatment components, such as close-coupled catalysts, electrically heated catalysts, and hydrocarbon adsorber systems.
• Thermal management studies for exhaust systems, including insulation design and exhaust gas heat recovery.
• Evaluation of fuels and lubricants under cold climate conditions to support product development and regulatory compliance.

FAQ

Why is the low-temperature test conducted at minus 7 degrees Celsius?

Minus 7 degrees Celsius represents a statistically common cold climate condition in temperate regions and produces consistently reproducible cold-start behavior. Regulatory authorities in China, Europe, and the United States adopted this temperature range because it reliably triggers elevated cold-start emissions while remaining achievable in standard environmental test chambers.

Which pollutants are limited in the low-temperature cold start emission test?

The primary regulated pollutants are carbon monoxide and hydrocarbons, because cold-start conditions disproportionately affect these species. Specific limit values, and whether additional pollutants such as NOx are included, depend on the applicable regulatory standard and vehicle fuel type.

How long must the vehicle soak before the test?

The vehicle is typically soaked at the test temperature for a minimum of 12 hours, with a maximum period of approximately 36 hours. The soak is considered complete only when engine oil and coolant temperatures are stabilized within the chamber temperature tolerance, guaranteeing a genuine cold start.

Do hybrid and electric vehicles require low-temperature cold start testing?

Hybrid electric vehicles with internal combustion engines are subject to low-temperature testing, taking into account their charge-sustaining and charge-depleting operating modes. Pure battery electric vehicles do not produce tailpipe emissions and are therefore exempt, although they may be subject to separate low-temperature range and energy consumption evaluations.

What equipment is required to perform the test?

A compliant laboratory requires a climatic chamber capable of maintaining the specified temperature, a chassis dynamometer suitable for WLTC or equivalent cycles, a constant volume sampling system, regulatory-grade exhaust gas analyzers, traceable calibration gases, and data acquisition systems for ambient and vehicle parameter monitoring.

How does the low-temperature test differ from the standard type approval emission test?

The standard type approval test, such as the Type I WLTC procedure, is conducted at normal laboratory temperature of approximately 23 degrees Celsius after a shorter soak. The low-temperature test uses a significantly colder ambient condition, a longer soak, and focuses specifically on cold-start performance, with emission limits set specifically for this severe condition.