Low-Temperature Cold-Start Exhaust Emission Testing: Standards, Procedures, and Applications

Low-temperature cold-start exhaust emission testing measures the pollutants a vehicle emits immediately after engine start-up at sub-zero ambient temperatures, when the catalyst is cold and emissions are at their highest. This guide explains the regulatory basis, test samples, measured pollutants, laboratory methods, and practical applications of this essential automotive compliance and development test.

Technical Overview

Low-temperature cold-start exhaust emission testing is a chassis-dynamometer laboratory procedure in which a complete vehicle is thermally stabilized at a low ambient temperature—typically −7 °C (20 °F)—and then started from a fully cold state and driven over a regulated driving cycle. The mass of pollutants emitted at the tailpipe is measured continuously or via dilute-exhaust sampling bags and compared against regulatory limits or engineering targets.

Cold start is the most emission-intensive phase of vehicle operation. At low ambient temperatures the problem is amplified by several compounding factors:

• The three-way catalyst or diesel oxidation catalyst is far below its light-off temperature (typically 250–400 °C) and converts almost nothing during the first minutes of operation.
• Calibration strategies use fuel enrichment and retarded ignition to accelerate catalyst heating, increasing raw hydrocarbon (HC) and carbon monoxide (CO) output.
• Cold, viscous lubricating oil and dense intake air raise friction losses and fuel consumption.
• Battery and starter performance degrade, extending cranking and warm-up time.

Consequently, emissions during a low-temperature cold start can be several times higher than those measured in the standard Type I warm-start compliance test. Regulators therefore mandate dedicated low-temperature procedures, including the US EPA 20 °F (−7 °C) FTP-75 test under 40 CFR Part 86, the EU Type 6 test (−7 °C) harmonized with UN Regulation No. 154 and WLTP, and the corresponding Type VI provisions in China 6 (GB 18352.6). Emerging legislation such as Euro 7 proposes stricter temperatures and an expanded pollutant scope, making this test increasingly important for market access.

Testing Samples

Gasoline Light-Duty Vehicles

The primary test article is a complete, production-representative passenger car or light commercial vehicle equipped with port fuel injection or direct injection, a three-way catalyst, and possibly a gasoline particulate filter. Vehicles must be run in, typically within a defined mileage window, and must be mechanically sound with no malfunction indicators active.

Diesel and Compression-Ignition Vehicles

Diesel vehicles are tested with their full aftertreatment system, including diesel oxidation catalysts, diesel particulate filters, selective catalytic reduction units, and exhaust gas recirculation systems, since low temperature strongly affects NOx conversion and particulate behavior.

Hybrid and Alternative-Fuel Vehicles

Hybrid electric vehicles undergo the same low-temperature procedure, with the high-voltage battery conditioned to a specified state of charge and temperature. Vehicles running on ethanol blends, natural gas, or LPG may be tested with appropriate reference fuels to characterize fuel-specific cold-start behavior.

Research Samples

Beyond complete vehicles, the method supports development testing of catalyst bricks, electrically heated catalysts, hydrocarbon traps, engine calibrations, fuel formulations, and low-viscosity lubricants on dynamometer-mounted engines or mule vehicles.

Testing Items

The test quantifies regulated gaseous and particulate emissions over the complete cold-start cycle, reported as mass per unit distance (mg/km or g/mile):

• Total hydrocarbons (THC), non-methane hydrocarbons (NMHC), or non-methane organic gases (NMOG)
• Carbon monoxide (CO)
• Nitrogen oxides (NOx), including NO2 fraction where required
• Carbon dioxide (CO2) and calculated fuel consumption
• Particulate matter (PM) mass on gravimetric filters
• Particle number (PN), typically counted at 23 nm and above
• Optional research measurements: methane, nitrous oxide (N2O), ammonia (NH3), and formaldehyde

Supplementary engineering outputs include second-by-second modal emission traces, catalyst inlet and outlet temperatures, time or distance to catalyst light-off, and lambda or air-fuel ratio profiles during enrichment events.

Testing Methods

Vehicle Preparation and Cold Soak

The vehicle first undergoes a preconditioning drive and refueling with the specified emission test fuel. It is then parked in a climate-controlled soak room or chamber where it is stabilized at the target temperature—commonly −7 °C with a tolerance of a few degrees—for a minimum of 12 hours and up to 36 hours, ensuring the engine block, coolant, oil, catalyst, and drivetrain are uniformly cold. Battery condition is documented, and where permitted the battery may be removed and conditioned separately.

Chassis Dynamometer and Climatic Chamber Setup

The test is performed on a chassis dynamometer inside a climatic chamber maintained at the specified ambient temperature. Road load coefficients, established by coastdown or from certification data, simulate aerodynamic and rolling resistance, and the effective inertia is set to match the vehicle test mass. A cooling fan directed at the radiator and a 4WD-capable roller set support realistic thermal behavior, and tire pressures are set per the applicable standard.

Cold Start and Cycle Execution

The engine is cranked according to the owner's manual and the driving cycle begins immediately or after a short idle period, depending on the regulation. The driver follows the speed trace of the applicable cycle—such as the WLTC under WLTP-based regulations or the FTP-75 in the United States—keeping speed deviation within the permitted tolerance. Any stall, cycle deviation, or sampling malfunction can invalidate the run, so skilled driver operation and automated driver-aid systems are essential.

Exhaust Sampling and Analysis

Diluted exhaust is sampled using a constant volume sampling (CVS) system with critical-flow venturis or a positive-displacement pump. A proportional sample fills Teflon bags for integrated analysis, while a proportional sample is drawn through pre-conditioned filters for PM mass and through a volatile-particle-removing dilution line into a condensation particle counter for PN. Analytical techniques include:

• Non-dispersive infrared (NDIR) analysis for CO and CO2
• Heated flame ionization detection (HFID) for total hydrocarbons
• Gas chromatography with FID or a non-methane cutter for methane and NMHC
• Chemiluminescence detection (CLD) for NOx
• Gravimetric microbalance weighing for PM mass

Data Processing and Quality Assurance

Mass emissions are computed from bag concentrations, total CVS volume, and component densities, with dilution-air background subtraction and drift correction applied as required. Quality assurance measures include span and zero calibration with certified reference gases, CVS verification by propane injection, leak checks, blank filter handling, and analyzer linearity verification. Repeatability is confirmed through duplicate runs, since cold-start results are inherently sensitive to soak uniformity and start behavior.

Applications

Low-temperature cold-start emission testing supports a wide range of regulatory and engineering objectives:

• Type approval and certification demonstrating compliance with US EPA, UNECE, Euro, and China 6 low-temperature emission limits before market launch.
• Powertrain calibration development, optimizing injection strategy, idle speed, spark retard, and catalyst heating logic to reduce cold enrichment without compromising driveability.
• Aftertreatment development, including close-coupled catalysts, electrically heated catalysts, and hydrocarbon traps engineered specifically for fast light-off.
• Fuel and lubricant research, comparing ethanol blends, volatility classes, and synthetic low-viscosity oils for cold-start emission benefits.
• Cold-climate product validation for export markets, ensuring robust operation from −7 °C down to arctic conditions of −30 °C or below in research programs.
• OBD calibration and in-service conformity investigations, verifying that monitor thresholds and real-vehicle behavior match certification assumptions.
• Academic and real-world correlation studies linking laboratory results with portable emissions measurement and real-driving emissions data.

FAQ

What ambient temperature is used for the test?

The regulatory standard for most markets is −7 °C (20 °F). Research and development programs frequently extend testing to −10 °C, −18 °C, or −30 °C to evaluate extreme cold-climate robustness beyond the certification point.

How long must the vehicle be cold-soaked before testing?

Regulations typically require a soak of at least 12 hours, with an acceptable window of roughly 12 to 36 hours, at the specified temperature to guarantee that the engine, fluids, catalyst, and drivetrain are uniformly cold before start-up.

Which driving cycle is used?

WLTP-based regulations use the WLTC cycle at −7 °C for the Type 6 test, while the United States uses the FTP-75 cycle at 20 °F. Each cycle defines a specific speed-time trace that the driver must follow within the permitted tolerance.

Why are cold-start emissions so much higher than warm-start emissions?

The catalytic converter is far below light-off temperature and initially converts almost no pollutants, while cold-start fuel enrichment, dense intake air, and viscous oil greatly increase raw engine-out emissions at exactly the moment conversion efficiency is lowest.

Are hybrid vehicles subject to low-temperature testing?

Yes. Hybrids follow the same procedure, with the traction battery brought to a defined state of charge and thermal condition so that engine start strategy and electric assist behavior are representative and repeatable.

What results are reported at the end of a test?

The laboratory reports distance-specific mass emissions for each pollutant, typically in mg/km or g/mile, along with cycle-integrated bag results, modal traces, catalyst temperatures, fuel consumption, and a statement of conformity with the applicable limit values.