Engine Cold Start Combustion Testing: Methods, Standards, and Applications

Engine cold start combustion testing evaluates how an internal combustion engine initiates and sustains stable combustion at low ambient temperatures, a condition in which fuel vaporization, lubricant viscosity, and electrical system performance are all significantly degraded. Because a large fraction of regulated tailpipe hydrocarbon and carbon monoxide emissions is produced during the first minutes of operation, cold start behavior is a critical focus of emissions legislation and engine calibration. This article explains the technical principles, sample types, measured parameters, test methods, and typical applications of cold start combustion testing performed in climate-controlled dynamometer facilities by accredited third-party laboratories.

Technical Overview

Cold start combustion testing is a specialized engine evaluation discipline that examines the ignition, flame development, and cycle-to-cycle combustion behavior of an engine when it is started after being soaked at ambient temperatures well below normal operating conditions. During a cold start, several physical factors work simultaneously against stable combustion: gasoline fuel vapor pressure drops sharply, cylinder walls are cold enough to quench early flames, intake charge motion is weak at low cranking speeds, and lubricant viscosity increases cranking torque demand. In diesel engines, lower compressed charge temperatures lengthen ignition delay and increase the risk of misfiring or excessive white smoke.

From an emissions perspective, the cold start phase is disproportionately important. Before the exhaust catalyst reaches its light-off temperature, essentially all hydrocarbons and carbon monoxide produced in the cylinders pass through to the tailpipe. Regulatory frameworks such as the UN ECE R83 Type VI low-temperature test, the corresponding Type VI requirement in China's GB 18352.6 emission standard, and the cold-temperature carbon monoxide test in the United States EPA regulations under 40 CFR Part 86 were all established specifically to control emissions during this phase. Consequently, cold start combustion testing serves both as a compliance tool and as a core development activity for fuel injection strategy, ignition timing, and catalyst heating calibration.

A complete cold start combustion test program typically combines three assessment dimensions: startability, meaning whether and how quickly the engine fires and reaches idle; combustion quality, meaning how stably the engine runs once firing begins; and emissions and warm-up behavior, meaning how quickly the aftertreatment system becomes functional. Data from these tests allow engineers to quantify the interaction between low-temperature physics and engine management strategies.

Testing Samples

Gasoline Engines

Port fuel injected and gasoline direct injected engines are the most common test subjects. GDI engines receive particular attention because cold-start fuel film formation on piston and liner surfaces strongly influences particulate number emissions and mixture quality during the first firing cycles.

Diesel Engines

Diesel engines are evaluated for cold start capability with emphasis on glow plug performance, injection pressure buildup, ignition delay, and white smoke behavior. Common rail systems with multiple pre-injection events are frequently characterized at temperatures down to -30 degrees Celsius or lower.

Alternative Fuel and Flex-Fuel Engines

Engines calibrated for ethanol blends such as E85, compressed natural gas, liquefied petroleum gas, or hydrogen-enriched fuels require dedicated cold start evaluation because fuel volatility and ignition characteristics differ substantially from conventional gasoline, especially for high-ethanol blends at low temperature.

Engine Systems and Components

Component-level cold start combustion testing is also performed on injectors, ignition coils and spark plugs, glow plugs, starter motors and batteries, engine control unit software calibrations, fuels, and lubricants, each of which can be the decisive factor in whether robust combustion is achieved.

Testing Items

Startability Parameters

• Minimum cranking speed and cranking speed stability
• Battery voltage and starter current draw during cranking
• Time to first combustion event and time to sustained firing
• Total start time from ignition key or start command to stable idle
• Number of stalled start attempts and restart behavior

Combustion Characteristics

• In-cylinder pressure traces resolved by crank angle for every cycle
• Indicated mean effective pressure (IMEP) for each cylinder and cycle
• Coefficient of variation of IMEP as the primary combustion stability metric
• Peak cylinder pressure, combustion phasing, and heat release rate analysis
• Misfire, partial burn, and knock detection during start and fast idle

Emissions During Cold Start and Warm-Up

• Total hydrocarbons measured with fast-response flame ionization detection
• Carbon monoxide, carbon dioxide, and oxides of nitrogen
• Particulate mass and particle number for direct injection engines
• Catalyst light-off time and conversion efficiency versus temperature

Thermal and Fuel Behavior

• Cold start fuel consumption and enrichment duration
• Coolant, oil, and metal temperature response during warm-up
• Fuel dilution of the oil film under prolonged enrichment conditions

Testing Methods

Testing is conducted in an environmental chamber or climatic dynamometer test cell capable of controlling ambient temperature from approximately -40 to +25 degrees Celsius with tight humidity control. The engine or complete vehicle is soaked at the target temperature, typically for 8 to 12 hours, to ensure thermal equilibrium of the block, coolant, oil, fuel, battery, and aftertreatment system, in accordance with the soak requirements defined in the applicable regulation.

Combustion instrumentation includes water-cooled piezoelectric in-cylinder pressure transducers in every cylinder, a crank angle encoder or optical degree wheel for angle-based resolution, charge amplifiers, and a combustion analysis system that computes IMEP, heat release, and cycle statistics in real time. Emissions are sampled continuously from the tailpipe using fast-response analyzers, since events of interest occur within the first few seconds. Electrical measurements capture battery terminal voltage, starter current, and cranking speed at high sampling rates.

Test procedures generally follow recognized frameworks, including the starting performance test methods of GB/T 12535, the low-temperature Type VI procedures of GB 18352.6 and UN ECE R83, the cold-temperature test procedures of 40 CFR Part 86, and emission measurement principles consistent with ISO 8178 for non-road applications. A typical execution sequence is as follows:

• Stabilize and characterize the engine at reference ambient temperature to establish baseline combustion behavior
• Soak the test article at the specified low temperature until full thermal equilibrium is achieved
• Execute the start event while recording pressure, speed, electrical, and emissions data from the first crank rotation onward
• Analyze the first 30 to 200 fired cycles for combustion stability, misfire rate, and enrichment behavior
• Repeat the soak and start sequence a statistically sufficient number of times to characterize variability
• Report cycle-resolved results against defined acceptance criteria for start time, stability, and emissions

Data reduction focuses on cycle-by-cycle analysis rather than averaged values, because the transition from cranking to firing to stable idle is inherently transient. Time-aligned correlation of cylinder pressure data with injection and ignition commands is essential for diagnosing the root cause of unstable or failed starts.

Applications

Cold start combustion testing supports a wide range of engineering and regulatory objectives across the automotive, off-highway, marine, and power generation industries:

• Emissions homologation and certification for low-temperature regulatory test types
• Development of cold start fueling strategies, injection phasing, and ignition retard for catalyst heating
• Validation of glow plug, injector, ignition, and starter system performance in cold climates
• Fuel and lubricant formulation studies, including ethanol blend behavior and low-viscosity oil effects
• Benchmarking of competitor engines and root cause analysis of field startability complaints
• Durability assessment of aftertreatment components exposed to repeated cold start enrichment
• Research into advanced combustion concepts such as homogeneous charge compression ignition and gasoline compression ignition at low temperature

FAQ

What ambient temperatures are typically used?

Regulatory low-temperature emission tests are commonly performed at approximately -7 degrees Celsius, while startability and combustion robustness development programs frequently extend to -18, -30, or -40 degrees Celsius for arctic market validation. The exact setpoints are selected based on the target market and applicable standard.

What is the difference between a startability test and a combustion test?

A startability test determines whether the engine starts within a defined time or cranking limit, while a combustion test goes further by analyzing cycle-resolved in-cylinder pressure to quantify how well each combustion event develops. Combustion testing reveals marginal or unstable cycles that a simple startability pass would hide.

Why is the coefficient of variation of IMEP important?

The coefficient of variation of IMEP is the accepted metric for cycle-to-cycle combustion stability. Values during the first fired cycles of a cold start are typically far higher than at warm idle, and tracking this metric allows calibration engineers to quantify how quickly combustion stabilizes and to compare different strategies objectively.

Which standards govern this testing?

Commonly referenced frameworks include GB/T 12535 for starting performance, GB 18352.6 and UN ECE R83 for low-temperature Type VI emission testing, 40 CFR Part 86 for cold-temperature testing in the United States, and ISO 8178 measurement principles for gaseous and particulate emissions. Accredited third-party laboratories select and combine these procedures according to the product type and destination market.

How many repeat tests are required?

Because cold start outcomes are stochastic, regulations and internal engineering standards typically require multiple soak-and-start repetitions per condition. Statistical evaluation across repeats is necessary to distinguish true calibration differences from random cycle-to-cycle variation.