Engine combustion exhaust gas analysis is a specialized testing discipline that measures regulated and unregulated pollutants emitted by internal combustion engines, including carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. Accredited third-party laboratories perform exhaust emission testing in accordance with recognized standards such as ISO 8178, GB/T 8190, and ASTM D6348 to support engine certification, research and development, and regulatory compliance. This article explains the sample types, testing items, analytical methods, and typical applications of engine combustion exhaust gas analysis.
Technical Overview
Engine combustion exhaust gas analysis is the systematic sampling, measurement, and characterization of the gaseous and particulate species present in the exhaust of internal combustion engines. The composition of exhaust gas reflects the completeness and efficiency of the combustion process, the quality of fuel-air mixing, the effectiveness of aftertreatment devices, and the overall emission performance of the engine. Quantitative exhaust analysis is therefore essential for emissions certification, combustion system development, and compliance verification across the automotive, non-road, marine, rail, and stationary power generation sectors.
From a combustion chemistry perspective, carbon dioxide and water vapor are the products of complete oxidation, while carbon monoxide, unburned hydrocarbons, soot, and partially oxygenated compounds indicate incomplete combustion. Nitrogen oxides form primarily at high flame temperatures through thermal fixation of atmospheric nitrogen, and sulfur dioxide originates from fuel-bound sulfur. Residual oxygen and the excess air coefficient (lambda) provide direct insight into the equivalence ratio actually achieved in the cylinder, making exhaust gas composition a powerful diagnostic indicator of engine behavior.
Exhaust emission measurement for reciprocating internal combustion engines is standardized internationally in the ISO 8178 series, which has been adopted in China as the GB/T 8190 series and in Japan as the JIS B 8008 series. Additional requirements for road-vehicle engines are defined in national regulations such as GB 17691 for heavy-duty diesel engines and GB 18352 for light-duty vehicles, while complementary analytical procedures are covered by ASTM and EN standards. Together, these frameworks define test cycles, sampling systems, analyzer specifications, and calculation methods that ensure results are reproducible and comparable between laboratories.
Testing Samples
Light-Duty and Heavy-Duty Road Vehicle Engines
Exhaust samples are collected from production and prototype gasoline, diesel, and hybrid vehicle engines operating on engine dynamometers or chassis dynamometers. Both steady-state and transient test cycles are used to evaluate emissions under certification-representative conditions, including cold-start and warm-start operation.
Non-Road Diesel and Gasoline Engines
Engines used in agricultural tractors, construction machinery, forklifts, generators, and forestry equipment are tested using the non-road cycles defined in ISO 8178-4, such as the C1, D1, D2, and G1-G3 cycles. These engines typically use partial-flow dilution sampling on engine test beds.
Natural Gas, LPG, and Dual-Fuel Engines
Gaseous-fueled and dual-fuel engines require measurement of methane and non-methane hydrocarbon fractions, because methane has a disproportionately high global warming potential. Dedicated heated flame ionization detection and gas chromatographic methods are applied to separate methane from other hydrocarbons.
Marine, Rail, and Stationary Engines
Large-bore medium-speed and slow-speed diesel and dual-fuel engines for marine propulsion, rail traction, and power generation are tested using the E2, E3, and D2 cycles, with measurements adapted to the IMO Tier III and equivalent regional regulatory frameworks.
Aftertreatment System Effluent
Exhaust samples upstream and downstream of diesel oxidation catalysts, diesel particulate filters, selective catalytic reduction systems, and three-way catalysts are analyzed to quantify conversion efficiencies, ammonia slip, and nitrous oxide formation across the aftertreatment chain.
Testing Items
Engine combustion exhaust gas analysis covers regulated pollutants, particulate emissions, unregulated compounds, and combustion diagnostics. Typical testing items include:
- Carbon monoxide (CO) and carbon dioxide (CO2) concentrations
- Total hydrocarbons (THC), methane (CH4), and non-methane hydrocarbons (NMHC)
- Nitrogen oxides (NOx) as the sum of nitric oxide (NO) and nitrogen dioxide (NO2)
- Nitrous oxide (N2O) and ammonia (NH3) slip
- Particulate matter (PM) mass concentration and brake-specific mass emissions
- Particle number (PN) concentration and size distribution
- Smoke opacity expressed as absorption coefficient and filter smoke number
- Exhaust oxygen (O2) content and excess air coefficient (lambda)
- Aldehydes and ketones, including formaldehyde and acetaldehyde
- Volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and sulfur dioxide (SO2)
- Brake-specific fuel consumption derived from the carbon balance method
Testing Methods
Sampling Systems: Raw, Partial-Flow, and Full-Flow Dilution
Raw exhaust sampling draws undiluted gas directly from the exhaust line through heated probes, and is used primarily for gaseous species and engine mapping. For particulate measurement, ISO 8178 prescribes either partial-flow dilution systems, in which a proportional fraction of exhaust is diluted with filtered air, or full-flow constant volume sampling (CVS), in which the entire exhaust stream is diluted and the total volume is metered. Correct dilution ratio control and proportional sampling are critical for valid mass emission calculations.
Gaseous Analyzer Technologies
Carbon monoxide and carbon dioxide are measured by non-dispersive infrared (NDIR) analyzers, while nitrogen oxides are quantified by chemiluminescence detection (CLD), which offers high selectivity and a wide dynamic range. Total hydrocarbons are measured with heated flame ionization detectors (HFID) maintained at approximately 190 degrees Celsius to prevent condensation of heavy hydrocarbons. Oxygen is determined by paramagnetic or electrochemical sensors, and Fourier transform infrared spectroscopy (FTIR) per ASTM D6348 enables simultaneous multi-component measurement of NO, NO2, N2O, NH3, and other trace species.
Particulate Matter and Smoke Measurement
Particulate mass is determined gravimetrically by collecting diluted exhaust on fluorocarbon-coated glass fiber filters in a temperature- and humidity-controlled weighing room, followed by conditioning and precision weighing. Particle number concentrations are measured with condensation particle counters in accordance with recognized particle measurement protocols. Smoke emissions from compression ignition engines are assessed using opacimeters compliant with ISO 11614 or filter-type smoke meters compliant with ISO 10054, and free-acceleration smoke tests on in-use engines are performed according to GB 3847.
Unregulated Emissions and Chemical Speciation
Carbonyl compounds are sampled by drawing exhaust through 2,4-dinitrophenylhydrazine (DNPH) cartridges, with subsequent high-performance liquid chromatography analysis following ASTM D5197. Volatile organic species and PAHs are collected on adsorbent tubes or filters and analyzed by gas chromatography-mass spectrometry, while individual light hydrocarbons and permanent gases can be separated by gas chromatography techniques consistent with ASTM D1946 principles.
Test Cycles and Portable Emissions Measurement
Steady-state cycles such as the C1, D2, and E3 modes of ISO 8178-4, together with world-harmonized transient cycles, are executed on calibrated engine dynamometers to generate weighted emission factors. For in-service conformity and real-world verification, portable emissions measurement systems (PEMS) are installed on the vehicle or machine to record NOx, PN, and gaseous concentrations under actual operating conditions, in line with methodologies reflected in ISO 16183 and regional in-service testing regulations.
Applications
Engine combustion exhaust gas analysis serves a wide range of engineering and regulatory purposes throughout the engine life cycle:
- Emissions certification and type-approval demonstration for road, non-road, marine, and stationary engines
- Combustion system development, including injection timing, boost, and exhaust gas recirculation calibration
- Aftertreatment development and aging evaluation for catalysts, particulate filters, and SCR systems
- In-service conformity testing and periodic technical inspection of engines in operation
- Fuel and lubricant research, including biodiesel, ethanol blends, hydrogen, methanol, and ammonia dual-fuel combustion studies
- Failure diagnosis, root-cause investigation, and benchmarking of engine emission performance
- Greenhouse gas accounting and carbon balance determination of fuel consumption
FAQ
What is engine combustion exhaust gas analysis?
It is the laboratory or on-site measurement of the gaseous and particulate constituents of engine exhaust to quantify emission levels, evaluate combustion quality, and demonstrate compliance with applicable emission standards.
Which standards govern exhaust emission testing?
The core methodology is defined by the ISO 8178 series (adopted in China as GB/T 8190 and in Japan as JIS B 8008), supplemented by GB 17691 and GB 18352 for vehicle engines, ASTM D6348 and ASTM D5197 for spectroscopic and carbonyl analysis, and ISO 11614 and ISO 10054 for smoke measurement.
Why are heated sampling lines required?
Heated lines and probes, typically maintained near 190 degrees Celsius, prevent condensation of water vapor and heavy hydrocarbons on the sampling path, which would otherwise cause analyte loss and biased results, particularly for hydrocarbon measurements.
What is the difference between raw and dilute exhaust measurement?
Raw measurement analyzes undiluted exhaust directly and is suited to gaseous species and steady-state diagnostics, whereas dilute measurement, using partial-flow or CVS systems, mimics atmospheric dilution and is mandatory for gravimetric particulate mass determination and transient cycle testing.
How are emission results reported?
Gaseous and particulate emissions are typically expressed as brake-specific values in grams per kilowatt-hour (g/kWh) calculated over the weighted test cycle, while supporting data are reported in ppm(v) or percent by volume, smoke as absorption coefficient in inverse meters or filter smoke number, and particle number as particles per kilowatt-hour.
Can exhaust analysis be performed outside the laboratory?
Yes, portable emissions measurement systems allow real-driving and in-field evaluation of engines installed on vehicles, machinery, vessels, and generator sets, providing representative data that complement controlled dynamometer testing.