Exhaust Gas Analyser

Overview

An exhaust gas analyser, also known as a vehicle emission gas analyser, is a specialised diagnostic and testing device used to measure the concentrations of various pollutants in motor vehicle exhaust emissions. It analyses the volumetric concentration of components such as carbon monoxide (CO), hydrocarbons (HC), carbon dioxide (CO2), nitrogen oxides (NOx), and oxygen (O2) in petrol vehicle exhaust, and automatically calculates the excess air ratio (lambda / λ value).

Exhaust gas analysers are widely utilised across automotive workshops, vehicle manufacturing plants, vehicle inspection centres, and environmental monitoring agencies. During periodic motor vehicle inspections, it serves as the core apparatus to determine whether petrol vehicle emissions comply with statutory standards; in workshop diagnostics, it helps technicians evaluate engine combustion efficiency and assess the operational health of catalytic converters and electronic engine management systems based on exhaust gas composition.

Working Principle

Exhaust gas analysers operate on multiple sensing technologies, with specific measurement principles applied to different gas components.

Non-Dispersive Infrared (NDIR) absorption is the primary method for measuring CO, CO2, and HC. The underlying principle relies on specific gas molecules absorbing infrared light at distinct wavelengths, where the level of absorption is directly proportional to the gas concentration. By measuring the degree of energy attenuation as infrared light passes through the exhaust sample, the instrument calculates the exact gas concentration. Devices utilising this principle feature a straightforward structure, long service life, and high measuring precision, making them the most widely adopted solution in automotive repair and inspection sectors.

The electrochemical method is mainly used to measure O2 and NO. Electrochemical sensors generate electrical signals through chemical reactions with target gases, producing a signal output that corresponds linearly with gas concentration. Such sensors are consumable items with a typical lifespan of under two years, but offer the advantages of compact dimensions, low replacement cost, and ease of servicing.

For high-precision laboratory testing, Flame Ionisation Detection (FID) can be used to measure HC, whilst Chemiluminescence Detection (CLD) is employed for NO/NOx analysis. While offering superior precision, these systems entail high equipment costs (reaching millions of RMB) and are largely confined to powertrain R&D and type-approval homologation.

Classification and Application Scenarios

Based on functional configurations, exhaust gas analysers are classified into 2-gas analysers (measuring CO and HC), 4-gas analysers (adding CO2 and O2), and 5-gas analysers (further adding NO), with higher configurations offering broader testing capabilities.

Equipment requirements vary depending on operational settings: vehicle inspection centres typically mandate 4-gas or 5-gas analysers; environmental enforcement units for roadside spot-checks also rely predominantly on 4-gas/5-gas units; dedicated petrol workshops require petrol-specific analysers; workshops catering to both petrol and diesel models can deploy separate exhaust gas analysers and smoke opacity meters, or opt for integrated dual-fuel test equipment.

In recent years, portable handheld analysers have gained increasing popularity. Compact and lightweight (approx. 1 kg) with built-in batteries providing over six hours of continuous operation, they are well suited for roadside enforcement and mobile in-vehicle testing. Certain advanced units feature companion tablets supporting wireless connectivity (exceeding 20 metres), automatic number plate recognition (ANPR), and 4G data transmission, enabling real-time integration with central motor vehicle management platforms.

Regulatory Standards for Periodic Vehicle Inspection

The operation of exhaust gas analysers is governed by stringent regulatory standards. GB 18285-2018 "Limits and measurement methods for emissions from petrol vehicles under two-speed idle conditions and short driving mode conditions" serves as the benchmark standard for petrol vehicle emission testing, stipulating that analysers must measure CO, HC, CO2, NO, and O2 concentrations whilst supporting real-time data logging and lambda calculation.

For periodic vehicle inspections, the primary testing methodologies comprise the two-speed idle test (performed with the vehicle in neutral at normal idle and high idle; simple and cost-effective, though unable to evaluate NOx) and the short driving mode test (conducted on a chassis dynamometer to simulate actual road driving cycles, delivering more realistic data and effectively capturing NOx emissions). Testing centres insert the analyser's sampling probe into the vehicle tailpipe to continuously sample exhaust gases under mandated test cycles, comparing measured values against statutory threshold limits to determine a pass or fail.

Analysers deployed at inspection institutions must also satisfy the requirements of HJ 1237-2021 "Specifications for Periodic Emission Inspection of Motor Vehicles" : the sampling hose length must not exceed 7.5 metres; NOx measurement via conversion furnaces must ensure verified converter efficiency; and self-diagnostic daily calibration check functions must be integrated.

Metrological Verification and Maintenance

The measuring accuracy of exhaust gas analysers is safeguarded through statutory metrological verification. Under JJG 688-2017 "Verification Regulation of Vehicle Exhaust Gas Analyzers" , authorised metrology authorities use certified reference standard gases to conduct periodic calibration audits. By comparing standard gas concentrations with instrument readouts, compliance is assessed and legally binding verification certificates are issued.

During routine usage, sensor accuracy naturally degrades over time. Electrochemical NO and O2 sensors generally have an effective service life of 1 to 1.5 years; using them beyond their lifespan leads to reduced accuracy and sluggish response times. Even an instrument certified as compliant will yield inaccurate test results if routine maintenance and filter upkeep are neglected.