Recommended Practice for Improving Measurement Quality of Particle Number Counting Devices
The scope of this document focuses on the setup and use of solid particle number (SPN) counting devices in both engine development and certification environments. The document reviews best practices for collecting and measuring samples from both raw and diluted exhaust gas streams across several sample probe insertion locations relative to installed aftertreatment devices. Appropriate dilution techniques, sample transfer processes, and temperature management techniques are included. Finally, performance and validation checks are covered to ensure that long-term degradation and instrument failures can be identified quickly.
The particle size range targeted in this document is >23 nm, which aligns with the present EU SPN regulations and targets only solid particles. Current commonly available measurement devices are designed for counting efficiencies of 50% at 23 nm and 90% counting efficiencies at 40 nm so the contents of this document primarily address these particle sizes. It is expected that future regulations will target a lower 10 nm size cut-point. Consequently, a short section has been added discussing best practices that a user may need to consider when measuring particles in this size range.
Since the introduction of the first particulate mass emissions standard for heavy-duty diesel engines in 1988, engine and vehicle manufacturers have substantially reduced particulate matter (PM) mass emissions from modern internal combustion engines. PM emissions standard levels have decreased from 0.6 g/bhp-h to the current 0.01 g/bhp-h over the U.S. Heavy Duty Federal Test Procedure (FTP). This reduction in PM emissions resulted mainly from improvements in fuel-air combustion, the introduction of high-pressure fuel injection systems, fuel property improvements, and exhaust aftertreatment.
The introduction of catalyzed wall-flow particulate filters for compression ignition (CI) engines lowered the tailpipe particulate mass to very low levels. Testing, verifying, and certifying the improvements has largely been accomplished with PM sampling systems that collect PM mass on a pre-weighed 47 mm teflon filter during the prescribed test cycle, which is then weighed after sample collection and compared to the regulatory standard. This method requires the use of sensitive analytical balance instrumentation and may have reached a practical lower limit. The collected mass includes a number of contributors including ash, solid carbon particles, sulfates, nitrate compounds, and a mix of heavy hydrocarbons known as the soluble organic fraction (SOF). The smallest solid carbon components have been identified as primary soot particles, which can range in geometric mean diameter size from several nanometers to tens of micrometers. The interrelationship between these soot particles, human health effects, and poor urban air quality largely drove the development of instrumentation designed to measure the concentration of particles in the air. The European Union Particulate Measurement Program (PMP) was commissioned to investigate and recommend the best and most cost-effective way to measure the number of particles emitted by an engine during a given test cycle. Many of the recommendations by this group were codified into European Union Law/Regulation under UNECE R-83 with Standard Limits first included in EURO 5+ in 2011 for light-duty diesel passenger cars. Heavy-duty diesel engines were regulated by EU under Stage V and Euro 6.
As a result, multiple instrument manufacturers have introduced devices that measure solid particle number (SPN) under the EURO 5+ rules. As the use of SPN counters have become more wide-spread in both R&D and certification, a growing need has developed for a commonly understood recommended practice for the setup and use of these measurement devices. This recommended practice provides a guide for SPN measurement using the currently available measurement devices.