Research Projects

Project at a Glance

Project Status: complete

Title: Study to define cold and hot start emissions final investigative report.

Principal Investigator / Author(s): Arnold, Thomas E

Contractor: GM Powertrain Division, GM Corp.

Contract Number: 93-322

Research Program Area: Emissions Monitoring & Control

Topic Areas: Mobile Sources & Fuels


The objective of this study is to collect emission test data and other vehicle related parameters that can be utilized to improve the capability of the California Air Resources Board's (ARB) Emission Inventory Model EMFAC. Specifically, the data will enhance the model's ability to estimate start emissions under real-world conditions which are not included in the current methodology. Data collected in this study will be analyzed in depth by the ARB staff to determine the effect of engine cranking, catalyst light-off and cool-down times, engine-off duration, driving mode and fuel type on start emissions at different ambient temperatures.

Until recently, emissions data for estimating start emissions were collected by testing vehicles over the Federal Test Procedure (FTP). Start emissions were calculated using the three bags of the FTP, i.e., bag 1 (cold start), bag 2 (stabilized), and bag 3 (hot start). However, this method has since been reevaluated by the ARB and found inadequate for modeling starts.

The FTP, developed over two decades ago, includes a series of accelerations, decelerations, idles, and cruises originally intended to simulate a typical trip. However, contemporary driving patterns involve conditions not encountered during the FTP. In ARB's previous methodology, cold start emissions of a catalyst-equipped vehicle were calculated using bag 1 (performed after an overnight soak), and were applied to all cold soaks greater than 60 minutes. Similarly, hot start emissions, calculated using bag 3 (performed after a 10-minute soak), were applied to all hot starts less than 60 minutes. Recent studies by the ARB instead indicate that start emissions are best modeled as a continuous function of soak time. The ARB has since revised the start emission calculations methodology in an attempt to address these inadequacies.

The purpose of this project is to supplement data used for the revised methodology by testing vehicles of various ages and technologies at various engine-off times and driving modes. In addition, this project required testing vehicles with different fuels and under different ambient temperatures not required by the FTP. The results will be used to estimate what effects various fuel formulations and temperature combinations may have on start emissions. Overall, the shortcomings of the current methodology will be addressed and will thus improve the ARB's capability for modeling emission factors related to starts.


For questions regarding research reports, contact: Heather Choi at (916) 322-3893

Stay involved, sign up with ARB's Research Email Listserver