This effort aims to establish an amendment to Global Technical Regulation (gtr) No. 4 (WHDC) with respect to pollutant and CO2 emissions from heavy duty hybrids. The initial focus is on a Hardware-in-the-Loop (HILS) approach, which starts from the vehicle speed pattern of the WHVC vehicle cycle and simulates powertrain and vehicle components to result in a hybrid specific engine cycle for pollutant emissions and CO2 testing and measurement, and which allows using the test cell environment, data evaluation procedures and emissions calculations specified in gtr No. 4. This effort may lead to a second phase to establish a chassis dyno test procedure, as an alternative to HILS, which would involve specifying new requirements with respect to test cell environment, data evaluation procedures and emissions calculations. The informal group developing this regulatory language expects to finalize its work in March 2013 for GRPE review the following June (setting the stage for possible WP.29 adoption in November 2013 or March 2014).