A heavy-duty engine testing project involving Cummins Engine Company, Southwest Research Institute (SwRI), and West Virginia University (WVU) has been completed. This project evaluated the transient exhaust gas emissions rate of Cummins N-14 heavy-duty diesel engines converted to natural gas. Three heavy-duty N-14 diesel engines were converted to run on natural gas using a lean burn strategy by SwRI and are in field service in Santa Barbara Air Pollution Control District (SBAPCD). Two of the engines were tested under a steady-state cycle that simulates the U.S. heavy-duty transient cycle. The third engine was tested at the WVU Engine Research Laboratory following the U.S. Federal Test Procedure (FTP). However, at WVU, lean burn combustion strategy was shifted rich of stoichiometric during idling time of the FTP test. This may have caused the engine to produce more total hydrocarbons (THC) and carbon monoxide (CO).For the simulated FTP tests at SwRI, the engines showed nitrogen oxides (NOx) emissions of 2.17 g/bhp-hr, THC emissions of 4.1 g/bhp-hr, and CO of 2.1 g/bhp-hr. At WVU, NOx emission was 2.33 g/bhp-hr, THC was 12.05 g/bhp-hr, and CO emission was 12.66 g/bhp-hr.