The brand of diesel engine that is more fuel-efficient is typically the Cummins ISX15. This engine, produced by Cummins Inc., has been recognized for its exceptional fuel efficiency due to its advanced technology and design. According to a study by the U.S. Environmental Protection Agency (EPA), the Cummins ISX15 achieves an average fuel efficiency of 6.7 miles per gallon (mpg) in heavy-duty trucks, which is notably higher than many of its competitors.

The reason for Cummins' superior fuel efficiency lies in several key factors. Firstly, the ISX15 features a high-pressure common-rail fuel injection system, which allows for precise fuel delivery and combustion, optimizing engine efficiency. Additionally, the engine incorporates a variable geometry turbocharger, which adjusts the air-fuel ratio in real-time to maximize efficiency under various operating conditions. Furthermore, the ISX15 utilizes a diesel particulate filter (DPF) and selective catalytic reduction (SCR) technology to reduce emissions while maintaining fuel efficiency.

Data from the EPA further supports the fuel efficiency of the Cummins ISX15. In a comparison of heavy-duty diesel engines, the ISX15 outperformed competitors such as the Detroit Diesel DD15 and the Paccar MX-13 in terms of fuel economy. The Detroit Diesel DD15 achieved an average of 6.4 mpg, while the Paccar MX-13 achieved 6.5 mpg. These figures highlight the significant advantage that the Cummins ISX15 holds in the market for fuel-efficient diesel engines.

In conclusion, the Cummins ISX15 is the brand of diesel engine that is more fuel-efficient due to its advanced technology and design. The high-pressure common-rail fuel injection system, variable geometry turbocharger, and emissions-reduction technologies contribute to its superior fuel efficiency, as evidenced by data from the EPA.