The global push towards reducing green-house gas and criteria pollutant emissions is leading to tighter emission standards for heavy-duty engines. Among the most stringent of these standards are the California Air Resource Board (CARB) 2024+ HD Omnibus regulations adopted by the agency in August 2020. The CARB 2024+ HD Omnibus regulations require up to 90% reduction in NOx emissions along with updated compliance testing methods for on-road heavy-duty engines. Subsequently, the agency announced development of new Tier 5 standards for off-road engines in November 2021. The Tier 5 standards aim to reduce NOx/PM emissions by 90%/75% respectively from Tier 4 final levels, along with introduction of greenhouse gas emission standards for CO2/CH4/N2O/NH3. Furthermore, CARB is also considering similar updates on compliance testing as those implemented in 2024+ HD Omnibus regulations including, low-load cycle, idle emissions and 3-bin moving average in-use testing. While multiple technologies have already been developed for on-road engines to meet the 2024+ HD Omnibus regulations, they cannot be directly applied to off-road engines due to unique requirements of diverse machine applications, high durability, high reliability, packaging for visibility/turning radius, initial machine cost, transient response, and high low-end torque. A model-based approach is therefore necessary to evaluate tradeoffs in multiple engine and aftertreatment technology concepts to develop a modular, scalable, robust and cost-effective solution for meeting the proposed Tier 5 standards.
Following the extensive evaluations previously conducted by the authors on technology solutions for meeting on-road 2024+ HD Omnibus emission regulation, a model-based approach using GT-SUITE has been presented in this study to evaluate engine and aftertreatment technology packages for meeting the proposed off-road Tier 5 emission regulation. A validated engine and aftertreatment model of the baseline 228kW diesel engine was modified to investigate multiple advanced engine and aftertreatment technologies such as downspeeding, exhaust gas recirculation (EGR) pump, electric turbocharger (E-Turbo), variable geometry turbochargers (VGT), exhaust variable valve timing (Ex-VVT), dual urea dosing, closed couple catalysts and electric heater (EH) on off-road engine duty cycles. The technology packages were compared over Non-Road Transient Cycle (NRTC) cycle that is currently used for off-road engine certification. Key focus was placed on reducing engine out NOx during cold start, aftertreatment warmup, aftertreatment temperature-hold and NOx/CO2 tradeoff. Additional cycles that were investigated included a CARB recommended low load cycle, extended idle and a real-world challenge cycle for in-use compliance assessment.