Engineers unlock path to meeting proposed Tier 5 standards on off-road diesel engines

Engineers at the Southwest Research Institute (SwRI) have demonstrated a diesel engine system designed to help off-highway vehicles meet strict Tier 5 emissions standards, which are currently being proposed by the California Air Resources Board (CARB).

The project proves that major environmental reductions remain possible, without reducing overall engine capability or requiring complete powertrain overhauls.

Currently, the construction and earthmoving industries do not possess a cost-effective, commercially ready hardware solution to meet the proposed Tier 5 restrictions, which target significant cuts to nitrogen oxides (NOx) and particulate matter (PM).

To find a practical compliance path, researchers conducted extensive laboratory testing on a commercially available, four-cylinder, 55-kilowatt diesel engine.

The development team focused heavily on improving performance through a strategic combination of hardware modifications and optimized engine calibration, rather than designing an entirely new power unit from scratch.

A primary mechanism utilized in the testing was exhaust gas recirculation (EGR). This process works by redirecting a portion of the exhaust gases back into the engine cylinders, lowering combustion temperatures and curbing the formation of NOx.

To manage PM emissions while preserving the original horsepower output, engineers paired the hardware upgrades with precise engine calibration.

Calibration acts as the central control for how fuel gets delivered, dictate engine responses, and manage overall efficiency under differing operational stresses.

An automated testing system accelerated the development window, mapping out engine behavior across hundreds of operational data points to pinpoint the most effective settings.

The modified engine successfully met the proposed Tier 5 targets during two critical standard industry certification procedures.

These validation steps included the Ramped Modal Cycle (RMC) and the Non-Road Transient Cycle (NRTC), both of which assess emissions under varied operating conditions.

In the RMC demonstrations, the modified system reduced NOx emissions by 22 percent and non-methane hydrocarbon (NMHC) emissions by 56 percent, compared to the draft Tier 5 targets.

During the NRTC evaluations, the system showed a 28 percent reduction in NOx and a 50 percent reduction in NMHC against the proposed limits.

The achieved metrics verify that heavy machinery can run substantially cleaner through innovative engine calibration and minimal hardware changes, offering a viable blueprint for equipment manufacturers preparing for future regulatory shifts.