CARB study investigates low-emission off-road diesel engines
30 June 2021
At a CARB hosted research seminar on June 28, 2021, Chris Sharp from SwRI summarized results for a CARB funded study (Contract 18RD006) to characterize emissions of two Tier 4 final off-road diesel engines with similar power ratings but different aftertreatment configurations. Testing was carried out over the current certification cycles (NRTC and RMC) as well as a low load cycle. The project also included a survey of the off-road market in California and as well as modeling work intended to identify potential engine and aftertreatment configurations to further reduce emissions.
The project sets the stage for extending the CARB-funded on-highway low NOx technology demonstration project at SwRI into the off-road sector and, potentially, for the development of next stage, Tier 5 California/US emission standards for off-road engines.
In 2019, there were about 660,000 off-road machines in California. Agricultural tractors below 100 hp (75 kW) accounted for 47% of these machines. Agricultural tractors over 100 hp, skid steer loaders, compact track loaders, backhoe loaders and wheeled loaders represented about 28% while the remainder consisted of about a dozen categories.
A survey of engine certification families showed a near even split between low power (19≤kW<56) and higher power (56≤kW<560). For families below 56 kW, 95% used EGR for in-cylinder NOx control, none used SCR and 50% used no DPF (DOC only). Above 56 kW, 85% used EGR for in cylinder NOx control, all used SCR, 20% did not use an ammonia slip catalyst (ASC) and 60% used no DPF.
The engine testing portion of the project was supported by John Deere who provided two engines. An aftertreatment configuration consisting of a DOC+DPF+SCR was found on a 6068HFC09 6.8 L 6 cylinder with series turbocharging, cooled EGR and available in ratings from 168-224 kW. A non-DPF aftertreatment system composed of a DOC+SCR was found on a 4045HFC04 4.5 L 4 cylinder with a wastegated turbocharger, cooled EGR and available from 63-104 kW.
The low load cycle used in the project, referred to as Low Load Application Cycle (LLAC), was based on application cycles that Deere uses for product development. While not intended to represent a full Low Load Cycle development process, the LLAC did allow emissions to be measured over realistic low load conditions that the test engines would likely see in use. The LLAC was run hot. Engine operation over the LLAC fell outside the NTE zone for the engines tested and would therefore not be subject to scrutiny under the current regulations.
Emission results are shown in Table 1. PN emissions, not regulated in the USA, were measured as per the Euro Stage V procedure. While both systems comfortably met the Tier 4 final requirements, several observations are noteworthy:
- Emissions of NOx over the LLAC were considerably higher that the Tier 4 final limit and about 10 to 20 times higher than from the hot start NRTC. The average duty cycle on the LLAC is 15% of maximum power and the SCR catalyst moves in and out of light-off on multiple occasions. Average NOx conversion over the LLAC was 67% for the 6.8 L engine and 82% for the 4.5 L engine.
- For PM and PN emissions, the DPF-equipped engine yielded PM emissions that were one order of magnitude and PN emissions that were two to three orders of magnitude lower than the non-DPF engine.
| Test Cycle | 6.8 L DPF engine | 4.5 L non-DPF engine | ||||
|---|---|---|---|---|---|---|
| NOx | PM | PN | NOx | PM | PN | |
| g/kWh | g/kWh | #/kWh | g/kWh | g/kWh | #/kWh | |
| Cold NRTC | 0.39 | 0.001 | 3.7 × 1012 | 0.66 | 0.011 | 6.2 × 1013 |
| Hot NRTC | 0.08 | 0.0005 | 4.9 × 1010 | 0.12 | 0.009 | 5.5 × 1013 |
| Composite NRTC | 0.10 | 0.0004 | 4.1 × 1011 | 0.16 | 0.009 | 5.6 × 1013 |
| RMC C1 | 0.08 | 0.0003 | 2.9 × 1010 | 0.05 | 0.009 | 5.0 × 1013 |
| RMC D2 | 0.02 | 0.0004 | 1.1 × 1011 | 0.02 | 0.005 | 4.9 × 1013 |
| LLAC | 1.8 | 0.0003 | 6.3 × 1010 | 1.3 | 0.009 | 2.9 × 1013 |
Modeling work to identify possible solutions to reduce low load NOx emissions examined a dual SCR system with dual dosing and a modified conventional DOC+DPF+SCR+ASC system. When the conventional system with a “Generation 4” SCR catalyst (more low temperature activity) and having a volume of 80% of the baseline SCR catalyst was combined with minor modification to the engine calibration to lower engine-out NOx and improve exhaust gas thermal management, tailpipe NOx emissions were estimated to be 0.022 g/kWh and 0.14 g/kWh over the LLAC and cold start NRTC respectively for the 6.8 L engine. This is a considerable reduction compared to the 1.8 g/kWh and 0.39 g/kWh shown Table 1. Little impact on CO2 emissions was noted. While the dual dosing system provided additional benefit on the cold start NRTC, it would present considerable cost and packaging challenges for the off-road sector.
The project also characterized non-regulated emissions of VOCs, carbonyls, PAHs and nitro-PAHs, trace elements, inorganic ions, water soluble organic carbon, EC/OC as well as semi-volatile and intermediate volatility organic compounds (SVOC/IVOC).
Source: CARB