Retrofitting Tier 2 Engines: The Most Practical Path to OEL Compliance in South African Underground Mines
Underground mines across South Africa are adapting to one of the most significant regulatory shifts in recent years. From 26 June 2025, amendments to the Mine Health and Safety Act will enforce a strict Occupational Exposure Limit (OEL) for Diesel Particulate Matter (DPM) of 0.1 mg/m³ (100 µg/m³), a requirement designed to protect the health and safety of underground workers
While new Tier 4 Final and Stage V engines or battery-electric vehicles may seem like the obvious future, the reality in South African mining is more complex: full fleet replacement is expensive, disruptive, and often impractical.
For most operations, the fastest, most cost-effective and lowest-risk strategy is clear:
Upgrade the engines you already have.
At HJS Emission Technology South Africa, we specialise in helping mines achieve OEL compliance by retrofitting existing Tier 2 engines with advanced, field-proven emission-control systems engineered specifically for harsh underground conditions.
Why Retrofitting Tier 2 Engines Works
Tier 2 engines are still the backbone of most underground fleets—they are robust, mechanically straightforward, and built for long service life. With the right after-treatment technology, they can be transformed into low-emission power units capable of meeting today’s strict OEL limits.
Why not simply replace them?
Higher-tier engines are excellent technologies but come with challenges:
- High purchase and installation costs
- Complex electronics, dosing systems and diagnostics
- Sensitivity to fuel sulphur, dust, vibration and heat
- Need for upgraded cooling, ventilation, and DEF/AdBlue systems
- Longer downtime for commissioning and re-certification
- Shortage of local technical skills and spare parts
Retrofitting avoids these barriers entirely while delivering the same—or in underground conditions, even better—compliance outcomes.
Compliance Happens Underground, Not in a Lab
Laboratory certification for Tier 4/Stage V engines does not guarantee that those engines will remain compliant underground. Real-world conditions, load spikes, dust, ventilation variation, steep gradients, create emissions behaviour that cannot be replicated in controlled testing.
That is why compliance-based emissions management is now the preferred approach in underground mines worldwide. HJS SA supports this through:
Emissions-Based Maintenance (EBM)
Tracking raw-gas emissions upstream of the after-treatment system to detect failing injectors, turbo wear, and combustion issues early.
Portable Emissions Measurement Systems (PEMS)
Real-time underground emissions testing to verify OEL compliance under the mine’s specific operating conditions.
Certification shows what an engine achieved once. Compliance shows what it is achieving today.
HJS SMF® Technology – Purpose-Built for Underground Mining
HJS Emission Technology in Germany has engineered a retrofit system specifically for Tier 2 underground machines. Through HJS SA, these systems are installed, supported and serviced locally.
Key features of the HJS retrofit solution:
SMF® (Sintered Metal Filter) Technology
- Up to 99% DPM reduction
- Stainless-steel, corrosion-resistant filter media
- Cleanable and reusable
- No fuel burners or additives
- Exceptionallylow backpressure
Optional SCR Modules
- Up to 95% NOx reduction
- Compact, modular integration
- Designed for intermittent, variable load cycles
Intelligent Emission Control Unit (ECU)
- Manages regeneration
- Integrates with vehiclemonitoring systems
- Enables predictive maintenance
Modular, Retrofit-Friendly Design
- No modifications to engine cooling
- No need to change ECU mapping
- Fast installation and low downtime
Full Local Support
- Engineering, diagnostics, installations and training
- Local stockholding of filters, sensors and spares
- Backed by HJS Germany’s global engineering centre
Business and Operational Advantages
Why mines choose retrofits:
Factor | Tier 4/Stage V Engine Replacement | Tier 2 Retrofit with HJS |
Capital cost | Very high | Moderate |
Downtime | Long | Minimal |
Complexity | High (multiple ECUs, sensors, software) | Low |
Maintenance | Specialist skills required | Straightforward |
Fleet consistency | Mixed technologies | Standardised platform |
Compliance verification | Certified in lab | Measured underground |
Risk | Higher | Lower |
Carbon footprint | New engine manufacturing | Asset reuse |
The result: Faster compliance. Lower risk. Better return on investment.
Conclusion: A Smarter, Faster Path to Cleaner Underground Air
Retrofitting Tier 2 engines with HJS emission-control systems has become the practical and sustainable choice for South African mines preparing to comply to the OEL that came into effect in June 2025.
It provides:
- Measurable DPM compliance
- Lower capital expenditure
- Minimal operational disruption
- Proven, rugged technology
- Full local engineering and support
- Better ESG performance through asset reuse
At HJS Emission Technology South Africa, our mission is simple: Help mines achieve compliance confidently, sustainably, and without unnecessary cost or complexity.
We welcome the opportunity to assess your fleet, conduct emissions testing, and design a retrofit strategy tailored to your mine.
Clean Air Is Our Mission.
