Overview#
The Johnson Matthey CompactSCRT is a factory-assembled integrated aftertreatment system that combines DPF, DOC, and SCR into a single-lift module — achieving full EPA Tier 4 Final emissions compliance for diesel generators. With up to 97% NOx reduction and 90%+ PM reduction, it's the most comprehensive aftertreatment solution for generators that need both particulate and NOx control.
Johnson Matthey's Stationary Emissions Control division (headquartered in Huntington Beach, CA) has 30+ years of DPF manufacturing experience and an installed base exceeding 3.4 GW of SCR-treated stationary engine power worldwide.
Technology stack#
The CompactSCRT integrates three aftertreatment stages:
- DOC (Diesel Oxidation Catalyst) — oxidizes CO and hydrocarbons, raises exhaust temperature
- CRT+ DPF (Continuously Regenerating Trap) — captures particulate matter, regenerates passively using NO2 from the DOC
- SCR (Selective Catalytic Reduction) — injects DEF to reduce NOx by up to 97%
Optional additions:
- Ammonia Slip Catalyst (ASC) — prevents unreacted ammonia from exiting
- Integrated silencer — 40-45 dBA attenuation (hospital-grade options)
- SootAlert monitor — DPF operational readiness tracking
CompactSCRT vs Rypos: different approaches#
| Johnson Matthey CompactSCRT | Rypos RH-DPF | |
|---|---|---|
| Technologies | DPF + SCR + DOC | DPF only |
| NOx reduction | Up to 97% | None (PM only) |
| PM reduction | 90%+ | Up to 95% |
| Regeneration | Passive (exhaust heat) | Active (electric heating) |
| DEF required | Yes | No |
| Best for | Prime power, peaking, high-runtime | Standby (low runtime, low exhaust temp) |
| Maintenance complexity | Higher (SCR + DEF + DPF ash) | Lower (DPF only, electric regen) |
| Cost | Higher | Lower |
For California standby generators that run <100 hours/year: Rypos is often more practical (active regen works at any load). For prime power or peaking generators with significant runtime: CompactSCRT provides comprehensive PM + NOx reduction.
Our service experience#
We encounter Johnson Matthey aftertreatment primarily on larger prime-power and peaking installations where the generator runs enough hours to sustain passive DPF regeneration. For standby-only generators (our primary market), we more commonly specify Rypos for PM compliance because the active electric regen doesn't depend on exhaust temperature. However, when air district permits require NOx reduction in addition to PM (increasingly common for new large diesel installations in BAAQMD territory), CompactSCRT or similar integrated systems become necessary.