Why crankcase blow-by is harder to filter than you think
Every internal combustion engine produces blow-by gas — a mixture of combustion gases, unburnt fuel, and oil mist that escapes past the piston rings into the crankcase. In a typical diesel generator or gas engine, blow-by represents 0.5–1.5% of total airflow, but its composition makes it one of the most aggressive gas streams a coalescing filter will ever encounter.
Unlike the relatively clean oil aerosol in a vacuum pump exhaust, engine blow-by contains a cocktail of contaminants:
- Oil mist and aerosol: Sub-micron droplets (0.1–1.0 µm) of lubricating oil, often partially degraded by heat
- Soot and carbon particles: Hard, abrasive particles from incomplete combustion — particularly heavy in diesel engines
- Acidic condensates: Water vapour combined with sulphur and nitrogen oxides forms corrosive acids (sulphuric, nitric)
- Unburnt hydrocarbons: Fuel vapour and partially combusted compounds that can degrade filter media
Why standard elements fail here
Standard coalescing elements designed for compressed air or vacuum pump duty simply cannot cope with the soot loading and acidic conditions in engine blow-by. Using the wrong element leads to service intervals of just a few hundred hours instead of thousands.
CCV vs. OCV — two approaches, different filtration demands
| Feature | Closed Crankcase Ventilation (CCV) | Open Crankcase Ventilation (OCV) |
|---|---|---|
| Exhaust routing | Filtered gas returned to engine intake | Filtered gas vented to atmosphere |
| Primary driver | Emissions compliance (TIER, STAGE V, TA Luft) | Equipment protection, intake cleanliness |
| Oil recovery | Coalesced oil returned to sump | Coalesced oil returned to sump or waste |
| Back-pressure sensitivity | Critical — excessive restriction affects engine | Moderate |
| Efficiency required | Typically >95% at 0.3 µm | Variable — depends on regulations |
| Typical application | Data centres, hospitals, marine | Remote installations, ventilated engine rooms |
CCV impact on turbochargers
In CCV systems, filtered gas re-enters the engine intake upstream of the turbocharger. Any oil or soot that passes through coats turbocharger blades, fouls intercoolers, and accelerates wear. Industry data suggests 15–20% of turbocharger failures trace back to inadequate crankcase ventilation filtration.
What determines element service life?
The number one complaint about crankcase ventilation filters is premature element failure. The root causes are almost always the same:
Soot loading
Soot accumulates permanently in the media. Unlike liquid, it cannot drain. As soot blocks coalescence pathways, back-pressure rises.
Acidic condensate attack
Below the dew point (cold starts, low loads), acids form that degrade glass-fibre media and bonding resins over time.
Wrong element specification
Standard compressed air coalescing elements lack the soot-holding capacity and chemical resistance for engine blow-by.
No pre-separation
Without upstream pre-separation, bulk soot reaches the fine coalescing media directly, shortening life dramatically.
Selecting the right coalescing element for CCV duty
| Selection Criterion | What to Consider |
|---|---|
| Media type | Borosilicate glass-fibre with acid-resistant binder. Avoid cellulose-based media in high-soot or acidic conditions. |
| Soot-holding capacity | Progressive-density depth media with graduated pore structure provides higher capacity than uniform elements. |
| Drainage | Coalesced oil must drain freely. Correct element orientation and adequately sized housing drain connections are essential. |
| Temperature resistance | Blow-by can reach 120–150 °C in turbocharged engines. All materials must tolerate sustained thermal exposure. |
| Chemical compatibility | Resistance to pH 2–4 condensates, hydrocarbon solvents, and degraded lubricating oil. |
| Housing back-pressure | CCV systems are very sensitive to restriction. Low clean-element ΔP leaves margin for soot loading. |
System design best practices
Pre-separation
Install an inertial or centrifugal pre-separator upstream of the coalescing filter for diesel and high-soot applications.
Oversize the housing
Reduces media velocity, lowers ΔP, and extends service intervals. Account for increased blow-by as piston rings wear.
Monitor differential pressure
A DP gauge or transmitter provides the only reliable indication of element condition. Time-based schedules are unreliable.
Manage temperature
Keep gas above the acid dew point (~120 °C for diesel). Insulated or heated housings may be needed in cold climates.
Oil drain routing
Ensure coalesced oil drains continuously by gravity back to the crankcase. Drain lines must be adequately sized and free of restrictions. In CCV systems, a check valve prevents intake vacuum from pulling air back through the drain.
Applications where CCV filtration is critical
- Stationary power generation: Diesel and gas generators running 24/7 in data centres, hospitals, and critical infrastructure. Enclosed locations demand CCV with high-efficiency filtration to meet TA Luft.
- Biogas and landfill gas engines: Contaminants in the fuel (siloxanes, H₂S, moisture) make blow-by particularly aggressive. Accelerated soot and acidic condensate production.
- Marine propulsion and auxiliary: IMO TIER III and local port regulations increasingly require closed crankcase ventilation. Vibration and vessel motion add mechanical stress.
- CHP / cogeneration: Combined heat and power in urban environments with strict emission requirements and limited maintenance windows.
- Mining and construction: Enclosed or semi-enclosed engine operation in tunnels, mines, and confined spaces requires effective blow-by filtration.
The connection to vacuum pump filtration
Same technology, tougher conditions
Crankcase ventilation filtration uses the same fundamental coalescing technology as vacuum pump exhaust filtration. The difference: engine blow-by adds soot, acids, and higher temperatures, demanding more robust elements and often a pre-separation stage. Our RF-CS series elements and RF-H-420/456 series housings cover both applications — with stainless steel RF-H-420S–456S rated to 200 °C for elevated-temperature blow-by.
Key Takeaway
When CCV filter element life is disappointingly short, the cause is almost always one of four factors: excessive soot loading, acidic condensate attack, wrong element specification, or inadequate pre-separation. All of these are fixable with the right element selection and system design.
Specify the right CCV filter
Enter your engine type, operating conditions, and flow rate — the Engineering Tool recommends compatible housings and elements for your application.
If you are experiencing premature element failure or specifying a new system, contact our filtration specialists. We will review your operating conditions and recommend the appropriate element type, efficiency grade, and system design improvements.
