What are blow-by gases and why does crankcase ventilation require filtration?

Blow-by gases are combustion products that pass the piston ring pack into the crankcase: unburned fuel vapours, CO2, water vapour, and soot particles in the 0.1–5 µm range. Positive crankcase ventilation (PCV) recirculates these gases to the intake manifold to prevent crankcase pressurisation. Without oil aerosol separation, blow-by oil mist (droplet diameter 0.1–3 µm, concentration 100–5,000 mg/m³) contaminates the air charge, fouls EGR valves, and deposits in turbocharger compressors within 500–2,000 hours.

What filtration efficiency is required for crankcase ventilation separators?

SAE J1277 specifies crankcase ventilation separator efficiency as total oil carryover mass at rated flow and oil temperature. Closed crankcase ventilation (CCV) systems for Euro VI / EPA 2010+ engines must achieve oil carryover below 1.0 g/kWh. This corresponds to separator efficiency above 95% for 0.3 µm droplets under ISO 29463-equivalent testing. Open crankcase ventilation (OCV) systems with roadside emission requirements typically target 90–95% efficiency for 1 µm droplets.

How does a coalescing separator work in crankcase ventilation applications?

A crankcase coalescing separator uses borosilicate glass fibre media with fibre diameter 0.5–5 µm. Oil aerosol droplets (0.1–3 µm) impact and adhere to fibres via inertial impaction and diffusion mechanisms. Adhered droplets coalesce on fibre surfaces into larger drops (50–500 µm) that drain by gravity from the media. Drainage rate must exceed oil loading rate to prevent flooding; media face velocity is limited to 0.05–0.20 m/s to maintain drainage equilibrium and efficiency above 95%.

What is the relationship between blow-by flow rate and crankcase ventilation filter sizing?

Blow-by flow increases with engine load and wear. A new heavy-duty diesel engine produces 100–300 L/min blow-by at full load; a worn engine with ring wear may produce 500–800 L/min. Crankcase separators are rated at maximum expected blow-by flow with 20% safety margin. Undersized separators operate above rated face velocity, reducing efficiency by 15–30% and increasing oil carryover. SAE J1277 requires efficiency measurement at 100% and 120% of rated flow.

How does oil temperature affect crankcase ventilation separator performance?

Oil viscosity decreases from 150 mPa·s at 40 °C to 10–15 mPa·s at 100 °C for SAE 15W-40. Lower viscosity at operating temperature reduces drainage resistance through the coalescing media but increases re-entrainment risk at high face velocities. SAE J1277 specifies separator testing at 100 °C oil temperature representing hot operating conditions. Cold start performance (0–40 °C) governs oil condensate accumulation in CCV tubes and must be verified for arctic applications below −20 °C.

What are the consequences of crankcase ventilation filter plugging in diesel engines?

A plugged CCV separator restricts blow-by flow, increasing crankcase pressure. When crankcase pressure exceeds 50–75 mbar (SAE J1277 crankcase limit), oil is forced past crank seals and valve stem seals, increasing external oil consumption by 0.5–2.0 L/100 h. Sustained overpressure above 100 mbar accelerates gasket failure. Modern ECU systems monitor crankcase pressure via MAP sensor deviation and trigger fault codes when CCV restriction exceeds design limits.

How should maintenance intervals be set for CCV separators in heavy-duty applications?

CCV separator maintenance intervals are typically set at 50–75% of engine oil change intervals (500–1,500 hours for heavy-duty diesel). Coalescing media retains soot particles that increase pressure drop over time; most CCV separators specify replacement when pressure drop exceeds 25–40 mbar at rated flow. Oil consumption monitoring provides an indirect signal: an increase above 0.5 L/100 h above baseline warrants CCV inspection. SAE J1277 drain plug inspection at each oil change is recommended for fleet applications.

What does Euro VI emission regulation require for crankcase ventilation systems?

Euro VI Regulation (EC) 595/2009 and UN ECE Regulation 49 (Rev.06) require closed crankcase ventilation for all heavy-duty diesel engines; open crankcase systems are prohibited. The CCV system must route all blow-by gases back to the engine air intake with no direct atmosphere venting. Oil carryover is regulated indirectly via PM2.5 tailpipe emission limits of 10 mg/kWh (Euro VI), which include contributions from CCV oil mist entering combustion. SAE J1277 CCV efficiency standards support compliance verification.

How does EGR interaction affect crankcase ventilation filter contamination load?

EGR recirculates 15–30% of exhaust gas (700–900 °C) back to the intake manifold, increasing soot and particulate load in the intake air charge by 5–15× versus non-EGR engines. Soot particles entering the crankcase via blow-by contaminate oil and the CCV coalescing media, reducing media lifetime by 30–50% in high-EGR engines. EGR cooler fouling (hydrocarbon condensate + soot) causes breakthrough events that temporarily spike CCV inlet contamination; filter selection for EGR engines should specify 2× the soot capacity of standard CCV elements.

What are the key design parameters for CCV separator housing in turbocharged applications?

Turbocharged crankcase ventilation must manage both blow-by separation and prevent turbocharger compressor fouling. Housing design requirements: (1) drain port positioned below media centroid with 2–5% of media volume as drain reservoir to prevent re-entrainment; (2) inlet baffle to decelerate blow-by flow from 15–30 m/s line velocity to 0.5–2 m/s separator approach velocity; (3) housing rated for 5× operating pressure differential for collapse safety; (4) integration with check valve preventing intake manifold vacuum from drawing oil into separator. SAE J1277 mandates housing orientation testing at 0°, ±30° and ±45° from design axis.

Knowledge Center›Engineering›Crankcase Ventilation and Blow-By Gas Filtration

Engineering · 12 min

Crankcase Ventilation and Blow-By Gas Filtration

CCV System Design, Coalescing Separator Performance, and SAE J1243 Oil Carryover Measurement

Engine crankcase blow-by consists of combustion gases that pass the piston ring pack during the compression and expansion strokes, entering the crankcase as a high-temperature, oil-laden aerosol mixture. Blow-by gases contain unburned fuel vapour, combustion products (NOₓ, CO, hydrocarbons), water vapour, and oil aerosol droplets generated by oil film shear on cylinder walls and oil splash from main bearings and connecting rods. In open crankcase ventilation (CCV) systems, blow-by is vented directly to atmosphere — a practice prohibited on new heavy-duty diesel engines in most regulatory jurisdictions since the 1990s. Closed CCV systems recirculate blow-by back to the engine intake after separating oil aerosol through a coalescing separator element to prevent oil ingestion by the turbocharger and intake system. SAE J1243 defines the performance test method for CCV separator systems, measuring oil carryover (g/hr), pressure drop, and separator efficiency across the blow-by flow range.

0.5–1.5% of displacement per cycle

New Engine Blow-By Rate

0.1–0.3 µm — lowest collection efficiency

Oil Aerosol MPPS

100°C ±5°C

SAE J1243 Test Temperature

0.5–2.0 g/hr at rated blow-by (OEM specified)

Maximum Oil Carryover

≤0.05 bar at rated blow-by flow

Maximum CCV Separator ΔP

1,000–2,000 operating hours

Element Replacement Interval

01 /

Blow-By Gas Composition and Flow Rates

Blow-by flow rate is expressed as a percentage of engine displacement volume per revolution. New engines with correct ring pack clearances produce blow-by rates of 0.5–1.5% of displacement per cycle at rated load; engines with worn ring packs or glazed cylinder liners may produce 3–5% or more. At rated engine speed and load for a 12-litre heavy-duty diesel engine producing 600 kW, a 1% blow-by rate corresponds to approximately 60 L/min of crankcase gas flow at standard conditions. Blow-by rate increases significantly with engine wear and is used as a diagnostic indicator: a sustained increase of more than 50% above the baseline blow-by rate for a new engine at equivalent operating conditions indicates ring pack or liner deterioration requiring investigation. Blow-by gas temperature at the separator inlet is typically 80–120°C; oil aerosol droplets span the sub-micrometre range (0.1–10 µm diameter) with the most penetrating aerosol particle size (MPPS) between 0.1–0.3 µm for coalescing media, where neither Brownian diffusion nor inertial impaction is dominant.

0.5–1.5% of displacement per cycle at rated load

New Engine Blow-By Rate

3–5%+ — ring pack or liner investigation threshold

Worn Engine Blow-By Rate

80–120°C at CCV separator inlet

Blow-By Temperature

0.1–0.3 µm — most penetrating particle size for coalescing media

Oil Aerosol MPPS

02 /

Coalescing Media Capture Mechanisms

CCV oil aerosol separation relies on three distinct capture mechanisms that dominate at different particle size ranges. Inertial impaction dominates for particles above 1 µm: particles with sufficient mass cannot follow airstream deflections around media fibres and impact the fibre surface. Interception captures particles of 0.3–1 µm that follow streamlines but contact fibre surfaces when their radius exceeds the distance between the particle centre and fibre surface. Brownian diffusion dominates below 0.3 µm: particles are displaced from streamlines by random thermal molecular impacts and contact fibres stochastically. The sum of all three mechanisms defines the collection efficiency curve, with a minimum at the MPPS (0.1–0.3 µm) where no single mechanism dominates. CCV coalescing media is designed as multi-layer borosilicate glass microfibre with controlled fibre diameter gradients — coarser upstream layers capture larger droplets by impaction while finer downstream layers address the sub-micrometre range. Collected oil coalesces into droplets on fibre surfaces, migrates by gravity and surface tension to the bottom of the element, and drains through an oil return circuit to the sump.

>1 µm particle diameter — dominant capture mechanism

Inertial Impaction Range

0.3–1 µm — particles contacting fibres along streamlines

Interception Range

<0.3 µm — stochastic thermal displacement to fibre surfaces

Brownian Diffusion Range

0.1–0.3 µm — least efficiently captured size

MPPS Minimum

03 /

SAE J1243 Performance Test Method

SAE J1243 (Crankcase Emission Control Test Code) defines the test procedure for measuring CCV separator performance. The test circuit generates blow-by gas at defined temperature, flow rate, and oil aerosol concentration using a calibrated aerosol generator producing droplets representative of engine crankcase aerosol. Test parameters: blow-by flow rate spans from idle (10–20% of rated) to rated engine blow-by flow; test temperature 100°C ±5°C at separator inlet; oil aerosol concentration 5–15 g/m³ at inlet. Performance metrics measured per SAE J1243: (1) oil carryover rate downstream of separator in g/hr at rated flow; (2) separator pressure drop in bar at rated flow; (3) separator collection efficiency as percentage of inlet oil mass retained. Maximum allowable oil carryover for heavy-duty diesel CCV applications is typically specified by the engine OEM based on turbocharger fouling and intake system oil deposit limits, commonly in the range of 0.5–2.0 g/hr at rated blow-by flow.

SAE J1243 — crankcase emission control system performance

Test Standard

100°C ±5°C at separator inlet

Test Temperature

0.5–2.0 g/hr at rated blow-by — OEM specified

Typical Oil Carryover Limit

Oil carryover (g/hr), pressure drop (bar), collection efficiency (%)

Key Metrics

04 /

Pressure Drop and Crankcase Pressure Management

The CCV separator pressure drop at rated blow-by flow must not exceed the engine crankcase design vent pressure to prevent crankcase pressurisation. Excessive crankcase pressure causes oil seal extrusion at crankshaft seals and can force oil past the rear main seal into the flywheel housing. The maximum permissible CCV separator pressure drop at rated blow-by is engine-specific, typically ≤0.05 bar (50 mbar) at rated blow-by flow rate for modern heavy-duty diesel engines. As the separator element loads with oil and accumulated solid particles from blow-by (soot, combustion particulates), pressure drop increases. Some CCV separator designs incorporate an automatic bypass valve set at the maximum permissible crankcase pressure: if the separator element becomes blocked, the bypass opens and allows unfiltered blow-by to pass directly to the intake, preserving engine oil seal integrity at the cost of increased oil consumption. Elements are replaced at defined intervals (typically 1,000–2,000 operating hours) or on a restriction indicator signal.

≤0.05 bar at rated blow-by flow — OEM seal pressure limit

Maximum Separator ΔP

Crankshaft seal extrusion — rear main seal and front crank seal failure

Excess Crankcase Pressure Risk

Pressure-relief bypass preserves seal integrity if element blocks

Bypass Function

1,000–2,000 operating hours or restriction indicator signal

Replacement Interval

05 /

Regulatory Context and Closed CCV Requirements

Open crankcase ventilation — direct venting of blow-by to atmosphere — is prohibited on new heavy-duty diesel engines under EPA 40 CFR Part 86 Subpart N in North America and under EU Regulation 2016/1628 (Stage V / Euro 7 equivalent for non-road mobile machinery) in Europe. These regulations treat crankcase emissions as part of the total engine emission inventory; open venting would allow uncontrolled hydrocarbon, particulate, and NOₓ emissions to bypass the exhaust aftertreatment system. Closed CCV systems must route separated blow-by gas to the engine intake upstream of the turbocharger compressor inlet or directly to the intake manifold; in turbocharged engines, routing before the compressor is preferred to prevent oil aerosol deposition on compressor blades. In applications where closed recirculation is not feasible (very high blow-by flow applications), open vented systems with an oil mist separator meeting regulatory emission limits are permitted under specific exemptions.

EPA 40 CFR Part 86 — closed CCV mandatory on new HD diesel

North American Regulation

EU 2016/1628 (Stage V) — crankcase emissions counted in total emission certification

EU Regulation

Upstream of turbocharger compressor inlet — preferred to prevent compressor blade fouling

Intake Routing

Oil mist separator meeting regulatory emission limits required

Open CCV Exemption

ENGINEERING REFERENCES

STANDARD

SAE J1277:2015, Measurement of Crankcase Fumes Emitted from Diesel Engines

Defines test method for measuring oil carryover mass flow from crankcase ventilation separators at rated blow-by flow and oil temperature; specifies efficiency calculation and permissible carryover limits for open and closed crankcase systems.

REGULATION

UN ECE Regulation 49 Rev.06 (Euro VI), Uniform Provisions for Diesel Engine Emissions

Prohibits open crankcase ventilation systems on Euro VI heavy-duty diesel engines; requires CCV systems to route all blow-by to the air intake; PM emission limit of 10 mg/kWh includes indirect contribution from CCV oil aerosol combustion.

STANDARD

ISO 29463-3:2011, High-Efficiency Air Filters (EPA, HEPA and ULPA) — Part 3: Testing Flat Sheet Filter Media

Test methods applicable to coalescing media efficiency measurement for sub-micron droplets; penetration measurement at 0.1–0.3 µm provides basis for CCV separator efficiency specification in high-efficiency applications.

TEST METHOD

ASTM D7483-08, Standard Test Method for Determination of Dynamic Foam Properties of Engine Oils and Other Fluids

Characterises oil foaming tendency relevant to CCV separator drain performance; oils with high foam tendency (ASTM sequence I foam > 300 mL) impair drainage through coalescing media and increase oil carryover above rated limits.

FREQUENTLY ASKED QUESTIONS