Evolving Requirements for Engine Air Induction Filters

May/June 2015 | Volume 34 No. 3

By Neville J. Bugli, Toledo Molding and Die, Inc.

Automotive “Air Induction Systems” (AIS) have been constantly evolving to deliver higher performance levels. The “Air Induction Filter” (AIF) within the AIS is an engineered component designed to specifically perform for a certain type and size of the AIS. Besides meeting a variety of engineering requirements, it must also meet packaging requirements, higher flow rates, lower system restriction, managing flow to the Mass Air Flow Sensor (MAFS) and serviceability. MAFS performances are continually driven to be more stringent to meet vehicle calibration levels. This in turn has also challenged air filter designs. Air filters have to be optimized to meet the following functions [1, 2, 5, 6]:

  • Remove airborne and road
  • Protect engine by reducing wear
  • Protect against Throttle Body
  • Protect MAFS to maintain vehicle calibration and performance
  • Manage flow to MAFS for proper calibration / fuel consumption
  • Shield and remove water (droplets)
  • Shield against snow
  • Remove Soot type (nano) contaminants
  • In some cases attenuate high frequency NVH
  • Manage flow loss / restriction to maximize engine power
  • Meet sub-system leakage requirements

Some of these challenges for AIF designs are discussed below.

Global Challenges for AIF

Majority of the automotive OEMs (Original Equipment Manufacturer) are rolling out “global vehicle programs” where the AIF have similar design features (or same global design & components) to lower part count and incorporate best in class standard practices. However, there are several global challenges to be addressed:

  • Engine durability
  • 150K miles for pass cars
  • 250K miles for trucks
  • Filtration requirements
  • Water & snow management
  • PZEV, LEV III & LEV II requirements
  • Service requirements
  • MAFS performance & flow management
  • Leakage requirements
  • Packaging environment
  • Using carryover designs
  • Driving environments
  • Driving conditions
  • Customer usage
  • Customer perception and expectations*
  • Availability of OEM & OES components
  • Use of aftermarket components
  • Servicing & miss-assembly challenges
  • Global supply base – customer support
  • Industry & quality standards
  • Reduce parts complexities
  • Overall value to customer
  • Styling/appearance.
  • Lower costs
  • Recycling requirements*

Topics listed above can be further discussed in more details but is outside the scope of this paper. Some of these topics have been further discussed in related papers listed in the references.

AIF Design Considerations

Engine AIF are primarily designed to effectively remove airborne contaminants in order to protect the engine. Optimum performance of AIF is mandatory to protect the engine throughout its service life. The engine requires a certain level of ingested air cleanliness to reduce engine wear and improve engine efficiency. Engine protection and wear depends on the type, size, and concentration of contaminants ingested. AIF have to perform under a variety of operating and environmental conditions. These factors need to be understood and analyzed. Table 1 briefly lists some key design factors and variables that affect AIF. These factors may not be all inclusive.

Table 1:

Designs Variables
Inertia Pre-separators Baffles, Louvres, Cyclonic, etc.
Pre-filters Reticulated foams, Felts, Fibrous bats, Screens, etc.
Air Filter Design Panel, Cylindrical, Conical, Multi-sided panel, Depth, Multi-layered foams
Pleat Design Pleat shape, pleat height, pleat spacing, embossing, pleat stiffness, etc.
Filter Media Paper media, felt media, combination, resin system, flammability, smolder, hydrophobic properties, etc.
Media Face Velocity Flow rate, filter size, media area, flow uniformity, media usage, etc.
Housing Design Filter orientation, inlet/outlet location, flow distribution, MAFS location, etc.
Packaging Space Vehicle styling, engine size, under-hood space, etc.
Operating Environment Hot & cold climates, tropical regions, high altitudes, high humidity, etc.
Driving Conditions Normal, severe, dirt roads, agricultural, mining, desert, etc.
Type of Contaminants Abrasive, dry, oily/sticky, wet (mud), snow, water, soot, industrial dusts, etc.
Contaminant Loading Light, medium, heavy
Contaminant Size Range Pebbles, coarse, fine, ultra-fine, etc. (Typically range ingested ; < 0.1 mm to Ôëñ65 mm) [3]
Dynamic Conditions Vehicle vibrations, WOT conditions, back flow, pulsating flow, etc.
Service Intervals 30K normal, severe usage, extended usage, etc.
Serviceability Ease of filter removal – overall design for service and assembly
Durability 150K miles durability for pass cars, 250K miles med trucks and ~1000K miles for heavy duty
Emissions MAFS contamination, MAFS performance, hydrocarbon adsorbers, etc.
Filter Seal Seal type (compression, radial, etc.), compression set, tear, durability, etc.

Factors listed in Table 1 are critical and could be major discussion topics by themselves. More information on some of these design factors can be found in the references. Typically, the performance of AIF is evaluated in the lab with control variables or factors, though these factors could very well be uncontrolled in the field. Nevertheless evaluating filter performance with controlled variables yield meaningful and relative data. Some controlled and uncontrolled factors are tabulated below in Table 2, as they would relate to the field environment.

Table 2: Some controlled and uncontrolled attributes for AIF designs:

Controlled factors Uncontrolled factors (Field Conditions)
Type of media Flow rate changes
Pleat spacing Environment
Media area Driving conditions
Filter design/shape Contaminant concentrations
Housing design Contaminant size
Packaging Types of contaminants
Power loss Dynamic conditions
Inlet/Outlet locations Flow Distribution
MAFS location Service intervals
Test flow rate Use of aftermarket filters & options
Test Dust Filter cleaning
Miss-assembly in-service


A) Filter Performance

Higher initial and overall collection efficiencies are required to reduce engine wear. Many automotive OEMs do not specify or require measuring the initial efficiency of the air filter. Most, if not all, automotive air filters will exhibit high overall or final efficiencies once loaded with test dusts [1, 2, 3, 4]. The overall efficiencies are typically in the range of ~98.5%+ for light duty and 99.99%+ for heavy duty [1, 2, 3, 4]. Table 3 breaks down the current efficiency levels for normal driving conditions. The table shows data only for ISO (International Standards Organization) fine test dust. Studies have shown that using ISO Fine test dust is more relevant to engine air filters compared to ISO Coarse test dust [1].

Table 3: Typical Gravimetric Efficiency levels of AIF:

Typical Initial & Final Efficiency (gravimetric efficiencies)- ISO Fine Test Dust ISO 12103-1
Normal Driving Conditions Pass Car Lt/Med Trucks / SUVs Heavy Duty
Initial Efficiency @ 20g of test dust ┬│ 98.5 % ┬│ 99% ┬│ 99.5% – 99.9%
Overall Efficiency @ termination (generally 2.5kPa) ┬│ 99.0 % ┬│ 99.5 % ┬│ 99.9 – 99.99%

Tables 4 and 5 show the expected dust holding capacity (DHC) required to meet normal service life. Many OEMs specify DHC based on historic data that may or may not be relevant to what may be required to meet a particular service life. Recently some OEMs are specifying service life requirements and the expectation is that the AIS supplier will design the required DHC to meet service life [2, 4].
Table 4: Expected DHC with ISO fine tests dust (ISO 12103-1) to meet desired service life:

Regions: North America, European Union
Normal Driving conditions Pass Car Lt Trucks / SUVs Med Trucks / Large SUVs Heavy Duty
Typical flow rate, g/s 180 180 Р300 300 300 Р625
30K miles service ┬│ 60 g ┬│ 105 g ÔëÑ 150 g ┬│ 500 g
45K miles service ┬│ 90 g ┬│ 158 g ÔëÑ 225 g ┬│ 800 g

Table 5: Expected DHC with ISO fine tests dust (ISO 12103-1) to meet desired service life:

Regions: South America, Africa, Middle East, China, Asia Pacific
Normal Driving conditions Pass Car Lt Trucks / SUVs Med Trucks / Large SUVs Heavy Duty
Typical flow rate, g/s 180 180 Р300 300 300 Р625
15K km service ┬│ 70 g ┬│ 115 g ÔëÑ165 g ┬│ 500 g
25K Km service ┬│ 110 g ┬│ 192 g ÔëÑ 275 g ┬│ 750 g

Traditionally, AIF are evaluated based on gravimetric (mass) measurements. In addition to gravimetric efficiencies, some OEMs are also measuring the fractional size efficiency of the filter. Fractional size efficiency of the filter is also very important as it relates to filter sensitivity at a particular particle size. Many automotive fluid flow filtration systems use fractional size efficiency as the primary measurement. For example fuel filters, oil filters, and cabin air filters are evaluated based on its ability to remove various particle sizes. However, there is now a new ISO technical standard “Fractional Efficiency Procedure ISO TS 19713-1 & -2,” which was specifically developed to measure fractional size efficiencies of AIF. Part 1 deal with fine particle sizes (0.3╬╝m to 5╬╝m optical diameter) and part 2 deals with coarse particle sizes (5╬╝m to 40╬╝m optical diameters).

B) Water & Snow Exposure

Water Soak

There are OEM specified water tests for AIF to measure their robustness. These tests typically involve submerging the AIF in water, allowing it to soak for a few hours, followed by a steady or pulsating airflow at engine rated requirements. The filters are then either dried by airflow or in an oven, followed by a pressure drop test and filter performance test (DHC and efficiency).

Water Ingestion

Some OEMs prefer to spray water to the AIF simulating a rainstorm and measure the amount of water ingested on clean side. There is a standard test method, “SAE J2554,” that may be used to generate a high concentration of water droplets simulating a rainstorm environment. The function of the AIF during the water ingestion test is to keep water slugs/droplets from reaching the engine. Excessive water slugs in the engine can cause a “hydraulic lock” condition. The filter mainly acts as a barrier to break the water flow into a fine mist that goes into the engine. A certain amount of fine water mist may be acceptable and tolerated by the engine. Table 6 shows rain droplet sizes in different types of rainfall. Water ingestion tests can also be performed in a wind tunnel.

The air filter after water soak/ingestion test is evaluated for the following:

a. Tearing of media
b. Holes in the media
c. Pleat deformation, excessive waviness, collapse, buckling, fold-over, etc. (see examples in Figure 1)
d. Pleat bunching
e. Seal tear or ripping (polyurethane seal)
f. Deformation or bending of filter element (excessive pressure drop)
g. Support screen (clean side – metal or plastic) separating from pleated pack

Table 6: Typical rain droplet sizes & terminal velocities:

Drop Size Terminal Velocity
Rain Type mm in m/sec miles/hr.
Light Rain (.04” per hour)
Small Drop 0.5 0.02 2.06 4.6
Large Drop 2.0 .08 6.49 14.4
Moderate Rain (.25” per hour)
Small Drop 1.0 .04 4.03 8.9
Large Drop 2.6 .10 7.57 16.1
Heavy Thundershower (1.0” per hour)
Small Drop 1.2 .05 4.64 10.3
Large Drop 4.0 .16 8.83 19.6
Largest Possible Drop 5.0 .20 9.09 20.2
Hailstone 10 0.4 10.0 22.2
Hailstone 40 1.6 20.0 44.4


Figure 1: Examples of pleat deformation after water soak followed by exposure to rated flow.

Figure 1: Examples of pleat deformation after water soak followed by exposure to rated flow.

Snow Ingestion/Packing

Snow ingestion and packing is also a concern for AIS systems. OEMs have recommended vehicle-level snow ingestion tests. The AIF prevents the flaky snow from entering into the engine. The snow tends to pack around the AIF (dirty side) but is still porous enough for the air to pass through (although the engine loses some power). In a worst-case scenario, the snow packing is so dense that the engine is starved of air and the vehicle stalls. To prevent this condition, the AIS is designed with secondary air inlets and in some cases a heated wire mesh to melt some snow so air can pass into the engine. In the field, snow-packing tests are performed under very specific conditions with very low humidity levels. Snow, when wet, is not recommended for the test. Snow packing tests can also be performed in the wind tunnel. Figure 2 shows pictures of conical & panel AIF after a snow ingestion test. Observe how the snow packs around the filter element during the test. Basically the snow packing creates a mold of the air cleaner.

Figure 2: Typical Snow Ingestion / Packing Test

Figure 2: Typical Snow Ingestion / Packing Test

MAFS Performance

MAFS being one of the emissions components is assigned performance level targets that it needs to maintain in order to be robust. The MAFS performance targets are set by the vehicle calibration team. The AIF design has a significant impact on MAFS performance. How the air filter interacts with the air cleaner geometry to disturb the airflow has a significant effect on MAFS signal to noise (S/N) and MAFS (dQ/Q) variation (measurement error). The type of AIF interaction includes:

  • Clean filter
  • Blocked filter (OEM specified test)
  • Water soaked filter (OEM specified test)
  • Loaded filter (Test dust or road contaminants)

A typical MAFS S/N target level is ~2.5% to 3% when evaluating the whole AIS on a bench. Some years ago, the target was ~5%, however with increasing emissions requirements the targets are getting stringent. Similarly, a typical MAFS dQ/Q variation target is ~┬▒3% to ┬▒5%. Some Key AIF design parameters to control for optimal MAFS performance are briefly listed below.

1. Filter media (included pre-filter media) should be very uniform.
2. Filter media (included pre-filter media) should not shed any fibers and/or resins when exposed to flow.
3. Pleats have to be rigid and stable against pulsating flow conditions.
4. Uniform pleat spacing, embossing and pleat count control is mandatory (Figure 3).
5. Pleats should be aligned parallel to flow direction.
6. Glue beads on dirty or clean side (reduce pleat flutter) should be well controlled and straight (Figure 3).
7. Filter pleated pack should utilizing the full cross section/footprint of cover/tray is desired. Filter packs having air gap between pleat ends and tray (hanging seal – packaging driven to accommodate the air filter in the housing) may not be robust for MAFS performance (Figure 4).
8. Designing a filter support (using metal or plastic screen) generally improves MAFS performance. However depending on AIS design this may not always be true.
9. Design AIF to the recommended media face velocities. Higher media face velocities (sadly is the norm today) tend to create higher flow turbulence and impacts MAFS performance (especially S/N).
10. Blocked filter performance will depend on the AIS air cleaner design and is system specific. On occasion engineers have also evaluated dust-loaded filters (real world) on MAFS performance. Blocked filter tend to be a very severe test.
11. Filter sealing system should be robust to hold the filter in place and minimize leakage to acceptable limits as required by OEMs.
12. When using polyurethane sealing system, the blowing of polyurethane in the mold needs to be controlled (Figure 5).

Figure 3: An example of good pleats, pleat spacing and glue beads.

Figure 3: An example of good pleats, pleat spacing and glue beads.

Figure 4: Large air gaps between tray wall and pleated pack pleat ends should be avoided.

Figure 4: Large air gaps between tray wall and pleated pack pleat ends should be avoided.










Figure 5: Good control of polyurethane seal blowing during molding process.

Figure 5: Good control of polyurethane seal blowing during molding process.

Figure 6: Example of a typical AIS system showing key components for leakage.

Figure 6: Example of a typical AIS system showing key components for leakage.












Leakage tests are generally specified by the customer for the complete AIS. Leakage is measured on the clean side. The filter seal material and sealing system has a significant impact on meeting leakage requirements. Figure 6 shows potential air leakage paths on a typical AIS design. The biggest leakage contributors are the filter seal area (panel filters) and the attachment of the clean air tube to air cleaner cover. There are several types of leakage tests typically performed based on customer preference. The type or method of leakage test is generally specified in the customer SOR (Statement of Requirements) or ES (Engineering Specifications) requirements.

1. Vacuum decay test – leak down test (most difficult to meet)
2. Dust leakage test – generally performed in a dust chamber with flow)
3. Soap bubble test – system under vacuum
4. Soap bubble test – system under pressure

The leak down test is the most challenging and difficult to meet depending on test parameters. Very small leakage rates are measured in the range of ~0.05% to 0.012% of engine max flow rates at extreme temperature soaks.


About the Company

Toledo Molding and Die, Inc. (TMD) is a Full Service Tier 1 & 2 automotive supplier for Engine Powertrain and Vehicle interior systems and components. TMD designs, develops and manufactures Air Induction Systems, Full Instrument Panels, Vehicle Interior Plastics, Fluid Reservoirs and HVAC Ducts. TMD is fully integrated with injection molding, blow molding, tooling, prototypes and testing (accredited by A2LA) expertise.


For more information contact:
Neville Bugli, Engineering Manager, Core & Filtration Products
Toledo Molding & Die, Inc.
47912 Halyard Drive, Suite 106
Plymouth, MI 48170
Tel: 248-739-9627


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6. N. Bugli, “Service Life Expectations and Filtration Performance of Engine Air Cleaners”, Paper presented at the SAE Brazil International Congress, Sao Paulo, October 3 – 5, 2000. SAE technical paper # 2000-01-3317.
7. Willig J. T., “Designing urethane leak test seals,” Mechanical Engineering, December 1993, pp.78-80.