Marty Barris
Donaldson Company, Inc.

Introduction

Over the course of the past two decades considerable attention has been paid to reducing the gaseous and particulate matter (PM) emissions from diesel engine tailpipes. This has resulted in reductions in emissions, especially since the pre-1991 time frame. As we look ahead to further significant emissions reductions based on the U.S. Environmental Protection Agency (EPA) standards set for 2004 and proposed for 2007-2011, we will eventually reach the point where other sources from the diesel engine will become more important when considering overall engine emission reductions.

One such source is the PM emissions from an engine's crankcase breather. Today, in most turbocharged, aftercooled diesel engines, the crankcase breather is vented to the atmosphere, often using a downward directed draft tube. While a rudimentary filter is often employed, substantial PM is released to the atmosphere. PM emissions as high as 0.7 g/bhp-hr can be found during idle conditions for today's modern engines. This exceeds the level of PM currently emitted from that same engine's tailpipe.

This article presents a perspective on the emissions issue regarding crankcase ventilation from diesel engines, and describes one means of providing a closed crankcase filtration system. The use of such a closed system virtually eliminates engine "blow-by" gases as an airborne PM emissions source.

Regulations

This source of emissions has not escaped the attention of engine manufacturers and emission regulators in other areas of the world. Certain European engine manufacturers are voluntarily implementing closed breather systems for on-highway and off-highway diesel engines, as a means to promote cleaner diesel operation. More closed systems were added in 2000 and expectations are for this to increase in the upcoming years. Emission regulations in Japan, published by JAMA (Japan Automobile Manufacturers Association, Inc.), require the use of closed systems starting in the year 2002. Other vehicle weight categories are added through the year 2004. The EPA has proposed closed crankcase engine operation for the 2007 regulations. The keys to regulation compliance are an understanding of the technical issues and having available a means of meeting the standards.

Crankcase Ventilation Issue

As described previously, nearly all turbocharged and aftercooled diesel engines vent their blow-by gases from their crankcase. The reciprocating movement of the pistons and the slight pressure leakage past the piston rings form these gases. As stated, these emissions can reach levels as high as 0.7 g/bhp-hr. Mass flow levels vary greatly by engine manufacturer, engine type, breather location, and power output. It is also a function of engine speed and load. Emissions rates determined in laboratory tests for several engines are summarized below. Blow-by mass flow rates can vary from nearly 20 g/hr to less than 1 g/hr, depending on the conditions mentioned above. In terms of a maximum tolerable level of blow-by mass flow, a desirable level specified by several engine OEMs is <0.5 g/hr. This reflects consideration of turbocharger and aftercooler fouling. Engines utilizing EGR systems may be even more susceptible to closed crankcase mass flow.

Table 1 lists ranges of mass and volume flows for rated and idle conditions of a typical modern heavy-duty diesel engine. The engine detailed below is a 1999 production design. Its blow-by volume and mass flow are functions of speed and load as indicated in Figure 1 and Figure 2.

Figure 1. Blow-by volume flow for a typical modern heavy-duty diesel engine

Figure 2. Blow-by mass flow for a typical modern heavy-duty diesel engine

Typical engines produce 140 to 300 l/min (5-11 cubic ft/min) of blow-by gas volume flow at rated conditions. This increases over an engine's life, due to piston ring and cylinder wear. It can reach levels as high as 1120 l/min (40 cubic ft/min). A general rule of thumb is approximately 0.5 l/min (1 cubic ft/hr) of blow-by per rated engine horsepower. Therefore, a 500 hp engine produces 0.5 times 500 or about 250 l/min (8-9 cubic ft/min) of blow-by. While this may appear small compared to the volume flow of intake or exhaust flow (only 0.4-0.8 percent), on a mass basis it represents a substantial portion of emitted particulate matter.

Blow-by Physical/Chemical Characterization

The emitted blow-by aerosol consists mainly of oil droplets, with some carbon and traces of wear debris and fugitive dust. All contents of the lube oil sump are likely to be represented in the emitted aerosol. Particle sizes range from 0.1 to 3 micrometers, with most of the mass distribution falling between 0.5 to 2 micrometers. Particle size distribution measurements place more than 90 percent of the particle number at less than 3 micrometers, 75 percent less than 2 micrometers, and 40 percent less than 1 micrometers. Such a size distribution means that this aerosol is highly respirable as defined by the American Congress of Government Industrial Hygienists' PM10 curve.

Closing the Loop

As discussed, closed loop routing and consumption of these blow-by gases can foul turbochargers and aftercoolers. This is not a large issue for non-turbocharged engines commonly used in automobiles, as PCV systems have been reliably used for many years. In turbocharged engines, such exposure may be tolerable when using straight-weight lube oil. However, its lubricating properties are inferior. The use of multi-weight oil provides superior lubrication, but its viscosity index modifiers (high molecular weight hydrocarbon compounds) can contribute to excessive fouling on compressor blades and on aftercooler surfaces. This results in a measurable loss of boost pressure. The operator is forced to increase engine load in an attempt to regain the desired power output, often resulting in a "lugged engine" condition. The engine's tailpipe emissions increase from this condition and as a result of higher intake manifold temperatures.

Crankcase Emissions Control Technology Option

One solution to this dilemma is a multi-stage filter system (Spiracle^TM) designed to collect, coalesce, and return the emitted lube oil to the engine's sump.

Figure 3. The Spiracle^TM multi-stage filter system

The filtered gases are then returned to the intake system, while balancing the differential pressures involved. The system thus consists of a filter housing and pressure regulator. A pressure relief valve and oil check valve completes the system.

Figure 4. Diagram of the multi-stage filter system process

Service lives of 500 to 1500 hours or more can be designed into the system. This results from optimizing the size of the system and service life goal with the aerosol generation rate and the separation efficiency requirements. After this period, its replaceable filter must be changed to maintain appropriate system pressures and filtration efficiency. Over the course of 500 hours, on the order of 2 liters (1/2 gallon) of oil are returned to the sump. The 2-stage filter's efficiency is generally >90 percent when new, falling to a final level of approximately 75-80 percent at the end of its useful life. It has been demonstrated to easily provide the <0.5 g/hr level that is acceptable to many engine manufacturers.

Conclusion

As efforts worldwide focus on reducing emissions from heavy-duty diesel engines, the need to address crankcase emissions is becoming increasingly apparent. Donaldson's multi-stage filter system technology represents one means of achieving that balance of dramatically reduced lube oil aerosol emissions without impacting engine performance or durability. This technology is being marketed this year for Euro III applications. It is anticipated that this system approach will also be available for applications in North America and Japan as a best available control technology.

For more details, contact Donaldson's Marty Barris at (952) 887-3436.