Crankcase ventilation
The crankcase ventilation ( KGE ) is a device in reciprocating piston engines that ensures orderly pressure conditions in the crankcase in relation to the surrounding atmosphere. Another task is the collection and environmentally friendly removal of the leak gases that arise in the engine.
Tasks and functionality
In piston engines with a closed crankcase, there are deviations from atmospheric pressure not only in the working spaces but also below the pistons. These are due, on the one hand, to the volume changes caused by the running pistons (especially in single and two-cylinder engines) and, on the other hand, to the gases from the work process that collect in the crankcase.
In internal combustion engines , these occur in the crankcase basically blow to gases. Since this forms a closed space, the pressure would rise steadily without venting. To avoid this, the blowby gases, which contain combustion products and unburned hydrocarbons, are discharged from the crankcase. The ideal relative crankcase pressure is in the slightly negative range around - 2 mbar , since under these conditions the engine does not tend to "exude" lubricating oil. If the negative pressure is significantly higher (the value is engine-specific and depends on the design of the sealing compound), there is a risk that air contaminated with dirt particles will be sucked in via the shaft seals and seals on the crankcase. That would lead to increased wear on internal components. When bleeding, oil droplets that are generated by rotating components are inevitably entrained from the crankcase. The oil content in the discharged gas depends heavily on the mean pressure, the pairing of the cylinder, piston ring and liner, and on the extraction point (position and cross section) in the engine.
There are different versions of the crankcase ventilation:
Closed crankcase ventilation
With closed crankcase ventilation, the blowby gases are introduced into the engine's intake system via a ventilation line. The negative pressure in the intake tract also creates a negative pressure in the crankcase in most operating states. When the engine is charged, it is initiated before the turbocharger . The gases from the crankcase are also sucked in. As a rule, the crankcase pressure is set automatically by a pressure control valve, but sometimes also manually by means of a throttle valve / ball valve in large engines in stationary operation. This version has the following advantages and disadvantages:
| advantages | disadvantage |
|---|---|
| Increase in efficiency by recycling unburned hydrocarbons | Without or with an inadequate oil mist separator : Contamination of the intake tract ( LMM , throttle valve, valves), particularly problematic in turbocharged engines with charge air cooling , since the efficiency of these components is severely limited by the build-up of deposits / contamination => performance losses |
| Environmental benefits by reducing overall engine emissions | Without or with an inadequate oil mist separator: poisoning and clogging of the exhaust gas aftertreatment systems (in the case of CHP units also clogging of the exhaust gas heat exchanger) due to oil additives and increased oil ash formation |
| When using oil mist separators, an exhaust fan is usually not necessary | With insufficient oil mist separator: too high crankcase pressure |
| Clean engine compartment | Without or with an inadequate oil mist separator: Increase in the tendency to knock , as oil mist can considerably reduce knock resistance |
| Reduction of oil consumption |
The requirements for oil mist separation are very high so that optimal function and longevity of the components is guaranteed in modern and especially turbocharged engines. In passenger car engines, oil mist separation systems integrated into the cylinder head cover are often used according to the impact and / or cyclone separation principle. These offer relatively low separation rates and often cause problems. That is why centrifugal separators , which are driven electrically or via oil pressure, are now sometimes used. A disadvantage of centrifugal separators is that they hardly separate oil droplets smaller than 1 µm. Through downsizing and ever higher charging an increase in the resulting mean pressure . Increasing mean effective pressure and an increase in oil temperature serve to increase efficiency and thus reduce fuel consumption. However, these also lead to a reduction in the size of the oil droplets and the proportion of fine oil droplets increases sharply. With the appropriate design, filter systems based on coalescence filters offer the best separation rates. Oil particles up to 0.1 µm (this is also the detection limit for oil particles) can be separated. They also offer the advantage that they work optimally regardless of the operating conditions of the engine. With stationary operated gas engines for CHP applications, the requirements for the oil mist separation of the blowby gases are highest. The service life of the gas engine is over 40,000 operating hours (for comparison: a car engine is designed for a service life of around 5000 operating hours). Due to the design for maximum efficiency at the full load point, even the smallest deposits on the compressor wheel and housing of the turbocharger can lead to significant losses. As a result, the engine no longer reaches full load and the overall efficiency of the CHP is considerably reduced. MWM and MTU Friedrichshafen , among others, use highly efficient oil mist separators based on coalescence filters in engines with closed crankcase ventilation in their stationary gas engines. Here, residual oil quantities of <1.0 mg / m³ and degrees of separation over 99.9% are achieved during the entire service life of the filter elements (up to around 8,000 hours).
Open crankcase ventilation
As the name suggests, here the blowby gases are discharged from the crankcase into the atmosphere. This version is particularly widespread in shipping and in countries with low environmental regulations. Since the use of an oil mist separator is not absolutely necessary here, this version can be made very inexpensive. Problems in engine operation usually do not arise, even without an oil mist separator. The positive crankcase pressure due to the lack of suction and the resulting oil leakage are accepted. Since the implementation of the closed crankcase ventilation - due to inadequate or incorrectly designed oil mist separators - often leads to problems, it is not uncommon for a conversion to open crankcase ventilation to be carried out. In many cases this is illegal. The biggest disadvantage of open crankcase ventilation is the associated pollution . Another disadvantage is the need to use a regulated suction fan in conjunction with an oil mist separator, since the costs for such a system are quite high. The fan is required to overcome the pressure loss inherent in an oil mist separator.
Environmental aspects
For reasons of environmental protection, a closed crankcase ventilation has been required by law for car engines for a long time. The manufacturers of truck engines are also increasingly focusing on environmental protection and thus on closed crankcase systems. However, this is often not the case in other internal combustion engine applications (e.g., marine, power generation, off-highway, etc.). The use of an oil mist separator in open versions is not yet legally regulated in all areas of application. As a result, engine oil can escape unhindered into the atmosphere. Shipping accounts for a particularly high proportion of this output. A cruise ship z. B. emits around 700 liters of oil per year through the crankcase ventilation of the engines for propulsion and power generation. These then enter the sea unhindered. The closed crankcase ventilation is the best solution from an environmental point of view. All pollutants from the crankcase are fed into the combustion process and automatically included in the overall emissions. With highly efficient oil mist separation (residual oil quantities <5.0 mg / m³), the oil emissions can be reduced by up to 99.9% even in engines with open crankcase ventilation. One example of this are the gas engines from Rolls-Royce Marine, some of which are supplied with such oil mist separators.
Quality aspects
The crankcase ventilation attracted public attention when in 2003 engine damage to the VW Lupo and the largely identical Seat Arosa occurred due to icing in the crankcase ventilation in winter. To avoid this, the ventilation lines are heated in the affected car models.
Under unfavorable circumstances (e.g. bearing damage, combustion misfires, piston ring breakage, too high an oil level, too high a speed), blowby gases can be flammable and lead to a crankcase explosion or a continuous diesel engine .
literature
- Durst, Michael / Klein, Gunnar M: Filtration in Vehicles, Chapter 17: Crankcase Ventilation. 1st edition 2006, expert Verlag, ISBN 978-3-8169-2660-3
- Richard van Basshuysen, Fred Schäfer (Ed.): Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives; Chapter 7.7: Crankcase ventilation. 3. Edition. Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden 2005, ISBN 3-528-23933-6
Individual evidence
- ↑ Marcus Efler: CAR: caught ice cold. In: Focus Online . January 20, 2003, accessed October 14, 2018 .