The present invention relates to a separator. In particular, the present invention relates to a separator for separating particulate, liquid and aerosol contaminants from a fluid stream. Certain embodiments of the present invention relate to a separator for separating particulate, liquid and aerosol contaminants from a blow-by gas stream within a reciprocating engine. Separators in accordance with particular embodiments of the present invention incorporate mechanisms for regulating the pressure within a crankcase ventilation system. Embodiments of the present invention provide a pump assisted integral separator and regulator suitable for use in a crankcase ventilation system.
Blow-by gas within a reciprocating engine is generated as a by-product of the combustion process. During combustion, some of the mixture of combustion gases escape past piston rings or other seals and enter the engine crankcase outside of the pistons. The term “blow-by” refers to the fact that the gas has blown past the piston seals. The flow level of blow-by gas is dependent upon several factors, for example the engine displacement, the effectiveness of the piston cylinder seals and the power output of the engine. Blow-by gas typically has the following components: oil (as both a liquid and an aerosol, with aerosol droplets in the range 0.1 μm to 10 μm), soot particles, nitrous oxides (NOx), hydrocarbons (both gaseous hydrocarbons and gaseous aldehydes), carbon monoxide, carbon dioxide, oxygen, water and other gaseous air components.
If blow-by gas is retained within a crankcase with no outlet the pressure within the crankcase rises until the pressure is relieved by leakage of crankcase oil elsewhere within the engine, for example at the crankcase seals, dipstick seals or turbocharger seals. Such a leak may result in damage to the engine.
In order to prevent such damage, and excessive loss of oil, it is known to provide an outlet valve that allows the blow-by gas to be vented to the atmosphere. However, with increasing environmental awareness generally, and within the motor industry in particular, it is becoming unacceptable to allow blow-by gas to be vented to atmosphere due to the discharge of oil and other contaminants from within the crankcase. Furthermore, such venting increases the speed at which crankcase oil is consumed.
Consequently, it is known to filter the blow-by gas. The filtered blow-by gas may then be vented to the atmosphere as before (in an open loop system). Separated oil is returned to the sump via a drain hose. The blow-by gas may pass through a filtering medium or another known form of gas contaminant separator to remove oil, soot and other contaminants to protect engine components from fouling and any resultant reduction in performance or failure of a component. In order to avoid unacceptably high engine crankcase pressures, such a separator must not have a flow pressure differential higher than an allowable limit which is defined by the engine manufacturer in order to avoid oil leakage from the engine crankcase and other seals. Typically an upper limit of between 5 mbar and 50 mbar is set.
By returning the cleaned blow-by gas to the air intake of an engine (to form a closed loop system) it is ensured that no oil aerosols remaining after separation are vented to atmosphere. For such systems (known as Closed Crankcase Ventilation systems) the small vacuum created by the engine air intake results in the requirement for a separate pressure regulator to prevent negative pressures being translated to engine at some transient speed and load conditions.
Where cleaned blow-by gas is returned to the air intake of an engine via a turbo-charger system it is necessary to comply with the specifications for how clean the air must be from the turbo-charger manufacturer. For instance, a typical maximum oil contamination rate for turbo-chargers is 0.2 g per hour. This requirement can further increase the required separation efficiency.
The maximum gravimetric efficiency of known separators having a pressure differential within the range defined by either an open or a closed crankcase ventilation system have been measured and are known by those in the industry. Generally 70%-80% of oil aerosols can be removed by mass. The application of two separators in series, each utilising a portion of the available pressure differential has been found to yield no significant improvement in overall efficiency.
There is an increasing demand for higher separation efficiency in both open and closed loop systems. For instance, an overall oil separation efficiency of greater than 98% measured by mass (gravimetric) for particles collected using an absolute measurement filter is required by many engine manufacturers. Utilising state of the art equipment, the fractional efficiency (that is, the separation performance of the device at any given particle size) can be measured for particle sizes larger than around 0.03 μm. The particle challenge characteristics of the engine (that is, the fractional makeup of the contaminants) can similarly be measured. In some cases an efficiency requirement is given for specific particle sizes as small as 0.2 μm, which may be as high as 85%. Furthermore, emissions legislation in Europe and the US are incrementally increasing the required separation efficiency such that it will soon be necessary to achieve 99% gravimetric separation efficiencies.
Separation using filter mediums is undesirable as such filters have a finite lifespan before they become clogged and must be replaced. Engine manufacturers and end users in general prefer to only use engine components that can remain in place for the life of the engine. While fit for life separators are known, typically only powered centrifugal separators and electrostatic precipitators have hitherto been able to achieve the required levels of separation efficiency. Such separators are costly to manufacture, consume electrical power, or have moving parts which may be prone to wear. Low cost, fit for life impactor separators (where separation occurs as a contaminated gas stream is incident upon an impactor plate transverse to the gas flow) are not usually able to achieve the required separation efficiency. Impactor separators are also referred to in the art as inertial gas-liquid impactor separators. It is known to use inertial gas-liquid impactor separators in both open and closed crankcase ventilation systems. Contaminants are removed from the fluid stream by accelerating the fluid to a high velocity through a slit, nozzle or other orifice and directing the fluid stream against an impactor plate to cause a sharp directional change.
WO-2009/037496-A2 in the name Parker Hannifin (UK) Ltd discloses a separator for separating contaminants from a fluid stream. The separator comprises: a chamber, a first inlet for receiving a first fluid stream, the first inlet having a convergent nozzle for accelerating the first fluid stream and a second inlet for receiving a second fluid stream including entrained contaminants, for instance blow-by gas. The second inlet is arranged relative to the first inlet such that the first fluid stream can entrain and accelerate the second fluid stream forming a combined fluid stream within the chamber. A surface is coupled to the chamber and arranged such that the surface can cause a deviation in the course of the combined fluid stream incident upon it such that contaminants are separated from the combined fluid stream.
According to this known form of separator, contaminants can be removed from a fluid stream to a high level of efficiency without the need for driven or moving parts. The separator is suitable for separating contaminants from a gas stream such as a blow-by gas stream derived from an internal combustion engine. The first fluid stream may be derived from a turbo compressor or other source of compressed air within a vehicle engine and serves to draw the blow-by gas from the crankcase of an engine. The first fluid stream forms an area of reduced pressure in the chamber which draws in the blow-by gas. Such a separator may be a fit for life separator owing to the absence of moving parts that may fail or filter mediums that would be prone to clogging and require frequent replacement.
For separators having an impaction surface arranged to cause separation by deflecting the fluid stream, the separation efficiency can be increased by providing a nozzle through which the fluid stream passes. The nozzle causes the fluid stream to be accelerated such that the fluid stream is incident upon the impaction surface at a higher velocity. It is desirable to apply a nozzle with the smallest possible cross sectional area in order to achieve the highest velocity and separation efficiencies. An undesirable consequence of this is that there is a higher pressure drop created across the separator. In order to prevent the crankcase pressure increasing to unacceptable levels, the minimum size of the nozzle and consequently the performance of the separator is limited. To control crankcase pressure within acceptable limits a pressure regulator must also be added either upstream or downstream of the separator.
Such inertial separators as described above, having fixed section nozzles produce an air-stream having a uniform velocity across the impactor face. Due to the difference in inertia of different sized particles a characteristic fractional separation efficiency profile results with the smallest particles having significantly lower chances of successful separation compared to larger and heavier particles.
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