Diesel Engine Scavenge Process
Types of scavenging
There are two distinct types of scavenging arrangements;
The main difference between the two types is that uniflow requires an exhaust valve or piston to operate. Loop or cross flow relies on the piston to open and close exhaust ports
After ignition of the fuel the piston travels down the liner uncovering firstly the exhaust ports. The exhaust gas at a pressure above atmospheric is expelled. This is often referred to as blowdown and its effect can be seen on the power card for all the types of scavenging as a rapid drop in cylinder pressure towards the end of the cycle.
The method of loop scavenging is similar to the cross flow except the exhaust and scavenge ports may be found on the same side
The scavenge air enters through the scavenge ports in the lower part of the cylinder liner, the exhaust gas is expelled through the centrally mounted exhaust valve in the cylinder cover. The scavenge ports are angled to generating a rotational movement of the rising column of air.
Air is forced out of the cylinder by the rising piston leading to low flow resistance, the effect is often compared to squeezing the contents out of a tube.
Disadvantages of the loop/crossflow method of scavenging
The greatest disadvantage of this system, and the one that has led to the abandonment of its usage where once it was widespread is its inefficiency in clearing the cylinder of all combustion products. Following the exhaust blowdown the scavenge ports are opened. The period available for scavenging is limited to the recovering of the exhaust ports and is only at its most effective until the closing of the scavenge ports. Therefore, high air velocities are designed in, the air entering through steeply angled shaped ports. The possibility exists for the scavenge air to shortcut directly to the exhaust ports, a situation which worsens with blockage of the scavenge ports due to carbon build up. Due to the inefficiencies above there is a high volume of scavenge air requirement with this design. This has led to the complicated underpiston effect designs to augment the turboblower output with some engine power being absorbed dropping cycle efficiency.
A problem with having fixed ports is that difficulty is encountered with port timing. On the piston down stroke the exhaust port is opened followed by the scavenge port to make effective use of blowdown. However, the same timing for closing the ports means that the effective compression stroke is reduced. To try to remedy this differing means of closing the exhaust ports before the piston covered the ports was tried, One such method was by engine driven rotating valves which opened and closed the exhaust ports. All the designs increased complexity and often proved unreliable due to the arduous conditions they had to operate in.
To prevent exhaust gas entering the cylinder under the piston as the piston moves up to TDC, extended piston skirts are fitted. This adds to the reciprocating mass and increases load on the crosshead bearing. The small amount of side thrust not absorbed by the crosshead is spread over the larger of the skirt reducing loading and wear on the liner, however, problems of increased lubrication requirements for the increased surface area largely negate any advantage.
The requirement for both the exhaust and scavenge ports being fitted into the liner makes for a more complicated design with increased liner lubrication difficulties especially in way of the exhaust ports. This region suffering not only the washing away effect of the gas flow but also contamination from combustion products and increased temperature. Cylinder lubricating oil volume demand is therefore higher with this design.
In an attempt to improve the scavenge efficiency shaped pistons have been used which produce a combustion chamber shape not the best for efficient combustion. Asymmetrical piston designs can also lead to excessive thermal loading and complicated strengthening and cooling designs.
One advantage of the Loop method of scavenging is that it does away with the requirement for an exhaust valve or opposed piston. This means that all the extra running gear associated with this can be omitted. That means, simpler cylinder cover design, simpler and less stressed camshaft and camshaft drive train, and a shorter piston reducing reciprocating mass.
Where an exhaust valve is fitted in the cover, there is increase thermal stressing especially in way of the valve where higher temperatures are encountered.
Additional advantages of the Uniflow method
The fitting of an exhaust valve does give a major advantage in that the timing of opening and closing of the valve can be altered which is used to its fullest with modern designs with 'Variable exhaust timing' control fitted. This means that the effect of the scavenge air inertia entering the cylinder, and optimising the closing of the valve to increase the effective compression (which starts when the exhaust valve is closed) can all be taken into account for the varying loads and engine speeds.
The increased scavenging efficiency with this type of scavenging creates greater scope for increased economy and so all modern designs are based on this design. The opposed piston design once in favour due to its inherent dynamic load balancing has now largely disappeared due to its increased mechanical complexity.
There is a minimum air demand as the ingressing air pushes the combustion products ahead of it with little requirement for scavenging by dilution. As the air flow is symmetrical rising up the liner the thermal influencing on the liner walls, cylinder covers and piston crowns is also symmetrical. this allows simpler oil cooling of the piston crown
The reduced number of ports (no exhaust ports), and reduced size of the scavenge ports (due to reduced air volume requirements), this reduces the problems of liner lubrication allowing reduced oil consumption.
Oily carbon deposits build up in the scavenge trunking during normal operation of the engine. Drains are provided in order to remove such deposits and so help keep the scavenge trunking reasonably clean
Scavenge drains system
Under certain conditions the deposits may dry out and ignite. Piston blow past due to sticking or broken piston rings, or even excessive liner wear, is a major cause of the problem. Faulty combustion due to late injection or incorrect atomisation may also be responsible as may blow back through the scavenge ports caused by a restriction in the exhaust.
Some of the smaller bore RND engines has sighting ports fitted to the scavenge air space under the piston. The amount of blowpast could be seen and a close watch kept on the condition of any deposits.
On the occasions when the blowpast started to become excessive, slowing the engine down for a short period would settle the engine. Subsequent speeding often went with much reduced blowpast. In this way many potential fires where averted. Unfortunately these sigthting ports are no longer fitted.
In all cases the increased temperature leads to a drying out of the oily deposits in the air box and subsequently ignition takes place. A scavenge fire can cause serious damage to the piston rod diaphragm gland as well as leading to possible distortion of the air box and cracking of the liner. Tie rod tension will almost certainly be affected.
The worst case scenario for a scavenge fire is it leading to a crankcase explosion.( B&W on some designs fit a cooling jacket between the air box and crankcase to help prevent this.)
The fire may also spread outside the scavenge box due to relief doors leaking or oil deposits on the hot casing igniting. For these reasons a scavenge fire should be dealt with as quickly as possible.
B&W and other engine builders recommend that in the event of a fire the engine should be slowed as soon as possible and preferably stopped. The turning gear should be put in and the engine continuously turned to prevent seizure. Air supply should be cut off by enclosing the turbocharger inlets, for mechanically operated exhaust valves the gas side should also be operated, (hydraulically operated exhaust valves will self close after a few minutes). The individual isolating valves on the scavenge drains line are close to prevent the fire transferring to other boxes. Boundary cooling may be employed.
Fixed extinguishing mechanism should be used at the earliest possible time. When the fire is extinguished the air box is allowed to cool, then the air boxes should be well cleaned, the tie-rod tension checked. The causes of the fire should be ascertained and remedied.
Fire fighting media
Carbon dioxide- will put out a fire but supply is limited. Susceptible to loss if dampers do not effective prevent air flow
Dry powder- will cover the burning carbon and oil but is messy. As the fire may still smoulder below the powder care must be taken when the scavenge doors are removed as the powder layer may be blown away. Steam-plentiful and effective
Water spray- perhaps the ideal solution giving quick effective cooling effect to the fire.
Indications that a fire is imminent are a reduction in flow through the drains and a temperature rise in the scavenge. This temperature rise can be detected by temperature wires or mats.
When a fire is in progress there is a slow down of the engine with the blowers surging as the fire consumes the oxygen. Sparks are seen issuing from the drains and there is a temperature rise in the scavenge space.
Scavenge belt relief door
Fitted to both ends of the scavenge belt and set to lift slightly above the maximum normal working scavenge air pressure
For Combustion of a fuel, an adequate quantity of air is required. For a Turbocharger system capacity should be sufficient to ensure that the air demand is met when the turbocharger is not at its optimum.
In a four stroke diesel engine, this air is induced during a down stroke in one of the two engine cycles per power stroke. The exhaust gasses are removed by the preceding upstroke .
For a two stroke no such cycle for scavenging and air replenishment exists. Instead, air under pressure is supplied at the end of the power stroke providing a new charge of air and removing the exhaust gasses. The period allowed for scavenging is limited as the longer the exhaust port or valve remains open so the shorter the travel of piston is available for compression. The greater the mass of air that can be supplied, the more efficient the scavenging process will be, and also the greater mass of air will be available for the combustion of an equally greater mass of fuel. The mass of air is increased by increasing the pressure at which it is supplied.
Pressure charging can be obtained by a number of means including scavenge pumps, chain driven rotary blowers and exhaust gas driven blowers.
Exhaust gas driven blowers or Turbochargers make use of gas in the cylinder which theoretically could be expanded further, the power that would be developed could be used for driving an engine driven scavenge pump. In practice it is more efficient to use this exhaust gas in the turbocharger as further expansion of the gas would require an increased stroke. Increased stroke would mean increased engine height with problems of crankshaft construction, cylinder lubrication and effective scavenging coming into play. The work that could be extracted from this low pressure gas would be limited and more efficiently extracted in a rotary machine.
Centrifugal turbochargers are generally cheaper to produce than axial flow. In addition for smaller sized radial units the effects of blade leakage are less important They are very common in automotive systems were they are suited to the manufacture of large volumes of standard design. Axial flow may be selected even when there are centrifugal alternatives as it is better suited to individual modifications and is able to operate better on heavy fuels.
A turbocharger is made basically in two linked parts, the gas side and the air side.
The gas side is made out of cast iron, is in tow parts and is generally water cooled. The turbine inlet casing carries the nozzle blade shroud ring and forms the bearing housing. The turbine outlet casing forms the main part of the blower which includes the mountings. In addition it forms a shroud for the shaft and contains bled air passageways for supplying air to the labyrinths seals.
The air side casing is also in two parts but is made of aluminium alloy. The inlet casing may be arranged to draw air form the engine room or from the deck , both methods via a filter and silencer arrangement. The advantage of drawing air form outside the engine room is that it will tend to be cooler and less humid. An advantage of drawing from the engineroom would be simpler ducting arrangements and that the engine room tends to be slightly pressurised.
The main parts of the Compressor are the Compressor wheel (made up from a separate Inducer and Impeller on larger designs), the diffuser, and the air inlet and outlet casing.
With the wheel rotating a unit of air massing the compressor wheel experiences circumferential velocity (v)at its distance from the wheel centreline (radius r). A radial velocity is experienced of value v2/r which causes it to move radially outwards. The unit of air leaves the compressor with a resultant velocity the angle of incidence of which should, by careful design, match the inducer inlet angle. This leads to maximum compressor efficiency.
The effects of frictional losses, whether due to surface imperfections or fouling of the compressor wheel will result in changing the angle of incidence and thus a drop in efficiency
Takes place if the air mass delivered by the blower falls at a faster rate than the air pressure of delivery. With all blowers it is possible to produce a graph showing the effect. Surging gives an unpleasant noise. The initial action in order to prevent a blower surging is to reduce engine load. Blower efficiency is highest closer to the surge line and so if a high efficiency is demanded there is little leeway against surging. In practice the fitting of blowers is a compromise between a reasonable blower efficiency and an acceptable degree of safeguard against surging.
Surging is a condition whereby an imbalance in demand and supply of air from the turbocharger causes a rapid deceleration. This is accompanied by a loud barking noise and vibration. It was not uncommon on pulse systems in heavy weather, it is less prevalent in modern constant pressure designs but may begin due to reasons explained later.
The normal characteristic of a turbocharger running at constant speed is one of reducing possible pressure ratio for increasing air flow demands. This characteristic is exaggerated when frictional losses are taken into account. As described above from maximum efficiency the air leaving the compressor wheel should enter the inducer at an optimal angle. Failure to do so leads to losses and a characteristic shown. It should be noted that this shows a relationship at a specific instant of Turbocharger speed. It would be possible to plot many lines of constant speed on the graph. The point at which surging occurs could be plotted for each and a surge line drawn. Moving the plant operating line towards the surge line can lead to an increase in turbocharger efficiency.
The stable operating point is at A though which passes the respective engine operating line ( this line indicates the relationship the engine requires between Air flow and pressure), the unstable point leading to surging is at B.
If the air flow through the turbocharger reduces The effect would be a decrease in pressure at the receiver. However the pressure ratio of the turbocharger (running at constant speed) would Increase. The effect of this is to return the system back to its stable point A.
For an engine operating on the line passing through B then the effects of a reduced air flow will be a corresponding reduction in compressor pressure ratio. The engine however requires increased air flow which the turbocharger cannot supply and the result is surging. Theoretically this effect begins where the constant pressure line is flat.
Conditions leading to Surging
Turbochargers are generally specified in relation to set ambient operating conditions
and then matched to engine load requirements. Deviation away from this due to such
things as changes in ambient conditions and changes in engine speed/load relationship
has to be taken into account.
It is very unusual for a modern turbocharger to such. However surging may begin after several years of stable operation.
The above is a very simplified description of the operation of the compressor and how surging occurs. No doubt I will be receiving barrages of complaints from turbocharger specialist. In case you still cannot get it try to understand you are looking at a specific point in time and looks only at the turbocharger running at a constant speed
This may again be thought of two parts; the gas side and shaft and the air compressor side. They are usually made of two materials as the conditions that the wheels operate in is very different. The advantage of making the compressor end of a lighter aluminium alloy material rather than using the same material throughout, is that it reduces the total mass of the rotor , is more easily cast into intricate shapes, and the rotational inertia is reduced.
Must be capable of maintaining strength at high temperatures so material is usually a chromium steel. The rotor for a smaller blower may be a single piece forging but for a larger blower it may consist of two separate sections of shaft and turbine wheel with bolted connection.
The impeller is made of an aluminium alloy and for larger compressors may have a separate inducer section at the eye. Whatever the form of construction it must preserve the rotor balance and that means refitting in the same position after removal from the rotor. This is usually achieved by having one of the connection splines larger than the others.
The blades shown above are twisted and tapered to allow for the increased blade velocity with increased radius Blades must be capable of withstanding the high exhaust temperatures and also the highly corrosive environment of the exhaust gas. Stainless steel is frequently used. They are mounted axially in the disc using inverted fir tree root or similar e.g. 'T' piece or bulb roots. Locking strips are provided to prevent axial movement of the blades in the disc due to the axial gas force.
The blades are not force fit into the disc but are relatively loose.
The blades are made lose fit for the following reasons;
For larger blades lacing wires are used as a means of dampening vibration by the friction acting between the wire and the blade material at the hole. The wire is normally fitted about 1/3 of the way from the tip, it may pass through all blades or batches and is crimped to hold it in place. Dampening due to friction and stiffening up because of the connection of a number of blades avoid vibration.
The main problem with lacing wire, usually of wrought iron, is that it breaks and sections fall out resulting in an unbalanced rotor.
Balance of the rotor is essential in order to avoid vibration and blade damage due to impact, corrosion, erosion and deposit build up all cause problems.
Blade Wear and its affect of blower Speed
Most main engine turbochargers are water cooled in order to keep temperatures reasonable. On the most modern of turbochargers this cooling water has been reduced in quantity to that is required for cooling the bearings. The space between the compressor and turbine being filled with insulation material.
There are some smaller blower designs which by design can be cooled by air flow. As no cooling jacket is required it is convenient do place the bearings in between the turbine and compressor wheels. this allow for better rotor support. The larger blowers have the bearings placed at the coolest part of the charger, at the ends of the rotor within cooling jackets. This has the advantage of making them more readily accessible.
Plain white metal bearings may be used , these have an indefinite life but require lube oil to be supplied at pressure. they also require a header system to supply oil in the event of the main supply pump failure. A common system is by supplying from the main engine lube oil system via a header system similar to that employed with steam turbines.
Plain bearing Lube oil system
Care should be taken to ensure that the bearings are adequately protected when the engine is stopped as the blower is liable to turn due to natural draught (although modern engines having hydraulic exhaust valve actuation are not susceptible to this as the all valves close after a short period of inactivity). Locking the blower, isolating the blower from the scavenge belt by use of a slide valve, putting covers over the blower suction or continuation of supply of lube oil after engine stoppage may be used.
Ball or Roller bearings require elasto-hydrodynamic lubrication and may be supplied by means of a shaft driven gear pump from an integral sump. The gear pump is operated by rotation of the rotator. The bearing housing as a cooling water jacket.
ball and roller bearings have a definite life and must be changed on a running hours bases, typically every 15,000 Hrs. This means that they should be placed in a readily accessible position. The transmission of vibration is dampened out by the use of radial and axial springs between the bearing carrier and the casing.. These can consist of leaf springs wrapped around the bearing and fitted at the bearing ends.
An axial thrust is generated by the passage of the exhaust gas over the turbine . This must be balanced out . For turbochargers fitted with plain bearings a double-sided thrust is fitted at both ends. This takes the form of a collar on the rotor acting on white metalled 'Mitchell' type segments. Double-sided thrusts are fitted to locate the turbine during rolling and pitching. Generous oil quantities are supplied to bearings in order to allow for cooling as well as lubrication
These are provided at each ends of the rotor and between the turbine and compressor and serve to prevent the passage of exhaust gas and also to prevent oil laden air being drawn into the eye of the impeller from the bearing. Oil seals in the from of thrower plates are also fitted at the bearings to prevent the passage of oil along the shaft.
Labyrinth seals consist of projections on the rotor which almost touch the casing.
Principle of the Labyrinth Gland
The leakage of steam is reduced by the use of labyrinths, these provide a torturous path for the gas to follow to exit the turbine reducing the pressure across a series of fine clearances
Within the cavity where the flow is turbulent, the velocity of the gas is increased with an associated drop in pressure. The kinetic energy is the dissipated by the change in direction, turbulence and eddy currents.
Air is bled from the compressor end into the middle of the Turbine glands, this air expands in both directions and provides a very effective seal. The flow of air in the centre gland also aids cooling and minimises the heat transmission form the turbine wheel.
Care must be taken to ensure that deposits do nit build up in the seals otherwise its effectiveness is lost. Also there is a possibility of 'rub' occurring
Timing of scavenging on ported liner on two stroke slow speed
The scavenge and exhaust period can be divided into three periods starting as the piston travels down the cylinder and uncovers first the exhaust port followed by the scavenge air port.
1, Blowdown-Exhaust port is open and cylinder pressure falls to or below the scavenge
2, Scavenge- Incoming air forces the exhaust gasses and any unburned fuel out
3, Post scavenge- Exhaust only is open, some air is lost during this period. For ported exhausts this is unavoidable due to design of the liner with the exhaust ports above the scavenge. Some loss of compression therefore occurs, on the Sulzer RD an attempt was made to limit the blowdown by the use of a rotary exhaust valve. This proved very unreliable and was omitted on the later RND.
Modern Slow Speeds make use of exhaust valves, and with the most modern the exhaust valve timing is variable dependant on load and to some point fuel type.
Blower corrosion can take place on the gas, water or air sides. As most water cooled blowers make use o the engine cooling systems the same problems and solutions exist as in the jacket water system. In general with modern systems there are few problems if treatment quality and quantity is maintained. On the gas side deposits depend upon the quality of fuel and combustion. Carbon from poor combustion, sulphur products from the fuel, Vanadium Pentoxide from the fuel and Calcium Sulphate from the alkaline additives in cylinder oil all result in deposits and/or corrosion. Correct attention to operating conditions and matching of cylinder oil alkalinity to sulphur content will minimise the problem. Pitting corrosion and scale formation will lead to imbalance. On the air side there is a lesser risk but pitting oxidation of aluminium can take place in the prescience of salt spray. If air is taken from the deck there is greater risk than if it is drawn from the engine room because the oil mist in the engine room causes a protective film to form on the aluminium surface. Regular cleaning of parts is essential to maintain efficiency, minimise corrosion and ensure balance.
Out of service cleaning is relatively straight forward but requires the blower to be stripped down and time might not allow that. Light deposits on the air side may be easily wiped away, but gas side deposits require the rotor and nozzle blades to be 'boiled' for about 12 hours in clean water or water containing chemical; care must be taken in handling chemicals and 'special shipboard mixtures' should be avoided as they can be highly corrosive resulting in damage tot he rotor. In service cleaning provides an alternative.
For the airside this usually consists of injecting a limited quantity of water into the eye of the impeller, the water droplets then wipe the oily film from the surface but often deposits this on the cooler from where it must also be removed. If heavy deposits do form on the impeller and volute and then the risk of surging will increase. The usual in service cleaning method for blower gas side employs water but it is also possible to make use of ground rice or walnut shells, Whichever method is used care must be exercised.
In service water washing of the gas side requires the blower speed to be reduced to half or below ( 3000 rpm for a medium sized slow speed ), in order to avoid impact damage by the water droplets
The casing drain must be open and known to be clear. Water is injected via an air atomiser nozzle into the gas flow. The flow rate is controlled by means of a pressure gauge and orifice plate. The basic principle is that the water droplets impinging on the blades has a shot blasting effect. Observation of the water flowing from the drains will indicate when sufficient water has been injected. On completion the blower speed should be increased gradually to prevent thermal shocking, ensure all the water in the gas side casing is removed , and to prevent damage due to any unbalance caused by partially dislodged deposits.
The injection of nutshells and rice can take place at full load.
Modern trends in Turbocharger design for Large slow speeds
With the search for ever increasing plant efficiency and power/size ratios, greater demands are made of the Turbocharger. Some manufactures have answered this by the use of totally water free blowers , these are fitted with plain bearings and supplied from the main engine lubrication system.
Running the aluminium alloy impeller above the aging temperature (190-200oC) threatens a reduction in material strength. This temperature can easily be reached at pressure ratio is of 3.7 and above depending on suction air temperatures. Progressive creep deformation can occur above 160oC requiring careful consideration of stress on the blades. ABB turbos have available an aluminium compressor with a pressure ratio of 4.6 units new designs.
For higher pressure ratios stainless steel or Titanium is used where pressure ratios of up to 5.2 have been possible.
A typical modern design has plain bearings supplied by oil from the main lubrication systems or from a dedicated external system. The casing is entirely uncooled relying instead on the lubrication oil to be splashed around the generously sized bearing space to cool the areas adjacent to the bearings
Variable geometry nozzle rings are available which adjust blade angles depending on load.
The blades are high chord (thick section) meaning that lacing wires can be omitted. Special attention has to be made on the shaft fit arrangement alloyed aluminium compressor wheel as the rotational speeds of 500 m/s create high centrifugal stresses. The number of blades in the volute is matched to the number of blades on the compressor to reduce noise
The thrust bearing which is subjected to high loading is mounted outside the radial bearing on the compressor end for ease of maintenance
The idea of supplying air under pressure to the engine dates back to Dr Rudolf Diesel in 1896. The use of a turbocharger to achieve this dates to 1925 to work on 'pulse system' carried out by A.Buschi. This system feeds exhaust gas through small diameter pipes to the turbocharger turbine. Cylinders whose timing and firing order means there is no effect on each others scavengeing can be connected into a single entry
In the early days turbocharger efficiency was not high enough to supply the scavenging needs of two stroke engines and engine driven scavenge pumps had to be used
Shown above is a constant pressure turbocharger in series with a scavenge pump. This system saw use in Gotaverken engines aound 1970. The scavenge pump is a double acting LP air compressors driven either by the main engine crosshead through levers or by an additional crank section in the main crankshaft. There capacity is about 1.5 times the swept volume and they absorb about 5% of the engine power.
They have a low mechanical efficiency and increase the length of the engine.
A rotary compressor of the positive displacement type (Rootes blower) is directly connected to the engine by chain drive. It absorbs about 6% of the power and increases the charge air pressure by about 0.3 bar.
Gas driven blowers rely on the available enthalpy of the gas so there operation is very much load dependent. They absorb no power from the engine but slightly effect operation by causing a back pressure on the exhaust.
The pulse (constant volume) system was employed at the same time by B&W. The advantageous of this system is that no scavenge assist is needed at part loads. The disadvantage is reduced turbocharger efficiency as the blade and nozzle angles have to be a compromise because of the varying gas velocity and pressure at inlet.
Shown above is a variation on the scavenge piston assisted engine. In this the underside of the piston is used as the scavenge pump. In full load conditions air passes through the cooler, into camber A, the pressurerised air then passes through the non-return valves into chamber B, additional compression as the piston travels down the liner occurs and the air then passes onto the scavenge manifold indicated by C. In low load conditions the auxiliary electric driven blower operates drawing air from A and passing it out via non return valves in chamber D.
The use of uniflow scavenging and smaller (and thus with a lower rotational inertia) high efficiency turbochargers has meant that the requirement for energy sapping underpiston effect is negated
This is the modern layout with an auxiliary blower assisting the turbocharger at lower loads. In addition starting air may be supplied to the larger sized blowers to start them turning when the engine is first started.
Impulse( pulse,blowdown,constant volume)
This is the oldest turbocharger system for marine diesels, and can trace its lineage back to the conversion from mechanically driven to exhaust gas driven superchargers.
In the impulse system, the exhausts from each cylinder is led to the turbocharger via short length small diameter pipes. The small volume of this pipe means there is a large variation in exhaust gas pressure over the cycle.
Best use is made of the high temperature and pressure gas during the blowdown period. On opening of the exhaust valve this hot gas is expelled forcing the cooler gases through the turbine, following this initial surge the scavenged gas being forced out by the scavenge air is of lower pressure and temperature. There is therefore, a wide variation in conditions of the gas at entry to the turboblower. Due to the restricted outlet of the exhaust, the exhaust valve must be opened relatively early to ensure that the pressure has fallen sufficiently before the scavenging air is introduced.
By connecting three cylinders to each blower a 10% increase in the useful energy is extracted form the gases. It is essential that the pulse do not interfere with each other and hence careful attention has to be paid to firing order (which can lead to problems of torsional vibrations on the crankshaft). The graph above shows the ideal where three cylinders are led to the turboblower, the cylinders are firing 120o apart, and the exhaust valve is open for a longer period leading to overlap. Often complicated exhaust geometry is required to ensure maximum efficiency.
The main advantage of this system is that best use is made of the available energy from the exhaust gas at part load to a point that auxiliary blowers of any sort are usually omitted except where fitted for emergency use. The system also responds rapidly to load changes
Due to the fact that maximum efficiency occurs when only three cylinders are connected to a turboblower, several blowers are required for multiple cylinder plants. This leads to increased cost and maintenance requirements. Where four cylinders are fed to a turboblower, it is normal to use 'Split entry' where the cylinders are split into pairs each feeding a separate inlet and nozzle chamber in the blower. Where say five blowers feed two turboblowers the central cylinder gases are split feeding both blowers simultaneously. This system is often called the 'Balanced' system as the turboblowers are kept the same size rather than having a large and small blower being fed by three and two cylinders respectively, so reducing spare gear requirements.
There are three main points of note with this system;
The energy available at the turbine can be increased by 2% by advancing the point of exhaust port/valve opening by one degree. Therefore it is essential that exhaust valves should open as rapidly as possible
As the exhaust gas pressure drop between the cylinder and the exhaust pipe increases, so throttling losses increase. So the manifold is kept as small as possible.
As gas pressure builds up in the pipe so the period of blowdown is increased.
Tuned system-This refers to a pulse system designed to reduce air loss from the cylinder when the scavenge air valves/ports are closed and the exhaust valve still open. On opening of an adjacent cylinder the blowdown causes a shock wave ahead of it, by careful attention to pipe geometry and timing this shock wave can be utilised in pushing back the scavenge air into the cylinder without the exhaust gas from the adjacent cylinder actually entering the scavenged cylinder before the exhaust valve shuts.
The pulses of the exhaust gas are evened out by leading the gases into a volume chamber. Therefore the gases enter the turbine with little fluctuation in temperature and pressure. The exhaust manifold must be well insulated and the turboblower can be sighted at any convenient location. Also the pipework leading from the exhaust valves can be very much simplified so reducing costs
Due to the stability of the conditions of the gas entering the turbine, the blades and blade angles can be optimised for maximum efficiency. Due to this, a constant pressure system can offer about a 5% increase in efficiency over a pulse system fitted engine, also due to the increased efficiency of the blower more energy is available at outlet from the blower for utilisation in the waste heat recovery or power turbine. The stability in the gas flow has an added advantage in that the loading on the rotating parts and the bearings is reduced.
Another reason why constant pressure systems are more efficient is that exhaust valve opening can be made later in the stroke as the high pressure blowdown of the exhaust gas is not required and also there is less resistance to the outflow of the exhaust gas. These effects can be shown on the working diagram;
It can be seen from the diagram that more work is available with the constant pressure process as indicate by the shaded area. Scavenging will take place at a higher pressure for the constant pressure process, however, compression work is reduced as indicated by the lower curve as the cylinder has been purged with the scavenging air longer and so the charge is at a lower temperature. Due to this reduced temperature of the charge, the final compression pressure is lower. This means that fuel injection can be retarded so increasing the pressure rise for the same peak pressure. As the period of this pressure rise is reduced so thermal efficiency is increased.
The main disadvantage of this system is that some means of assisting the scavenging is required at part load. This normally takes the form of an electrically driven blower which is sighted in the scavenge manifold. This blower draws air in over the turboblower compressor and compresses it discharging directly in the scavenge manifold. Drawing air in over the turboblower assists with inertia. Another disadvantage is the possibility of back leakage into the cylinder under low load conditions.
Although it would be possible to design a constant pressure system using only one Turboblower, more generally more than one is fitted for the following reasons;
It can be note that improvements to efficiency can not be gained by altering the exhaust opening timing
Comparison of blading of the constant pressure and the pulse turbine
It can be seen that the blades of the pulse turbine have a very pronounced 'Bull nose' which is required to cope with the varying relative gas inlet angles caused by the varying gas speed at outlet from the nozzle. The blade is also made thicker to cope with the shock loading at each pressure pulse.
Timing variation due to supercharging
Unsupercharged four stroke
Air inlet opens 25o BTDC
Air inlet closes 25o ATDC
Exhaust opens 42o BBDC
Exhaust closes TDC Overlap occurs at the beginning of the cycle with exhaust open until TDC and air inlet open from 25 degrees before this, allowing cooling of crown and exhaust valve
Supercharged four stroke
Air inlet opens 55o BTDC
Air inlet closes 28o ATDC
Exhaust opens 42o BBDC
Exhaust closes 35o ATDC
Valve overlap has increased to 90o , this allows increased cooling effect on the crown and exhaust valves necessary with the increase in fuel burnt.
Two stage systems
The purpose of this system is an attempt to utilise the advantages of both systems. Although the inherent disadvantages are also present. The turbochargers are of different sizes because of different flow rates and pressure levels
Another purpose for two stage systems is the production of scavenge air at an increased pressure. For a single unit there is a limit to the maximum pressure that can be generated governed by the work that can be recovered from the exhaust gas and the volume of air required for scavenging i.e. for a set volume of air, the required energy to compress increases with final pressure.
Greater scavenge pressures allow for increased mass of air for combustion for the same bore/stroke, hence more fuel can be burnt more efficiently.
A second method of two stage Turbocharging involves two turbines mounted within the same casing. Both the air and exhaust gas flows through the turbines in series. The second stage turbine has larger blades to cope with the expanded gas. The compressor attached to the second stage turbine acts as the first stage of air compression. An intercooler is fitted between the stages.
Balanced system-variation on pulse
Divided Gas flow-split entry blower( allows more than three exhaust pulses per blower)
benefits in improved efficiency can be gained by reducing the number of bends in the pipework associated with the turboblower. A diffuser considerably reduces the pressure loss associated with the two bends. Straight pipes avoid the pressure loss due to bends.
Failure to ensure filters, blower components, pipework and downstream items such as exhaust gas boilers clean can lead to compressor surging
By increasing the pressure of the air the density is also increased. This allows more fuel per cycle stroke to be burnt increasing the power of the engine per unit size.
Associated with the increase in pressure of air passing through the compressor of the turbocharger is an increase in temperature.
From the general gas equation;
p.v = m.R.T
p = pressure
v = volume
m = mass
R = gas constant
T = absolute temperature
m/v = p /R.T
It can be seen that should the should the absolute temperature increase at the same rate as the pressure then the effects of compression is negated. Air coolers are therefore used, these are situated down stream from the compressor, before the air enters the scavenging ports.
The coolant is generally sea water circulated through finned tubes, the cross section of which may be of various shapes including round and oval. The thin fins are normally soldered on to the tubes but if the tubes are round they may be expanded into the fins. Flat sided an oval tubes are soldered into the tube plates whilst round tubes are expanded. An allowance must be made for expansion, in some cases the tubes are formed into a 'U'-shape, in other one end of the tube is fixed, the other end sits in a floating tube plate able to move in the casing and sealed by o-rings.
An ultimate limit is placed on the degree of cooling, this being to prevent thermal shocking especially of the piston crown, prevent excessively increasing cylinder lube oil viscosity and keeping above the dew point.
At the dew point water droplets will form which can scour the cylinder lub oil of the liner walls. Where engines operate close to or below the dew point then water separators are fitted downstream of the cooler. Grid type separators consisting of a pair of angled blades and relying on the higher inertia of the water droplets can remove upto 85% of the water.
Air cooler grid type water separator