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 (constsnt 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 passess 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 auxilliary electric diven 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 auxilliary 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.
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.
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.
It can be note that improvements to efficiency can not be gained by altering the exhaust opening timing
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.
Constant pressure
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.
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;
Comparison of blading of the constant pressure and the pulse turbine
Timing variation due to supercharging
| 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.
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.
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, piework and downstream items such as exhaust gas boilers clean can lead to compressor surging
Two stage systems
Other types
Balanced system-variation on pulse
Divided Gas flow-split entry blower( allows more than three exhaust pulses per blower)
System layout
