A turbo charger (sometimes called a turbo blower) can be fitted to both two and four stroke engines to increase the volumetric efficiency and thus their power output.
The advantage of a turbo charger is that fuel consumption is lower than that of a normally aspirated engine of the same power output.
In addition, the turbo charger utilises the exhaust gases of the engine so no power from the engine is required to drive it.
The turbo charger causes a larger mass of air into the cylinder to that of a same cubic capacity normally aspirated engine. This allows for a proportional increase in the amount of fuel that can be injected and burnt in the cylinder thereby providing an increase in the power output of the engine.
Components of a turbo charger
The components of a turbo charger are shown below.
It has a rotor shaft which has exhaust gas turbine blades on one end and air compressor blades on the other end.
The exhaust gas turbine blades are housed in a casing which is attached to the exhaust manifold and to the exhaust pipe. Some casings are fresh water cooled to minimise the heat radiated out into the engine space. This allows for a cooler engine space, cooler air entering the engine air intake and therefore more power again. A nozzle ring is fitted inside the casing to direct the flow of exhaust gases to the turbine blades.
The air compressor blades are also housed in a casing which has an air cleaner on the intake side and is connected to the intake manifold on the discharge side. Where an engine is after cooled, the discharge side is connected to the after cooler which is then connected to the intake manifold.
Both the above casings are attached to a centre casing which contains the bearings, seals and method of lubrication.
Bearings and lubrication
The shaft may rotate in white metal bearings which can be lubricated from the engine driven oil pump. This method of lubrication also allows the oil to remove some of the heat in the turbo charger. One bearing locates the shaft and takes the small residual thrust, the other bearing allows the shaft to move longitudinally to accommodate the differential thermal expansion of casings and shafting.
Alternatively, the smaller turbo chargers usually incorporate a ball bearing for positioning at the compressor end and a roller bearing to accommodate axial expansion at the turbine end of the rotor shaft. The bearings may have their own reservoir which forms part of the turbo charger. These reservoirs usually have round oil level sight glasses with two horizontal lines marked to indicate the high and low levels. Seals are fitted to retain the oil.
Operation of the turbo charger on a diesel engine
In a four stroke engine, exhaust gases flow from each cylinder into the exhaust manifold and then past the turbine blades of the turbo charger. With the engine running at full speed, the turbo charger can obtain speeds up to 100,000 revolutions per minute (rpm).
The air compressor blades will revolve at the same speed. Air is drawn through the air cleaner and forced under pressure into the intake manifold. When the inlet valve opens on the induction stroke, with the piston descending in its cylinder, air is forced into the cylinder.
It is necessary to reduce the turbo charger speed in stages or slowly for two reasons:
1. If the engine speed is reduced from full engine speed to stop quickly and the bearings of the turbo charger are lubricated by the main engine driven lubricating oil pump, the engine, on stopping, will cease to supply the lubricating oil to the turbo charger bearings. Because of its high speed, it will take some time for the turbo charger to come to rest and the bearings could be damaged.
2. The exhaust gas side of the turbo charger operates at a very high temperature. It is preferable to reduce the temperature gradually rather than quickly to prevent unequal contraction of the turbo charger parts as it slows down.
Monitoring the performance
Normally, as part of the purchase of a new engine, the engine distributor or dealer will do an installation and pre-run check. The following will be recorded:
· The speed of the turbo charger at a nominated engine speed.
· Air flow in.
· Air flow out.
· Air pressure after the compressor blades.
· Exhaust gas flow.
The flow of air going into the turbo charger is important. The air is taken from the engine room so sufficient ventilation to the engine room is required to ensure there is enough for the engine as well as cooling the engine room.
The exhaust gas flow is also important. It ensures the installation of the exhaust piping is within limits and not restricting the performance of the engine.
As the above is recorded, checks can always be carried out and readings compared with the initial ones.