The Turbo Charger

By turbo charging an engine, the following advantages are obtained:

        Increased power for an engine of the same size OR reduction in size for an engine with the same power output.

           

            Reduced specific fuel oil consumption - mechanical, thermal and scavenge efficiencies are improved due to less cylinders, greater air supply and use of exhaust gasses.

       

    Thermal loading is reduced due to shorter more efficient burning period for the fuel leading to less exacting cylinder conditions.

      

    The turbocharger consists of a single stage impulse turbine connected to a centrifugal impeller via a shaft.

     The turbine is driven by the engine exhaust gas, which enters via the       gas inlet casing. The gas expands through a nozzle ring where the   pressure energy of the gas is converted to kinetic energy. This high    velocity gas is directed onto the turbine blades where it drives the turbine    wheel, and thus the compressor at high speeds (10 -15000 rpm). The exhaust gas then passes through the outlet casing to the exhaust uptakes.

     On the air side air is drawn in through  filters, and enters the compressor wheel  axially where it is accelerated to high velocity. The air exits the impeller radially and passes through a diffuser, where some of the kinetic energy gets converted to pressure energy. The air passes to the volute casing where a further energy conversion takes place. The air is cooled before passing to the engine inlet manifold or scavenge air 

       The turbine casing is of cast iron. Some casings are water cooled which complicates the casting. Water cooled casings are necessary for turbochargers with ball and roller bearings with their own integral LO supply (to keep the LO cool). Modern turbochargers with externally lubricated journal bearings have uncooled casings. This leads to greater overall efficiency as less heat energy is rejected to cooling water and is available for the exhaust gas boiler.

       

     The compressor impeller is of aluminium alloy or the more expensive titanium. Manufactured from a single casting it is located on the rotor shaft by splines. Aluminium impellers have a limited life, due to creep, which is dictated by the final air temperature. Often the temperature of air leaving the impeller can be as high as 200°C.

           The life of the impeller under these circumstances may be limited to about 70000 hours. To extend the life, air temperatures must be reduced. One way of achieving this is to draw the air from outside where the ambient air temperature is below that of the engine room.

            Efficient filtration and separation to remove water droplets is essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of salt water. 

      Efficient filtration and separation to remove water droplets is essential and the impeller will have to be coated to prevent corrosion accelerated by the possible presence of salt water.

    The air casing is also of aluminium alloy and is in two parts.

         Bearings are either of the ball or roller type or plain white metal journals. The ball and roller bearings are mounted in resilient mountings incorporating spring damping to prevent damage due to vibration. These bearings have their own integral oil pumps and oil supply, and have a limited life (8000 hrs).

            Plain journal bearings are lubricated from the main engine oil supply or from a separate system incorporating drain tank, cooler and pumps. Oil is supplied in sufficient quantity to cool as well as lubricate. The system may incorporate a header tank arrangement to supply oil to the bearings whilst the turbocharger comes to rest should the oil supply fail. A thrust arrangement is required to locate and hold the rotor axially in the casing. In normal operation the thrust is towards the compressor end.

  

            Labyrinth seals or glands are fitted to the shaft and casing to prevent the leakage of exhaust gas into the turbine end bearing, or to prevent oil being drawn into the compressor. To assist in the sealing effect, air from the compressor volute casing is led into a space within the gland.

  

            A vent to atmosphere at the end of the labyrinth gives a guide to the efficiency of the turbine end gland. Discoloring of the oil on a rotor fitted with a roller bearing will also indicate a failure in the turbine end gland.

            A labyrinth arrangement is also fitted to the back of the compressor impeller to restrict the leakage of air to the gas side

        A two stroke crosshead engine must be supplied with air above atmospheric pressure for it to work. Although the first turbochargers were developed for aero engines in the first world war, it was not until the 1950s that large two stroke engines were turbocharged.

  

            Before then the pressurised air needed to "scavenge" the cylinders of the exhaust gases and supply the charge of air for the next combustion cycle was provided by mechanically driven compressors (Roots Blowers), or by using the space under the piston as a reciprocating compressor (Under Piston Scavenging). This of course meant that the engine was supplying the work to compress the air, which meant that the useful work obtained from the engine was decreased by this amount.

         Engine powers have increased phenomenally in the past 20 years. In 1980 an engine delivering 15000kW was a powerful engine. Today's largest engines are capable of delivering over 4 times this amount. This is due not only to improved materials and manufacturing techniques, but also to the improvements and developments in the design of the turbochargers fitted to these engines.

            The amount of useful energy that an engine can produce is dependant on  two factors; The amount of fuel that can be burnt per cycle and the efficiency of the engine.

    

     Fuel consists mainly of Carbon and Hydrogen. By burning the fuel in oxygen the energy in the fuel is released and converted into work and heat. The more fuel that can be burnt per cycle, the more energy released.

     However, to burn more fuel, the amount of air supplied must also be increased. For example, a 10 cylinder engine with a bore of 850mm and a stroke of 2.35m must burn 1kg of fuel per revolution to deliver 38500kW when running at 105 RPM. (assuming 50% efficiency). This means that each cylinder burns 0.1 kg fuel per stroke. To ensure that the fuel is burnt completely it is supplied with 200% more air than theoretically required. Because it takes about 14kg of air to supply the theoretical oxygen to burn 1kg of fuel, 4.2kg of air must be supplied into each cylinder to burn the 0.1kg of fuel.

         A lot of this air is used up scavenging (clearing out) exhaust gas from the cylinder. The air also helps cool down the liner and exhaust valve. As the piston moves up the cylinder on the compression stroke and the exhaust valve closes, the cylinder must contain more than the theoretical mass of air to to supply the oxygen to burn the fuel completely (about 100% or 2.8kg)

            2.8kg of air at atmospheric pressure and 25ºC occupies a volume of  2.4m3. The volume of the cylinder of the engine in our example is about 0.74m3 after the exhaust valve closes and compression begins. Because the temperature of the air delivered into the engine is raised to about 50ºC, it can be calculated that to supply the oxygen required for combustion, the air must be supplied at 3.5 × atmospheric pressure or 2.5 bar gauge pressure.

         NOTE: These figures are approximate and for illustration only. Manufacturers quote the specific fuel oil consumption of their engines in g/kWh. These figures are obtained from testbed readings under near perfect conditions. Quoted figures range between 165 and 175g /kWh. The actual specific fuel consumption obtained is going to depend on the efficiency of the engine and the calorific value of the fuel used.

         About 35% of the total heat energy in the fuel is wasted to the exhaust gases. The Turbocharger uses some of this energy (about 7% of the total energy or 20% of the waste heat) to drive a single wheel turbine. The turbine is fixed to the same shaft as a rotary compressor wheel. Air is drawn in, compressed and, because compression raises the temperature of the air, it is cooled down to reduce its volume. It is then delivered to the engine cylinders via the air manifold or scavenge air receiver.

           The speed of the turbocharger is variable depending on the engine load. At full power the turbocharger may be rotating at speeds of 10000RPM

    

                               

 MATERIALS

       Gas Casing: Cast Iron (may be water cooled)

       Nozzle ring and blades: Chromium nickel alloy or a nimonic alloy.

       Compressor casing: Aluminium alloy

       Compressor Wheel: Aluminium alloy, titanium or stainless steel