Operational Information

The Two Stroke Crosshead Engine

The Turbo Charger
 

 

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 220% more air than theoretically required. Because it takes about 14kg of air to supply the theoretical oxygen to burn 1kg of fuel, 4.5kg of air must be supplied into each cylinder to burn the 0.1kg of fuel.

Some 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 (about 3.7 kg) to to supply the oxygen to burn the fuel completely.

3.7kg of air at atmospheric pressure and 30ēC occupies a volume of  3.2m3. The volume of the cylinder of the engine in our example is about 1.2m3 after the exhaust valve closes and compression begins. Because the temperature of the air delivered into the engine is raised to about 70ēC as it enters the engine, it can be calculated that to supply the oxygen required for combustion, the air must be supplied at 3 × atmospheric pressure or 2 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.

Turbochargers on a Sulzer RTA96

 

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

 

Aux. Blower on a MAN B&W 2 stroke engine

STARTING THE ENGINE

Because the engine needs to be supplied with air when starting up and running at low speeds, an auxiliary  blower powered by an electric motor is provided. This automatically cuts out when the charge air supplied by the turbocharger is sufficient to supply the engine on its own.

 

 

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