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What are the differences between reciprocating engines, gas turbines and microturbines?(2)

- Feb 16, 2018 -

What are the differences between reciprocating engines, gas turbines and microturbines?(2)

This really should be self evident from the question...A reciprocating engine has no turbine.  Instead it has "Pistons" running back and forth inside closely fitting tubes called "Cylinders", which are sealed at one end (at the top of the Cylinder, usually called the "Cylinder Head").  To ensure a near gas-tight fit the Piston is fitted with springy metal rings called "Piston Rings".  
The rapidly expanding gas created from burning fuel in the top of the Cylinder pushes downwards against the Piston, which in turn pushes against a "Connecting Rod", which itself is connected to the Piston at one end (the "little end") and to a Crank Shaft at the other end (the "big end") and thus the movement of the Piston along the Cylinder (the "Stroke") causes the Crank Shaft to turn, imparting rotary motion which can be used to drive things.  One complete 360 Degree rotation of the Crank Shaft is called a "Revolution".
Because the downward force from the Piston is not smoothly applied (it comes as a series of sharp pulses) there are counter weights on the Crank Shaft to help balance the moving parts and a heavy metal disk called a Flywheel is fitted to the rear end of the Crank Shaft to help smooth out the rotation between the power strokes.  However,  it is virtually impossible to eliminate the vibration the reciprocating movement causes but by adding more cylinders and it can be reduced. 
Reciprocating engines can be either "two stroke" or "four stroke".  A two stroke engine uses the Piston itself as a Valve, controlling the inlet of fuel and air, and the exit of exhaust gases, via "Ports" in the Cylinder wall as the Piston passes them.  Two stroke engines have one power stroke for each revolution of the Crank Shaft, which takes two strokes of the piston to complete, hence the name.
A "four stroke" engine has inverted-mushroom shaped Valves in the Cylinder Head to control the inlet of fuel and air and the exit of exhaust gases.  The Valves are usually kept closed with powerful springs and opened either by being pushed down indirectly by a "Valve Rocker" connected to the "Cam Shaft" via a "Push Rod", or directly via an "Overhead Cam Shaft".  There are separate Valves for the fuel and air mixture (called "Inlet Valves") and for the exit of exhaust gases (called "Exhaust Valves").  The shape and position of the various "Cams" on the Cam Shaft control which Valves open and close and when they do so.  Four stroke engines only have one power stroke for every two revolutions of the Crank Shaft, which takes four strokes of the Piston to complete, again, hence the name.  Because of the extra complexity of Four stroke engines they are therefore heavier and produce less power than a similar two stroke engine of the same Cylinder capacity. (the capacity of a Cylinder is called its "Cubic Capacity", or "CC" for short. Usually measured in fractions of a Litre, or in America, in "Cubic Inches").
But whilst this may make two stroke engines seem much better than four stroke engines, two stroke engines also have some major disadvantages compared to four stroke engines.
 Two Stroke engines are lubricated by mixing an oil with the fuel (called "Two Stroke Oil") and burning it inside the Cylinder.  This produces an exhaust of white smoke, full of unburnt oil which not only pollutes the environment but is very dangerous to Human health when inhaled.  Also, as the Piston repeatedly passes the ports in the Cylinder wall, it wears away the both the Piston and the Piston Rings...Two stroke engines typically use 50-70% more fuel than four stroke engines of the same output power and they wear out much faster too, making them too unreliable for most uses.  They tend to be used only where lightweight and compact size is essential (In Chainsaws, and Brush-cutters, for instance).   Conversely,  four stroke engines are lubricated either by oil held in a "Sump" under the engine, which is pumped to roughly where its needed in the engine via an external Oil Pump, (called a "Wet Sump" design) or in highly controlled amounts via precision nozzles at specific critical points within the engine via a more complex computer controlled Oil Pump (Called a "Dry Sump" design).  The former makes up the majority of four stroke designs as its relatively cheap and simple to produce with the latter being reserved for very expensive, high power applications such as in automotive racing cars.
Gas Turbines on the other hand have no reciprocating parts at all, only rotating parts.  Their operation is very simple to explain...At the front of the Gas Turbines engine is an air compressor, usually comprised of several disks of rotating blades fitted to the central shaft.  This is used to compress the air that is sucked into the engine.  Behind the compressor section is the combustion section where the compressed air is mixed with fuel and burnt inside special units called "Combustion Chambers".  The rapidly expanding gas which is produced is passed through a turbine, also connected to the central shaft and then through a constricted outlet called the "Jet Pipe", which produces a fast jet of gas, which produces the engines "Thrust".  As the gas passes through the turbine its turns the central shaft and thus the compressor too...Its a positive feedback loop!   Reduction gearing from the central shaft can be used to turn a propeller on the front of the engine. (This is known as a "Turbo-Prop"), or to power external machinery such the tracks of a Tank (which is known as a "Turbo-shaft").
 Gas Turbines produce virtually no vibration and are extremely reliable, but they do use lots of fuel to operate and they require very expensive, "highly exotic" materials for the construction of the turbine and associated components that have to resist both extreme stresses and very high temperatures, which makes Gas Turbines extremely expensive to produce and beyond the budget of the majority of people.
Micro Turbines are simply scaled-down normal-sized Gas Turbines. Many are still experimenting and trying to perfect this technology because when you scale down the compressor and turbine of a full-sized Gas Turbine the components have to rotate exponentially faster in order to work effectively and this greatly increases the mechanical stresses on the miniature components too.  For instance, the central shaft in a full-size Gas Turbine might need to rotate to a maximum of about 60,000 RPM, but the shaft in a micro Gas Turbine might have to rotate to 500,000 RPM or more!  The compressor in a micro Gas Turbine is usually only some 20mm in diameter.


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