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Engine Principles : |
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In a typical internal combustion engine, as
the piston descends on the induction stroke a pressure lower than
atmosphere is created in the cylinder. Since the inlet valve is also open
at this time, air flows from outside the engine through the intake system
and into the cylinders via the head ports. This is because air will flow
from a region of high pressure to a region of lower pressure. Now if you
picture the piston at the bottom of its travel (i.e. Bottom Dead Centre or
BDC) in theory the most air that can be contained in the cylinder is the
volume of the entire cylinder and combustion chamber. This would be termed
100% volumetric efficiency as the entire cylinder is now full of air at
the same pressure as atmosphere. Now for simplicity if we assume the inlet
valve is now closed, the piston now ascends compressing the air in the
cylinder. The resulting compressed air (and fuel) is where the engine
obtains it's energy from, once the spark plug has ignited, causing the
mixture to burn and expand, generating a downward force on the piston
causing the crank to rotate.
So now we understand how this works how can we increase the force on the piston and hence the power. Well one way is to increase engine capacity, so that each induction stroke of the engine draws more air in, and hence there is more compressed air/fuel to ignite to extract more energy. The other method is forced induction, or in this case turbocharging. By utilising the energy that would otherwise be wasted in the hot exhaust gases, we converge the exhaust manifold into one venturi that then flows through a vaned wheel called the turbo exhaust wheel. (Picture a wind mill to grasp the concept). This exhaust wheel is connected via a shaft to another vaned wheel, called the turbo compressor wheel that is housed separately. As exhaust gases flow, the exhaust wheel spins, which in turn spins the compressor wheel. The compressor wheel is arranged so that the intake to the wheel draws from atmosphere. The outlet from the compressor wheel is connected to the engine intake. Now if you remember I mentioned the maximum amount of air we can obtain in the cylinder is the volume of the entire cylinder at atmospheric pressure when the piston is at BDC. What we actually do by turbocharging is effectively raise atmospheric pressure so that the pressure differential between the low pressure in the cylinder and outside is greater. This results in a greater volume of air entering the cylinder. What it actually amounts to is the air is already semi compressed before it enters the cylinder so that although the cylinder volume is always the same, once the piston has risen and compressed the air/fuel mixture, there is a greater pressure and hence more energy can be extracted which results in more power. Using this method it is possible in road engines to achieve up to 300% volumetric efficiency, or in Laymans terms equivalent to an engine of 3 times the capacity. |
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