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Diesel engines for Locomotives and Atkinson & Miller Cycle with Toyota
Some diesel locomotives, cars and more use Miller Cycle Engines.
In the Atkinson (or Miller) cycle the engine can behave like two volumes. If it is operating as Atkinson, not as Otto, the suction valve is actually not closed as soon as movement begin the cylinder to TDC.
Let's make some calculations:
Cylinder Volume: h.π.r ^ 2/4 (Engine Cylinder Volume = Cylinder Volume x Number Of Cylinders)
h: stroke, TDC-BDC.
r : cylinder radius (bore is the diameter of cylender, radius=bore/2 and radius^2=bore/4)
π: complex number. So, the cylinder volume can not calculate clearly, only approximately.
So, what is the compression ratio?
(Combustion Chamber Volume + Cylinder Volume) / Combustion Chamber Volume
This gives us the compression ratio and the efficiency that the engine can produce. We calculated the cylinder volume above, how do we calculate the volume of the combustion chamber?
When the piston is in the TDC, it is the space between the top of the piston and the cylinder head.
From the formula above, the approximate value can be calculated, and the indentations and protrusions in that area must also be taken into account.
Following these methods, we can calculate some technical details of an engine for the Otto cycle.
In Atkinson, or Miller cycles, the suction valves do not close directly (through valve timing) when the piston begins to move towards to TDC. Thus, a part of the air sucked into the cylinder is pushed back into the suction channel (an overfeed system in the Miller cycle pushes the air back to the cylinder direction.). As a result, the engine compression ratio decreases as the combustion chamber volume increases (may calculate with the formula above). Namely, system behaves as have two different volume in cylinder as suction and compression.
Expansion Ratio = (Expansion Stroke Volume + Combustion Chamber Volume) / Combustion Chamber Volume
Compression Ratio = (Compression Stroke Volume + Combustion Chamber Volume) / Combustion Chamber Volume
So, it sucks less fuel.
So Mr. Atkinson has actually reduced unnecessarily engine efficiency.
No!, of course not.
Okay, the above is correct and Atkinson's cycle causes a reduction of engine efficiency. So if the engine is 4 cylinders and a totally 1798 cc (assume about 1800 cc), Atkinson will have 25% lower volume in the cycle, because the compression ratio has decreased. This mean relatively low power in Atkinson cycle because there is no supercharger like Miller cycle.
So, what did Mr. Atkinson think?
In Otto engine, when the piston begins to move into the direction, the closure of the valves means that there will be more and more compressions, and thus a pressure build-up to resist the movement of the piston to the TDC. So, apart from friction forces, this force which makes the movement of the piston difficult must be taken into account as well. If the unit is multi-cylinder, it will be the another cylinder at the time of ignition (work) attempting to overcome the air trapped in a cylinder at the compressed. On the other hand, electrical support systems (Hybrid) let to use Atkinson cycle instead of supercharger.
If we consider a four-stroke and four-cylinder engine, a cylinder has to defeat its own resistance (friction, inertia) as well as the resistance of the other 3 cylinders when doing a job. In other words, a part of the power generated by a cylinder at the time of work is used and wasted in this way. When we think that principle, "when the pressure of the gases increases, their temperature increases too", the thermal efficiency is increased because the compression is reduced. This means a reduction in emissions.
On the other hand, the ignition time of the piston is also of great importance. Because the linear force generated by the ignition is transferred to the flywheel with the help of the piston, piston rod and crank. If the piston and therefore the piston rod are on the same axis as the crank center point (if the torque arm angle is zero), the first force to be generated in the ignition is transferred vertically to the crankshaft. Whereas, when the crank-piston rod at the point of downward stroke (after passing the middle point in the + direction), the force will turn into a much larger rotating movement when the firing occurs. The detail revealed here is that the torque arm is relatively longer at the time of ignition. This enables more efficient power generation.
Special thanks to James Atkinson and Ralph Miller and Toyota and Toyota Canada Inc.
Video Music: MSG
Regards,
Hakan N. Öztürk
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