Engine Crankcase Case Study
The crankcase used in Internal combustion engines is a four-bar linkage (the piston and cylinder wall are the fourth bar). It’s a concept that dates back hundreds of years. A four-stroke engine uses two rotations of the shaft to complete the four strokes of intake, compression, power, and exhaust. Because of the geometry of the crankcase, the compression ratio and the power ratio are equal. There is a limit to how much the air-fuel mix can be compressed making the compression ratio limit the expansion ratio. Too bad, because more energy could be derived with a higher expansion ratio. About half the available energy in the cylinder is lost when the exhaust valve opens. (You also pay extra for a muffler that can dissipate that much energy).
An alternative to the standard crankcase, the Atkinson engine harvests the otherwise lost energy by using a six-bar (or more) linkage to derive more power from the expansion cycle. Atkinson engines (or Atkinson Cycle) are becoming more popular due to greater efficiency demands. The criticisms of the Atkinson Engine are that it the size of the linkage system is very large, lowering the energy density. Second pressing the piston onto the cylinder walls creates friction, and wear. The engine may lack durability.
21Geo strives for a “Precise Cycle” where the best-case efficiency defines the geometry. Using 21Geo’s Precise Motion Technology, the cycle can be defined precisely. The height of the piston is defined as an array of heights corresponding to shaft rotations. Unfortunate restrictions like requiring the compression ratio equal the power ratio are removed.
Moving forward with the design of the cycle. Below are the design requirements.
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The four strokes will be completed in 360 degrees of shaft rotation.
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The power stroke will be twice as long as the compression stroke.
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The exhaust, intake and compression strokes will take place in 240 degrees of shaft rotation, leaving 120 degrees for the power stroke.
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The mechanical advantage of the power stroke will vary to ensure constant torque. (This requires a force over distance curve which is not available at this time).
The transformations from a legacy crankcase can be seen in the figure below.
Requirement 2 enables the engine to use more of the available energy. It’s similar to the Atkinson Engine in this respect. Efficiency increases of 40% are possible. Improved efficiency means lower emissions. No only pollutants, but also CO2.
Requirement 3 enables reducing the number of cylinders from 4 to 3 since with three cylinders there will always be a cylinder driving power to the shaft. Fewer cylinder means fewer parts, like manifolds, valves, cams, and pistons. There is less cooling infrastructure as well.
Requirement 4 ensures smooth operation of the engine. Currently, the most common method to improve the smoothness of an engine is to add more cylinders, making the power delivery overlap from cylinder to cylinder. Six- and eight-cylinder engines are common. Luxury cars may have twelve cylinders. By engineering smooth output, only a few cylinders are needed. It’s conceivable that a 120-degree engine performs as well as a V12.
The Precise Cycle crankcase replacement is shown below.
The next transformation for the engine will be to further compress the exhaust, intake and compression cycles to 180 degrees of shaft rotation, leaving 180 degrees for the power stroke. This further reduces the number of cylinders to two. The piston height as a function of shaft rotation is shown below:
The 180-degree engine Precise Cycle crankcase replacement is shown below.
While the 120 and 180 engines could be built, the next revision will make them much better. Check back for the improvements in 2019.