Wrist Pin Offset: Piston slap and internal friction
INTRODUCTION
Engines have been the single most important driver for the industrial revolution. James watt invented steam engine where energy of the fluid converted into kinetic energy of the locomotive. Progressively, this led to Otto cycle and Diesel cycle after various modifications to power cycles which led to discovery of Otto engines or Constant volume combustion engines and Diesel engines or constant pressure combustion engines. Now, the combustion was enclosed and hence the name Internal combustion engines.
The combustion took place in the chamber, this chemical energy which was released was directed to act on the piston and due to force imbalance would move downwards. Piston would have the linear movement, but what was required at the wheels of the vehicle was the rotation. Hence the linear movement was converted to rotation by litany of components. The connecting rod connects the Piston pin with the crank pin. Connecting rod is the single most, by design, mechanically complicated as it has to transmit both linear and rotation motion. The converted rotary motion is then fed to crank and in turn to the crankshaft. The motion, thus obtained is passed on to the Transmission equiments which includes Gears, Clutches and Wheel.
However, this simple arrangement comes with several complications. This mechanism involves lot of moving components and hence useful energy is lost in overcoming friction and thus reduces net total energy obtained. Secondly, each component are subjected to enormous stress and cyclic loading and hence the engine is constrained. One, cannot increase the Engine speed or Stroke length at the will. Since piston makes contact with the liner, piston skirt (Lower portion of the pison), makes the slight contact with the liner and results in screeching noise and friction. This phenomenon is known as Piston slap. This would result in constant eroding of the liner and increase in clearance between piston and liner
Secondly, the thrust forces on the piston that is, horizontal force acting on piston when piston reaches maximum and minimum volume is high which restricts piston design. One easy way of overcoming is to add strokelength but that in turn constricts mean piston speed and hence limits the speed. This would also contribute to increased friction and lower power output.
Hence, an easy solution without revising entire engine design is to introduce an offset in the piston pin bore. In former cases, piston pin bore used to align along the piston cylindrical axis, now with the offset, it results in lower angularity at the TDC and BDC and results in lesser thrust forces. It has also been noticed that Piston slap is also considerably reduced. This produces same effect as adding the stroke length. However, beyond certain value, it doesnt produce any significant improvement, piston pin offset, thus is a very strong function of connecting rod length and crank throw.
The maximum volume for the offset piston doesnt occur at the BDC and Minimum doesnt occur at the TDC.
MOTION BODY ANALYSIS
GEOMETRY
Piston, Piston pin, Connecting rod, End cap, Crank is designed as follows. They are then assembled.
The piston without the pin offset is as shown below. Piston, Piston pin bore and crank axis are aligned.
PISTON PIN OFFSET
By convention, for the clockwise rotation of the crankshaft, when piston pin bore lies right of piston axis, it is deemed negative and if piston pin bore lies to the left of piston axis, it is deemed positive.
Negative Piston offset
Positive Piston offset
MOTION SIMULATION
The three cases are setup, one with 0mm offset, 10mm positive offset, -10mm offset
The motor is setup to run at 2000RPM, Contacts is established between end cap and crank, connecting rod and crank, piston and piston pin, piston pin and connecting rod.
Gravity is turned in Y axis and numerical calculation is performed.
RESULTS AND PLOTS
Piston displacement is plotted as shown. 3 traces are compared
Further, when the plot is taken a deeper look
DISCUSSION ON PLOTS AND RESULTS
1) From the above graph, offset moves the maximum and minimum volume points slightly away from the BDC and TDC
2) Compression ratio seems to change a bit
3) Piston pin offset provides same effect as that of increasing stroke length without constraining design limits
4) For the clockwise motion of crankshaft, positive offset is generally provided as it would reduce noise due to piston slap, reduce friction losses.
5) Simulation video can be found in this link: https://www.youtube.com/watch?v=vaxHGpAmqD4