Multi-Body Dynamics of Piston Assembly using Motion Analysis

Aim:- To Study Motion Analysis of Piston Assembly

Objective:-

1. To create 3D models of different components of the piston assembly.

2. To run motion study analysis at 2000 RPM crank speed for the following cases -

a. Zero mm Wrist Pin offset

b. 10 mm positive wrist pin offset

c. 10 mm negative wrist pin offset

3. Obtain the Linear displacement of the Piston Head in all the three cases. Then export the graphs into Excel. Then superimpose all the three graphs into a single graph.

Introduction:-

A reciprocating engine, also often known as a piston engine, is typically a heat engine (although there are also pneumatic and hydraulic reciprocating engines) that uses one or more reciprocating pistons to convert pressure into a rotating motion. An internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the connecting rod and onto the crankshaft.

Each piston is inside a cylinder, into which a gas is introduced, either already under pressure (e.g. steam engine), or heated inside the cylinder either by the ignition of a fuel-air mixture (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (Stirling engine). The hot gases expand, pushing the piston to the bottom of the cylinder. This position is also known as the Bottom Dead Center (BDC), or where the piston forms the largest volume in the cylinder. The piston is returned to the cylinder top (Top Dead Centre) (TDC) by a flywheel, the power from other pistons connected to the same shaft, or (in a double-acting cylinder) by the same process acting on the other side of the piston. This is where the piston forms the smallest volume in the cylinder. In most types, the expanded or "exhausted" gases are removed from the cylinder by this stroke. The stroke is simply the distance between the TDC and the BDC, or the greatest distance that the piston can travel in one direction.

3D Models :-

Piston Head: A piston is a component of reciprocating engines. It is located in a cylinder and is made gas-tight by piston rings. Its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In two-stroke engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall. The piston reciprocates inside the engine due to the forces generated inside the combustion chamber. This reciprocating motion is responsible for the intake and exhaust of the gases from the engine cylinder. The piston receives the explosive forces generated due to the burning of fuel and converts it into mechanical energy by rotating the crankshaft. The piston is connected to the crankshaft through a connecting rod. It is generally made of aluminium alloys.

Connecting rod: A connecting rod also called a con rod, is the part of a piston engine that connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.

Endcap: The big end of the connecting rod connects to the crankpin journal to provide a pivot point on the crankshaft. Connecting rods are produced as a one-piece or two-piece components. A rod cap is the removable section of a two-piece connecting rod that provides a bearing surface for the crankpin journal. The rod cap is attached to the connecting rod with two cap screws for installation and removal from the crankshaft.

Gudgeon pin/Wrist pin: The connecting rod is attached to the piston by a swiveling gudgeon pin/wrist pin. The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. A few designs use a 'fully floating' design that is loose in both components. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall, usually by circlips.

Crankshaft: A crankshaft is a rotating shaft which (in conjunction with the connecting rods) converts the reciprocating motion of the pistons into rotational motion. Crankshafts are commonly used in internal combustion engines and consist of a series of cranks and crankpins to which the connecting rods are attached.

The crankshaft rotates within the engine block through the use of main bearings, and the crankpins rotate within the connecting rods using rod bearings. Crankshafts are usually made from metal, with most modern crankshafts being constructed using forged steel.

Motion Analysis:-

For all the cases:

Motion analysis is calculated when the piston was at the top dead center (TDC) and the crank was at zero degrees.

A torque/motor was provided for the axis of the crank with 2000 rpm in the clockwise direction. Solid-body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact was selected and 12000 fps were provided for a smooth transition and to obtain results more accurately and were solved by running calculations.

Study 1- Crankshaft speed is 2000 RPM clockwise with zero wrist pin offset

Zero offset means the axis of the wrist pin and the crankshaft is linear vertically.

Plots/Graphs:-

For zero offset, maximum piston displacement is 302.50mm.

Study 2- Crankshaft speed is 2000 RPM clockwise with wrist pin positive offset of 10mm

Positive offset here means that the axis of the wrist pin is at 10mm left to the linear axis(piston motion axis) with the axis of the crankshaft.

Plots/Graphs:-

For positive offset, maximum piston displacement is 302.47 mm.

Study 3- Crankshaft speed is 2000 RPM clockwise with wrist pin negative offset of 10mm

The negative offset here means that the axis of the wrist pin is at 10mm right to the linear axis(piston motion axis) with the axis of the crankshaft.

Plots/Graphs:-

For negative offset, maximum piston displacement is 302.47 mm.

Superimposed data:-

The plots obtained from each case were exported to an excel sheet for analyzing data values and superimposing them for comparison. On observing this graph, we analyzed that number of cycles of zero offset is less than others that is the number of times the piston reaches TDC for a given time period. The piston displacement for zero offset is maximum (302.50mm) as compared to positive and negative offset graphs.

Simulation:-

Google drive link -

https://drive.google.com/drive/folders/17CYRmLPZPTR0d_gEJp7vk5YqzItl7Fy-?usp=sharing

Conclusion:-

1. When the crank angle is at 0 degrees, the piston is positioned at the top dead center(TDC) at which the displacement is maximum. After half of the rotation (at 180 degrees), the piston is at the bottom dead center(BDC) and the displacement is minimum and in this way, the cycle follows the sinusoidal path as shown in the graph. The angular velocity of the crank almost remains constant throughout the motion.

2. Wrist pin offset minimizes connecting rod angularity when cylinder pressure is at its highest. Reduced angularity leads to lower piston thrust forces, hence lower frictional losses during the period of maximum cylinder pressure, especially at the beginning of the power stroke when pressure rises sharply about 12-15 degrees after TDC.

3. Stroke length is decreased for offset condition. This reduces the piston slapping and also reduces the friction loss thus increasing the power output of the engine. A maximum stroke length can be observed at a zero offset case.

4. Wrist pin offset improves the noise characteristics of the engine by reducing the piston slap at the top and bottom dead center. This is a major NVH (noise vibration and harshness) concern for production engineers who want to eliminate alarming noises.

5. On comparing piston displacement for three cases, we observed that the maximum piston displacement for zero offsets is maximum(302.50 mm) than negative offset(302.47mm) and positive offset(302.47mm).

Reference:-

https://en.wikipedia.org/wiki/Reciprocating_engine

http://blog.wiseco.com/what-is-wrist-pin-offset#:~:text=Wrist%20pin%20offset%20is%20a,and%20aftermarket%20piston%20engineers%20alike.&text=On%20the%20surface%20it%20seems,of%20any%20internal%20combustion%20engine.

Tags- #piston #pistonanalysis #pistonassembly #engine #motionanalysis #simulation #inference #enginekinematics #tdc #bdc #automomobile #automobiles #vehicles #postprocessing #solidworks #dassaultsystems