MBD Simulation on a Piston Assembly

Aim:

Modelling and assembly of Piston Assembly as well as Calculating Linear displacement of the Piston Head in all three cases of gudgeon offset with help of motion analysis and compare results.

Introduction:

Piston assembly consists of a piston, crank, connecting rod and piston pin known as a gudgeon pin. Functions of piston assembly are given below:

It transmits gas force generated during combustion to the connecting rod and crankshaft.

As the piston is connected to the small end of the connecting rod with the help of a gudgeon pin.

It guides the connecting rod when gas force is transmitted.

Theory:

Piston parts, unlike designs and materials, do not vary across different pistons.

They remain the same with differences only in their sizes.

Piston components extend all the way from the part bordering the combustion chamber to the end that connects to the crankshaft.

Collectively, these parts make the piston assembly.

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1. Piston Ring

Piston Ring Definition

Piston rings are pieces of split rings that mount on a recessed area of the piston. There are usually 3 piston rings in a typical car engine. The number varies, and a piston can even have one ring. The areas or surfaces between these rings are called piston ring lands. The ring mounting grooves are designed to maintain the piston ring position, and you may hear something like tapered construction.

The split design of the piston ring has several benefits. It allows for spring action, which helps the rings to maintain the correct piston ring gap. The split also makes piston ring installation easy. To ensure spring is constant under heat, load, pressures, and other conditions, manufacturers prefer cast iron or pieces of steel for piston rings material.

Piston Ring Function

The primary function of piston rings is to seal off the combustion chamber and regulate lubricating oil usage. The rings also serve to conduct heat away to the cylinder bore. As mentioned before, most vehicle engine pistons feature three rings; two upper compression rings and a lower oil ring. For clarity, the different rings are explained below.

• Compression ring– this is the top side ring and nearest the combustion chamber. It is also called the gas or pressure ring. The ring prevents combustion gases from leaking. Compression rings also help transfer heat from the piston to the walls of the cylinder.
  
  
• Scraper/wiper ring– this is located between the compression and oil rings. It features a tapered surface and serves the function of both rings: to seal the combustion chamber and to wipe the oil off the walls of the piston cylinder.
  
  
• Oil control ring– the piston oil ring is the lower ring on the piston. It consists of two thin surfaces, with holes all around. The slots allow oil to flow back into the sump. As its name suggests, the function of the piston oil control ring is to remove excess oil from the cylinder walls. It does that as the piston works back and forth.

2. Piston Skirt

Piston Skirt Definition

The skirt of a piston refers to the cylindrical material mounted on the round section of a piston. The part is usually made from cast iron material due to its excellent wear resistance and self-lubricating properties. The skirt contains the grooves to mount the piston oil ring as well as compression rings. Piston skirts come in different styles to suit specific applications.

Piston Skirt Function

The skirt guides the piston as it travels up and down the cylinder. Its design helps the piston to overcome the side forces created by the changing angle of the connecting rod. If the skirt is worn, obtaining a proper piston seal for efficient combustion would prove difficult.

The piston would also rock in the cylinder uncontrollably and cause a piston slap. When that happens, you may hear the infamous piston slap noise, especially during cold starts. Piston slap will often go away after the engine has warmed. That is because the resulting expansion closes the gap between piston and cylinder.

If the noise does not stop, the cylinder may need to be tightened, among other measures. Otherwise, the problem does not present much danger, and you may drive your car if the noise only appears when starting the engine. 

There are two main types of piston skirts:

Full skirt

It is also known as a solid skirt. The full skirt features a tubular shape. It is commonly used in the engines of large automobiles.

Slipper skirt

The type of piston skirt is used on the pistons of motorcycles and some cars. It has part of the skirt cut away to leave only the surfaces on the back and front of the cylinder wall. This helps to reduce weight and minimize the contact area between the cylinder wall and the piston.

3. Piston Pin

Piston Pin Definition

Also known as a wrist pin or Gudgeon pin, the piston pin is the hollow or solid shaft in the skirt section. The piston rod pivots on this pin, held in the piston ring bushing. For tensile strength, piston pins are usually built from alloy steel and machined to fit the piston bearing. Holes in the connecting rod deliver oil to the wrist pin, helping to reduce friction.

Piston pin assemblies and mounting styles vary. They can be categorized into 3 designs: free to revolve in both piston and connecting rod, clamped to connecting rod, and rigidly mounted to piston bosses.

Piston Pin Function

A piston pin forms the connection or pivotal point of the pistons and connecting rod. They provide bearing support and help pistons to function properly. In other words, the pin facilitates the back and forth movement of the piston.

As we have seen, piston pins use three methods to mount on the piston assembly. These give rise to the following types of pins.

• Stationary/fixed pin– the pin attaches to the bosses of the piston via a screw. The piston rod then pivots on the pin.
  
  
• Semi floating– the pin attaches to the connecting rod in the middle, and the pin ends move freely within the piston bearing and at the bosses. 
  
  
• Full-floating– in this pin type, the pin is not attached to the pin or piston connecting rod. Instead, it is secured by plugs, clips, or snap rings attached to the piston bosses. The pin can then oscillate at the bosses as well as the rod.
  
  

4. Piston Head/Crown

Piston Head Definition

Also known as the piston crown or dome, the head of a piston is its top surface. It is the part that comes into contact with the combustion gases. As a result, it gets heated to extremely high temperatures. To prevent melting, piston head parts are made using special alloys, among them steel alloy.

A piston head is usually built with channels and cavities. These help to create a swirl that improves combustion. Different piston head types are used in different engines. Reasons for the differences vary. The preferred piston head design depends on many factors, such as expected performance and engine type.

Piston Head Functions

The crown, as it is also known, forms the surface that takes in the pressures, temperatures, and other stresses of the expanding gases. Among the purposes it serves are:

• Creating swirl to make combustion uniform and regulate knocking
  
  
• Act as a heat barrier between the combustion chamber and the lower piston parts
  
  
• Contain the pressures resulting from a knock in the cylinder
  
  

5. Connecting Rod

Connecting Rod Definition

Often shortened to conrod or rod, the connecting rod is one of the most important piston parts. It links the piston to the engine’s crankshaft and moves the piston in and out of the combustion chamber. Connecting rods have to bear a lot of mechanical strain and should be sturdy enough. For this reason, the parts are mostly forged, although casting is also frequently used.

Automotive piston manufacturers often prefer steel to manufacture these rods. Alloy steel is also a popular conrod material, especially for high-performance engines. For milder engines, aluminium may be preferred for its lightweight nature. The rods of small engines such as scooters can even be made from iron.

Connecting Rod Function

The conrod rotates the crankshaft, producing the motion that enables a vehicle to move. In some engines, the piston rod features a hole or bore to deliver lubricating oil to the cylinder walls and wrist pin. Manufacturers make conrods in various designs. Versions include cracked joint, milled joint, straight and angled separation rod, and the parallel and tapered rod design.

The connecting rod is divided into several parts. They are:

• Small end– it is the smaller end of the rod. It consists of the red-eye and piston bushing. The small end links to the piston via the piston pin.
  
  
• Big end– the big end is the part opposite the small end of the conrod. It connects to the crankshaft and features a slit design to allow it to be mounted.
  
  
• Connecting rod beam– this is the section between the small and big parts of the rod. Usually a double T construction, the beam may or may not contain an oil bore to channel lubricant to the cylinder.

6. Connecting Rod Bolt

Connecting Rod Bolt Definition

Also in the list of piston parts is the conrod bolt. These bolts clamp the rod to the crankshaft. The bottom end of the rod bolts bolt is rod caps and bearings, held in place by a nut. A cotter pin on the nut prevents the assembly from coming undone.

Conrod bolts are usually made from steel. In applications where weight reduction is an essential feature, aluminium bolts are common. The rod can also be made from nickel. Nickel connecting rod bolts are stronger and are mostly used in heavy-duty rods.

Connecting Rod Bolt Function

As we have seen, the bolts secure the connecting rod to the crankshaft. They help the rod to withstand the strain caused by the rotating crankshaft. In their absence, the rod would break down affect the working of other engine parts. The rods guide every piston stroke, ensuring smooth engine operation.

Conrod bolts are made with the ability to bend a little under the action of the piston and crankshaft movements. That protects the rod caps from collapsing due to the extreme strain caused by the moving crankshaft and piston.

Connecting rod bolts come in a variety of designs. They can be hexagonal, round, flat or embossed design. Some come threaded others unthreaded. Threaded bolts make the best parts due to their firmer grip.

7. Piston Bearings

The bearings are piston parts that are located at the points where pivotal rotation takes place. They are usually semicircular metal pieces that fit in the bores of these points. Piston bearings include the shells found at the big end where the rod connects to the crankshaft. There are also bearings at the small end where the rod links to the piston.

Piston bearings are usually manufactured using composite metals like lead copper, silicone aluminium, and others. The bearings are often coated to improve hardness and enable them to bear the load of the piston and connecting rod movements.

Modelling parts

Piston

Crank

Connecting rod

End Cap

Wrist pin

Assembled position parts

Animation of motion analysis

Motion analysis of piston assembly 

The crank speed 2000 RPM fixed in the piston assembly

linear displacement will by varying the centre of the axis of the piston head and the crank

The linear displacement is found with the Piston Head 

Case 1

When the centre of the axis of the piston head and the crank are coincident with each other

The displacement piston

the piston is displaced with a mean position of 252 mm from the fixed plane the maximum displacement is 89mm

Then export the graphs into Excel.

Case 2

When the centre of the axis of the piston head and the crank are Positive Offset by 10mm when it rotates at 2000RPM in a clockwise direction

 The displacement piston

The piston is displaced with a mean position of 252 mm from the fixed plane the maximum displacement is 90mm

Now, export the graphs into Excel. 

Case 3

When the centre of the axis of the piston head and the crank are Negative Offset by 10mm when it rotates at 2000RPM in a clockwise direction

 The displacement piston

the piston is displaced with a mean position of 252 mm from the fixed plane the maximum displacement is 90mm

Then export the graphs into Excel.

Results and explanation:

Then Superimpose all the three graphs into a single graph. 

the graph shows no major change. But some minute changes in displacement and frequency 

Observation and the conclusion:

Thus the 3D modelling of the Piston assembly is done and the motion analysis is carried out to get some useful plots for further analysis.