IC Engine Connecting Rod

Introduction

The connecting rod is one of the most critical components in an internal combustion diesel engine. It is under constant stress whether the engine is running or not and thus, requires extremely careful design, manufacturing, operation and maintenance for a long and incident-free service life.

In this article, we provide an in-depth explanation of the connecting rod functions, types, parts, materials, and stresses.

What is a Connecting Rod?

In a trunk-piston engine, a connecting rod is a part that connects the piston to the crankshaft. It converts the reciprocal motion of the piston into the rotation of the crankshaft.

In long reciprocating engines (such as large 2-stroke marine engines), the connecting rod connects the crosshead and the crankshaft. In these engines, instead of the piston, the con rod converts the linear motion of the crosshead into the rotational motion of the crankshaft.

The piston transfers the power linearly to the crosshead through a piston rod and the crosshead in turn transfers it to the crankshaft through the connecting rod.

 

Related read: Main Engine Crankshaft Types and Materials Explained

 

Connecting Rod Stresses

The con rod rotates at the same speed as the crankshaft. But in addition to the rotation, it undergoes massive tensile (during starting/stopping), compressive stresses (during running), shear stresses as well as bending stresses (due to centrifugal forces during rotation).

Factors such as compression ratio, camshaft overlap, charge air pressure, ignition timing, engine power, speed, torque, etc. affect the overall stress on connecting rods.

All these factors create high possibilities of its failure. As a result, material selection, design, fabrication and regular monitoring are very important in the case of the connecting rod.

Parts of a Connecting Rod

The connecting rod is one component but we can divide it into its sub-components as follows:

1. Big end
2. Small end
3. Rod
4. Bush
5. Bearing insert
6. Bolt and nut

Connecting rod in a trunk-piston engine – Image by Abouttools, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Big end

The big end of the connecting rod is the section that connects to the crankshaft’s crank pin. Special nuts and bolts are used to ensure a secure connection.

Small end

The small end of the connecting rod connects to the crosshead in long-stroke reciprocating engines, whereas in the case of trunk piston engines, it connects directly to the piston through the gudgeon pin (also sometimes known as piston pin or wrist pin).

Large marine engines have crossheads whereas smaller engines such as in generators have a direct connection between the connecting rod and the piston.

Rod

The rod is the central portion of the connecting rod that connects the big end to the small end. It may have the shape of an I-beam or an H-beam.

Using an I-beam ensures that the rod can support greater loads while reducing deflections for the same amount of material.

The rod also has central holes meant for passing lubrication oil from one end to the other, especially in engines that have oil-cooled pistons.

H-beam rods are generally more expensive, more rigid and meant for higher compressive forces. They cost more than I-beam rods as their manufacturing is sometimes more complex.

Bush

We use a bush bearing on the small end of the connecting rod to facilitate smooth movement between the piston and the small end. The bush operates under continuous metal-to-metal contact with the piston pin. It must also be noted that the load on the small end bush is considerably higher than most other bearings in the engine. Thus, these bushes must be designed to withstand high cyclic loading.

Bearing insert

Bearing inserts are semi-circular pieces of low-friction material placed between the connecting rod and the crankshaft at the big end. Two of these make up a complete 360-degree buffer between the crankshaft and the con rod.

Bearing inserts prevent metal-to-metal contact and also help maintain an oil film between them. These advantages increase the durability of both the crankshaft as well as the connecting rod.

Bolt and nut

Bolts and nuts secure the connecting rod at the big end as well as the small end (when connected to a crosshead). These bolts are the most stressed fasteners in any engine. They must endure fluctuating tensile stresses, bending stresses and shear stresses during operation.

These bolts are the most stressed fasteners in any engine.

Due to the cyclic nature of the stresses, connecting rod bolts are vulnerable to failure, especially in medium-speed engines. Diligent preventive and condition-based maintenance reduces the possibility of failure.

Connecting rod material

The material selection for connecting rods is a crucial process. We want our connecting rod to have high tensile and compressive strength and to resist fatigue failure as well.

Based on the application, the material for a connecting rod can vary a lot. 

Steel is the most commonly used connecting rod material in automotive applications.

For high compressive forces, aluminium alloy or cast nodular steel are good choices. When we need to work with high tensile forces, cast, forged or fabricated steel is more suitable.

When cost is a concern and lower performance is satisfactory, cast iron works well as it is cheaper. It is used in applications such as motor scooters.

On the other hand, when the cost is not a concern, titanium fares well as it reduces weight along with delivering high performance. For light connecting rods, we may also use aluminium alloys such as T6-2024 or T651-7075.

Other than the above-mentioned materials, connecting rods may also be made from iron-based sintered metal, micro-alloyed steel and spheroidized graphite cast iron.

Types of Connecting Rods

Based on their manufacturing method, we can divide connecting rods into four types. These are:

1. Cast rod
2. Forged rod
3. Powdered metal rod
4. Billet rod

 

Cast rod

Cast rods are connecting rods fabricated from the casting process. Casting is generally easier and cheaper than alternatives. These rods are adequate for low-power, low-speed applications that do not require very high-performance levels.

Using cast rods helps manufacturers keep costs down and keep their equipment affordable for the majority of customers.

However, they must never be used for high-speed applications as the disadvantages of using a cast rod in high-performance situations far outweigh its advantages.

Forged rod

As the name suggests, forged rods are made from the forging process. The process aligns a metal’s grain structure to the rod shape and improves mechanical properties such as ductility, impact toughness, and fatigue strength.

These are all very desirable properties in a connecting rod. The strength of forged steel rods is more than powdered metal rods. These rods can sustain higher compression ratios while meeting the rigid requirements of size tolerance and weight.

Powdered metal rod

Powdered metal rods are fast becoming the most common type of connecting rods in the automotive industry. They offer a cost-effective and high-performance alternative to traditional cast or machined rods.

Powdered metal rods required no additional machining, which improves the profitability of the process despite having a higher raw material cost than casting.

A powdered metal or sintered metal connecting rod is manufactured by pressure moulding metal powder and heating it in a furnace to enable metal bonding.

For even better mechanical properties, powdered metal rods are put through the forging process. This improves their yield strength, tensile strength, density, elongation capability, toughness, and fatigue resistance.

Billet rod

Billet rods are made from a single billet of flat forged steel. They are designed in CAD programs and then cut using CNC methods such as waterjet, laser cutting, etc.

As a result, billet rods can be manufactured in various sizes and designs. We don’t require tooling, dies or retooling for design changes. The same equipment can manufacture different types of connecting rods.

Billet rods, however, retain the original grain structure. This reduces the overall strength as the grains do not go around the big end but rather stay straight as in the original billet.

Also, as billet rods are typically manufactured in limited batches that have been customised, they require more time for design, setup and finishing operations. This makes billet rods less affordable than forged rods made from the same material.

Conclusion

Connecting rod failure is a single point of failure in any engine. Their failure can render an engine useless and may lead to irreparable damage. In certain situations, such as factories and ships, engine failure can lead to consequential incidents and jeopardize the safety of personnel, property and the environment.

The connecting rod is therefore one of the most crucial engine parts. The proper design and maintenance of connecting rod can delay or completely prevent its failure. It is, therefore, very important to ensure that their condition is being monitored on a regular basis.