A Connecting Rod is an important part of an internal combustion engine. It connects the piston to the Crankshaft. The rod turns the piston’s straight up-and-down motion into the crankshaft’s rotation, which powers the engine.
Key Functions of the Connecting Rod
- Motion Conversion: The connecting rod changes the piston’s vertical motion into the crankshaft’s rotating motion, which moves the vehicle.
- Force Transmission: It transfers forces from the piston to the crankshaft, handling tension, compression, and other stresses.
- Pivoting and Rotation: The connecting rod allows movement at both ends. This helps the engine run smoothly and transfers power effectively.
- Engine Balance and Timing: The connecting rod’s movement ensures the engine’s timing is correct, helping it sync with other parts like the camshaft.
- Smooth Piston Movement: It keeps the piston moving smoothly, preventing knocking and ensuring steady motion.
- Power and Fuel Efficiency: The connecting rod transfers power accurately, improving fuel efficiency and the engine’s power output.
Parts of the Connecting Rod
- Big End: The larger end connects to the crankshaft’s pin. It is secured with special nuts and bolts.
- Small End: The smaller end connects to the piston with a wrist pin (or gudgeon pin) and includes a bearing to reduce wear and friction.
- Shank: This is the main body of the rod. It connects the big and small ends and is designed to be strong but lightweight.
Materials and Design Considerations
- Connecting rods are made from materials like micro-alloyed steel, sintered metals, high-quality aluminum, and for high-performance engines, CFRP (Carbon Fiber Reinforced Polymer) and titanium.
- The rod’s design, including its length and shape (like I-beam or H-beam), affects the engine’s performance and durability.
Stress Factors on the Connecting Rod
- The connecting rod faces tension, compression, bending, and other stresses as the engine operates. The engine’s compression ratio, camshaft overlap, and speed all impact the stress placed on the rod.
By understanding these points, it’s clear how the connecting rod helps the engine run smoothly and efficiently.
Primary Functions of the Connecting Rod
The connecting rod plays an important role in the engine, helping it work properly by performing several key functions.
Transmit Motion from Piston to Crankshaft
The connecting rod changes the linear motion of the piston into the rotary motion of the crankshaft. This is necessary to generate the rotational energy that powers the vehicle.
Support Engine Balance and Timing
For proper engine balance, the connecting rod needs to be matched with others of similar weight. This balance helps smooth engine operation by distributing the reciprocating weights evenly. It also helps synchronize the engine’s timing with the Camshaft and other parts, making the combustion cycle more efficient.
Ensure Smooth Piston Movement
The connecting rod ensures consistent and smooth piston movement. By connecting the piston pin to the crank pin, it allows the rod to swing and adjust its angle as the piston moves up and down. This keeps the piston’s path stable inside the cylinder, reducing engine knocking and improving overall performance.
Components of a Connecting Rod
The connecting rod is a vital engine part. It consists of several components that help it work well under high mechanical stress.
Small End
The small end connects to the piston through a piston pin (or wrist pin). This pin allows the piston to move back and forth. Spring clips or piston pin locks keep the pin in place.
Big End
The big end attaches to the crankpin journal on the crankshaft. This creates another important pivot point. It often has a bearing surface and may include a removable rod cap in two-piece rods. The cap is secured with cap screws or special nuts and bolts, ensuring a solid connection to the crankshaft.
Shank
The shank is the main body of the connecting rod, linking the small and big ends. Its job is to transfer forces from the piston to the crankshaft. The shank can come in different shapes like rectangular, tubular, or circular, depending on the design of the rod.
Shoulders
The shoulders help provide support and stability under heavy stress. They prevent distortion and maintain the rod’s strength.
Bearing Inserts
At the big end, the connecting rod has bearing inserts to ensure smooth movement. These inserts are attached to the bearing cap and fit tightly around the crankshaft.
Bush Bearings
Both ends of the connecting rod use bush Bearings. The small end has a phosphor bronze bush, while the big end has a crank-bearing shell, allowing the rod to move smoothly and efficiently.
Bolt and Nut
Bolts and nuts secure the connecting rod to the crankshaft on both sides of the big end. These fasteners ensure a strong, reliable connection to handle engine forces.
Types of Connecting Rods
Connecting rods are essential parts in engine design, and their types affect performance. Here are some common types used in modern engines:
One-Piece Connecting Rods
These rods are made from a single piece of material, providing solid and reliable construction. They are widely used in most engine types, especially where ease of maintenance and strength are important.
Two-Piece Connecting Rods
These rods have a removable cap, which makes maintenance and replacement easier. One big advantage of the two-piece design, like the Thunder Rod by Transcend, is that it moves the piston’s pivot point from the piston itself to the rod. This lets the piston move faster and farther, improving torque and dynamic compression by 25 to 30 percent.
I-Beam Connecting Rods
- Design: Shaped like a capital ‘I’, these rods are common in standard engine applications.
- Use: They are cost-effective and strong enough for most engine types, including standard ones.
- Features: These rods are usually heavier than H-beam rods but can be light and strong if made from high-strength steel. High-performance versions can support up to 1,000 hp.
H-Beam Connecting Rods
- Design: These rods resemble a capital ‘H’ and are mainly used in high-power, low-speed engines.
- Use: Perfect for turbocharged or compressor-based engines, they provide better performance under high stress.
- Features: H-beam rods are stronger, handle compression and bending stresses better, and are more expensive to make due to their complex shape.
Diesel Engine Connecting Rods
- Design: These rods do not follow the I-beam or H-beam shapes, but are known for their durability.
- Use: Mainly used in diesel engines, where reliability and high torque endurance are crucial.
- Features: They are built to handle the low rpm and high torque typical of diesel engines.
Hybrid Connecting Rods
- Design: A mix of I-beam and H-beam designs, combining the benefits of both.
- Use: Mainly used in high-performance racing engines, providing a good balance of weight and strength.
- Features: These rods are strong, resistant to cracking, and lightweight, making them ideal for extreme racing.
Primary and Secondary Connecting Rods
- Design: Found in V-type engines, where a primary rod and an auxiliary rod are used. The auxiliary rod attaches to the primary rod, allowing staggered cylinders without increasing engine length.
- Use: They are helpful in making efficient engine designs for V-type configurations.
Parallel Connecting Rods
- Design: These rods are placed side by side on the same crank pin.
- Use: Mainly used in V-type engines, where the cylinders are staggered along the crankshaft’s length.
Each type of connecting rod serves a specific role depending on the engine’s design and performance needs. The choice of rod depends on factors like power, cost, and the expected load on the engine.
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Materials Used
Connecting rods are made from different materials, each with its own advantages and disadvantages depending on the use.
Forged Steel
Forged steel is the most common material for connecting rods in cars. It has excellent strength because its grain structure is aligned during the forging process. Common alloys include 42CrMo4, 43CrMo4, 44csr4, C-70, EN-8D, SAE1141. These alloys improve properties like ductility, impact toughness, and fatigue strength. Forged steel rods are 37% stronger and last much longer (about 100 times longer) than powder metal rods.
Billet Steel
Billet steel is used in high-performance engines, especially 4340 billet or forged steel, which is a chrome molly alloy known for its strength. The strength and durability of 4340 steel can change depending on how it’s heat-treated.
Aluminum Alloy
Aluminum alloys such as T6-2024 or T651-7075 are popular for their light weight and ability to absorb impact. These materials are often used in performance vehicles where reducing weight is a priority. However, aluminum alloys are usually less durable than steel and not suitable for high-strength or long-lasting applications.
Titanium
Titanium is a high-performance material that is ideal when weight reduction is essential, and cost is not a major concern. Titanium rods have an excellent strength-to-weight ratio, making them perfect for situations where both performance and weight matter.
Cast Iron
Cast iron is a cheaper option used in lower-performance applications like motor scooters. It’s not recommended for high-speed or high-performance engines because of its low strength and durability.
Powder Metal
Powder metal or sintered metal connecting rods are a cost-effective choice for mass production. These rods are made by pressing metal powder into molds and heating it. While they perform well, their yield strength and tensile strength are lower than forged steel, and their fatigue strength is about 27% lower.
Other Materials
Other materials include iron-based sintered metal, micro-alloyed steel, and spheroidized graphite cast iron. These are chosen based on engine needs, balancing cost, performance, and durability.
Design Considerations
The design of connecting rods is crucial for engine performance. Several factors need to be considered to optimize efficiency, durability, and balance.
Connecting Rod Length
The length of the connecting rod affects the rod-to-stroke ratio, which in turn impacts engine performance. A longer connecting rod can improve performance by slowing piston acceleration near top dead center (TDC). This leads to more time at TDC, helping the cylinder fill better, especially at high RPMs.
However, there are trade-offs. While longer rods reduce side thrust on the piston, they may increase the overall engine height and weight. For example, in the Mitsubishi 4G63 engine, using a stock 150mm connecting rod with a modified 100mm stroke crankshaft reduces the rod-to-stroke ratio from 1.70 to 1.50. To get a better ratio, a longer rod (e.g., 156mm) can be used, especially with the taller deck height of the 4G64 block.
Connecting Rod Weight
The weight of the connecting rod is also important. Lighter connecting rods can boost engine power and reduce vibration. A lighter rod lowers reciprocating mass, which improves engine efficiency and smoothness.
When designing connecting rods, it’s vital to match their weights closely for engine balance. The rods should be within a couple of grams of each other, and the big end weights should match to avoid secondary imbalance and vibrations.
Beyond overall weight, the distribution of mass in the rod matters. The mass of the big end and the small end (pin end) should be balanced separately since part of the rod mass rotates while the other part moves up and down.
Balancing Connecting Rods
Balancing becomes more complex for V engines. The bob weight calculation takes into account the combined mass of the piston, conrod, bearing shells, and ring set. This calculation ensures the engine remains balanced and operates smoothly.
Maintenance and Replacement
Maintaining and replacing the connecting rod is important for the engine’s longevity and performance. Here are the steps for proper maintenance and replacement.
Access and Removal
- Rotate the flywheel to reach the bolts on the big end of the connecting rod.
- Use a 5/16″ socket and ratchet to remove the bolts.
- This will allow you to detach the big end bearing cap and disconnect the connecting rod from the crankshaft.
Component Removal
- Remove the big end bearing cap to detach the oil stirring arm connected to the connecting rod.
- Be careful not to damage the piston compression rings, as they are delicate.
Additional Steps
- For some engines, you may need to remove the cylinder head to access the connecting rod and piston.
- You may also need to remove the oil pan, oil pump screen, timing chain, and timing cover.
Reinsertion of Components
- Reinsert the lubricated pin into the connecting rod’s head.
- Add the shims and roller cage, making sure all components are aligned.
Alignment and Balancing
- Make sure the connecting rod is centered and balanced before reassembly.
- You may use an OMC Rod Cap Alignment Fixture/Tool for accurate alignment, but some mechanics manage without it.
Torque Specifications
- When reattaching the connecting rod cap, use a 5/16″ – 12-point socket for high torque (e.g., 372 in. lbs).
Final Checks
- Double-check that all bolts are tightly secured and that the crankshaft is clean before reassembling other engine components.
- Ensure the crank webs are clean and that the pin is inserted straight to avoid damaging the crankshaft.
The connecting rod does more than just link engine parts—it plays a key role in performance. It keeps everything working together, ensuring smooth power delivery. When we understand its function, we see the precision behind every engine. Even small parts make a big difference, showing us the value of paying attention to details.