An extraneous knock at the bottom of the cylinder block during a cold start often indicates critical wear of the main liners crankshaft or violation of clearances in the lubrication system. It is this node, along with camshaft, forms the kinematic basis of any piston internal combustion engine, converting the reciprocating motion of the pistons into rotational motion, which is transmitted to the wheels. Understanding which shafts are in the engine is necessary to correctly diagnose knocks, vibrations and gas distribution problems, since failure of any of these elements leads to expensive overhauls.
In modern power units, be it classic V-shaped eight or an in-line four, the design of the crank mechanism (CVM) and gas distribution mechanism (GRM) may differ significantly. However, the basic set of rotating elements remains unchanged. In this article we will analyze in detail the structure of each shaft, their mutual influence on the operation of the motor and typical symptoms indicating the need for intervention by a mechanic.
Crankshaft: the heart of the crank mechanism
The main element responsible for energy conversion is crankshaft. It takes the forces from the connecting rods moving up and down and transforms them into torque. The design of this shaft is a complex system of main and connecting rod journals connected by cheeks. In modern engines such as Toyota Camry or BMW 3-series, crankshafts are often forged or cast from ductile iron to improve fatigue resistance.
On the crankshaft counterweights there are holes for attaching the flywheel and oil pump drive gear. It is important to note that it is through this shaft that the entire cylinder-piston group is lubricated: oil under pressure is supplied to the main journals, then through internal channels it enters the connecting rod journals and is sprayed onto the cylinder walls. Violation of this process, for example, due to a clogged oil receiver, leads to rapid rotation of the liners.
- 🔧 Root necks — supporting surfaces lying in the beds of the cylinder block.
- 🔧 Crankpins — places of attachment of connecting rods, offset relative to the axis of rotation.
- 🔧 Counterweights - massive parts of the cheeks that balance the rotating masses.
- 🔧 Flange - rear part for attaching the flywheel.
⚠️ Attention: The appearance of a metallic knock, the frequency of which increases in proportion to engine speed, is a direct indication of problems with the crankshaft liners. Operating the engine with this symptom is prohibited.
Camshaft and valve control
The second critical element is camshaft (camshaft), which synchronizes the opening and closing of the intake and exhaust valves. It is driven from the crankshaft by means of a chain, belt or gears, ensuring strict compliance with the engine strokes. In modern engines with a system VVT-i, CVVT or Vanos A phase shifter is installed at the end of the camshaft, changing valve opening angles to optimize power and efficiency.
The camshaft cams have a complex profile, which determines the valve lift height and the duration of its opening. Wear on the cams or camshaft beds leads to loss of compression, engine vibration and increased fuel consumption. In overhead camshaft (OHC) engines, this element also often drives the oil and vacuum pumps.
Camshaft lubrication system design
Oil is supplied to the camshaft under pressure through channels in the cylinder head. Particular attention should be paid to the condition of hydraulic compensators, if they are provided for in the design, since their jamming can cause shock loads on the cams.
There are several camshaft layouts. Simple motors use a single shaft (SOHC) to control all the valves. More powerful versions use a DOHC design with two shafts: one for the intake valves and the other for the exhaust valves. This allows for better filling of the cylinders with mixture at high speeds.
Balance shafts: combating vibration
In second-order inline four-cylinder engines, vibrations often occur due to uneven rotation of the crankshaft and the movement of the pistons. To extinguish them, engineers implement balancing shafts. Usually there are two of them, they are located in the cylinder block or crankcase and rotate in opposite directions at twice the speed of the crankshaft.
These shafts have special eccentric weights that create inertial forces that are opposite to the vibration forces of the engine. This makes the motor run smoother and quieter. However, the presence of additional shafts complicates the design and requires a high-quality lubrication system, since balancer bearings are often friction pairs that are sensitive to oil starvation.
If the balancing shaft drive belt breaks (if it is structurally connected to the timing belt) or their bearings jam, the timing belt may break, which will lead to the valves meeting the pistons. Therefore, the condition of the balancer drive system must be given no less attention than the main timing belt.
Auxiliary shafts and their functions
In addition to the main elements of the crankshaft and timing belt, the engine also contains other shafts. These include oil pump shaft, which ensures the circulation of lubricant. In some designs it is driven directly from the crankshaft, in others it is driven by a separate circuit or even an electric motor. The service life of the entire engine depends on its serviceability.
Also worth mentioning are the turbocharger shafts in supercharged engines. Although technically part of the intake/exhaust system, the turbine shaft rotates at enormous speeds (up to 200,000 rpm) and requires ideal lubrication and cooling. A jammed turbine shaft can lead to oil leaking into the intake manifold and water hammer.
- 🛢️ Oil pump shaft - creates pressure in the lubrication system.
- 💨 Turbocharger shaft — transfers the energy of the exhaust gases to the compressor.
- ⚙️ Intermediate shaft - used in some timing circuits to transmit rotation.
⚠️ Attention: When replacing the timing belt on engines with balancing shafts, be sure to check the tension of their drive. An assembly error can lead to engine destruction after several thousand kilometers.
Comparison of shaft materials and designs
The choice of material for the manufacture of shafts depends on the load and cost of the engine. For mass-produced cars, cast iron is most often used, while forged steel is preferred in sports cars and diesel engines. Below is a table comparing the main characteristics of the elements.
| Shaft type | Material | Main function | Resource (km) |
|---|---|---|---|
| Cranked | Forged steel / Cast iron | Motion conversion | 300 000+ |
| Distribution | Cemented steel | Valve control | 250 000+ |
| Balancing | Steel | Vibration Dampening | 200 000+ |
| Turbocharger | Heat-resistant alloy | Air injection | 150 000+ |
It is worth noting that heat treatment The surface finish of the camshaft and crankshaft journals is a key factor in their longevity. The hardness of the surface layer allows it to withstand friction even under boundary lubrication conditions.
Diagnostics and symptoms of malfunctions
You can determine which shaft requires attention by the nature of the noise and the conditions under which it occurs. The knock of the crankshaft is usually dull and intensifies under load. The camshaft knock is louder and more frequent, often heard in the upper part of the engine. The vibration of the balancers is felt as a trembling of the body at idle, which disappears when the speed increases.
☑️ Diagnosis of engine knocks
For accurate diagnostics, it is necessary to use a motor tester and analyze oscillograms of pressure in the cylinders and the operation of shaft position sensors. Phase mismatch between the crankshaft and camshaft, recorded by sensors CKP and CMP, often indicates a stretched timing chain or a faulty phase shifter.
Use only motor oils recommended by the manufacturer. Insufficient viscosity or low levels of anti-wear additives (zinc, phosphorus) will accelerate wear on the camshaft cams and crankshaft bearings.
Replacement process and important nuances
Replacing shafts is a labor-intensive operation that requires complete or partial disassembly of the engine. When installing a new crankshaft, it is necessary to bore the block for repair bearings and grind the journals, if they are not new. Camshafts are often replaced as an assembly with the housing or require replacement of hydraulic lifters and rockers.
It is critical to maintain the tightening torque of the bearing cap bolts. Over-tightening will lead to deformation of the shafts and scuffing, and under-tightening will lead to knocking and rapid destruction of the liners. After assembly, it is necessary to perform the procedure of bleeding the lubrication system and, in some cases, adapting the phase shifters through a diagnostic scanner.
The quality of engine oil and the timeliness of its replacement affect the service life of engine shafts by 80%. Saving on oil always leads to expensive repairs of the crankshaft and timing belt.
How often should the timing belt connected to the shafts be changed?
The replacement interval depends on the engine model and usually ranges from 60,000 to 120,000 km. However, if there are signs of wear (cracks, delamination), replacement must be done immediately, regardless of mileage. A broken belt on most modern engines leads to bent valves.
Is it possible to drive with a knocking engine?
Strongly not recommended. Knocking indicates the presence of gaps and shock loads. Every minute a knocking engine runs brings you closer to the point where rebuilding is no longer economically feasible. In the case of the crankshaft, the count can go on in kilometers.
Does the quality of gasoline affect shaft wear?
Indirectly - yes. Detonation from poor fuel creates shock loads on the piston group and crankshaft, accelerating their fatigue failure. Also, products of incomplete combustion can contaminate the oil, impairing its lubricating properties.
What is “oil starvation” of shafts?
This is an operating mode in which the rubbing pairs (liner-neck, cam-pusher) do not receive a sufficient amount of oil at the required pressure. Causes: low oil level, faulty pump, clogged filter or channels. The result is rapid heating and rotation of the liners.