What is Crankshaft?
The crankshaft is a moving part of the internal combustion engine (ICE). Its main function is to transform the linear motion of the piston into rotational motion. The pistons are connected to the crankshaft through the connecting rods. The crankshaft is mounted within the engine block.
A crankshaft is a shaft driven by a crank mechanism, consisting of a series of cranks and crankpins to which the connecting rods of an engine are attached. It is a mechanical part able to perform a conversion between reciprocating motion and rotational motion.
In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion, whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion.
In order to do the conversion between two motions, the crankshaft has “crank throws” or “crankpins”, additional bearing surfaces whose axis is offset from that of the crank, to which the “big ends” of the connecting rods from each cylinder attach.
It is typically connected to a flywheel to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsional vibrations often caused along the length of the crankshaft by the cylinder’s farthest from the output end acting on the torsional elasticity of the metal.
Parts of Crankshaft
Following are the main parts of the crankshaft with its diagram:
- Main journals
- Crank web
- Thrust washers
- Oil passage and oil seals
- Flywheel mounting flange
1. Crank Pin
The crankpin is a mechanical part of an engine. Which allows the connecting rod to be attached to the crankshaft very firmly.
The surface of the crankpin is cylindrical, to give the rotative force to the large end of the connecting rod. These are also known as connecting rod journals.
2. Main Journals
Journals have been attached to the engine block. These bearings hold the crankshaft and provide it to rotate inside the engine block. This bearing is such as a plain bearing or journal bearings. The main bearings vary from engine to engine, often according to the forces given by the engine.
3. Crank Web
The Crank web is the most essential part of the crankshaft. The Crank web connects the crankshaft to the main bearing journals.
The counterweights are a type of weight, that applies opposite force, which provides balance and stability to the crankshaft. These are mounted on the crank web.
The reason for installing counterweights in the crankshaft is, they can eliminate the reaction caused by rotation. And it is very helpful for achieving the higher RPM and makes the engine run easily.
5. Thrust Washers
At some points, two or more thrust washers are provided to stop the crankshaft from moving lengthways. These thrust washers assemble among the machined surfaces in the web and the crankshaft saddle.
With the help of thrust washers, it can be easily maintained the gap and helps to reduce the lateral movement of the crankshaft. In many engines, these are made as part of the main bearings, usually, older types, use separate washers.
6. Oil Passage and Oil Seals
Crankshaft oil passage passes oil from main bearing journals to the big end journals. Normally the hole is drilled on the crank web. When the crankpin is in an upward position and combustion forces push the connecting rod to a downward position, it allows oil to enter between journal and bearing.
The crankshaft has some extend beyond the crankcase on both ends. This causes oil to leak from these ends. So, preventing oil from these openings, oil seals are provided. There are two main oil seals are connected at the front end and rear end.
- Front End Oil Seals: They are very similar to the rear end oil seals. However, its failure is less destructive and it is more easily accessible. The front oil seal will be set behind the pulley and timing gear.
- Rear End Oil Seals: They are placed within the main journals and flywheels. This is pushed into a hole between the engine block and the oil pan. The oil seal has a shaped lip that is kept tightly into the crankshaft by a spring called a garter spring.
7. Flywheel Mounting Flange
In most cases, the crankshaft attaches to the flywheel through the flanges. The diameter of the crankshaft wheel end is larger than the other end. This gives a flange face to mount the flywheel.
Construction and Working of Crankshaft
The crankshaft is fitted into the engine block through its main journals. The connecting rods are fixed on the conrod journals of the crankshaft. On opposite sides of the conrod journals, the crankshaft has counterweights that compensate outer moments, minimizes internal moments, and thus reduces vibration amplitudes and bearing stresses.
At one end of the crankshaft, the flywheel is connected, and on the other end the valve timing gearing.
The number of main journals and conrod journals depends on the number of cylinders and the type of the engine (V-type, straight, etc.). On both main journals and conrod journals, the crankshaft has lubrication orifices (oil bore) through which oil flows when the engine is running.
The engine torque is not continuous because it’s produced only when each piston is on an expansion cycle. Due to this, a flywheel is mounted onto the crankshaft in order to smooth the engine torque and reduce vibrations.
On a V-type engine on the same conrod journals, two connecting rods are mounted. Because of this arrangement, a V-engine, for the same number of cylinders, is more compact than a straight engine. The length of a V6 engine is shorter than the length of a straight 6 cylinders (L6) engine.
Between the crankshaft and the engine block, on the main journals, crankshaft bearings are fitted. Their role is to reduce friction through a layer of anti-friction material that comes into contact with the engine block mounts.
Two types of crankshafts are produced, cast, and forged. The counterweights can be also forged directly onto the crankshaft or bolted-on (fixed with threaded bolts).
All the pistons of the internal combustion engine are transmitting their forces to the crankshaft. From the mechanical point of view, the crankshaft has to withstand high torsional forces, bending forces, pressures, and vibrations.
Crankshaft Position Sensors
A crank sensor is an electronic device used in an internal combustion engine, both petrol, and diesel, to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control the fuel injection or the ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on petrol engines.
The crank sensor can be used in combination with a similar camshaft position sensor to monitor the relationship between the pistons and valves in the engine, which is particularly important in engines with variable valve timing.
This method is also used to “synchronize” a four-stroke engine upon starting, allowing the management system to know when to inject the fuel. It is also commonly used as the primary source for the measurement of engine speed in revolutions per minute.
Common mounting locations include the main crank pulley, the flywheel, the camshaft, or the crankshaft itself. This sensor is one of the two most important sensors in modern-day engines, together with the camshaft position sensor.
As the fuel injection (diesel engines) or spark ignition (petrol engines) is usually timed from the crank sensor position signal, the failing sensor will cause an engine not to start or will cut out while running. Engine speed indicator takes speed indication also from this sensor.
Types of sensors
There are several types of sensors that can be used: the inductive sensor, Hall Effect sensor, magnetoresistive sensor, and optical sensor. Inductive sensors have the simplest construction and are usually purely passive devices.
Hall effect and magnetoresistive sensors have the advantage over inductive sensors in that they can detect static (non-changing) magnetic fields. Optical sensors do not have great resistance against fouling but are able to provide the most precise edge detection.
Some engines, such as GM’s Premium V family, use crank position sensors that read a reluctor ring integral to the harmonic balancer. This is a much more accurate method of determining the position of the crankshaft and allows the computer to determine, within a few degrees, the exact position of the crankshaft (and thereby all connected components) at any given time.
The function of Position Sensors
The functional objective for the crankshaft position sensor is to determine the position and/or rotational speed (RPM) of the crank. Engine Control Units use the information transmitted by the sensor to control parameters such as ignition timing and fuel injection timing.
In diesel, the sensor will control the fuel injection. The sensor output may also be related to other sensor data including the cam position to derive the current combustion cycle, this is very important for the starting of a four-stroke engine.
Sometimes, the sensor may become burnt or worn out – or just die of old age at high mileage. One likely cause of crankshaft position sensor failure is exposure to extreme heat. Others are vibration causing a wire to fracture or corrosion on the pins of harness connectors.
Many modern crankshaft sensors are sealed units and therefore will not be damaged by water or other fluids. When it goes wrong, it stops transmitting the signal which contains the vital data for the ignition and other parts in the system.
A bad crank position sensor can worsen the way the engine idles or the acceleration behavior. If the engine is revved up with a bad or faulty sensor, it may cause misfiring, motor vibration, or backfires. Acceleration might be hesitant, and abnormal shaking during engine idle might occur. In the worst case, the car may not start.
The first sign of crankshaft sensor failure, usually, is the refusal of the engine to start when hot but will start again once the engine has cooled.
One detail of some designs is the “three-wire” inductive crank sensor whereby the third wire is actually just a co-axial shield around the two main sensor wires to prevent them from picking up stray electrical pulses from elsewhere in the vehicle engine bay.
Another type of crank sensor is used on bicycles to monitor the position of the crankset, usually for the cadence readout of a cyclo-computer. These are usually reed switches mounted on the bicycle frame with a corresponding magnet attached to one of the pedal’s crankset arms.
Common Crankshaft & Crankshaft Sensor Failure Symptoms
- Check engine light comes on
- Engine not starting
- Poor performance
- Engine stalling
- Increased fuel consumption
- Inconsistent acceleration
- Sudden drops in RPM
Common Causes of Failure
Over time, any sensor will fail either from accidents, power issues, or normal wear. Due to crankshaft or camshaft position sensor failure, an engine might cut, die while driving or refuse to start.
A faulty sensor may lead to catastrophic engine failure.