What Is A Crankshaft

What Is A Crankshaft? | Auto Mechanics 101

When invented in 1872, internal combustion engines were much simpler. Thanks to constant innovation in the automotive engineering world, car engines are a lot more complicated these days. Many high-tech components are precisely working in unison to create always more power while staying as efficient as possible. However, one of the components that hasn’t changed much is the crankshaft. Materials used to produce them have evolved, the design was improved but the basic operating principle is the same. Nonetheless, the crankshaft is one of the most vital parts of your engine and when crankshaft problems arise, it’s never a good thing. To help you understand how crankshafts work, here’s an article explaining what is a crankshaft, what it does, what happens when it goes bad as well as what you need to do if you have a faulty crankshaft on your vehicle.

What is a crankshaft?

Speaking simply, a car’s engine generates power thanks to the controlled burning of the fuel inside the cylinders. The explosions then push the piston down, making the crankshaft turn. The rotational movement is then transferred to the transmission and to the wheels, ultimately making your car move.

The crankshaft is a rotating shaft that consists of a series of cranks and crankpins. It’s used to convert the back-and-forth linear movement of the pistons into a rotational one.

When the pistons move up and down, not only does the crankshaft transfers power to the transmission but it also makes every other moving component move. The crankshaft is basically the center of the engine.


Internal components of a car engine

What are crankshafts made of?

In most cars, crankshafts are made via casting or forging. They are usually single-piece shafts, although some engines have crankshafts made from more than one piece.

Cast iron crankshafts are suitable for smaller, cheaper and less powerful engines. This process is simpler and cheaper yet it has its downsides. Cast iron crankshafts, however, are heavier, thus inadequate for high-performance engines.

On the other hand, forged crankshafts are lighter, have better inherent damping and higher structural rigidity than steel ones. Contrary to camshafts which are still primarily iron cast due to high manufacturing costs of steel forging, forged crankshafts are the most common ones in modern cars. The reason for that is because crankshafts don’t require high alloy steels like camshafts do, which makes the manufacturing process much cheaper.

Finally, crankshafts can also be CNC machined. This process is better for low volume and bespoke engines because of the higher material costs. During the CNC grinding process, a single billet of steel is transformed into a crankshaft which will then need additional heat treatment. Machined steel crankshafts are naturally considered superior to forged crankshafts.

During the manufacturing process, crankshafts are additionally hardened to improve strength and make them fatigue resistant. These processes usually include induction hardening for the bearing surfaces, and a radius rolled at the ends of main and crankpin bearings. For high performance machined crankshafts, nitridation is used instead of induction hardening, and micro finishing is used to prevent cracking.

Exploded view of a crankshaft

Crankshaft design

The earliest crankshafts come from the Ancient Roman ages and their basic design remains the same in internal more recent combustion engines. However, technological developments prompted major changes and improvements to the original concept.

In an internal combustion engine, a crankshaft consists of the main journals and the same number of rod journals as the number of pistons. As their name implies, rod journals are attached to connecting rods of the pistons. The distance between the rod journal axis and the main journal axis determines the stroke length of the pistons.

To ensure stable movement around the main axis, a car’s crankshaft has counterweights that balance out the forces generated through the rotation. The invention of crankshaft counterweights is what allows car engines to achieve higher RPM. Some high-performance engines even use counterweights made out of heavy metals like tungsten and lead.

The crankshaft itself is inside of the engine block and rotates thanks to the main bearings.

Some V-configuration engines use flying arms instead. This design reduces engine weight and length but compromises the crankshaft’s rigidity as it has fewer main bearings than conventionally designed crankshafts.


How crankshafts work?

A crankshaft and a camshaft might look similar or even sound the same, but there’s a big difference between the two. The two shafts work in conjunction and are essential for generating and controlling the forces that power the engine.

With that in mind, the main purpose of the crankshaft is to convert the reciprocating motion of the cylinders into rotary motion. A crankshaft rotates halfway when a piston makes an upward motion, and halfway when it makes a downward motion, thus completing its revolution. In modern four-stroke motors, a crankshaft will make two revolutions during one power stroke.

The movement of the crankshaft is transferred to the camshaft via the timing belt or timing chain, making sure that both of them turn at the exact same time.

Common crankshaft issues

Apart from fatigue from age and use, the most common crankshaft issue is actually related to a sensor. A faulty crankshaft sensor is the most common problem you’ll encounter, and it can be a pest. Luckily, it’s easy to fix and it’s most often a cheap replacement that won’t keep your car sitting at the mechanics for long.

Modern cars use a crankshaft position sensor to monitor the rotational speed and position of the crankshaft. These parameters are important to make sure that other components work the engine move in sync with the crank. For example, the spark created by the spark plug and the opening of the valves needs to be in perfect sync with the crankshaft for the engine to run. Once the CKP sensor sends an incorrect signal, the PCM won’t be able to time the spark accordingly anymore.

Most common symptoms of a failing crankshaft sensor

Check engine light
As soon as the PCM detects a discrepancy between the crankshaft and the camshaft signals, it will record a DTC code. When it happens, the check engine light will come up on your dashboard right away. Of course, there can be various reasons for the CEL to illuminate. Do not assume that the CKP sensor is faulty on your car right away. Scan the recorded code using an OBD2 scanner first. Since most engines can work around a faulty crank sensor, it might be the only way to detect a failure.

Slow accelerator response
If a crankshaft sensor is faulty, it will not send the correct information to the ECU. Due to this information gap, you might face a delayed response when you press the gas.

Hard starts
Longer and erratic starts are a common symptom of a dead crankshaft position sensor. Electrical issues could also cause this symptom, so make sure to rule them out first. Once the CKP sensor stops working, the lack of a crank signal will force the PCM to rely on the cam sensor signal instead. That might cause the engine to take a couple more rotation before it fires up.

Engine misfiring
A faulty crankshaft position sensor will likely cause your car to misfire, followed by backfiring from the exhaust. The reason for this is the inability of the sensor to read the correct information about the position of the piston. Consequently, it will often cause sparks to come too early or too late. When it happens, the engine might run rough or stall.

To sum it all up…

‘Luckily, crankshafts don’t need frequent replacements. On most cars, the crankshaft should last longer than the rest of the engine. When a crankshaft-related problem arises, the cause is usually another related component. It’s rarely the crankshaft itself. It can be either a bad sensor, faulty balancer, damaged connecting rods, etc. In all cases, removing and replacing a crankshaft is not an easy task. Such a job often requires removing and disassembling the whole engine. If you intend to perform any kind of work on your car’s crankshaft, make sure to get your hands on your car’s repair manual first. Being able to follow the recommended removal and installation procedures will not only speed things up but it will also prevent any further crankshaft-related problem.

Good luck!

About Nishal Singh

Car enthusiast since before I could drive.

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