The crankcase is a fundamental component of an internal combustion engine. It is located below the engine’s cylinders and encapsulates the crankshaft, pistons, and various lubrication components, ensuring their protection, proper function, and longevity.
This sealed chamber plays a pivotal role in maintaining the engine’s integrity, containing lubricating oil, controlling pressure, and facilitating the efficient conversion of fuel into mechanical power.
What is a Crankcase In an Engine?
A crankcase is the housing for the crankshaft in a reciprocating internal combustion engine. In most modern engines, the crankcase is integrated into the engine block. It also functions as housing and protects the engine parts against dust, water, and splashing mud.
The crankcase stores lubricating oil required for lubricating the engine parts. The size of a crankcase is sufficiently large as it accommodates the revolving crankshaft with the connecting rod.
Various accessories like a carburetor, fuel pump, generator, water pump, air cleaner, starting motor, fan, oil filter, oil body of cooler, etc. are also mounted on the crankcase.
The crankcase not only gives support to the engine parts and engine mountings, but also withstands the loads caused by piston thrust, gas pressure, primary and secondary forces, couples, etc.
Therefore, the crankcase must be strong to withstand these loads and pressures. When the cylinder block and the crankcase are cast together in one unit, grey cast iron is used because it has rigidity, low cost, and high wear resistance.
Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/air mixture passing through the crankcase before entering the cylinder. This design of the engine does not include an oil sump in the crankcase.
Four-stroke engines typically have an oil sump at the bottom of the crankcase and the majority of the engine’s oil is held within the crankcase. The fuel/air mixture does not pass through the crankcase in a four-stroke engine; however, a small amount of exhaust gasses often enters as “blow-by” from the combustion chamber.
An open-crank engine has no crankcase. This design was used in early engines and remains in use in some large diesel engines, such as used in ships.
Why are Crankcases important?
Initially, considering that each part of the engine functions in coordination with each other to work efficiently, this would produce a significant amount of pressure within the engine. Hence, with iron being a key part of the casting material, this is inextricably useful for the engine, an area of high stress and pressure concentration.
Due to a diesel engine having a concentration of high-pressures, it is crucial to have effective ventilation. For instance, gases can get released and leak into the crankcase, because of too much pressure inside the engine’s combustion chamber.
These combustion gases, contain impurities that can further mix with the oil, forming greasy and thick sediment within the crankcase. If this happens, this can corrode and damage the components inside the engine. Hence, a crankcase’s ventilation system helps prevent gas and oil mixing by venting the gas and replacing with fresh air.
Additionally, the pressure that is created inside the cylinder during the combustion process goes against the piston’s motion during the power stroke, which means the engine loses a certain amount of power working against the pressures within the crankcase.
Therefore, an engines vacuum system is designed to remove the gases to limit crankcase pressure pumps, further increasing the power output of the engine.
Furthermore, the crankcase is designed to be the core structural component of the engine, a component where all other mechanisms are held together. The design of a crankcase ensures structural robustness, as well as increases the efficiency of oil and pressure passages.
Types of Crankcases
Crankcase-compression
Many two-stroke engines use a crankcase-compression design, where a partial vacuum draws the fuel/air mixture into the engine as the piston moves upwards. Then as the piston travels downward, the inlet port is uncovered and the compressed fuel/air mixture is pushed from the crankcase into the combustion chamber.
Crankcase-compression designs are often used in small petrol (gasoline) engines for motorcycles, generator sets and garden equipment. This design has also been used in some small diesel engines, however it is less common.
Both sides of the piston are used as working surfaces: the upper side is the power piston, the lower side acts as a pump. Therefore an inlet valve is not required. Unlike other types of engines, there is no supply of oil to the crankcase, because it handles the fuel/air mixture. Instead, two-stroke oil is mixed with the fuel used by the engine and burned in the combustion chamber.
Lubricating crankcase
Large two-stroke engines do not use crankcase compression, but instead a separate scavenge blower or supercharger to draw the fuel/air mixture into the compression chamber. Therefore the crankcases are similar to a four-stroke engine in that they are solely used for lubrication purposes.
Advantages of Crankcase
The advantages include:
- The crankcase serves as a protective enclosure, safeguarding the connecting rod and crankshaft from potential contamination and damage.
- It effectively blocks the ingress of debris, preventing it from reaching the critical crankshaft and connecting rod components.
- In intricate engine designs, the crankcase plays a vital role in facilitating the circulation of oil, ensuring proper lubrication.
- It functions as a pressurisation chamber, aiding in the management of the fuel-air mixture within the engine.
- This essential engine component establishes a sealed environment, effectively shielding the rotating elements from moisture and dirt infiltration.