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De Dion-Bouton engine (circa 1905) with a crankcase formed from separate castings of the upper and lower halves De Dion-Bouton engine (Rankin Kennedy, Modern Engines, Vol III).jpg
De Dion-Bouton engine (circa 1905) with a crankcase formed from separate castings of the upper and lower halves

In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block.


Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/air mixture passing through the crankcase before entering the cylinder(s). 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 enter as "blow-by" from the combustion chamber.

The crankcase often forms the lower half of the main bearing journals (with the bearing caps forming the other half), although in some engines the crankcase completely surrounds the main bearing journals.

An open-crank engine has no crankcase. This design was used in early engines and remains in use in some large marine diesel engines.

Two-stroke engines

Two-stroke crankcase-compression engine Two-Stroke Engine.gif
Two-stroke crankcase-compression engine


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. [2]

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.

Four-stroke engines

Four-stroke engine- oil shown in yellow at the bottom 4StrokeEngine Ortho 3D Small.gif
Four-stroke engine- oil shown in yellow at the bottom

Most four-stroke engines use a crankcase that contains the engine's lubricating oil, as either a wet sump system or the less common dry sump system. Unlike a two-stroke (crankcase-compression) engine, the crankcase in a four-stroke engine is not used for the fuel/air mixture.

Oil circulation

Engine oil is recirculated around a four-stroke engine (rather than burning it as happens in a two-stroke engine) and much of this occurs within the crankcase. Oil is stored either at the bottom of the crankcase (in a wet sump engine) or in a separate reservoir (in a dry sump system). [3] From here the oil is pressurized by an oil pump (and usually passes through an oil filter) before it is squirted into the crankshaft and connecting rod bearings and onto the cylinder walls, and eventually drips off into the bottom of the crankcase. [4]

Even in a wet sump system, the crankshaft has minimal contact with the sump oil. Otherwise, the high-speed rotation of the crankshaft would cause the oil to froth, making it difficult for the oil pump to move the oil, which can starve the engine of lubrication. [5] Oil from the sump may splash onto the crankshaft due to g-forces or bumpy roads, which is referred to as windage. [6]

Ventilation of combustion gasses

Although the piston rings are intended to seal the combustion chamber from the crankcase, it is normal for some combustion gases to escape around the piston rings and enter the crankcase. This phenomenon is known as blow-by. [7] If these gases accumulated within the crankcase, it would cause unwanted pressurisation of the crankcase, contamination of the oil and rust from condensation. [8] To prevent this, modern engines use a crankcase ventilation system to expel the combustion gases from the crankcase. In most cases, the gases are passed through to the intake manifold.

Open-crank engines

Gardner 0 stationary engine (a plate acts as a safety shield but the crankshaft is not fully enclosed). Gardner 0 engine, Abergavenny steam rally, 2015.jpg
Gardner 0 stationary engine (a plate acts as a safety shield but the crankshaft is not fully enclosed).

Early engines were of the "open-crank" style, that is, there was no enclosed crankcase. The crankshaft and associated parts were open to the environment. That made for a messy environment, because oil spray from the moving parts was not contained. Another disadvantage was that dirt and dust could get into moving engine parts, causing excessive wear and possible malfunction of the engine. Frequent cleaning of the engine was required to keep it in normal working order.

Some two-stroke diesel engines, such as the large slow-speed engines used in ships, have the crankcase as a separate space from the cylinders, or as an open crank. The spaces between the crosshead piston and the crankshaft, may be largely open for maintenance access.

See also

Related Research Articles

Two-stroke engine Internal combustion engine type

A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during one power cycle, this power cycle being completed in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.

Exhaust gas recirculation NOx reduction technique used in gasoline and diesel engines

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat, in order to reduce peak in-cylinder temperatures. The actual amount of recirculated exhaust gas varies with the engine operating parameters.

A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.

Four-stroke engine Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

Within piston engines, a wet sump is part of a lubrication system whereby the crankcase sump is used as an integral oil reservoir. An alternative system is the dry sump, whereby oil is pumped from a shallow sump into an external reservoir.

Injection pump

An Injection Pump is the device that pumps fuel into the cylinders of a diesel engine. Traditionally, the injection pump was driven indirectly from the crankshaft by gears, chains or a toothed belt that also drives the camshaft. It rotates at half crankshaft speed in a conventional four-stroke diesel engine. Its timing is such that the fuel is injected only very slightly before top dead centre of that cylinder's compression stroke. It is also common for the pump belt on gasoline engines to be driven directly from the camshaft. In some systems injection pressures can be as high as 620 bar.

Bourke engine Type of internal combustion engine

The Bourke engine was an attempt by Russell Bourke, in the 1920s, to improve the two-stroke internal combustion engine. Despite finishing his design and building several working engines, the onset of World War II, lack of test results, and the poor health of his wife compounded to prevent his engine from ever coming successfully to market. The main claimed virtues of the design are that it has only two moving parts, is lightweight, has two power pulses per revolution, and does not need oil mixed into the fuel.

Crankcase ventilation system System to relieve pressure in a combustion engines crankcase

A crankcase ventilation system removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source.

Hot-bulb engine Internal combustion engine

The hot-bulb engine is a type of internal combustion engine in which fuel ignites by coming in contact with a red-hot metal surface inside a bulb, followed by the introduction of air (oxygen) compressed into the hot-bulb chamber by the rising piston. There is some ignition when the fuel is introduced, but it quickly uses up the available oxygen in the bulb. Vigorous ignition takes place only when sufficient oxygen is supplied to the hot-bulb chamber on the compression stroke of the engine.

Hit-and-miss engine

A hit-and-miss engine or Hit 'N' Miss is a type of stationary internal combustion engine that is controlled by a governor to only fire at a set speed. They are usually 4-stroke but 2-stroke versions were made. It was conceived in the late 19th century and produced by various companies from the 1890s through approximately the 1940s. The name comes from the speed control on these engines: they fire ("hit") only when operating at or below a set speed, and cycle without firing ("miss") when they exceed their set speed. This is as compared to the "throttle governed" method of speed control. The sound made when the engine is running without a load is a distinctive "Snort POP whoosh whoosh whoosh whoosh snort POP" as the engine fires and then coasts until the speed decreases and it fires again to maintain its average speed. The snorting is caused by the atmospheric intake valve used on many of these engines.

Free-piston engine

A free-piston engine is a linear, 'crankless' internal combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion chamber gases, a rebound device and a load device.

Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.

Total-loss oiling system Engine lubrication system

A total-loss oiling system is an engine lubrication system whereby oil is introduced into the engine, and then either burned or ejected overboard. Now rare in four-stroke engines, total loss oiling is still used in many two-stroke engines.

Fiat AN.1 1920s Italian piston aircraft engine

The Fiat AN.1 was an experimental Italian water-cooled diesel straight six cylinder aircraft engine from the late 1920s.

Internal combustion engine Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to. This replaced the external combustion engine for applications where the weight or size of an engine was more important.

MWM AKD 112 Z Motor vehicle engine

The MWM AKD 112 Z is an air-cooled two-cylinder inline diesel engine produced by MWM from 1955 – 1960. One, three and four cylinder variants of the same engine family were also produced by MWM.

4 VD 14,5/12-1 SRW Motor vehicle engine

The 4 VD 14,5/12-1 SRW is an inline four-cylinder diesel engine produced by the VEB IFA Motorenwerke Nordhausen from 1967 to 1990. The engine was one of the standard modular engines for agricultural and industrial use in the Comecon-countries. Approximately one million units were made.

Mercedes-Benz OM 138 Motor vehicle engine

The Mercedes-Benz OM 138 is a diesel engine manufactured by Daimler-Benz. In total, 5,719 units were produced between 1935 and 1940. It was the first diesel engine especially developed and made for a passenger car. The first vehicle powered by the OM 138 was the Mercedes-Benz W 138. The light Mercedes-Benz trucks L 1100 and L 1500 as well as the bus O 1500 were also offered with the OM 138 as an alternative to the standard Otto engine.

The Diesel Air Dair 100 is an opposed-piston diesel aircraft engine, designed and produced by Diesel Air Ltd of Olney, Buckinghamshire for use in airships, home-built kitplanes and light aircraft. The prototype was built in the 1990s and exhibited it at PFA airshows. Although Diesel Air engines have been fitted to an AT-10 airship and to a Luscombe 8A monoplane, production numbers have been very limited.


  1. Kennedy, Rankin (1905). The De Dion-Bouton Engine and Cars. The Book of Modern Engines and Power Generators (1912 ed.). London: Caxton. pp. 78–89.
  2. "How Two-stroke Engines Work". 1 April 2000. Retrieved 27 September 2019.
  3. "Why do some engines use a dry sump oil system?". 1 April 2000. Retrieved 27 September 2019.
  4. "How Car Engines Work". 5 April 2000. Retrieved 27 September 2019.
  5. "Dear Tom and Ray - October 1996". Archived from the original on 26 September 2011.
  6. Jeff Huneycutt. "Oil Pans For Power". Retrieved 16 November 2006.
  7. "Dear Tom and Ray - September 1999". Archived from the original on 26 September 2011.
  8. "Dear Tom and Ray - January 2001". Archived from the original on 28 September 2009.