Monobloc engine
Encyclopedia
A monobloc or en bloc engine is an internal-combustion piston engine where some of the major components: cylinder head
, cylinder block
, or crankcase
are formed, usually by casting
, as a single integral unit, rather than being assembled later. This has the advantages of improving mechanical stiffness, and also improving the reliability of the sealing between them.
Monobloc techniques date back to the beginnings of the internal combustion engine
. Use of the term has changed over time, usually to address the most pressing mechanical problem affecting the engines of its day. There have thus been three distinct uses of the technique:
In most cases, any use of the term describes a deliberate single-unit construction, opposed to the more common contemporary practice. Where the monobloc technique has later become the norm, the specific term falls from favour. It is now usual and un-noteworthy practice to use monobloc cylinders and crankcases, but a monobloc head (for a water-cooled inline engine at least) would be regarded as peculiar and obsolescent.
The head gasket
is the most highly stressed static seal in an engine and was a source of considerable trouble in early years. The monobloc cylinder head forms both cylinder and head in one unit, thus avoiding the need for a seal.
Along with head gasket failure, one of the least reliable parts of the early petrol engine was the exhaust valve, which tended to fail by overheating and burning. A monobloc head could provide good water cooling, thus reduced valve wear, as it could extend the water jacket uninterrupted around both head and cylinder. Engines with gaskets required a metal-to-metal contact face here, disrupting water flow.
The drawback to the monobloc head is that access to the inside of the combustion chamber (the upper volume of the cylinder) becomes difficult. Access through the cylinder bore is restricted for machining the valve seats, or simply for inserting angled valves. An even more serious restriction is that for the maintenance task of de-coking and re-grinding the valve seats, a regular task on older engines. Rather than removing the cylinder head from above, the mechanic must now remove pistons, connecting rods and the entire crankshaft from beneath.
One solution to this for side-valve engines was to place a screwed plug directly above each valve, and to access the valves through this (illustrated). The tapered threads of the screwed plug provided a reliable seal. For low-powered engines this was a popular solution for some years. It was difficult to cool this plug, as the water jacket didn't extend into the plug. As performance increased, it also became important to have better combustion chamber designs with less "dead space". One solution was to place the spark plug
in the centre of this plug, which at least made use of the space. However this also placed the spark plug further away from the main combustion chamber, leading to long flame paths and slower ignition.
During World War I
, development of the internal combustion engine progressed enormously. After the war, as civilian car production recommenced, the monobloc cylinder head was required less frequently. Only high-performance cars such as the Leyland Eight
of 1920 persisted with it. Bentley
and Bugatti
were other racing marques who notably adhered to them, through the 1920s and into the 1930s.
Aircraft engines at this time were beginning to use high supercharging pressures, increasing the stress on their head gaskets. Engines such as the Rolls-Royce Buzzard
used monobloc heads for reliability.
The last engines to make widespread use of monobloc cylinder heads were large air-cooled aircraft radial engine
s, such as the Wasp Major. These have individual cylinder barrels, so access as a monobloc is less restricted than on inline engine. As they are also of high specific power and require the utmost reliability, the advantages of the monobloc remained attractive.
The difficulties of machining, and maintaining, a monobloc cylinder head were always a severe drawback to it. As head gaskets became able to handle the heat and pressure necessary, the technique went out of use. It is almost unknown today.
s to allow for water jackets, but not both simultaneously. Most early engines, particularly those with more than four cylinders, had their cylinders cast as pairs or triplets of cylinders, then bolted to a single crankcase.
As casting techniques improved, the entire cylinder block of four, six or even eight cylinders could be cast as one. This was a simpler construction, thus cheaper to make, and the communal water jacket permitted closer spacing between cylinders. This also improved the mechanical stiffness of the engine, against bending and the increasingly important torsional twist, as cylinder numbers and engine lengths increased. In the context of aircraft engines, the non-monobloc precursor to monobloc cylinders was a construction where the cylinders (or at least their liners) were cast as individuals, and the outer water jacket was applied later from copper or steel sheet. This complex construction was expensive, but lightweight, and so it was only widely used for aircraft.
V engine
s remained with a separate block casting for each bank
. The complex ducting required for inlet manifold
s between the banks were too complicated to cast otherwise. For economy, a few engines, such as the V12 Pierce-Arrow
, were designed to use identical castings for each bank, left and right. Some rare engines, such as the Lancia
22½° narrow-angle V12 of 1919, did use a single block casting for both banks.
Modern cylinders, except for air-cooled engines and some V engine
s, are now universally cast as a single cylinder block.
Most engines, except some V engines, are now a monobloc of crankcase and cylinder block.
s use a monobloc design where the cylinder head, block, and half the crankcase share the same casting, termed 'uniblock' by Honda. One reason for this, apart from cost, is to produce an overall lower engine height. Being an air-cooled OHC design, this is possible thanks to current aluminum casting techniques and lack of complex hollow spaces for liquid cooling. The valves are vertical, so as to permit assembly in this confined space. On the other hand, performing basic repairs becomes so time-consuming that the engine can be considered disposable. Commercial-duty Honda GX-family engines (and their many popular knock-offs) have a more conventional design of a single crankcase and cylinder casting, with a separate cylinder head.
Honda produces many other head-block-crankcase monoblocs under a variety of different names, such as the GXV-series. They may all be externally identified by a gasket which would bisect the crankshaft on an approximately 45° angle.
Exhaust valve failure is common and, owing to the monobloc design, so labour-intensive to repair that the engine is normally discarded.
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
, cylinder block
Cylinder block
A cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
, or crankcase
Crankcase
In an internal combustion engine of the reciprocating type, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder, which in a multicylinder engine are usually integrated into one or several cylinder blocks...
are formed, usually by casting
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
, as a single integral unit, rather than being assembled later. This has the advantages of improving mechanical stiffness, and also improving the reliability of the sealing between them.
Monobloc techniques date back to the beginnings of the internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
. Use of the term has changed over time, usually to address the most pressing mechanical problem affecting the engines of its day. There have thus been three distinct uses of the technique:
- Cylinder head and cylinder
- Cylinder block
- Cylinder block and crankcase
In most cases, any use of the term describes a deliberate single-unit construction, opposed to the more common contemporary practice. Where the monobloc technique has later become the norm, the specific term falls from favour. It is now usual and un-noteworthy practice to use monobloc cylinders and crankcases, but a monobloc head (for a water-cooled inline engine at least) would be regarded as peculiar and obsolescent.
Cylinder head
Head gasket
A head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in...
is the most highly stressed static seal in an engine and was a source of considerable trouble in early years. The monobloc cylinder head forms both cylinder and head in one unit, thus avoiding the need for a seal.
Along with head gasket failure, one of the least reliable parts of the early petrol engine was the exhaust valve, which tended to fail by overheating and burning. A monobloc head could provide good water cooling, thus reduced valve wear, as it could extend the water jacket uninterrupted around both head and cylinder. Engines with gaskets required a metal-to-metal contact face here, disrupting water flow.
The drawback to the monobloc head is that access to the inside of the combustion chamber (the upper volume of the cylinder) becomes difficult. Access through the cylinder bore is restricted for machining the valve seats, or simply for inserting angled valves. An even more serious restriction is that for the maintenance task of de-coking and re-grinding the valve seats, a regular task on older engines. Rather than removing the cylinder head from above, the mechanic must now remove pistons, connecting rods and the entire crankshaft from beneath.
One solution to this for side-valve engines was to place a screwed plug directly above each valve, and to access the valves through this (illustrated). The tapered threads of the screwed plug provided a reliable seal. For low-powered engines this was a popular solution for some years. It was difficult to cool this plug, as the water jacket didn't extend into the plug. As performance increased, it also became important to have better combustion chamber designs with less "dead space". One solution was to place the spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
in the centre of this plug, which at least made use of the space. However this also placed the spark plug further away from the main combustion chamber, leading to long flame paths and slower ignition.
During World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
, development of the internal combustion engine progressed enormously. After the war, as civilian car production recommenced, the monobloc cylinder head was required less frequently. Only high-performance cars such as the Leyland Eight
Leyland Eight
The Leyland Eight was a luxury car produced by Leyland Motors from 1920 to 1923. The car was designed by the chief engineer of Leyland Motors, J.G. Parry-Thomas and his assistant Reid Railton, and was intended to be the finest car available...
of 1920 persisted with it. Bentley
Bentley 3 Litre
The 3 Litre was the sports car that put Bentley on the automotive map. It was a large car compared to the tiny, lightweight Bugattis then dominating racing, but its innovative technology and strength made up for its weight...
and Bugatti
Bugatti
Automobiles E. Bugatti was a French car manufacturer founded in 1909 in Molsheim, Alsace, as a manufacturer of high-performance automobiles by Italian-born Ettore Bugatti....
were other racing marques who notably adhered to them, through the 1920s and into the 1930s.
Aircraft engines at this time were beginning to use high supercharging pressures, increasing the stress on their head gaskets. Engines such as the Rolls-Royce Buzzard
Rolls-Royce Buzzard
|-See also:-Bibliography:* Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6....
used monobloc heads for reliability.
The last engines to make widespread use of monobloc cylinder heads were large air-cooled aircraft radial engine
Radial engine
The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel...
s, such as the Wasp Major. These have individual cylinder barrels, so access as a monobloc is less restricted than on inline engine. As they are also of high specific power and require the utmost reliability, the advantages of the monobloc remained attractive.
The difficulties of machining, and maintaining, a monobloc cylinder head were always a severe drawback to it. As head gaskets became able to handle the heat and pressure necessary, the technique went out of use. It is almost unknown today.
Cylinder block
Casting technology at the dawn of the internal combustion engine could reliably cast either large castings, or castings with complex internal coreCore (manufacturing)
A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in injection molding.An intriguing example...
s to allow for water jackets, but not both simultaneously. Most early engines, particularly those with more than four cylinders, had their cylinders cast as pairs or triplets of cylinders, then bolted to a single crankcase.
As casting techniques improved, the entire cylinder block of four, six or even eight cylinders could be cast as one. This was a simpler construction, thus cheaper to make, and the communal water jacket permitted closer spacing between cylinders. This also improved the mechanical stiffness of the engine, against bending and the increasingly important torsional twist, as cylinder numbers and engine lengths increased. In the context of aircraft engines, the non-monobloc precursor to monobloc cylinders was a construction where the cylinders (or at least their liners) were cast as individuals, and the outer water jacket was applied later from copper or steel sheet. This complex construction was expensive, but lightweight, and so it was only widely used for aircraft.
V engine
V engine
A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...
s remained with a separate block casting for each bank
Cylinder bank
Internal combustion piston engines are usually arranged so that the cylinders are in lines parallel to the crankshaft. Where they are in a single line, this is referred to as an inline or straight engine....
. The complex ducting required for inlet manifold
Inlet manifold
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...
s between the banks were too complicated to cast otherwise. For economy, a few engines, such as the V12 Pierce-Arrow
Pierce-Arrow
Pierce-Arrow was an American automobile manufacturer based in Buffalo, New York, which was active from 1901-1938. Although best known for its expensive luxury cars, Pierce-Arrow also manufactured commercial trucks, fire trucks, camp trailers, motorcycles, and bicycles.-Early history:The forerunner...
, were designed to use identical castings for each bank, left and right. Some rare engines, such as the Lancia
Lancia
Lancia Automobiles S.p.A. is an Italian automobile manufacturer founded in 1906 by Vincenzo Lancia and which became part of the Fiat Group in 1969. The company has a long history of producing distinctive cars and also has a strong rally heritage. Some modern Lancias are seen as presenting a more...
22½° narrow-angle V12 of 1919, did use a single block casting for both banks.
Modern cylinders, except for air-cooled engines and some V engine
V engine
A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...
s, are now universally cast as a single cylinder block.
Crankcase
As casting improved and cylinder blocks became a monobloc, it also became possible to cast both cylinders and crankcase as one unit. The main reason for this was to improve stiffness of the engine construction, reducing vibration and permitting higher speeds.Most engines, except some V engines, are now a monobloc of crankcase and cylinder block.
Modern engines - Combined block, head and crankcase
Light-duty consumer-grade Honda GC-family small engineSmall engine
A small engine is the general term for a wide range of internal combustion engines used to power lawn mowers, generators, concrete mixers and many other machines that require independent power sources. Most small engines are single-cylinder, V-twin, or flat-twin units...
s use a monobloc design where the cylinder head, block, and half the crankcase share the same casting, termed 'uniblock' by Honda. One reason for this, apart from cost, is to produce an overall lower engine height. Being an air-cooled OHC design, this is possible thanks to current aluminum casting techniques and lack of complex hollow spaces for liquid cooling. The valves are vertical, so as to permit assembly in this confined space. On the other hand, performing basic repairs becomes so time-consuming that the engine can be considered disposable. Commercial-duty Honda GX-family engines (and their many popular knock-offs) have a more conventional design of a single crankcase and cylinder casting, with a separate cylinder head.
Honda produces many other head-block-crankcase monoblocs under a variety of different names, such as the GXV-series. They may all be externally identified by a gasket which would bisect the crankshaft on an approximately 45° angle.
Exhaust valve failure is common and, owing to the monobloc design, so labour-intensive to repair that the engine is normally discarded.