Reverse-flow cylinder head
Encyclopedia
A reverse-flow or non-crossflow cylinder head
is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction in order to exit the head. This is in contrast to the crossflow cylinder head
design.
(if applicable) are heated by the exhaust. This heating reduces the density
of the inlet charge and hence the volumetric efficiency
of the engine. In a spark-ignition Petrol engine
the heat also increases the likely hood of pre-ignition or detonation
which limits the allowable compression ratio
reducing both power
and efficiency
.
s and fasteners. This brings with it the problem that the exhaust ports have a tighter turn radius. This problem is somewhat offset by the larger port. Another popular solution, as used in the Leyland Mini and Holden 6 cylinder engines is the siamesed port. In this configuration one large port feeds 2 adjacent cylinders
. The gain in area comes from effectively removing the material between 2 adjacent ports. This solution encourages charge robbing, where one cylinder "robs" the charge from the port leaving the next one with less mixture. This happens because the 2 cylinders which share the port are not equally spaced in terms of firing order
. For example, the Leyland mini with its 1-3-4-2 firing order has the 1 and 2 inlets siamesed and the 3 and 4 inlets siamesed. First the number 3 sucks the mixture out of the port, then there is less left for number 4. While number 1 and 2 are sucking the port again fills with mixture and the process repeats leaving the number 1 and 4 cylinders always lacking mixture. Also the siamese port can make both acoustic and inertial ram tuning less effective due to the irregular pulse. One should keep in mind that larger ports are only necessary at higher rpm and that small ports are desirable at low rpm to improve air speed. Due to charge robbing and lower air speeds, large siamesed ports are more suited to high rpm race motors.
The heat problem can be minimized by staggering the ports in terms of height and using heat proof wraps and coatings on the exhaust header to the point that it poses a negligible problem. The heat can also be used as an advantage.
as a result.
Costs can be reduced in production engines by casting
the inlet and exhaust manifolds as one unit. This also transfers further heat to the inlet eliminating the need for manifold heating and other related devices. Such an engine is overall simpler and also has improved cold starting.
The main advantage of the reverse-flow cylinder head is that both the entering inlet charge end the exiting exhaust gas cause a tendency to swirl in the same direction in the combustion chamber
. In a crossflow head the inlet and exhaust gases promote swirl in opposite directions so that during overlap the swirl changes directions. The constant swirl during overlap which results in a reverse-flow cylinder head promotes better mixing and hence better scavenging of the end gas. The fact that the inlet charge must change direction before going out the exhaust makes it less likely that fresh mixture will make it out the exhaust before mixing during overlap. Overall this improves volumetric efficiency and reduces emissions.
is used, large port flow is not as important as it is when an engine is naturally aspirated. This means that the generally inferior flow of a reverse-flow head is less of a disadvantage. In the early days of turbo charging a reverse-flow head allowed the compressor
outlet of a turbocharger
to blow directly into the inlet manifold with either a blow-through or draw-through carburettor and no intercooler
. This allowed the use of shorter inlet plumbing which decreased turbo lag and lowered flow restriction. Modern turbocharged configurations utilizing intercoolers and fuel injection
are more difficult to plumb up to a reverse-flow head and are ideally suited to a cross flow head where the turbo is on the exhaust side of the engine, the charge crosses through an intercooler in front of the engine and into the inlet manifold
on the other side.
and Ford Inline 6
fans. In fact some Australian Ford enthusiasts consider the 250 2V reverse-flow head to be superior to the Honda
designed cross-flow head that replaced it. The reverse-flow head is a great example of the progress of modern engineering and the influence of trends on engineering
design.
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...
is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction in order to exit the head. This is in contrast to the crossflow cylinder head
Crossflow cylinder head
A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head...
design.
Disadvantages
The reverse flow design is generally considered to be inferior to a crossflow design in terms of ultimate engineering potential for two reasons. Firstly, there is limited space when inlet and exhaust ports are arranged in a line on one side of the head meaning a reduction in port area compared to a crossflow head. This mainly affects power delivery at high rpm by limiting airflow. Secondly, since inlet and exhaust manifolds are both on the same side of the engine and in close proximity, the inlet manifold and carburetorCarburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
(if applicable) are heated by the exhaust. This heating reduces the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the inlet charge and hence the volumetric efficiency
Volumetric efficiency
Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio of what quantity of fuel and air actually enters the cylinder during induction to the...
of the engine. In a spark-ignition Petrol engine
Petrol engine
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....
the heat also increases the likely hood of pre-ignition or detonation
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
which limits the allowable compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
reducing both power
Motive power
In thermodynamics, motive power is an agency, as water or steam, used to impart motion. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover. The term may also define something, as a locomotive or a...
and efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
.
Solutions
Engineering has found a number of solutions to the first problem such as staggering the ports by placing the inlet ports on a higher level than the exhausts. This way larger ports can be used while still leaving enough room for flangeFlange
A flange is an external or internal ridge, or rim , for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel...
s and fasteners. This brings with it the problem that the exhaust ports have a tighter turn radius. This problem is somewhat offset by the larger port. Another popular solution, as used in the Leyland Mini and Holden 6 cylinder engines is the siamesed port. In this configuration one large port feeds 2 adjacent cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
. The gain in area comes from effectively removing the material between 2 adjacent ports. This solution encourages charge robbing, where one cylinder "robs" the charge from the port leaving the next one with less mixture. This happens because the 2 cylinders which share the port are not equally spaced in terms of firing order
Firing order
The firing order is the sequence of power delivery of each cylinder in a multi-cylinder reciprocating engine.This is achieved by sparking of the spark plugs in a gasoline engine in the correct order, or by the sequence of fuel injection in a Diesel engine...
. For example, the Leyland mini with its 1-3-4-2 firing order has the 1 and 2 inlets siamesed and the 3 and 4 inlets siamesed. First the number 3 sucks the mixture out of the port, then there is less left for number 4. While number 1 and 2 are sucking the port again fills with mixture and the process repeats leaving the number 1 and 4 cylinders always lacking mixture. Also the siamese port can make both acoustic and inertial ram tuning less effective due to the irregular pulse. One should keep in mind that larger ports are only necessary at higher rpm and that small ports are desirable at low rpm to improve air speed. Due to charge robbing and lower air speeds, large siamesed ports are more suited to high rpm race motors.
The heat problem can be minimized by staggering the ports in terms of height and using heat proof wraps and coatings on the exhaust header to the point that it poses a negligible problem. The heat can also be used as an advantage.
Advantages
In carbureted engines, poorly atomized fuel reduces efficiency and power at lower rpm (at higher rpm the large air speed keeps the mixture in suspension). The inlet manifold of a reverse-flow cylinder head may be connected to the exhaust by a heat riser to transfer further heat, improving low rpm response and emissionsExhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
as a result.
Costs can be reduced in production engines 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...
the inlet and exhaust manifolds as one unit. This also transfers further heat to the inlet eliminating the need for manifold heating and other related devices. Such an engine is overall simpler and also has improved cold starting.
The main advantage of the reverse-flow cylinder head is that both the entering inlet charge end the exiting exhaust gas cause a tendency to swirl in the same direction in the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
. In a crossflow head the inlet and exhaust gases promote swirl in opposite directions so that during overlap the swirl changes directions. The constant swirl during overlap which results in a reverse-flow cylinder head promotes better mixing and hence better scavenging of the end gas. The fact that the inlet charge must change direction before going out the exhaust makes it less likely that fresh mixture will make it out the exhaust before mixing during overlap. Overall this improves volumetric efficiency and reduces emissions.
Forced Induction
When forced inductionForced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...
is used, large port flow is not as important as it is when an engine is naturally aspirated. This means that the generally inferior flow of a reverse-flow head is less of a disadvantage. In the early days of turbo charging a reverse-flow head allowed the compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
outlet of a turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
to blow directly into the inlet manifold with either a blow-through or draw-through carburettor and no intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
. This allowed the use of shorter inlet plumbing which decreased turbo lag and lowered flow restriction. Modern turbocharged configurations utilizing intercoolers and fuel injection
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
are more difficult to plumb up to a reverse-flow head and are ideally suited to a cross flow head where the turbo is on the exhaust side of the engine, the charge crosses through an intercooler in front of the engine and into the 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...
on the other side.
Summary of Uses
The reverse flow head was ideally suited to a production carbureted engine due to its low rpm performance and ease of manufacture. The design could be modified for high performance by porting (in particular siamese) and isolating the inlet manifold from the exhaust header. The configuration is also perfect for carbureted non-intercooled turbo-charging. However, the advent of fuel injection and electronic ignition has made most of the reverse-flow heads advantages redundant in a modern engine and as a result the design has lost its popularity. The reverse-flow head still enjoys immense popularity among enthusiasts including Leyland Mini, HoldenHolden
GM Holden Ltd is an automaker that operates in Australia, based in Port Melbourne, Victoria. The company was founded in 1856 as a saddlery manufacturer. In 1908 it moved into the automotive field, before becoming a subsidiary of the U.S.-based General Motors in 1931...
and Ford Inline 6
Straight-6
The straight-six engine or inline-six engine is a six-cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase...
fans. In fact some Australian Ford enthusiasts consider the 250 2V reverse-flow head to be superior to the Honda
Honda
is a Japanese public multinational corporation primarily known as a manufacturer of automobiles and motorcycles.Honda has been the world's largest motorcycle manufacturer since 1959, as well as the world's largest manufacturer of internal combustion engines measured by volume, producing more than...
designed cross-flow head that replaced it. The reverse-flow head is a great example of the progress of modern engineering and the influence of trends on engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
design.