Stratified charge engine
Encyclopedia
In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal combustion engines.
Conventionally, a four-stroke (petrol or gasoline) Otto cycle
engine is fuelled by drawing a mixture of air and fuel into the combustion chamber
during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a spark plug
at a predetermined moment near the top of the compression stroke.
In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx
.
If the Otto cycle
is abandoned, however, and fuel is injected
directly into the combustion-chamber during the compression stroke, the petrol engine is liberated from a number of its limitations.
First, a higher mechanical compression ratio
(or, with supercharged engines, maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock.
The engine may also run on a much leaner overall air/fuel ratio, using stratified charge.
Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder.
A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in a high-quality combustion of the much weaker mixture elsewhere in the cylinder.
Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty.
With the further impetus of tightening emissions legislation, the motor industry in Europe and north America has now switched completely to direct fuelling for the new petrol engines it is introducing.
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density
, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles.
History
The principle of injecting fuel directly into the combustion-chamber at the moment at which combustion is required to start was invented by Rudolf Diesel
, but it has been used to good effect in petrol engines for a long time. The Mercedes 300SL 'Gullwing' of 1952 used direct fuelling, though Mercedes-Benz subsequently switched to port fuelling for other models.
Honda
's CVCC
engine, released in the early 1970s models of Civic
, then Accord
and City
later in the decade, is a form of stratified charge engine that had wide market acceptance for considerable time. The CVCC system had conventional inlet and exhaust valves and a third, supplementary, inlet valve that charged an area around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate. At ignition a series of flame fronts shot into the very lean main charge, through the perforations, ensuring complete ignition. In the Honda City Turbo such engines produced a high power-to-weight ratio
at engine speeds of 7,000 rpm
and above.
Jaguar Cars in the 1980s developed the Jaguar V12 engine
, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ
12 and Jaguar XJS
models and used a stratified charge design called the 'May Fireball' in order to reduce the engine's very heavy fuel consumption.
Conventionally, a four-stroke (petrol or gasoline) Otto cycle
Otto cycle
An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....
engine is fuelled by drawing a mixture of air and fuel into 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...
during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a 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...
at a predetermined moment near the top of the compression stroke.
In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx
NOx
NOx is a generic term for the mono-nitrogen oxides NO and NO2 . They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures...
.
If the Otto cycle
Otto cycle
An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....
is abandoned, however, and fuel is injected
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....
directly into the combustion-chamber during the compression stroke, the petrol engine is liberated from a number of its limitations.
First, a higher mechanical 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...
(or, with supercharged engines, maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock.
The engine may also run on a much leaner overall air/fuel ratio, using stratified charge.
Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder.
A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in a high-quality combustion of the much weaker mixture elsewhere in the cylinder.
Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty.
With the further impetus of tightening emissions legislation, the motor industry in Europe and north America has now switched completely to direct fuelling for the new petrol engines it is introducing.
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles.
History
The principle of injecting fuel directly into the combustion-chamber at the moment at which combustion is required to start was invented by Rudolf Diesel
Rudolf Diesel
Rudolf Christian Karl Diesel was a German inventor and mechanical engineer, famous for the invention of the diesel engine.-Early life:Diesel was born in Paris, France in 1858 the second of three children of Theodor and Elise Diesel. His parents were Bavarian immigrants living in Paris. Theodor...
, but it has been used to good effect in petrol engines for a long time. The Mercedes 300SL 'Gullwing' of 1952 used direct fuelling, though Mercedes-Benz subsequently switched to port fuelling for other models.
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...
's CVCC
CVCC
CVCC is a trademark by the Honda Motor Company for an engine with reduced automotive emissions, which stood for "Compound Vortex Controlled Combustion". This technology allowed Honda's cars to meet United States emission standards in the 1970s without a catalytic converter...
engine, released in the early 1970s models of Civic
Honda Civic
The Honda Civic is a line of subcompact and subsequently compact cars made and manufactured by Honda. The Civic, along with the Accord and Prelude, comprised Honda's vehicles sold in North America until the 1990s, when the model lineup was expanded...
, then Accord
Honda Accord
The Honda Accord is a series of compact, mid-size and full-size automobiles manufactured by Honda since 1976, and sold in a majority of automotive markets throughout the world....
and City
Honda City
The Honda City is a subcompact car manufactured by the Japanese manufacturer Honda since 1981. Originally made for the Japanese, European and Australasian markets, the City was retired without replacement in 1994...
later in the decade, is a form of stratified charge engine that had wide market acceptance for considerable time. The CVCC system had conventional inlet and exhaust valves and a third, supplementary, inlet valve that charged an area around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate. At ignition a series of flame fronts shot into the very lean main charge, through the perforations, ensuring complete ignition. In the Honda City Turbo such engines produced a high power-to-weight ratio
Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
at engine speeds of 7,000 rpm
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
and above.
Jaguar Cars in the 1980s developed the Jaguar V12 engine
Jaguar V12 engine
Jaguar V12 piston engine was one of the premier powerplants of the 1970s and 1980s. It was first seen in the Series 3 Jaguar E-type of 1971 and was based loosely on an earlier design intended for a Le Mans car, the ill-fated Jaguar XJ13. The V12 was only Jaguar's second engine design to go into...
, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ
Jaguar XJ
Jaguar XJ is the designation that has been used for a series of luxury saloon cars sold under the British Jaguar marque. The first XJ was launched in 1968 and the designation has been used for successive Jaguar flagship models since then. The original model was the last Jaguar saloon to have had...
12 and Jaguar XJS
Jaguar XJS
The Jaguar XJ-S is a luxury grand tourer produced by the British manufacturer Jaguar from 1975 to 1996. The XJ-S replaced the E-Type in September 1975, and was based on the XJ saloon. It had been developed as the XK-F, though it was very different in character from its predecessor...
models and used a stratified charge design called the 'May Fireball' in order to reduce the engine's very heavy fuel consumption.
See also
- Lean burnLean burnLean burn refers to the use of lean mixtures in an internal combustion engine. The air-fuel ratios can be as high as 65:1, so the mixture has considerably less fuel in comparison to the stoichiometric combustion ratio ....
- Turbo fuel stratified injectionTurbo fuel stratified injectionTurbo fuel stratified injection denotes a type of forced-aspiration engine where the fuel is pressure-injected straight into the combustion chamber...
- Four-stroke engine