Stratified charge engine

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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.

Contents

Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fueled 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, meaning it contains the exact amount of air necessary for complete combustion of the fuel. This gives stable combustion, but it places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a much leaner mixture (less fuel or more air) with a homogeneous charge results in slower combustion and a higher engine temperature; this impacts on power and emissions, notably increasing nitrogen oxides or NOx.

In simple terms a stratified charge engine creates a richer mixture of fuel near the spark and a leaner mixture throughout the rest of the combustion chamber. The rich mixture ignites easily and in turn ignites the lean mixture throughout the rest of the chamber; ultimately allowing the engine to use a leaner mixture thus improving efficiency while ensuring complete combustion.

Advantages

Higher compression ratio

A higher mechanical compression ratio, or dynamic compression ratio with forced induction, can be used to improve thermodynamic efficiency. Because fuel is not injected into the combustion chamber until just before the combustion is required to begin, there is little risk of pre-ignition or engine knock.

Leaner burn

The engine can also run on a much leaner overall air/fuel ratio, using stratified charge, in which a small charge of a rich fuel mixture is ignited first and used to improve combustion of a larger charge of a lean fuel mixture.

Disadvantages

Disadvantages include:

Combustion management

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. [1] 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.

Charge stratification can also be achieved where there is no 'in cylinder' stratification: the inlet mixture can be so lean that it is unable to be ignited by the limited energy provided by a conventional spark plug. This exceptionally lean mixture can, however, be ignited by the use of a conventional mixture strength of 12-15:1, in the case of a petrol fuelled engine, being fed into a small combustion chamber adjacent to and connected to the main lean-mixture chamber. The large flame front from this burning mixture is sufficient to combust the charge. It can be seen from this method of charge stratification that the lean charge is 'burnt' and the engine utilising this form of stratification is no longer subject to ' knock' or uncontrolled combustion. The fuel being burnt in the lean charge is therefore not 'knock' or octane restricted. This type of stratification therefore can utilise a wide variety of fuels; the specific energy output being dependent only on the calorific value of the fuel.

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.

Comparison with diesel engine

It is worth comparing contemporary directly fueled petrol engines with direct-injection diesel engines. 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.

This comparison of 'burn' rates is a rather simplistic view. Although petrol and diesel engines appear similar in operation, the two types operate on entirely different principles. In earlier manufactured editions the external characteristics were obvious. Most petrol engines were carbureted, sucking the fuel/air mixture into the engine, while the diesel only sucked in air and the fuel was directly injected at high pressure into the cylinder. In the conventional four-stroke petrol engine the spark plug commences to ignite the mixture in the cylinder at up to forty degrees before top dead centre while the piston is still travelling up the bore. Within this movement of the piston up the bore, controlled combustion of the mixture takes place and the maximum pressure occurs just after top dead centre, with the pressure diminishing as the piston travels down the bore. i.e. the cylinder volume in relation to the cylinder pressure-time generation remains essentially constant over the combustion cycle. Diesel engine operation on the other hand inhales and compresses air only by the motion of the piston moving to top dead centre. At this point maximum cylinder pressure has been reached. The fuel is now injected into the cylinder and the fuel ' burn' or expansion is started at this point by the high temperature of the, now compressed, air. As the fuel burns it expands exerting pressure on the piston, which in turn develops torque at the crankshaft. It can be seen that the diesel engine is operating at constant pressure. As the gas expands the piston is also moving down the cylinder. By this process the piston and subsequently the crank experiences a greater torque, which is also exerted over a longer time interval than its petrol equivalent.

History

Brayton direct injecton 1887 Brayton direct injecton 1887.jpg
Brayton direct injecton 1887

The principle of injecting fuel directly into the combustion chamber at the moment at which combustion is required to start was first invented by George Brayton in 1887, but it has been used to good effect in petrol engines for a long time. Brayton describes his invention as follows: "I have discovered that heavy oils can be mechanically converted into a finely-divided condition within a firing portion of the cylinder, or in a communicating firing chamber." Another part reads "I have for the first time, so far as my knowledge extends, regulated speed by variably controlling the direct discharge of liquid fuel into the combustion chamber or cylinder into a finely-divided condition highly favorable to immediate combustion". This was the first engine to use a lean burn system to regulate engine speed / output. In this manner the engine fired on every power stroke and speed / output was controlled solely by the quantity of fuel injected.

Ricardo

Harry Ricardo first began working with the idea of a lean burn "stratified charge" engine in the early 1900s. In the 1920s he made improvements on his earlier designs.

Hesselman

An early example of gasoline direct injection was the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines used the ultra lean burn principle and injected the fuel in the end of the compression stroke and then ignited it with a spark plug, it was often started on gasoline and then switched over to run on diesel or kerosene. The Texaco Controlled Combustion System (TCCS) was a multifuel system developed in the 1950s which closely resembled the Hesselman design. The TCCS was tested in UPS delivery vans and was found to have an overall increase in economy of about 35%.

Honda

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 were 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

Jaguar Cars in the 1980s developed the Jaguar V12 engine, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ12 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..

Vespa

The Vespa ET2 scooter had a 50 cc two-stroke engine in which air was admitted through the transfer port and a rich fuel mixture was injected into the cylinder near the spark plug just before ignition. The injection system was purely mechanical, using a timed pumping cylinder and a non-return valve.

On its downward stroke it compresses the rich mixture to about 70 psi at which time the rising pressure raises a spring loaded poppet valve off its seat and the charge is squirted into the cylinder. There it is aimed at the spark plug area and ignited. The combustion pressure immediately shuts the spring-loaded poppet valve and from then on its (sic) just a "regular" stratified-charge ignition process with the flame front igniting those lean mixture areas in the cylinder. [2]

Volkswagen

Volkswagen currently uses stratified charge on its direct injection 1.0, 1.2, 1.4, 1.5, 1.8 and 2.0 litres TFSI (Turbo fuel stratified injection) gasoline engines, in combination with turbocharging.

Mercedes-Benz

Mercedes-Benz has been employing stratified charge engines with its Blue DIRECT system.

With the stratified-charge application, the 3.0L V-6 will continue to employ direct fuel injection, but the injectors have been redesigned to spray under higher pressure later in the intake stroke, just before compression, and the fuel is shaped to arrive in certain areas within the cylinder to optimize combustion. This strategy makes for an air-fuel mix within the chamber that is much leaner than with a conventional homogeneous-charge system that fills the chamber more uniformly before combustion.

Research

SAE International has published papers on experimental work with stratified charge engines. [3]

TFSI engines

Turbo fuel stratified injection (TFSI) is a trademark of the Volkswagen Group for a type of forced-aspiration ("turbo") engine where the fuel is pressure-injected straight into the combustion chamber in such a way as to create a stratified charge. FSI direct injection technology increases the torque and power of spark-ignition engines, makes them as much as 15 percent more economical and reduces exhaust emissions. [4]

Advantages

Some advantages of TFSI engines:

  1. Better fuel distribution and better fuel charge inside the combustion chamber
  2. During the injection process the fuel gets evaporated, cooling the cylinder chamber
  3. Cooling effect of the pressurized fuel allows for use of a lower octane fuel leading to a cost savings for the end user
  4. Higher compression ratios, which translates into more power
  5. Increased fuel combustion efficiency
  6. Higher power during pick-up of vehicle

Disadvantages

  1. Carbon build up behind the intake valves. Since fuel is directly injected inside the combustion chamber, it never gets a chance to wash any contaminants behind the valves. This results in excessive carbon build up over time, hindering performance. Some engines (like Toyota's Dynamic Force engines) combine direct injection with traditional multi port fuel injection to ameliorate this problem.
  2. More expensive - much higher pressure fuel pumps are required to inject the fuel directly into the cylinder. This requires fuel pressures of up to 200 bar, much greater than a traditional multiport injection setup (see direct injection) [5]

See also

Related Research Articles

<span class="mw-page-title-main">Compression ratio</span> Ratio of the volume of a combustion chamber from its largest capacity to its smallest capacity

The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.

<span class="mw-page-title-main">Diesel engine</span> Type of internal combustion engine

The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is called a compression-ignition engine. This contrasts with engines using spark plug-ignition of the air-fuel mixture, such as a petrol engine or a gas engine.

<span class="mw-page-title-main">Fuel injection</span> Feature of internal combustion engines

Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.

<span class="mw-page-title-main">Four-stroke engine</span> 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 a partial vacuum in the cylinder through its downward motion.
  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 port.

In spark-ignition internal combustion engines, knocking occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel–air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

Pre-ignition in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.

A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.

Indirect injection in an internal combustion engine is fuel injection where fuel is not directly injected into the combustion chamber.

Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air–fuel ratio may be as lean as 65:1. The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean-burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.

<span class="mw-page-title-main">Gasoline direct injection</span> Mixture formation system

Gasoline direct injection (GDI), also known as petrol direct injection (PDI), is a mixture formation system for internal combustion engines that run on gasoline (petrol), where fuel is injected into the combustion chamber. This is distinct from manifold injection systems, which inject fuel into the intake manifold.

Dieseling or engine run-on is a condition that can occur in spark-plug-ignited, gasoline-powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, drawing fuel through the carburetor, into the engine and igniting it without a spark.

Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction produces heat that can be transformed into work in a heat engine.

<span class="mw-page-title-main">Hot-bulb engine</span> Internal combustion engine

The hot-bulb engine, also known as a semi-diesel, 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.

<span class="mw-page-title-main">Model engine</span>

A model engine is a small internal combustion engine typically used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight, control line aircraft, or ground-running tether car model.

The term six-stroke engine has been applied to several alternative internal combustion engine designs that attempt to improve on traditional two-stroke and four-stroke engines. Claimed advantages may include increased fuel efficiency, reduced mechanical complexity, and/or reduced emissions. These engines can be divided into two groups based on the number of pistons that contribute to the six strokes.

<span class="mw-page-title-main">Two-stroke diesel engine</span> Engine type

A two-stroke diesel engine is a diesel engine that uses compression ignition in a two-stroke combustion cycle. It was invented by Hugo Güldner in 1899.

<span class="mw-page-title-main">Carbureted compression ignition model engine</span> Type of carbureted engine

A carbureted compression ignition model engine, popularly known as a model diesel engine, is a simple compression ignition engine made for model propulsion, usually model aircraft but also model boats. These are quite similar to the typical glow-plug engine that runs on a mixture of methanol-based fuels with a hot wire filament to provide ignition. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engines.

<span class="mw-page-title-main">Hesselman engine</span>

The Hesselman engine is a hybrid between a petrol engine and a diesel engine. It was designed and introduced in 1925 by Swedish engineer Jonas Hesselman.

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

<span class="mw-page-title-main">Internal combustion engine</span> 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. This process transforms chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.

References

  1. ↑ "32 (17) strat" (PDF). Archived from the original (PDF) on 2013-09-27. Retrieved 2014-05-10.
  2. ↑ "Motorcycle Online: Vespa ET2". 2005-07-28. Archived from the original on July 28, 2005. Retrieved 2014-05-10.{{cite web}}: CS1 maint: unfit URL (link)
  3. ↑ "Browse Papers on Stratified charge engines : Topic Results - SAE International". Topics.sae.org. Retrieved 2014-05-10.
  4. ↑ "Audi UK > Glossary > Engine & Driveline > FSI®". Archived from the original on April 28, 2009. Retrieved July 24, 2009.
  5. ↑ "Bosch Mobility Solutions".
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