Stratified charge

The stratified-charge or charge stratification is a method of Otto engine technology , wherein the fuel (for. Example, gasoline ) is prepared so that in the region of the spark plug an ignitable mixture ( lambda λ is = 0.5 to 1.0), while the remaining Combustion chamber has a very lean, difficult-to-ignite mixture ( λ = 1.5 to 3.0). The total air ratio for four-stroke engines is between 1.2 and 1.6. Only after ignition of the rich part of the mixture do conditions arise which also ignite the rest of the mixture. Stratified charge engines can be viewed as hybrid engines (not to be confused with hybrid drives ) because they combine the spark ignition feature of the gasoline engine with the inhomogeneity of the fuel-air mixture of the diesel engine .

In more recent literature, the usefulness of stratified charging is questioned, as it is associated with considerable design effort for the exhaust gas aftertreatment system and the need for high-quality, low-sulfur fuels (Super Plus), which make them usable worldwide only to a limited extent. The main motivation for the use of stratified charge was an expected lower fuel consumption and the associated lower carbon dioxide emissions; However, practical use shows that engines with stratified charge consume no less fuel and thus emit no less carbon dioxide than conventional engines.

Today's procedure

In the stratified charge process, direct injection with a qualitative control (similar to diesel engines) is primarily used today . This regulation results in fuel consumption advantages in the partial load and lower speed range, since there are no throttle losses through the throttle valve . Development goals are the design of the combustion process with regard to an improvement of the efficiency and the reduction of the legally regulated pollutant emissions ( hydrocarbons , nitrogen oxides , carbon monoxide ).

Honda brought in 1974 with the CVCC process (Compound Vortex Controlled Combustion) one on a carburetor and torch nozzle based stratification process in series.

Stratified charging is also implemented in slot-controlled two - stroke engines , especially reverse-scavenged engines , in order to reduce fresh charge losses. The principle, also known as flushing reservoir (SpV), has been known since the 1920s and is increasingly being used in series where valve-controlled engines are too heavy or prone to errors due to increasingly strict emissions regulations. The stratified charge is based on the mixture in either the transfer ports vorzulagern fresh air or old gas (temporal layering) or the outlet during the whole by means of a fresh air curtain charge exchange shield (spatial layering). In the temporal stratification, the fresh air reaches the transfer ports from the fresh air ducts via pockets in the piston skirt. The fresh air supply can also be controlled by means of non-return valves on the overflows. The total air ratio in mixture-lubricated engines is much richer than in four-stroke engines (lambda approx. 0.8 to 0.9).

technology

The development of stratified charge engines began with the planned tightening of the exhaust gas legislation in the early 1970s at all well-known car manufacturers and various research institutes. The main aim was to comply with the law without a catalyst if possible ; at that time these were still in the early stages of development.

The principle of a three-valve stratified charge concept was first mentioned in 1918 in literature and patent applications.

Two different stratified charge processes emerged (although, of course, overlaps were possible, such as the Newhall process, for example).

Version a

Mixture compressing process with subdivided combustion chamber (secondary chamber and main combustion chamber). The advantages of this process were the defined separation of the secondary chamber, which is supplied with a rich mixture, and the main combustion chamber, which is supplied with a lean mixture, and thus a reliable mode of operation in the entire operating range. On the other hand, the fissured combustion chamber with an unfavorable ratio of surface area to volume, higher hydrocarbon emissions, increased fuel consumption and reduced performance were disadvantageous.

The Porsche SKS (stratified charge chamber system) and the PCI process from Volkswagen (both with direct injection into the secondary chamber) as well as the Honda CVCC process (Compound Vortex Controlled Combustion), of which only the CVCC was briefly produced in series and in USA and Japan was sold. In addition, there were a few other processes, which differed mainly in the area of ancillary chambers due to their detailed solutions (including Ricardo, Nilov, Gussak, Nissan, Mercedes-Benz, General Motors).

Before direct injection was introduced into series production in passenger car diesel engines in the 1990s, this principle was used almost exclusively in the form of prechamber and swirl chamber engines.

Version B

Air-compressing process with modified combustion chamber and direct fuel injection. The advantages of this process were the undivided combustion chamber, the performance-increasing direct injection and the possibility of driving higher air ratios and thus operating the engine in a large operating range exclusively with quality control and thus largely avoiding throttle losses. The disadvantage, however, was that it was not possible to achieve stable stratification and thus reliable engine operation in the entire operating range of the engine.

The Hesselman process and the

MAN FM -,
Deutz AD (all-fuel direct injection),
Ford PROCO- (Programmed Combustion) and
Texaco TCCS process (Texaco Controlled-Combustion System),

but none of them got beyond the experimental stage and were mass-produced.

Only in the last few years has this process become topical again with modern injection technology. Known for example are the GDI (Gasoline Direct Injection) process from Mitsubishi and the FSI (Fuel Stratified Injection) process from Volkswagen , although with the latter, a mixture stratification is only implemented in a small part of the operating range and with increasing load and speed again at a homogeneous level stoichiometric mixture is driven. VW has given up this procedure because of technical and principle-related problems. All current FSI models are only run in homogeneous operation (status: 2009).

literature

Michael C. Turkish: 3-Valve Stratified Charge Engines: Evolvement, Analysis and Progression. SAE paper 741163; International Stratified Charge Engine Conference, October 30 - November 1, 1974, Troy, Michigan (USA).
W. Brandstetter: Recent work in the field of stratified charge engines . Report on the international SAE stratified charge conference 1974 in Troy, Michigan (USA). MTZ Motortechnische Zeitschrift, 36 (1975), 4, pp. 116-121.
W. Brandstetter: Status of the development of stratified charge engines. Association of German Engineers. VDI-Z, 118 (1976), No. 19, pp. 885-892.
Richard van Basshuysen (Ed.): Gasoline engine with direct injection - process systems development potential , 3rd edition, Springer Vieweg, Wiesbaden, 2013, ISBN 9783658014087

Web links

Problems of the procedure and the state of development ( Frankfurter Allgemeine Zeitung of August 17, 2007)

Individual evidence

↑ Richard van Basshuysen (Ed.): Otto engine with direct injection - process · systems · development · potential , 3rd edition, Springer Vieweg, Wiesbaden, 2013, ISBN 9783658014087 , p. 1
^ Van Basshuysen: Otto engine with direct injection , 3rd edition, p. 21 ff.
↑ see also FAZ of August 17, 2007: The future belongs to lean operations

[1] Richard van Basshuysen (Ed.): Otto engine with direct injection - process · systems · development · potential , 3rd edition, Springer Vieweg, Wiesbaden, 2013, ISBN 9783658014087 , p. 1

[2] Van Basshuysen: Otto engine with direct injection , 3rd edition, p. 21 ff.

[3] see also FAZ of August 17, 2007: The future belongs to lean operations