Homogeneous compression ignition

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Homogeneous charge compression ignition (engl. Homogeneous Charge Compression Ignition , abbreviated as HCCI , or Controlled Auto Ignition , abbreviated as CAI ), the concept referred to for an engine in which the combustion of a homogeneous mixture , takes place as a result of very high compression simultaneously throughout the combustion chamber. The goals of this development are to reduce consumption and pollutant emissions . This technology combines advantages from diesel and petrol technology.

Challenge

With this engine technology, the advantages of the diesel engine and gasoline engine are to be combined, but there are also challenges:

  • Compression ignition ( diesel engine ): Diesel engines are characterized by good fuel economy and, as a result, low fuel consumption with few pollutant emissions. The disadvantage of the diesel engine is the high mass of the engine with low power and increased design and manufacturing costs. The exhaust gas cleaning of the diesel engine is complex because it has an increased soot particle emission due to the injection pressure and does not work together with the three-way catalytic converter because of the over stoichiometric mixture.
  • Otto engine : Otto engines can be operated at a higher speed, which is why they offer more power with less mass, construction effort and manufacturing costs than diesel engines. The biggest disadvantage of the gasoline engine is the poor fuel economy, which leads to increased fuel consumption and poor exhaust gas behavior, but exhaust gas cleaning is easy to implement, since pollutants can be converted using a three-way catalytic converter in operating areas with a stoichiometric mixture.

functionality

At low and high loads, ignition is initiated via external ignition , as is usual with a gasoline engine . In the medium power range, the mixture combustion is initiated by HCCI. An electrically controlled camshaft adjustment regulates the extended valve control. Direct injection , compressor or turbocharging and variable compression are also used as required .

An HCCI engine works with a homogeneously (evenly) distributed mixture of fuel and air under very high pressure. Ignition is triggered by the temperature rising during compression (and any radicals remaining in the combustion chamber ). This ignition is otherwise desirable than the conventional gasoline engine and basis of the principle, which is why an ignition is not required by the spark plug in the HCCI region. Since HCCI operation is only possible in the partial load range , a spark plug is still required for mixed operation in engines using gasoline.

With diesel fuel powered engines operating with the HCCI process, run in the warm-up, idle and full load in the conventional diesel operation and at partial load according to the HCCI process. However, the HCCI range is more difficult to achieve with diesel fuel than with gasoline.

In the HCCI engine, the charge composition should be so uniform that combustion takes place simultaneously in the entire combustion chamber. This process can be controlled via several motor parameters. It is essential to avoid uncontrolled combustion because it leads to knocking combustion with a high pressure gradient. This can damage the engine and is also acoustically disruptive. An internal or external exhaust gas recirculation can be used to regulate the output and reduce the combustion speed ; this allows the combustion position to be controlled to a certain extent. In gasoline engines, internal exhaust gas recirculation is generally used to accelerate the ignition of the knock-proof fuel by free radicals. In diesel engines, exhaust gas is cooled and then recirculated to slow down the chemical reaction, as the lower temperature compensates for the high ignitability of diesel fuel. There are very high rates of pressure increase and peak pressures because the entire fuel-air mixture is converted almost simultaneously. Full load, but also higher partial load, cannot be operated in this way because this would place too much stress on the engine components.

Pollutants such as NO x and soot particles can be almost completely avoided thanks to the homogeneous compression ignition in the engine. Therefore, exhaust aftertreatment systems can be dispensed with if necessary. At the same time, the higher the compression, the higher the efficiency. However, emissions of carbon monoxide and unburned hydrocarbons cannot be avoided and are increased.

The compression ratio is always a compromise and depends on the fuel and the selected operating point. The ideal fuel for HCCI operation differs from the fuels previously traded. However, the previous fuels (petrol and diesel) can either be mixed or additives are used.

research

Companies and universities around the world are researching the possibilities of an engine with homogeneous compression ignition. The aim is, among other things, to enlarge the operating range of the homogeneous compression ignition, to improve power control and driving comfort, and to reduce emissions.

The prototype diesotto

The Diesotto engine (also Combined Combustion System or Combined Combustion Engines) is a Mercedes-Benz-specific name for engines with homogeneous compression ignition.

Technical specifications of the test engine from Daimler AG :

The Diesotto engine was shown for the first time in 2007 in the Mercedes-Benz F 700 research vehicle .

Series use

Honda used a homogeneous compression ignition process called Activated Radical Combustion for two-stroke engines , which ignites the mixture in the partial load range without spark ignition. This is achieved through an exhaust control, which has a knock sensor as an essential sensor element .

Since 2019, the Mazda3 has also been available as standard with a Skyactiv-X engine, which works according to the SPCCI system (Spark Controlled Compression Ignition).

Historical

Apart from the compression-ignition model engines, the first HCCI engine was the Lohmann auxiliary engine from 1949. Here, HCCI operation was achieved via variable compression.

literature

  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-23876-3 .
  • Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig 2000, ISBN 3-14-221500-X .

Individual evidence

  1. ↑ Mazda will build gasoline engines with compression ignition from 2019 heise.de, August 8, 2017, accessed on August 12, 2017
  2. ↑ Diesotto news from the diesel engine t-online.de, from January 27, 2014, accessed on August 12, 2017
  3. ↑ Honda CRM 250 AR Radikal-Kur motorradonline.de, December 29, 1997, accessed on August 12, 2017
  4. ↑ Test Honda CRM 250 AR A matter of the heart motorradonline.de, from March 13, 1998, accessed on August 12, 2017
  5. ↑ https://www.autozeitung.de/mazda/3/4-generation

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