Module7:Advanced Combustion Systems and Alternative Powerplants
  Lecture 33:HCCI and CAI Engines
 

HCCI/CAI ENGINES

Stringent emission  standards and the need to reduce greenhouse gas, CO2 emissions from vehicles has led to intensive research on new combustion systems namely, the homogeneous charge compression ignition (HCCI) or controlled autoignition (CAI) engines. These combustion concepts have the following features;

  • HCCI/CAI involves autoignition of very lean homogeneous mixtures of fuel and air so that the combustion temperatures are low.
  • Due to low combustion temperatures NOx formation is negligibly small. NOx  formation is  two orders of magnitude lower  than those from the current SI and CI engines
  • Very little soot is formed as the homogeneous charge is burnt.
  • High fuel efficiencies similar to  DI diesel engines can be obtained as very lean mixtures are burned.

 The first attempts to utilize HCCI/CAI combustion were made to control irregular and misfiring combustion in 2-stroke SI engines at light loads by Japanese researchers during late 1970s. Autoignition of the homogeneous charge was obtained by retaining large amounts of hot residual gas containing partially oxidized hydrocarbons and active chemical species in the cylinder.  Honda motors applied this form of combustion on a motorcycle engine prototype during mid-1990s, which was termed   as ‘Active Radical Combustion (ARC)’. Fuel economy improvements of about 30% and HC reduction of 50% were obtained compared to normal 2-stroke engine operation. The autoignition of lean homogeneous charge has been called by a variety of names such as Active Thermo Atmosphere Combustion (ATAC), Premixed Charge Compression Ignition (PCCI), Premixed Lean Diesel Combustion (PREDIC), Active Radical Combustion (ARC), Controlled Autoignition (CAI), Homogeneous Charge Compression Ignition (HCCI) etc.

HCCI v/s CAI

Application of autoignition of lean homogeneous charge has been studied in the conventional gasoline as well as diesel engines. The processes adopted to auto-ignite homogeneous charge and the objectives of its application to SI and CI engines are some what different
In the gasoline engines, external heating of intake charge or use of hot residual gas has been employed to cause controlled autoignition of high octane gasoline or natural gas –air mixtures. Therefore, the auto-ignited combustion process when applied to gasoline engines has been termed as controlled autoignition (CAI).  The main objective of CAI application to the gasoline engines is reduction in fuel consumption and NOx emissions.
In the conventional diesel engines, the fuel air mixture is heterogeneous and compression of air to high temperature is used to auto-ignite diesel fuel. The diesel fuel has low self-ignition temperature. In application of this concept to diesel engines, the main approach is to premix as much fuel as possible before autoignition without encountering negative effects of auto ignition on combustion parameters and emissions.  Autoignition may be caused by other forms of heating of fuel-air mixture in addition to compression heating. This process when applied to diesel engines is usually called as homogeneous charge compression ignition (HCCI). The main objective of HCCI application in the diesel engines is to reduce NOx and particulate emissions.
It may be noted that fundamentally the HCCI and CAI processes are the same.