01-07-2013, 04:13 PM
CAI ENGINES
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INTRODUCTION
CAI is abbreviation of Controlled Auto ignition.
Controlled auto-ignition (CAI) combustion technique is a newly developed low engine emission technology.
The first attempts to utilize CAI combustion were made to control irregular and misfiring combustion in 2-stroke SI engines at light loads by Japanese researchers during late 1970s.
One of the first reported works in CAI is attributed to Onishi et al. They developed a new technique, called “Active Thermo-Atmosphere Combustion (ATAC).”
The phenomenon of auto ignition, the curse of old petrol engines, is used here to cause instant ignition of the whole air-fuel mixture in the combustion chamber.
Meaning of CAI
It involves the auto-ignition and simultaneous combustion of a homogeneous air-fuel mixture.
In this chemical kinematics process is characterized by absence of flame propagation.
Rapid heat release rate.
Objectives of CAI
To reduce greenhouse gas such as CO2 .
To reduce NOx level in automobile exhaust.
In order to cope with strengthened global emission standards such as the SULEV (super ultra-low emission vehicle).
In order to meet CAFE (corporate average fuel economy) and the Kyoto Protocol, the demands for better fuel efficiency and lower emissions levels increased in the automotive industry.
Negative valve over lap
A negative valve overlap period is used to retain high amount of residual gases in the cylinder.
The exhaust valve is closed before the piston reaches top dead centre in the exhaust stroke and to prevent backflow of the burned gases in the intake system.
The intake valves open well after TDC.
Emissions with CAI
The emissions from an SI engine that operated in CAI mode at part loads are compared with the homogeneous stoichiometric SI engine using following:
Negative valve overlap ranging from 140-200º crank angle for CAI operation.
Residual gas fraction varied from 40 to 70%.
The intake air temperature was increased to 50º C by external heating.
Prototype of CAI
In 2007-2009, General Motors has demonstrated CAI with a modified 2.2 L engine installed in Opel Vectra and Saturn Aura. The engine operates in CAI mode at speeds below 60 miles per hour (97 km/h) or when cruising, switching to conventional spark-ignition when the throttle is opened, and produces fuel economy of 43 miles per imperial gallon (6.6 L/100 km) and carbon dioxide emissions of about 150 grams per kilometre, improving on 37 miles per imperial gallon (7.6 L/100 km) and 180 g/km of the conventional 2.2 L direct injection version.
Mercedes-Benz has developed a prototype engine called DiesOtto, with controlled auto ignition.
Volkswagen is developing engine for CAI operation called Gasoline Compression Ignition or GCI; it uses CAI when cruising and spark ignition when accelerating. This engine have been demonstrated in Touran prototypes, and the company expects them to be ready for production in about 2015.