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PROBLEMS FACED BY I.C ENGINES
1.Problem with S.I engine-poor part load
efficiency due to large losses during gas exchange, low combustion and thermo dynamical efficiency
2.Problem with C.I engine- Emissions of nitrogen oxides and particulates
HCCI ENGINE -AN IDEAL COMBINATION
HCCI ENGINE
Ideal combination of an engine type with efficiency of CI engine and very low emissions of that of SI engine with TWC

HCCI ENGINE PRINCIPLE
HCCI means that the fuel and air should be mixed before combustion starts and that the mixture is auto ignited due to the increase in temperature from the compression stroke.

HCCI is similar to SI in the sense that both engines use a premixed charge
HCCI is similar to CI engine both rely on auto ignition for combustion initiation
DIFFERENCE BETWEEN HCCI AND SI COMBUSTION
In S.I .Engine we have three zones
1 Burnt zone
2 Unburnt zone
3 Thin reaction zone
In hcci combustion entire mass in the cylinder reacts at once




There are two well defined flames and a sharp border between burned  and unburned zone
same engine when HCCI combustion was triggered by using regular gasoline A distributed chemiluminescence's image resulted
HCCI Combustion
REQUIREMENTS OF HCCI
a) The temperature after compression stroke should equal the auto ignition temperature of the fuel/air mixture.
(b) The mixture should be diluted enough to give reasonable burn rate
AUTO IGNITION TEMPERATURE OF FUELS AS A FUNCTION OF DILUTION

The auto ignition temperature has some correlation with the fuels’ resistance of knock in SI engines and thus the octane number.
With a too rich mixture the reactivity of the charge is too high. This means that the burn rate becomes Extremely high with richer mixtures. If an HCCI engine is run too rich the entire charge can be consumed within a fraction of a crank angle.
This gives rise to extreme pressure rise rates and hence mechanical stress and noise. With a high auto ignition temperature like that of natural gas, it is also possible that formation of NOx can be the load limiting factor.
NOx AS FUNCTION OF MAXIMUM TEMPERATURE

Very low emission levels are measured with ethanol.
If the combustion starts at a higher temperature like with natural gas, the temperature after combustion will also be higher for a given amount of heat released.
Partial oxidation of fuel to CO can occur at extremely lean mixtures
The oxidation of CO to CO2 requires a temperature of 1400-1500 K.
TEMPERATURE GOVERNANCE OF HCCI
Governed by 3 temperatures
Need to reach auto ignition temperature to get things started
Combustion should then increase the temperature to at least 1400 K
Temperature should not be increased to more than 1800 K to prevent NO formation
Laser based diagnostics were performed to analyze the difference in combustion between a perfectly homogeneous fuel/air mixture and one with small gradients
system setup with a laser generating a vertical laser sheet
The fuel distribution for the twocases
The fuel concentration with half the heat released with port fuel injection
Fuel distribution of at 50% heat released with mixing tank
LOCAL POSITIVE FEEDBACK IN TEMPERATURE
No concept of Flame propagationin HCCI
After studying numerous individual cycles it was concluded that the concept of flame propagation in HCCI could not be supported.
There will be a time lag between combustion starting point at different zones but new “hot-spots” show up randomly and the structures seen in the images are rather fixed i.e. do not move from image to image
Valve timing diagram for S.I engine
CYLINDER PRESSURE AS A FUNCTION OF CRANK ANGLE IN HCCI ENGINE
COMBUSTION CONTROLL IN HCCI
The HCCI engine require closed loop combustion control, CLCC. Closed loop control requires as always a sensor, control algorithm and control means.
The main parameter to control for HCCI is the combustion timing i.e. when in the cycle combustion
The rate of heat release for a range of timings
COMBUSTION PHASING
SIGNAL OBTAINED IN HCCI MODE
ADVANTAGES OF HCCI ENGINE
High compression ratio is used compression is fast, this gives high efficiency at low and part load as compared to S.I that has low efficiency at part load
Fuel consumption is very low. As very lean mixture is inducted into the cylinder, fuel economy is high
It produces very low amounts of NOR. the formation of NO strongly dependent on the combustion temperature
: It does not produce same levels of soot as the diesel engine because the charge is very well mixed and does not contain excess hid
DISADVANTAGES
The control combustion is more difficult in HCCI engines than in spark ignition or compression ignition engines.
The second problem with HCCI engines revolves around the factor that there is no triggering event, like a spark in SI or fuel injection in CI.
The fuel air mixture has to be lean to obtain the emission benefits, so HCCI is suitable for light and medium loads and speeds. To handle higher loads and speeds, more fuels need to be added to the mixture, but doing this would raise the combustion temperature and eliminates much of the environmental benefits
CONCLUSION
The Homogeneous Charge Compression Ignition, HCCI, combustion process is an interesting alternative to the conventional Spark Ignition and Compression Ignition processes.
The HCCI engine requires active control of the combustion process. Use of negative overlap is possible but often generates a limited operating range.
FUTURE
Faced with increasingly stringent government pollution standards as well as the realization that practical and affordably fuel cell technology is still many years’ away, researchers at the world’s major automakers and diesel engine manufacturers are working to determine whether HCCI technology will be technically and economically feasible.
. If so power plants based on this new combustion mode would serve as a potential bridge technology between today’s high emission diesel and gasoline fuelled piston engines and tomorrow’s ultra clean fuel cells