Homogeneous Charge Compression Ignition Enginehcci

8/13/2019 Homogeneous Charge Compression Ignition Enginehcci

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SEMINARon

An Experimental Study of HCCIEngine


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Contents Importance

Working principle

Starting of HCCI engines Control methods of HCCI

Dual mode transitions

Characteristics

Recent developments

Conclusion


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HCCI Importance

SI engines have very low NOx and PM emissions CI engines have high efficiency

Homogeneous Charge Compression Ignition (HCCI) is

a promising alternative combustion technology with

high efficiency and lower NOx and particulate matteremissions


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Principle HCCI is characterized by the fact that the fuel and air

are mixed before combustion starts and the mixtureauto-ignites as a result of the temperature increase inthe compression stroke

Optical diagnostics research shows that HCCI

combustion initiates simultaneously at multiple siteswithin the combustion chamber and that there is nodiscernable flame propagation.


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Comparison of SI and HCCI combustion

Spark Ignition HCCI


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HCCI Concept


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HCCI POTENTIAL

1. High efficiency, no knock limit on compression ratio.

2. Low NOx and no NOx after treatment systems required.

3. Low PM emissions, no need for PM filter.4. HCCI provides up to a 15-percent fuel savings, while

meeting current emissions standards.

5. HCCI engines can operate on gasoline, diesel fuel, andmost alternative fuels.

6. In regards to CI engines, the omission of throttle lossesimproves HCCI efficiency.


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HCCI BARRIERS1. The auto-ignition event is difficult to control,

unlike the ignition event in spark -ignition(SI)and diesel engines which are controlled byspark plugs and in-cylinder fuel injectors,respectively.

2. HCCI engines have a small power range,

constrained at low loads by lean flammabilitylimits and high loads by in-cylinder pressurerestrictions

3. High HC and CO emissions.


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Starting HCCI engines Charge does not readily auto ignite cold engines.

Early proposal was to start in SI mode and run in

HCCI mode. It involves the risk of knocking and cylinder failure at

high compression ratios.

Now intake air pre-heating with HE and burner

system allows startup in HCCI mode withconventional starter.


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Starting HCCI engines


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Control methods of HCCI combustion The spontaneous and simultaneous combustion of

fuel-air mixture need to be controlled.

No direct control methods possible as in SI or CI

engines. Various control methods are:

Variable compression ratio

Variable induction temperature

Variable valve actuation


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Control methods of HCCI combustion Variable compression ratio method

The geometric compression ratio can be changed with amovable plunger at the top of the cylinder head. This

concept used in diesel model aircraft engine.

Variable induction temperature The simplest method uses resistance heater to

vary inlet temperature. But this method is slow Now FTM (Fast Thermal Management) is used. It

is accomplished by rapidly varying the cycle tocycle intake charge temperature by rapid mixing.


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FTM Control method

Combustion timing can be controlled byadjusting balance of hot and cold flow


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Variable valve actuation (VVA)

This method gives finer control within combustion

chamber Involves controlling the effective pressure ratio. It

controls the point at which the intake valve closes.If the closure is after BDC, the effective volumeand hence compression ratio changes.

Control methods of HCCI combustion


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Control methods of HCCI combustion


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When auto-ignition occurs too early or withtoo much chemical energy, combustion is too

fast and high in-cylinder pressures candestroy an engine. For this reason, HCCI istypically operated at lean overall fuel mixtures

This restricts engine operation at high loads.


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Practical HCCI engines will need to switch toa conventional SI or diesel mode at very low

and high load conditions due to dilution limits

Two modes:

HCCI-DI dual mode

HCCI-SI dual mode


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SI mode transitions

It equips VVA and spark ignition system

Operates in HCCI mode at low to medium loads andswitches into SI mode at higher loads

Transition is not very stable and smooth


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DI-HCCI

Long ignition delay and rapid mixing are required toachieve diluted homogeneous mixture.

Combustion noise and NOx emissions were reducedsubstantially without an increase in PM.

Combustion phasing is controlled by injection timing.

Thus DI-HCCI proves to be promising alternative forconventional HCCI with good range of operation.


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Combustion characteristics


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Emission characteristics


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Engine performance


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Engine performance


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Recent developments in HCCI

Turbo charging initially proposed toincrease power

Challenges for turbo charging1. Exhaust gas temperatures low (300 to 350 c) because of

high compression ratio.

2. Post turbine exhaust gas temperature must be high enough

to preheat intake fuel-air mixture in HE.3. Low available compressor pressure ratio.


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Solution for turbo charging1. Use VGT (Variable Geometry Turbine) which allows for a

greater range of turbine nozzle area, better chance toachieve high boost.

2. Combining turbo charging and super charging may bebeneficial.

EGR (Exhaust Gas Re-circulation) Can be

adopted for higher efficiencies and lower HCand CO emissions.


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The exhaust has dual effects on HCCIcombustion. It dilutes the fresh charge, delaying ignition and reducing

the chemical energy and engine work.

Reduce the CO and HC emissions.


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HCCI prototypes

General Motors has demonstrated Opel Vectra and Saturn Aurawith modified HCCI engines.

Mercedes-Benz has developed a prototype engine called Dies Otto,with controlled auto ignition. It was displayed in its F 700 concept

car at the 2007 Frankfurt Auto Show Volkswagen are developing two types of engine for HCCI operation.

The first, called Combined Combustion System or CCS, is based onthe VW Group 2.0-litre diesel engine but uses homogenous intakecharge rather than traditional diesel injection. It requires the use ofsynthetic fuel to achieve maximum benefit. The second is called

Gasoline Compression Ignition or GCI; it uses HCCI when cruisingand spark ignition when accelerating. Both engines have beendemonstrated in Touran prototypes, and the company expectsthem to be ready for production in about 2015.


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HCCI prototypes

In May 2008, General Motors gave Auto Express access to aVauxhall Insignia prototype fitted with a 2.2-litre HCCI engine,which will be offered alongside their ecoFLEX range of small-

capacity, turbocharged petrol and diesel engines when the cargoes into production. Official figures are not yet available, butfuel economy is expected to be in the region of 43mpg withcarbon dioxide emissions of about 150 grams per kilometre,improving on the 37mpg and 180g/km produced by the current2.2-litre petrol engine. The new engine operates in HCCI mode

at low speeds or when cruising, switching to conventional spark-ignition when the throttle is opened


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Conclusions

HCCI-DI combustion with n-heptane/dieseldual fuel is a 3-stage combustion process

consisting of cool flame, HCCI combustionand diffusive combustion.

Increase of premixed ratio, shortens the NTC,increases the peak in-cylinder pressure and

temperature and rises the highest heatrelease rate of HCCI combustion phase


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Conclusions

NOx emissions decrease firstly at lowpremixed ratios and exhibit a trend of

increasing at higher premixed ratios.

Pre-mixed ratio has no significant effect onsoot emission and the soot emission couldremain at the same level but then have apeak value with a certain higher premixedratio relating to the equivalence ratio.


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Conclusions

The change of CO with premixed ratio ismainly depending on whether the premixed

equivalence ratio exceeds the critical value.UHC increases almost linearly with thepremixed ratio mainly due to the incompleteoxidation in the boundary layer and the

crevices.


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Conclusions

The IMEP increases with the increase ofpremixed ratio at low to medium loads.

The indicated thermal efficiency showsdeterioration at high load with large premixed

ratios.


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Thank you

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Homogeneous Charge Compression Ignition Enginehcci