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Progress and Recent Trends in Homogeneous Charge CompressionIgnition (HCCI) Engines
What Is HCCI?
An HCCI engine is a mix of both conventional spark-ignition and diesel compression ignition technology.
HCCI is characterized by the fact that the fuel and air are mixed before combustion starts and the mixture auto-ignites as a result of the temperature increase in the compression stroke.
The resulting spontaneous burn produces a flameless energy release in a large zone almost simultaneously -- very different than the spark/gasoline burn or the compression/diesel burn.
IMPORTANCE OF HCCI
Homogeneous Charge Compression Ignition (HCCI) is a promising alternative combustion technology with high efficiency and lower NOx and particulate matter emissions.
It can operate using a variety of fuels -- gasoline, diesel, natural gas, biofuels, hydrogen.
HCCI combustion incorporates the advantages of both spark ignition (SI) engines and compression ignition direct injection (CIDI) engines.
However, there are still tough challenges in the successful operation of HCCI engines, such as controlling the combustion phasing, extending the operating range, and high unburned hydrocarbon and CO emissions.
Massive research throughout the world has led to great progress in the control of HCCI combustion. The first thing paid attention to is that a great deal of fundamental theoretical research has been carried out.
Pioneering Researches In HCCI
Challenges Faced by HCCI
Difficulty in Combustion phasing control.
High levels of Noise, UHC and CO emissions
Operation Range
Cold Start
Homogeneous Mixture Preparation
Methodologies used in investigation of HCCI
Numerical Simulation
Zero Dimensional single zone model
Quasi Dimensional multi zone model
One Dimensional engine cycle
Multi Dimensional CFD
Optical Diagnostics
For In cylinder mixture formation
For Combustion Process
For Emission Process
Chemical Kinetics Model
Multi Dimensional CFD with Chemical Kinetics Models
Optical Diagnostics For In Cylinder Mixture Formation
Optical Diagnostics For Combustion Process
Effect Of fuels, additives and fuel modifications on HCCI chemical kinetics process
One of the advantages of HCCI combustion is its intrinsic fuel flexibility. HCCI combustion has little sensitivity to fuel characteristics such as lubricity and laminar flame speed. Fuels with any octane or cetane number can be burned, although the operating conditions must be adjusted to accommodate different fuels, which can impact efficiency
HCCI combustion can be controlled and the HCCI operating range can be extended by burning different octane number fuel at different engine modes, i.e. burning low/high octane number fuel at low/ high load mode respectively. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load.
Stratification Combustion- inhomogenity is an important path to achieve clean & high efficiency combustion in engines.
Ignition promoters and inhibitors are available.
Exhaust Gas Recirculation (EGR) into intake is the most practical means of controlling charge temperature in an HCCI engine.
EGR enhances combustion, control heat release, delay auto-ignition timing, and thus lower peak cylinder pressure.
Evolution In Control Strategies 
The spontaneous and simultaneous combustion of fuel-air mixture need to be controlled.
No direct control methods possible as in SI or CI engines.
In Diesel HCCI, it is difficult to prepare homogeneous mixture because of the lower volatility, higher viscosity and lower resistance to auto ignition of diesel fuel.
In Gasoline HCCI, high octane rating of the fuel means it needs high ignition temperature, which brings the difficulty of auto-ignition by compression.
Control Methods
Various control methods are:
Variable compression ratio
Exhaust Gas Recirculation
Variable valve actuation
Control Methods Explained
Variable compression ratio method (VCR)
The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head.
VCR changes in-cylinder pressure and density, which can produce effects on auto-ignition of fuel, and by which the in-cylinder temperature is controlled.
Exhaust Gas Recirculation (EGR)
EGR mixes with fresh air as diluter can lead to increase of specific heat capacity in the cylinder. Hence, Ignition delay becomes longer.
In addition, flame temperature after ignition decrease, which is beneficial in reducing NOx emissions.
Control Methods Explained
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 volume and hence compression ratio changes
Dual Mode Transitions
Practical HCCI engines will need to switch to a 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
Recent Developments
EGR (Exhaust Gas Re-circulation) Can be adopted for higher efficiencies and lower HC and CO emissions.
The exhaust has dual effects on HCCI combustion.
It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work.
Reduce the CO and HC emissions
HCCI Prototypes
General Motors has demonstrated Opel Vectra and Saturn Aura with 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 on the VW Group 2.0-litre diesel engine but uses homogenous intake charge rather than traditional diesel injection. It requires the use of synthetic fuel to achieve maximum benefit. The second is called Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating. Both engines have been demonstrated in Touran prototypes, and the company expects them to be ready for production in about 2015.
HCCI Prototypes
In May 2008, General Motors gave Auto Express access to a Vauxhall 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 car goes into production. Official figures are not yet available, but fuel economy is expected to be in the region of 43mpg with carbon dioxide emissions of about 150 grams per kilometer, improving on the 37mpg and 180g/km produced by the current 2.2-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
Conclusion
Four main results from pioneering investigations:
HCCI combustion demonstrates a strong potential to improve thermal efficiency of gasoline fuelled engines and reduce NOx and soot emissions of diesel fuelled engines.
Chemical kinetics has a dominating role in HCCI
Various challenges to HCCI are to be overcome
HCCI concept can be extended to all new engine combustion modes achieving high efficiency and low emissions.
Numerical Simulation has become a powerful tool in realizing HCCI and seeking control strategies for HCCI, and has higher flexibility and lower cost compared with engine experiments.
HCCI can be applied to a variety of fuels types and the choice of fuel will have a significant impact on both engine design and control strategies.
Utilizing charge inhomogenity is an important path to achieve clean and high efficiency combustion in engines. Stratification strategy has the potential to extend HCCI operation range to higher loads.
Application of Optical Diagnostics has greatly accelerated developments in many aspects of HCCI research, including combustion processes and emissions formation.
Essential factor needed to achieve diesel fuelled HCCI combustion is mixture control. For gasoline fuelled HCCI, it is by the use of large levels of residual exhaust gases.