Progress and Recent Trends in Homogeneous Charge CompressionIgnition (HCCI) Engines

Visakh R

CONTENTS What is HCCI? Importance of HCCI Pioneering Researches in HCCI Challenges Faced in HCCI Methodologies Used in Investigation of HCCI Evolution In Control Strategies Various Control Strategies Explained Dual Mode Transitions Recent Developments HCCI Prototypes Conclusion Reference

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 HCCISl.No AUTHOR Aim \ Input Output Result

1 Onishi et al

Investigate gasoline systems to increase combustion stability of two stroke engines.

Stable HCCI combustion between low and high loads at CR 7.5:1

Reduction in emissions and improved fuel economy

2 Noguchi et al

Spectroscopy on HCCI combustion, experimental work on two stroke engine

Species related to low temp auto ignition were found like O & HO2 radicals

Ignition at numerous points & no discernable flamefront.

3 Ryan et alInvestigate Diesel fueled HCCI. Used Port fuel injection.

Premature ignition and knocking;

Unburned HC emissions high; NOx reduced.

4 Mingfa yao et al

Evaluate the progress and current trends

Present state of research revealed

New strategies and challenges defined

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 Simulation1. Zero Dimensional single zone model2. Quasi Dimensional multi zone model3. One Dimensional engine cycle4. Multi Dimensional CFD Optical Diagnostics1. For In cylinder mixture formation2. For Combustion Process3. 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.

Contd…

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.

Contd… 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.

REFERENCE “Progress and recent trends in homogeneous charge

compression ignition engines” Ming Yao,Zhaolei Zheng, Haifeng Liu

“A parametric study on the emissions from an HCCI alternative combustion engine resulting from the auto-ignition of primary reference fuels” Hatim Machrafi , Simeon Cavadias, Jacques Amouroux.

“An experimental study of HCCI-DI combustion and emissions in a diesel engine with dual fuel” Junjun Ma, Xingcai Lü, Libin Ji, Zhen Huang

http://www-erd.llnl.gov/FuelsoftheFuture/pdf_files/hccirtc.pdf http://www.ecogeek.org/component/content/article/1631 http://www.greencarcongress.com/2004/05/keep_an_eye_on_

.html wikipedia.org

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