Homogeneous charge compression ignition
Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases chemical energy into a sensible form that can be transformed in an engine into work and heat.
HCCI has characteristics of the two most popular forms of combustion used in SI (spark ignition) engines: homogeneous charge spark ignition (gasoline engines) and CI engines: stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, the density and temperature of the mixture are raised by compression until the entire mixture reacts spontaneously. Stratified charge compression ignition also relies on temperature and density increase resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.
The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without an aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.
Recent research has shown that the use of two fuels with different reactivities (such as gasoline and diesel) can help solve some of the difficulties of controlling HCCI ignition and burn rates. RCCI or Reactivity Controlled Compression Ignition has been demonstrated to provide highly efficient, low emissions operation over wide load and speed ranges *.
HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for combustion to occur. Another example is the "diesel" model aircraft engine.
A mixture of fuel and air will ignite when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased by several different ways:
- High compression ratio
- Pre-heating of induction gases
- Forced induction
- Retained or re-inducted exhaust gases
Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures.
- HCCI provides up to a 30-percent fuel savings, while meeting current emissions standards.
- Since HCCI engines are fuel-lean, they can operate at a Diesel-like compression ratios (>15), thus achieving higher efficiencies than conventional spark-ignited gasoline engines.
- Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Actually, because peak temperatures are significantly lower than in typical spark ignited engines, NOx levels are almost negligible. Additionally, the premixed lean mixture does not produce soot.
- HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.
- In regards to gasoline engines, the omission of throttle losses improves HCCI efficiency.
- High in-cylinder peak pressures may cause damage to the engine.
- High heat release and pressure rise rates contribute to engine wear.
- The autoignition event is difficult to control, unlike the ignition event in spark ignition (SI) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively.
- HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.
- Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low in-cylinder temperatures) and trapped crevice gases, respectively.
Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult to control than other popular modern combustion engines, such as Spark Ignition (SI) and Diesel. In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel engines, combustion begins when the fuel is injected into compressed air. In both cases, the timing of combustion is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever the appropriate conditions are reached. This means that there is no well-defined combustion initiator that can be directly controlled. Engines can be designed so that the ignition conditions occur at a desirable timing. To achieve dynamic operation in an HCCI engine, the control system must change the conditions that induce combustion. Thus, the engine must control either the compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or re-inducted exhaust. Several control approaches are discussed below.
Variable compression ratio
There are several methods for modulating both the geometric and effective compression ratio. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This is the system used in "diesel" model aircraft engines. The effective compression ratio can be reduced from the geometric ratio by closing the intake valve either very late or very early with some form of variable valve actuation (i.e. variable valve timing permitting Miller cycle). Both of the approaches mentioned above require some amounts of energy to achieve fast responses. Additionally, implementation is expensive. Control of an HCCI engine using variable compression ratio strategies has been shown effective. The effect of compression ratio on HCCI combustion has also been studied extensively.
Variable induction temperature
In HCCI engines, the autoignition event is highly sensitive to temperature. Various methods have been developed which use temperature to control combustion timing. The simplest method uses resistance heaters to vary the inlet temperature, but this approach is slow (cannot change on a cycle-to-cycle basis). Another technique is known as fast thermal management (FTM). It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapidly mixing hot and cold air streams. It is also expensive to implement and has limited bandwidth associated with actuator energy.
Variable exhaust gas percentage
Exhaust gas can be very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work. Hot combustion products conversely will increase the temperature of the gases in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally.
Variable valve actuation
Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving finer control over the temperature-pressure-time history within the combustion chamber. VVA can achieve this via two distinct methods:
- Controlling the effective compression ratio: A variable duration VVA system on intake can control the point at which the intake valve closes. If this is retarded past bottom dead center (BDC), then the compression ratio will change, altering the in-cylinder pressure-time history prior to combustion.
- Controlling the amount of hot exhaust gas retained in the combustion chamber: A VVA system can be used to control the amount of hot internal exhaust gas recirculation (EGR) within the combustion chamber. This can be achieved with several methods, including valve re-opening and changes in valve overlap. By balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, it may be possible to control the in-cylinder temperature.
While electro-hydraulic and camless VVA systems can be used to give a great deal of control over the valve event, the componentry for such systems is currently complicated and expensive. Mechanical variable lift and duration systems, however, although still being more complex than a standard valvetrain, are far cheaper and less complicated. If the desired VVA characteristic is known, then it is relatively simple to configure such systems to achieve the necessary control over the valve lift curve. Also see variable valve timing.
Variable fuel ignition quality
Another means to extend the operating range is to control the onset of ignition and the heat release rate   by manipulating the fuel itself. This is usually carried out by adopting multiple fuels and blending them "on the fly" for the same engine . Examples could be blending of commercial gasoline and diesel fuels , adopting natural gas  or ethanol ". This can be achieved in a number of ways;
- Blending fuels upstream of the engine: Two fuels are mixed in the liquid phase, one with low resistance to ignition (such as diesel fuel) and a second with a greater resistance (gasoline), the timing of ignition is controlled by varying the compositional ratio of these fuels. Fuel is then delivered using either a port or direct injection event.
- Having two fuel circuits: Fuel A can be injected in the intake duct (port injection) and Fuel B using a direct injection (in-cylinder) event, the proportion of these fuels can be used to control ignition, heat release rate as well as exhaust gas emissions.
Direct Injection: PCCI or PPCI Combustion
Compression Ignition Direct Injection (CIDI) combustion is a well-established means of controlling ignition timing and heat release rate and is adopted in Diesel engines combustion. Partially Pre-mixed Charge Compression Ignition (PPCI) also known as Premixed Charge Compression Ignition (PCCI) is a compromise between achieving the control of CIDI combustion but with the exhaust gas emissions of HCCI, specifically soot . On a fundamental level, this means that the heat release rate is controlled preparing the combustible mixture in such a way that combustion occurs over a longer time duration and is less prone to knocking. This is done by timing the injection event such that the combustible mixture has a wider range of air/fuel ratios at the point of ignition, thus ignition occurs in different regions of the combustion chamber at different times - slowing the heat release rate. Furthermore this mixture is prepared such that when combustion occurs there are fewer rich pockets thus reducing the tendency for soot formation . The adoption of high EGR and adoption of diesel fuels with a greater resistance to ignition (more "gasoline like") enables longer mixing times prior to ignition and thus fewer rich pockets thus resulting in the possibility of both lower soot emissions and NOx  
High peak pressures and heat release rates
In a typical gasoline or diesel engine, combustion occurs via a flame. Hence at any point in time, only a fraction of the total fuel is burning. This results in low peak pressures and low energy release rates. In HCCI, however, the entire fuel/air mixture ignites and burns nearly simultaneously resulting in high peak pressures and high energy release rates. To withstand the higher pressures, the engine has to be structurally stronger and therefore heavier. Several strategies have been proposed to lower the rate of combustion. Two different fuels, with different autoignition properties, can be used to lower the combustion speed. However, this requires significant infrastructure to implement. Another approach uses dilution (i.e. with exhaust gases) to reduce the pressure and combustion rates at the cost of work production.
In both a spark ignition engine and diesel engine, power can be increased by introducing more fuel into the combustion chamber. These engines can withstand a boost in power because the heat release rate in these engines is slow. However, in HCCI engines the entire mixture burns nearly simultaneously. Increasing the fuel/air ratio will result in even higher peak pressures and heat release rates. In addition, many of the viable control strategies for HCCI require thermal preheating of the charge which reduces the density and hence the mass of the air/fuel charge in the combustion chamber, reducing power. These factors make increasing the power in HCCI engines challenging.
One way to increase power is to use fuels with different autoignition properties. This will lower the heat release rate and peak pressures and will make it possible to increase the equivalence ratio. Another way is to thermally stratify the charge so that different points in the compressed charge will have different temperatures and will burn at different times lowering the heat release rate making it possible to increase power. A third way is to run the engine in HCCI mode only at part load conditions and run it as a diesel or spark ignition engine at full or near full load conditions. Since much more research is required to successfully implement thermal stratification in the compressed charge, the last approach is being studied more intensively.
Because HCCI operates on lean mixtures, the peak temperatures are lower in comparison to spark ignition (SI) and Diesel engines. The low peak temperatures prevent the formation of NOx. This leads to NOx emissions at levels far less than those found in traditional engines. However, the low peak temperatures also lead to incomplete burning of fuel, especially near the walls of the combustion chamber. This leads to high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst would be effective at removing the regulated species because the exhaust is still oxygen rich.
Difference from Knock
Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in a spark ignited engine spontaneously ignite. The unburnt gas ahead of the flame is compressed as the flame propagates and the pressure in the combustion chamber rises. The high pressure and corresponding high temperature of unburnt reactants can cause them to spontaneously ignite. This causes a shock wave to traverse from the end gas region and an expansion wave to traverse into the end gas region. The two waves reflect off the boundaries of the combustion chamber and interact to produce high amplitude standing waves.
A similar ignition process occurs in HCCI. However, rather than part of the reactant mixture being ignited by compression ahead of a flame front, ignition in HCCI engines occurs due to piston compression. In HCCI, the entire reactant mixture ignites (nearly) simultaneously. Since there are very little or no pressure differences between the different regions of the gas, there is no shock wave propagation and hence no knocking. However at high loads (i.e. high fuel/air ratios), knocking is a possibility even in HCCI.
Simulation of HCCI Engines
The development of computational models for simulating combustion and heat release rates of HCCI engines has required the advancement of detailed chemistry models   . This is largely because ignition is most sensitive to chemical kinetics rather than turbulence/spray or spark processes as are typical in direct injection compression ignition or spark ignition engines. Computational models have demonstrated the importance of accounting for the fact that the in-cylinder mixture is actually in-homogeneous, particularly in terms of temperature. This in-homogeneity is driven by turbulent mixing and heat transfer from the combustion chamber walls, the amount of temperature stratification dictates the rate of heat release and thus tendency to knock . This limits the applicability of considering the in-cylinder mixture as a single zone resulting in the uptake of 3D computational fluid dynamics and faster solving probability density function modelling codes .
As of August 2007 there were no HCCI engines being produced in commercial scale. However several car manufacturers have fully functioning HCCI prototypes.
- General Motors has demonstrated HCCI with a modified Family II engine installed in Opel Vectra and Saturn Aura. GM is also researching smaller Family 0 engines for HCCI applications.
- Mercedes-Benz has developed a prototype engine called DiesOtto, 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 homogeneous 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.
- 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 kilometre, 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.
- In October 2005, the Wall Street Journal reported that Honda was developing an HCCI engine as part of an effort to produce a next generation hybrid car.
- Oxy-Gen Combustion, a UK-based Clean Technology company, has produced a full-load HCCI concept engine with the aid of Michelin and Shell 
Other Applications of HCCI Research
To date there have only been few prototype engines running in HCCI mode however the research efforts invested into HCCI research have disseminated into/resulted in direct advancements in fuel and engine development. Examples are;
- PCCI/PPCI combustion - A hybrid of HCCI and conventional diesel combustion offing more control over ignition and heat release rates with lower soot and NOx emissions .
- Advancements in fuel modelling - HCCI combustion is driven mainly by chemical kinetics rather than turbulent mixing or injection, this reduces the complexity of simulating the chemistry which results in fuel oxidation and emissions formation. This has led to increasing interest and development of chemical kinetics which describe hydrocarbon oxidation.
- Fuel blending applications- Due to the advancements in fuel modelling, it is now possible to carry out detailed simulations of hydrocarbon fuel oxidation, enabling simulations of practical fuels such as gasoline/diesel  and ethanol . Engineers can now blend fuels virtually and determine how they will perform in an engine context.
- Mercedes DiesOtto engine
- Internal combustion engine
- Gasoline engine
- Diesel engine
- Free-piston engine
- Variable valve timing
- Helical camshaft
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- ^ Controlling Heat Release Using Advanced Fuels
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- ^ a b c Blending practical fuels
- ^ Natural gas combustion
- ^ a b ethanol/gasoline blending
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- ^ a b c Emissions from PPCI engines
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- ^ srm suite - Advanced Combustion Simulator
- ^ ABG Tech analysis and driving impression: GM's HCCI Engine
- ^ 2007 Frankfurt Auto Show: Mercedes-Benz F 700
- ^ The German Car Blog: VW: Inside the secret laboratory
- ^ Auto Unleashed: Volkswagen's future eco-fiendly technologies
- ^ Auto Express: First drive of Vauxhall Vectra 2.2 HCCI
- ^ Wall Street Journal: Honda's Experimental Hybrid May Help in Race With Toyota
- Research, publications at Lund University, SE
- Research at Chalmers University of Technology, SE
- Research at Stanford University, USA
- Research, publications at University of Wisconsin, Madison, USA
- Research at University of California, Berkeley, USA
- Research at the University of Cambridge, UK
- Research at RWTH Aachen University, Germany
- Research at Eindhoven University of Technology, The Netherlands
- Advanced Simulation Technologies for HCCI/PPCI Combustion
- Advanced combustion concepts and bio-fuels for reduced CO2 emissions by cmcl innovations, February 2010.
- Impact of EGR on combustion and emissions in a CNG engine by cmcl innovations, March 2010.
- Improving engine stability and exhaust gas emissions through optimisation of control strategy: Examining the SI to HCCI transition by cmcl innovations, October 2010.
- Investigating fuel reforming in a HCCI engine with duel injection strategy by cmcl innovations, October 2010.
- Investigating the SI-HCCI transition multi-cycle multi-cylinder transient simulations by cmcl innovations, October 2010.
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