Homogeneous Charge Compression Ignition
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Homogeneous Charge Compression Ignition, or 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 translated by an engine into work and heat.
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[edit] Introduction
HCCI has characteristics of the two most popular forms of combustion used in IC engines: homogeneous charge spark ignition (gasoline engines) and 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.
[edit] History
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.
[edit] Operation
[edit] Methods
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 several different ways:
- High compression ratio
- Pre-heat induction gases
- Forced induction
- Retain or reinduct exhaust
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.
[edit] Advantages
- HCCI provides up to a 15-percent fuel savings, while meeting current emissions standards.[1]
- HCCI is closer to the ideal Otto cycle than spark-ignited combustion.
- Lean operation leads to higher efficiency than in spark-ignited gasoline engines
- Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. In fact, because peak temperatures are significantly lower than in typical spark ignited engines, NOx levels are almost negligible.
- Since HCCI runs throttleless, it eliminates throttling losses
[edit] Disadvantages
- High peak pressures
- High heat release rates
- Difficulty of control
- Limited power range
- High carbon monoxide and hydrocarbon pre-catalyst emissions
[edit] Control
Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult to control than other popular modern combustion methods.
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. An engine can be designed so that the ignition conditions occur at a desirable timing. However, this would only happen at one operating point. The engine could not change the amount of work it produces. This could work in a hybrid vehicle, but most engines must modulate their output to meet user demands dynamically.
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 reinducted exhaust.
Several approaches have been suggested for control:
[edit] Variable compression ratio
There are several methods of 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 and are expensive (no more true for the 2nd solution, the variable valve timing having been mastered). A 3rd proposed solution is being developed by the MCE-5 company (new rod).
[edit] Variable induction temperature
This technique is also known as fast thermal management. It is accomplished by rapidly varying the cycle to cycle intake charge temperature. It is also expensive to implement and has limited bandwidth associated with actuator energy.
[edit] Variable exhaust gas percentage
Exhaust gas can be very hot if retained or reinducted 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.
[edit] 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:
1. 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.
2. 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.
Whilst 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, whilst 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.
[edit] 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. 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 blends of fuel can be used, that will ignite at different times, resulting in lower combustion speed. The problem with this is the requirement to set up an infrastructure to supply the blended fuel. Alternatively, dilution, for example with exhaust, reduces the pressure and combustion rate at the cost of work production.
[edit] Power
In a gasoline engine, power can be increased by increasing the fuel/air charge. In a diesel engine, power can be increased by increasing the amount of fuel injected. The engines can withstand a boost in power because the heat release rate in these engines is slow. In HCCI however, the entire mixture burns nearly simultaneously. Increasing the fuel/air ratio will result in even higher peak pressures and heat release rates. Also, increasing the fuel/air ratio (also called the equivalence ratio) increases the danger of knock. 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 makes increasing the power in HCCI inherently challenging.
One way to increase power is to use different blends of fuel. 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.
[edit] Carbon Monoxide and Hydrocarbon emissions
Since HCCI operates on lean mixtures, the peak temperatures are lower in comparison to spark ignition and diesel engines. The low peak temperatures prevent the formation of NOx. However they 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 since the exhaust is still oxygen rich.
[edit] 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) simultaneaously. 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.
[edit] HCCI Prototypes
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 Opel Vectra and Saturn Aura with modified HCCI engines.[1]
- 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.[2]
- 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.[3][4]
- 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.[5]
[edit] References
- ^ 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
[edit] See also
- Mercedes DiesOtto engine
- Internal combustion engine
- Gasoline engine
- Diesel engine
- Variable valve timing
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