Homogeneous charge compression ignition


Homogeneous charge compression ignition is a form of internal combustion in which well-mixed fuel and oxidizer are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction produces heat that can be transformed into work in a heat engine.
HCCI combines characteristics of conventional gasoline engines and diesel engines. Gasoline engines combine homogeneous charge with spark ignition, abbreviated as HCSI. Modern direct injection diesel engines combine stratified charge with compression ignition, abbreviated as SCCI.
As in HCSI, HCCI injects fuel during the intake stroke. However, rather than using an electric discharge to ignite a portion of the mixture, HCCI raises density and temperature by compression until the entire mixture reacts spontaneously.
Stratified charge compression ignition also relies on temperature and density increase resulting from compression. However, it injects fuel later, during the compression stroke. Combustion occurs at the boundary of the fuel and air, producing higher emissions, but allowing a leaner and higher compression burn, producing greater efficiency.
Controlling HCCI requires microprocessor control and physical understanding of the ignition process. HCCI designs achieve gasoline engine-like emissions with diesel engine-like efficiency.
HCCI engines achieve extremely low levels of oxides of nitrogen emissions without a catalytic converter. Hydrocarbons and carbon monoxide emissions still require treatment to meet automobile emissions control regulations.
Recent research has shown that the hybrid fuels combining different reactivities can help in 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.

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. 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. Another example is the "diesel" model aircraft engine.

Operation

Methods

A mixture of fuel and air ignites when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased in several different ways:
  • Increasing 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.

Advantages

  • Since HCCI engines are fuel-lean, they can operate at diesel-like compression ratios, thus achieving 30% higher efficiencies than conventional SI gasoline engines.
  • Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Because peak temperatures are significantly lower than in typical SI engines, levels are almost negligible. Additionally, the technique does not produce soot.
  • HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.
  • HCCI avoids throttle losses, which further improves efficiency.

    Disadvantages

  • Achieving cold start capability.
  • High heat release and pressure rise rates contribute to engine wear.
  • Autoignition is difficult to control, unlike the ignition event in SI and diesel engines, which are controlled by spark plugs and in-cylinder fuel injectors, respectively.
  • HCCI engines have a small torque range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.
  • Carbon monoxide and hydrocarbon pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation and trapped crevice gases, respectively.

    Control

HCCI is more difficult to control than other combustion engines, such as 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 pre-compressed air. In both cases, combustion timing is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever sufficient pressure and temperature are reached. This means that no well-defined combustion initiator provides direct control. Engines must be designed so that ignition conditions occur at the desired timing. To achieve dynamic operation, the control system must manage the conditions that induce combustion. Options include 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.

Compression ratio

Two compression ratios are significant. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This system is 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 variable valve actuation. Both approaches require energy to achieve fast response. Additionally, implementation is expensive, but is effective. The effect of compression ratio on HCCI combustion has also been studied extensively.

Induction temperature

HCCI's autoignition event is highly sensitive to temperature. The simplest temperature control method uses resistance heaters to vary the inlet temperature, but this approach is too slow to change on a cycle-to-cycle frequency. Another technique is fast thermal management. It is accomplished by varying the intake charge temperature by mixing hot and cold air streams. It is fast enough to allow cycle-to-cycle control. It is also expensive to implement and has limited bandwidth associated with actuator energy.

Exhaust gas percentage

Exhaust gas is 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 output. Hot combustion products conversely increase gas temperature in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally.

Valve actuation

Variable valve actuation extends the HCCI operating region by giving finer control over the temperature-pressure-time envelope within the combustion chamber. VVA can achieve this via either:
  • Controlling the effective compression ratio: VVA on intake can control the point at which the intake valve closes. Retarding past bottom dead center changes the compression ratio, altering the in-cylinder pressure-time envelope.
  • Controlling the amount of hot exhaust gas retained in the combustion chamber: VVA can control the amount of hot EGR within the combustion chamber, either by valve re-opening or changes in valve overlap. Balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, makes it possible to control the in-cylinder temperature.
While electro-hydraulic and camless VVA systems offer control over the valve event, the componentry for such systems is currently complicated and expensive. Mechanical variable lift and duration systems, however, although more complex than a standard valvetrain, are cheaper and less complicated. It is relatively simple to configure such systems to achieve the necessary control over the valve lift curve.

Fuel mixture

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 blending multiple fuels "on the fly" for the same engine. Examples include blending of commercial gasoline and diesel fuels, adopting natural gas or ethanol. This can be achieved in a number of ways:
  • Upstream blending: Fuels are mixed in the liquid phase, one with low ignition resistance and a second with greater resistance. Ignition timing varies with the ratio of these fuels.
  • In-chamber blending: One fuel can be injected in the intake duct and the other directly into the cylinder.

    Direct Injection: PCCI or PPCI Combustion

Compression Ignition Direct Injection combustion is a well-established means of controlling ignition timing and heat release rate and is adopted in diesel engine combustion. Partially Pre-mixed Charge Compression Ignition also known as Premixed Charge Compression Ignition is a compromise offering the control of CIDI combustion with the reduced exhaust gas emissions of HCCI, specifically lower soot. The heat release rate is controlled by preparing the combustible mixture in such a way that combustion occurs over a longer time duration making it less prone to knocking. This is done by timing the injection event such that a range of air/fuel ratios spread across the combustion cylinder when ignition begins. Ignition occurs in different regions of the combustion chamber at different times - slowing the heat release rate. This mixture is designed to minimize the number of fuel-rich pockets, reducing soot formation. The adoption of high EGR and diesel fuels with a greater resistance to ignition enable longer mixing times before ignition and thus fewer rich pockets that produce soot and

Peak pressure and heat release rate

In a typical ICE, 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 over a much smaller time interval, resulting in high peak pressures and high energy release rates. To withstand the higher pressures, the engine has to be structurally stronger. Several strategies have been proposed to lower the rate of combustion and peak pressure. Mixing fuels, with different autoignition properties, can lower the combustion speed.
However, this requires significant infrastructure to implement. Another approach uses dilution to reduce the pressure and combustion rates.
In the divided combustion chamber approach , there are two cooperating combustion chambers: a small auxiliary and a big main.
A high compression ratio is used in the auxiliary combustion chamber.
A moderate compression ratio is used in the main combustion chamber wherein a homogeneous air-fuel mixture is compressed / heated near, yet below, the auto-ignition threshold.
The high compression ratio in the auxiliary combustion chamber causes the auto-ignition of the homogeneous lean air-fuel mixture therein ; the burnt gas bursts - through some "transfer ports", just before the TDC - into the main combustion chamber triggering its auto-ignition.
The engine needs not be structurally stronger.