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 energy that can be transformed in an engine into work and heat.
HCCI combines characteristics of conventional gasoline engine and diesel engines. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Modern direct injection diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI.
As in HCSI, HCCI injects fuel during the intake stroke. However, rather than using an electric discharge (spark) 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 (NO
x) without a catalytic converter. Hydrocarbons (unburnt fuels and oils) and carbon monoxide emissions still require treatment to meet automobile emissions control regulations.
Recent research has shown that the hybrid fuels combining different reactivities (such as gasoline and diesel) 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.
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.
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:
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 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.
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 (variable valve timing that enables the Miller cycle). 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.
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 (FTM). 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 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.
Variable valve actuation (VVA) 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:
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.
Another means to extend the operating range is to control the onset of ignition and the heat release rateby 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:
Compression Ignition Direct Injection (CIDI) 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 (PPCI) also known as Premixed Charge Compression Ignition (PCCI) is a compromise offering the control of CIDI combustion with the reduced exhaust gas emissions of HCCI, specifically lower soot. NO
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 (i.e. with exhaust gases) to reduce the pressure and combustion rates (and output).
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 (no spark plug required); 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.
In ICEs, 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 increasing the fuel/air ratio results in higher peak pressures and heat release rates. In addition, many viable HCCI control strategies require thermal preheating of the fuel, 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 technique is to use fuels with different autoignition properties. This lowers the heat release rate and peak pressures and makes it possible to increase the equivalence ratio. Another way is to thermally stratify the charge so that different points in the compressed charge have different temperatures and burn at different times, lowering the heat release rate and 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 SI engine at higher load conditions.
Because HCCI operates on lean mixtures, the peak temperature is much lower than that encountered in SI and diesel engines. This low peak temperature reduces the formation of NO
x, but it also leads to incomplete burning of fuel, especially near combustion chamber walls. This produces relatively high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst can remove the regulated species, because the exhaust is still oxygen-rich.
Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in an SI engine spontaneously ignite. This gas 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, thus forming a primitive thermoacoustic device where the resonance is amplified by the increased heat release during the wave travel similar to a Rijke tube.
A similar ignition process occurs in HCCI. However, rather than part of the reactant mixture igniting by compression ahead of a flame front, ignition in HCCI engines occurs due to piston compression more or less simultaneously in the bulk of the compressed charge. Little or no pressure differences occur between the different regions of the gas, eliminating any shock wave and knocking, but the rapid pressure rise is still present and desirable from the point of seeking maximum efficiency from near-ideal isochoric heat addition.
Computational models for simulating combustion and heat release rates of HCCI engines require detailed chemistry models.This is largely because ignition is more sensitive to chemical kinetics than to turbulence/spray or spark processes as are typical in SI and diesel 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 usefulness of considering the in-cylinder mixture as a single zone, resulting in the integration of 3D computational fluid dynamics codes such as Los Alamos National Laboratory's KIVA CFD code and faster solving probability density function modelling codes.
As of 2017, no HCCI engines were produced at commercial scale. However, several car manufacturers had functioning HCCI prototypes.
To date, few prototype engines run in HCCI mode, but HCCI research has resulted in advancements in fuel and engine development. Examples include:
In a combustion engine, the static compression ratio is calculated based on the relative volumes of the combustion chamber and the cylinder; that is, the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. The dynamic compression ratio is a more advanced calculation which also takes into account gasses entering and exiting the cylinder during the compression phase. The compression ratio is a fundamental specification for combustion engines.
The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion.
In engineering, the Miller cycle is a thermodynamic cycle used in a type of internal combustion engine. The Miller cycle was patented by Ralph Miller, an American engineer, US patent 2817322 dated Dec 24, 1957. The engine may be two- or four-stroke and may be run on diesel fuel, gases, or dual fuel.
A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NO
x) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. NO
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A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.
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Knocking in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
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Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1. The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean-burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.
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The SRM Engine Suite is an engineering software tool used for simulating fuels, combustion and exhaust gas emissions in internal combustion engine applications. It is used worldwide by leading IC engine development organisations and fuel companies. The software is developed, maintained and supported by CMCL Innovations, Cambridge, U.K.
An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful work. This replaced the external combustion engine for applications where weight or size of the engine is important.
One promising approach is that of Partially-Premixed Compression Ignition (PPCI) or Low Temperature Combustion (LTC) mode. Using this approach, smoke can be reduced in compression ignition engines by promoting the mixing of fuel and air prior to combustion. NOx levels can be reduced by reducing combustion temperatures by burning leaner, pre-mixed or using EGR, exhaust gas recirculation.