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 energy that can be transformed in an engine into work and heat.

Automotive-HCCI-Engine-Laboratory

HCCI combines characteristics of conventional gasoline engine and diesel engines. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. 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 Nitrogen oxide emissions (NO
x) without a catalytic converter. Unburned hydrocarbon and carbon monoxide emissions still require treatment to meet automotive emission 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.[1]

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:

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

Disadvantages

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 (variable valve timing that enables the Miller cycle). Both approaches require energy to achieve fast response. Additionally, implementation is expensive, but is effective.[9] The effect of compression ratio on HCCI combustion has also been studied extensively.[10]

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.[11] 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.[12] 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.[13]

Valve actuation

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.

Fuel mixture

Another means to extend the operating range is to control the onset of ignition and the heat release rate[14][15] by manipulating the fuel itself. This is usually carried out by blending multiple fuels "on the fly" for the same engine.[16] Examples include blending of commercial gasoline and diesel fuels,[17] adopting natural gas [18] or ethanol ".[19] This can be achieved in a number of ways:

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 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.[20] 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.[21] The adoption of high EGR and diesel fuels with a greater resistance to ignition (more "gasoline like") enable longer mixing times before ignition and thus fewer rich pockets that produce soot and NO
x[20][21]

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.[22] 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).[23]

Power

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.[24] 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.[25]

Emissions

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.

Difference from knock

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. However, at high loads (i.e. high fuel/air ratios), knocking is a possibility even in HCCI.

Simulation of HCCI Engines

Computational models for simulating combustion and heat release rates of HCCI engines require detailed chemistry models.[17][26] 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.[27] 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.[28][29]

Prototypes

As of 2012, no HCCI engines were produced at commercial scale. However, several car manufacturers had functioning HCCI prototypes.

Other Applications

To date, few prototype engines run in HCCI mode; however, HCCI research resulted in advancements in fuel and engine development. Examples include:

See also

References

  1. ↑ "College of Engineering @ The University of Wisconsin-Madison, initiatives in energy, health, nanotechnology, security, and information technology". Engr.wisc.edu. Retrieved 2014-03-31.
  2. ↑ Zhao, Fuquan; Asmus, Thomas W.; Assanis, Dennis N.; Dec, John E.; Eng, James A.; Najt, Paul M. (2003). Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues. Warrendale, PA, USA: Society of Automotive Engineers. pp. 11–12. ISBN 0-7680-1123-X.
  3. ↑ Warnatz, Jürgen; Maas, Ulrich; Dibble, Robert W. (2006). Combustion: Physical and Chemical Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation (4th ed.). Berlin, Germany: Springer. pp. 175–176. ISBN 3-540-25992-9.
  4. ↑ Dec, John E.; Epping, Kathy; Aceves, Salvador M.; Bechtold, Richard L. (2002). "The Potential of HCCI Combustion for High Efficiency and Low Emissions". Society of Automotive Engineers. 2002-01-1923.
  5. ↑ Baumgarten, Carsten (2006). Mixture Formation in Internal Combustion Engines: Mixture Formation in Internal Combustion Engines. Birkhäuser. pp. 263–264. ISBN 3-540-30835-0.
  6. ↑ Johansson, Rolf; Blom, Daniel; Karlsson, Maria; Ekholm, Kent; Tunestal, Per (2008). "HCCI Engine Modeling and Control using Conservation Principles". Society of Automotive Engineers. 2008-01-0789.
  7. ↑ Stanglmaier, Rudolf (1999). "Homogeneous Charge Compression Ignition (Hcci): Benefits, Compromises, and Future Engine Applications". Society of Automotive Engineers. 1999-01-3682.
  8. ↑ Aceves, Salvador M.; Flowers, Daniel L.; Espinosa-Loza, Francisco; Martinez-Frias, Joel; Dec, John E.; Sjöberg, Magnus; Dibble, Robert W.; Hessel, Randy P. (2004). "Spatial Analysis of Emissions Sources for Hcci Combustion At Low Loads Using a Multi-Zone Model". Society of Automotive Engineers. 2004-01-1910.
  9. ↑ Haraldsson, Goran; Hyvonen, Jari; Tunestal, Per; Johansson, Bengt (2002). "Hcci Combustion Phasing in a Multi-Cylinder Engine Using Variable Compression Ratio". Society of Automotive Engineers. 2002-01-2858.
  10. ↑ Pitz, William J.; SM Aceves; JR Smith; CK Westbrook (1999). "Compression ratio effect on methane HCCI combustion". Journal of Engineering for Gas Turbines and Power-Transactions of the ASME. 212 (3): 569–574.
  11. ↑ Flowers, Daniel L.; S. M. Aceves; J. Martinez-Frias; J. R. Smith; M. Y. Au; J. W. Girard; R. W. Dibble (2001). "Operation of a four-cylinder 1.9 l propane-fueled homogeneous charge compression ignition engine: Basic operating characteristics and cylinder-to-cylinder effects". Society of Automotive Engineers. 2001-01-1895.
  12. ↑ Haraldsson, Goran; Jari Hyvonen; Per Tunestal; Bengt Johansson (2004). "HCCI Closed-Loop Combustion Control Using Fast Thermal Management". Society of Automotive Engineers. 2004-01-0943.
  13. ↑ Au, Michael; Girard, J. W.; Dibble, R.; Aceves, D. F. S. M.; Martinez-Frias, J.; Smith, R.; Seibel, C.; Maas, U. (2001). "1.9-liter four-cylinder HCCI engine operation with exhaust gas recirculation". Society of Automotive Engineers. 2001-01-1894.
  14. ↑ Controlling Heat Release Using Advanced Fuels
  15. ↑ Smallbone, Andrew; Amit Bhave; Neal M. Morgan; Markus Kraft; Roger Cracknell; Gautam Kalghatgi (2010). "Simulating combustion of practical fuels and blends for modern engine applications using detailed chemical kinetics". Society of Automotive Engineers. 2010-01-0572.
  16. ↑ Sebastian, Mosbach; Ali M. Aldawood; Markus Kraft (2008). "Real-Time Evaluation of a Detailed Chemistry HCCI Engine Model Using a Tabulation Technique". Combustion Science and Technology. 180 (7): 1263–1277. doi:10.1080/00102200802049414.
  17. 1 2 3 Blending practical fuels
  18. ↑ Natural gas combustion
  19. 1 2 ethanol/gasoline blending
  20. 1 2 3 Kalghatgi, G; Hildingsson, L.; Johansson, B. (2009). "Low NOx and low smoke operation of a diesel engine using gasoline-like fuels". ASME ICES2009.
  21. 1 2 3 "Partially-Premixed Compression Ignition (PPCI) and Low-Temperature Combustion (LTC) modes". Cmcl. Archived from the original on 2012-03-09. Retrieved 2016-04-06. 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.
  22. ↑ Mack, J. Hunter; Daniel L. Flowers; Bruce A. Buchholz; Robert W. Dibble (2005). "Investigation of HCCI combustion of diethyl ether and ethanol mixtures using carbon 14 tracing and numerical simulations". Proceedings of the Combustion Institute. 30: 2693–2700. doi:10.1016/j.proci.2004.08.136.
  23. ↑ Choi, GH; SB Han; RW Dibble (2004). "Experimental study on homogeneous charge compression ignition engine operation with exhaust gas recirculation". International Journal of Automotive Technology. 5 (3): 195–200.
  24. ↑ Sjoberg, Magnus; John E. Dec; Nicholas P. Cernansky (2005). "Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in Hcci Engines, Based on Multi-Zone Modelling and Experiments". Society of Automotive Engineers. 2005-01-0113.
  25. ↑ Yang, Jialin; Todd Culp; Thomas Kenney (2002). "Development of a Gasoline Engine System Using Hcci Technology - The Concept and the Test Results". Society of Automotive Engineers. 2002-01-2832.
  26. ↑ "Combustion Chemistry". Pls.llnl.gov. Retrieved 2014-03-31.
    http://www.cmclinnovations.com/?page_id=67
  27. ↑ Maigaard, P; Fabian Mauss; Markus Kraft (2003). "Homogeneous Charge Compression Ignition Engine: A Simulation Study on the Effects of Inhomogeneities". Journal of Engineering for Gas Turbines and Power. 125: 466–471. doi:10.1115/1.1563240.
  28. ↑ srm engine suite software
  29. 1 2 Modeling combustion processes
  30. ↑ Sam Abuelsamid RSS feed. "ABG Tech analysis and driving impression: GM's HCCI Engine". Autobloggreen.com. Retrieved 2014-03-31.
    Sam Abuelsamid RSS feed. "GM's HCCI engines now run from idle to 60 mph!". Green.autoblog.com. Retrieved 2014-03-31.
    "GM exploring different valving strategies to extend HCCI operation for high loads; benefits of a Positive Valve Overlap approach". Green Car Congress. 2011-05-03. Retrieved 2014-03-31.
  31. ↑ Michael Scarlett (2008-05-21). "Vauxhall Vectra 2.2 HCCI". Auto Express. Retrieved 2014-03-31.
  32. ↑ "2007 Frankfurt Auto Show: Mercedes-Benz F 700". Edmunds. 2007-09-11. Archived from the original on 2007-10-12.
  33. ↑ Christian Steinert, The German Car Blog (2007-06-27). "VW: Inside the secret laboratory". The German Car Blog. Archived from the original on 2013-05-18. Retrieved 2014-03-31.
  34. ↑ "We Have Ignition: Hyundai's Experimental Gas Engine Runs Without Spark Plugs - Tech Dept.". www.caranddriver.com. Retrieved 2015-11-09.
  35. ↑ "(Korean) 현대자동차가 세계시장에 내놓을 비장의 무기". chosun.com. Chosun Ilbo. 2015-04-23. Retrieved 2015-11-09.
  36. ↑ Wall Street Journal: Honda's Experimental Hybrid May Help in Race With Toyota
  37. ↑ "Oxy-Gen Combustion". Oxy-Gen Combustion. Retrieved 2014-03-31.
  38. ↑ "Cleaner than electric? Mazda talks up gasoline engine fuel economy ambitions for SkyActiv 2". Phys.org. Retrieved 2014-03-31.
  39. ↑ Flynn, Malcolm (2015-11-09). "Mazda SkyActiv-R rotary could use compression ignition- Car News". CarsGuide. Australia. Retrieved 2016-08-08.

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