The Effect of Intake Temperature on HCCI Operation Using Negative Valve Overlap

Homogeneous Charge Compression Ignition (HCCI) combustion in a small-scale engine (25 cc) was experimentally examined. Historically, HCCI combustion has been studied in engines sized for passenger vehicles and trucks (approximately 500 cc and larger). HCCI combustion in large-scale engines is characterized by higher efficiency than SI and CI combustion. High rates of heat flux, due to large surface area to volume ratio and engine body material, initially prevented the small-scale engine from operating in HCCI mode. The high level of heat transfer was overcome and sustained small-scale HCCI operation was achieved with n-heptane fuel. Large-scale HCCI engines utilize ultralow equivalence ratios to achieve high efficiencies. The small-scale HCCI engine could not operate with equivalence ratios lower than 0.73. Performance characteristics including power, efficiency, and NOx emissions of the small-scale HCCI engine were poorer than when operated in SI mode. Recommendations to overcome high rates of heat flux and increase small-scale HCCI engine efficiency are presented.

<div class="section abstract"><div class="htmlview paragraph">Gasoline direct injection (GDI) technology is admitted to be one of the most effective measures to improve the fuel economy for the spark ignition (SI) engines. Homogeneous Charge Compression Ignition (HCCI) combustion has advantages of low fuel consumption and ultra low NO<sub>x</sub> emissions. But the difficulty in the autoignition control and the narrow operation region inhibit the practical application of this technology. A hybrid combustion mode which combines SI mode and HCCI mode in separated working regions was regarded as a promising technology for HCCI engines. In addition, monitoring and providing feedback to the in-cylinder combustion characteristics is generally considered to be an effective method to improve and to optimize the combustion process. A lot of combustion information is included in the ion current generated by the in-cylinder combustion, and hence the ion current detection technique is considered to be a potential combustion feedback method. A strategy used in the SI/HCCI combustion mode switch based on ion current was established and experimented in a GDI-HCCI engine assisted by negative valve overlap (NVO). During the combustion mode switch process, by adjusting the key factors within the switch process, such as injection mass, injection timing and throttle opening, the switch process was optimized. The optimized SI-HCCI mode switch, not only the smoothness was improved, cylinder pressure amplitude, indicated mean efficiency pressure (IMEP), HC and NO<sub>x</sub> emissions were stable, but also the fuel economy and the emission performance were promoted.</div></div>

<div class="htmlview paragraph">The reduction of NOx emissions required by the future Euro 6 standards leads engine manufacturers to develop Diesel Homogeneous Charge Compression Ignition (HCCI) combustion processes. Because this concept allows reducing both NOx and particulates simultaneously, it appears as a promising way to meet the next environmental challenges.</div> <div class="htmlview paragraph">Unfortunately, HCCI combustion often increases CO and HC emissions. Conventional oxidation catalyst technologies, currently used for Euro 4 vehicles, may not be able to convert these emissions because of the saturation of active catalytic sites. As a result, such increased CO and HC emissions have to be reduced under standard levels using innovative catalysts or emergent technologies.</div> <div class="htmlview paragraph">The work reported in this paper has been conducted within the framework of the PAGODE project (PSA, IFP, Chalmers University, APTL, CRF, Johnson Matthey and Supelec) and financed by the European Commission. The objectives are to develop advanced new catalyst formulation for efficient CO/HC conversion at low temperature, possibly coupled with new emerging technologies based on non-thermal plasma concept and adapted to the specific HCCI exhaust emissions.</div> <div class="htmlview paragraph">This paper presents one of the first steps of the study: the definition of the oxidation catalyst boundary conditions under HCCI combustion. For this, we perform a detailed characterisation of the CO and HC emissions on a NADI™ dual mode engine (HCCI at low load and switch to conventional combustion at high load), based on a 2.2-liter, 4-cylinder for passenger car.</div> <div class="htmlview paragraph">The regulated pollutant emissions (CO, THC) have been measured with standard gas analyzer devices. Moreover, hydrocarbon speciation has been done with a C1-C9 gas chromatograph (GC) coupled with a FID system. In addition, aldehydes and ketones emissions have been analyzed by means of a High Pressure Liquid Chromatograph (HPLC).</div> <div class="htmlview paragraph">The results show that HCCI combustion tends to form higher methane emissions than conventional combustion. Moreover, high EGR rates lead to higher oxygenated emissions, in particular formaldehyde. If aldehydes are easily converted when the catalyst is warm, methane is not oxidized at these temperatures.</div> <div class="htmlview paragraph">The next steps of the program will be the definition, the fabrication and the tests of an HCCI-dedicated efficient oxidation catalyst to comply with CO and HC emissions standards. Non-thermal plasma reactor will be also mounted in the exhaust line to help the oxidation reaction.</div>

A hybrid spark ignition/homogeneous charge compression ignition engine model for simulating the spark ignition to homogeneous charge compression ignition transition in a four-cylinder gasoline engine is presented. This hybrid spark ignition/homogeneous charge compression ignition model has been developed to provide help in understanding the complicated spark ignition to homogeneous charge compression ignition transition process and in tackling the control problems associated with it. The model was built in Matlab/Simulink, coupled with a tabulated stochastic reactor model and a detailed chemical kinetics mechanism. Typical computational times encountered in the simulation using this model were of the order of seconds for one engine cycle. The model was validated in steady-state spark ignition and homogeneous charge compression ignition operation by experimental data, followed by model-based parametric studies in homogeneous charge compression ignition operation. Then, a spark ignition to homogeneous charge compression ignition mode transition was simulated using the model. The results showed good agreement with experimental data, in terms of cylinder pressure, combustion phasing, air/fuel ratio, residual gas fraction and nitrogen oxides. The high computational efficiency and relatively high accuracy of this model enable its good applicability in experimental design and controller development, and potential use in hardware-in-loop applications.

<div class="htmlview paragraph">Homogenous charge compression ignition (HCCI) has been identified as a promising way to increase the efficiency of the spark-ignited engine, while maintaining low emissions. The challenge with HCCI combustion is excessive pressure rise rate, quantified here with Ringing Intensity. Turbocharging enables increased dilution of the charge and thus a reduction of the Ringing Intensity. The engine used is an SI four cylinder base with 2.2<i>L</i> displacement and is equipped with a turbocharger. Combustion phasing control is achieved with individual intake/ exhaust cam phasing. Fuel injection with spray guided design is used. Cycle resolved combustion state is monitored and used for controlling the engine either in closed or open loop where balancing of cylinder to cylinder variations has to be done to run the engine at high HCCI load. When load is increased the NOx levels rise, the engine is then run in stoichiometric HCCI mode to be able to use a simple three-way catalyst. The fuel used is 95 RON pump gasoline and injection strategies are evaluated in order to maintain low soot levels and high efficiency. Limitations and benefits on operating range are examined between 1000 and 3000 rpm. This paper investigates how to extend the HCCI range and how to reduce the high pressure rise rate with: increased boost from turbocharging, external EGR and different injection strategies. A higher boost pressure was found to extend the load range. It is shown that the limitation from high RI, NOx or soot is not the same in all engine speed and load points. By turbocharging the engine in HCCI mode there is greater flexibility to increase the range of practical operating points.</div>

<div class="section abstract"><div class="htmlview paragraph">The concept of hot surface assisted compression ignition (HSACI) was previously shown to allow for control of combustion timing and to enable combustion beyond the limits of pure homogeneous charge compression ignition (HCCI) combustion. This work investigates the potential of HSACI to extend the operating limits of a naturally aspirated single-cylinder natural gas fueled HCCI engine.</div><div class="htmlview paragraph">A zero-dimensional (0D) thermo-kinetic modeling framework was set up and coupled with the chemical reaction mechanism AramcoMech 1.3. The results of the 0D study show that reasonable ignition timings in the range 0-12°CA after top dead center (TDC) in HCCI can be expressed by constant volume ignition delays at TDC conditions of 9-15°CA. Simulations featuring the two-stage combustion in HSACI point out the capability of the initial heat release as a means to shorten bulk-gas ignition delay.</div><div class="htmlview paragraph">Engine trials were conducted to map the operating limits in HCCI and HSACI mode for an engine speed of 1400 1/min as a function of intake air temperature (148-173°C) and relative air-fuel ratio (λ = 2.0-3.0). Results show that HSACI extends the lean limit by more than ∆λ = 0.4 and reduces the minimum required intake temperature by at least 5 K compared to HCCI. Comparative experiments of HCCI and HSACI reveal that HSACI benefits from higher engine load, lower ringing intensity and lower NO<sub>x</sub> emissions without deteriorating efficiency. Experimental data concerning the occurrence of instable combustion in HCCI and the role of initial heat release in HSACI agree with the trends predicted by the 0D models.</div></div>

Homogeneous Charge Compression Ignition (HCCI) is a new combustion mode with higher thermal efficiency and lower emission. With the recent development of experimental and theoretical investigations, HCCI operation for high cetane number fuels has attracted more attention. As a reference fuel, n-heptane is very easy to be ignited for high cetane number and shows obviously two-stage heat release in homogeneous charge compression ignition process due to chemical properties. HC, CO and NOx emissions of n-heptane HCCI combustion with intake temperature and excess air coefficient are studied in this paper. Intake temperature and excess air coefficient have a great impact on HCCI combustion and provide important approaches to widen HCCI operating range.

This study investigates cycle-by-cycle variations in a gasoline fuelled, homogeneous charge compression ignition (HCCI) engine with internal exhaust gas recirculation. In order to study the effects of exhaust-fuel reactions occurring prior to the main combustion event fuel was injected directly into the cylinder at two selected timings during the negative valve overlap period. The engine was operated as both autonomous HCCI and spark assisted HCCI (SA-HCCI). The primary interest in this work was the operating region where the engine is switched between HCCI and spark ignition modes, thus operation with stoichiometric air–fuel mixture, which is typical for this region, was considered. Cycle-by-cycle variations in both combustion timing and indicated mean effective pressure (IMEP) were investigated. It was found that long-period oscillations of the IMEP occur when fuel injection is started at early stages of the negative valve overlap period, and that these can be suppressed by delaying the start of injection. This behaviour remained even when fuel injection was split into early and late-negative valve overlap injections. Spark assisted operation allowed eliminating late combustion cycles, thus improving thermal efficiency. However, characteristic patterns of IMEP variations were found to be the same for both HCCI and SA-HCCI operations, irrespective of the adopted negative valve overlap fuel injection strategy, as evidenced by using symbol-sequence statistics.

<div class="htmlview paragraph">Homogeneous-charge compression-ignition (HCCI) technology has high potential to significantly reduce fuel consumption and NOx emissions over PFI engines. Control of the HCCI combustion process over the full range of conventional PFI operating conditions, however, has been a challenge. This study describes an HCCI-SI dual-mode engine system proposal based on new approaches to optimize the engine performance. A 0.658L single-cylinder engine was built and tested using these concepts. The engine was operated in HCCI mode from idle to 5.5 bar NMEP and up to 4750 rpm. NSFC in HCCI mode was about 175 g/kWh over most of the operating range except at very low load or near the high load boundary. At a part load of 1500 rpm and an equivalent BMEP of 2.62 bar, net indicated fuel efficiency was 50% higher than PFI engines and 30% higher than a prototype SC-DISI engine. Net specific NOx emissions were lower than 0.1 g/kWh below 4 bar NMEP, approximately two orders of magnitude lower than those from both SC-DISI and PFI engines. Boosting the intake pressure at loads above 4 bar NMEP leaned the mixture, significantly reducing NOx emissions and expanding the HCCI high-load operating region. Engine operation at light loads increased CO emissions sharply, but with both intake throttling and advanced combustion timing, CO emissions were reduced. Other HCCI combustion control issues and strategies were also investigated, such as the available thermal energy for intake heating at light load. Unboosted, the engine was tested in SI mode up to 7 bar NMEP with fuel efficiency higher than conventional PFI engines. Boosted with 92 RON fuel, the engine was operated up to 11.7 bar GMEP.</div>

Evaluation of Homogeneous Charge Compression Ignition (HCCI) autoignition development through chemiluminescence imaging and Proper Orthogonal Decomposition

<div class="section abstract"><div class="htmlview paragraph">The carbon-neutral biodiesel is a promising renewable substitute for fossil diesel that renders the traditional oxides of nitrogen-particulate matter (NO<sub>x</sub>-PM) trade-off into a unidirectional NO<sub>x</sub> control problem. Low-temperature combustion (LTC) modes such as homogenous charge compression ignition (HCCI) are attractive for obtaining ultra-low NO<sub>x</sub> and PM emissions. Studies on utilizing biodiesel fuel for HCCI combustion mode are sparsely available. Moreover, biodiesel emulsions in the HCCI combustion mode have not been attempted so far. Based on this premise, the present work explored the potential to utilize biodiesel and its emulsions having 20% and 25% water by volume under HCCI operating conditions. Biodiesel was prepared from a non-edible Karanja oil. The biodiesel emulsions were prepared using a heated magnetic stirrer apparatus with 3% by volume of the raw Karanja oil as a surfactant. A production light-duty diesel engine is modified to run in external mixture preparation based HCCI mode. An air-preheater and a fuel vaporizer aid in vaporizing the low volatile biodiesel injected into the inner surface of the vaporizer at 300 bar pressure using a common rail direct injection (CRDI) system. The engine compression ratio was fixed at 15, the maximum operable value with biodiesel. The results show that the engine could be operated only up to 20% of the rated load with biodiesel. With biodiesel-water emulsion, 30% of the rated load could be achieved. The maximum cylinder pressure was higher, and the heat release rate was advanced with emulsions compared to neat biodiesel. The brake specific fuel consumption was reduced by 35%, and the brake thermal efficiency increased by 2.4%, along with ultra-low NO<sub>x</sub> and smoke emissions with emulsions. The present study shows the potential to utilize biodiesel-water emulsions in HCCI mode with reduced exhaust emissions. Suitable charge dilution methods must be adopted to extend the HCCI operating load range with emulsions.</div></div>

Diethyl ether as an ignition improver for biogas homogeneous charge compression ignition (HCCI) operation - An experimental investigation

With the advantages of ultra-low emissions of oxides of nitrogen (NOX) and high thermal efficiency, the homogeneous charge compression ignition (HCCI) mode applied to marine diesel engine is expected to be one of the technical solutions to meet the International Maritime Organization (IMO) MARPOL73/78 Convention-Annex VI Amendment Tier III requirement. According to the NOX chemical reaction mechanism, taking a marine diesel engine as the application object, the numerical study on the NOX formation characteristics of n-heptane for HCCI combustion process is performed. The results indicate that NO is usually the main component in the generation and emissions of NOX with the n-heptane HCCI mode. The combustor temperature plays more important role in the proportion of NO generation and emission. Compared with the experimental data of conventional marine diesel engine, the emission reduction rate of NOX can achieve an average of more than 95% in using HCCI technology.

<div class="htmlview paragraph">One major challenge in Homogeneous Charge Compression Ignition (HCCI) combustion is the difficulty in controlling the timing of auto-ignition which is dependant on mixture conditions. Understanding the effect of modifying the properties of the engine charge on the start of combustion is essential to be able to predict and control the auto-ignition timing. The purpose of this work is to develop a realtime model for predicting HCCI auto-ignition timing.</div> <div class="htmlview paragraph">The standard Livengood and Wu Knock-Integral Method (KIM) is modified to work with values that are easier to measure compared with the instantaneous in-cylinder parameters required in the original KIM. This modified Knock-Integral Method (MKIM) is developed and is then parameterized using HCCI Thermokinetic Kinetic Model (TKM) simulations for a single cylinder engine. Estimating the MKIM parameters is done using an off-line optimization technique. Once the parameters have been identified, the MKIM needs only the rate of Exhaust Gas Recirculated (EGR), equivalence ratio, intake manifold temperature and intake manifold pressure to predict auto-ignition timing. The MKIM is validated with the experimental data from the single cylinder engine in HCCI operation by varying equivalence ratio, EGR level, engine speed, and intake temperature for three different blends of Primary Reference Fuels (PRF) at octane values of 0, 10 and 20.</div>

<div class="htmlview paragraph">Homogeneous Charge Compression Ignition (HCCI) is effective for the simultaneous reduction of soot and NOx emissions from diesel engine. In general, high octane number and volatility fuels (gasoline components or gaseous fuels) are used for HCCI operation, because very lean mixture must be formed during ignition delay of the fuel. However, it is necessary to improve fuel injection systems, when these fuels are used in diesel engine.</div> <div class="htmlview paragraph">The purpose of the present study is the achievement of HCCI combustion in DI diesel engine without the large-scale improvements of engine components. Various high octane number fuels are mixed with diesel fuel as a base fuel, and the mixed fuels are directly applied to DI diesel engine. At first, the cylinder pressure and heat release rate of each mixed fuel are analyzed. The ignition delay of HCCI operation decreases with an increase in the operation load, although that of conventional diesel operation does not almost varied. In addition, the mixed fuel containing much high volatility fuel produces the higher peak of heat release rate of premixed combustion.</div> <div class="htmlview paragraph">The exhaust emissions (CO, CO<sub>2</sub>, THC, NOx, PM) in case of each mixed fuel are measured. Low NOx emission can be achieved by HCCI operation using the mixed fuels despite low volatility of base fuel. However, PM emission is relatively high in case of HCCI operation, and then PM is separated into SOF and ISOF. As a result, it can be confirmed that the high PM emission is derived from the increase in SOF, and the high SOF emission results from the base fuel in the mixed fuels.</div>