Closed Loop Control of SI/HCCI Combustion Mode Switch Based on Ion Current Feedback

<div class="htmlview paragraph">A naturally aspirated in-line six-cylinder 2.9-litre Volvo engine is operated in Homogeneous Charge Compression Ignition (HCCI) mode, using camshafts with low lift and short duration generating negative valve overlap. This implementation requires only minor modifications of the standard SI engine and allows SI operation outside the operating range of HCCI. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. A heat exchanger is utilized to heat or cool the intake air, not as a means of combustion control but in order to simulate realistic variations in ambient temperature. The combustion is monitored in real time using cylinder pressure sensors.</div> <div class="htmlview paragraph">HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. However, for a practical application the intake temperature will vary both geographically and from time to time. It is therefore very important to gain knowledge of how this variation affects the combustion process.</div> <div class="htmlview paragraph">The operating range in HCCI mode for this specific engine stretches from 650 rpm to 4000 rpm and from 1 bar IMEP to 4 bar IMEP. The emissions of NO<sub>x</sub> are approximately two orders of magnitude lower in HCCI mode compared to SI mode. The emissions of CO are slightly lower while the emissions of HC are slightly higher for HCCI operation. When run in HCCI mode the engine's efficiency is improved throughout the operating range by as much as 10 percentage units compared to SI operation. The effect of inlet air temperature variation on HCCI combustion through negative valve overlap is insignificant when the engine runs well inside of the HCCI operating range. The effect is however prominent when the engine is operated close to the border of the operating range.</div>

<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>

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="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>

<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>

<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>

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>

<div class="section abstract"><div class="htmlview paragraph">The Homogeneous Charge Compression Ignition (HCCI) combustion eliminates the issues of higher particulate matter and nitrogen oxides emissions that prevail in the traditional compression ignition (CI) combustion mode. The complete replacement of traditional fuels with renewable fuels for internal combustion engines is challenging because significant infrastructure changes in the production and delivery systems are required to ensure renewable fuel availability and economic feasibility. Thus, the use of renewable acetone blended with traditional gasoline has been proposed in the present study to smoothen the transition from the traditional CI to the HCCI engines. HCCI experiments were performed in a light-duty diesel engine at 1500 rpm rated speed. By varying the volumetric proportion of the acetone in the gasoline from 20% to 40%, the HCCI engine load range from 20%-60% was achieved, significantly higher than the limited diesel HCCI load range of 20%-38%. An ignition-quality enhancer, 2-Ethylhexyl nitrate, at 6 vol.%, was blended in the acetone-gasoline to prevent the higher cyclic IMEP fluctuations at the lower engine loads. The empirical correlation of the start of combustion (CA10) established on a rapid compression machine facility was validated for the HCCI engine. The combustion phasing (CA50) was predicted using CA10 and fresh air equivalence ratio. A double-Weibe function was developed to characterize specifically the high-temperature heat release rate during the HCCI combustion and determine the extent of its fast-burning core region. An in-depth energy balance study was conducted to elucidate the reason for the indicated thermal efficiency trends. The acetone-gasoline-fuelled HCCI engine generated smoke and nitrogen oxides emissions that were less than 0.0007 g/kW-h and 2.5 g/kW-h, respectively, at all the engine loads. This study concluded that lower exhaust emissions could be obtained without compromising engine performance when the acetone-gasoline blends are used as fuels for operating the HCCI engine.</div></div>

<div class="htmlview paragraph">To limit pollutants from transport and to reach a low level of exhaust emission, Low Temperature Combustion (LTC) modes such as Homogeneous Charge Compression Ignition (HCCI) are being studied. HCCI process is based on the auto-ignition of a highly diluted air/fuel mixture. It allows very low level of NO<sub>x</sub> and particulate emissions. But several problems must be solved: firstly the limited combustion range, the difficulty to control the combustion process, the noise but also the high HC and CO emissions. Two ways appear as very promising to improve HCCI combustion: engine technology and fuel formulation.</div> <div class="htmlview paragraph">In this work, attention is paid on initiation and HCCI combustion control through fuel formulation. This article shows the effect of acetal addition (and in particular butylal addition) in a Diesel base fuel.</div> <div class="htmlview paragraph">It seems to be difficult to have only an “HCCI engine” because the operation zone of the engine is too small. More likely, it will be a Diesel/HCCI engine. The advantage of acetal is to reduce significantly particulate emissions of a Diesel engine. This is an advantage for a Diesel/HCCI engine, when the engine works in Diesel combustion but it is necessary to get a better understanding of the influence of acetal addition in a Diesel base fuel during the HCCI combustion.</div> <div class="htmlview paragraph">Butylal can have a good potential to improve the HCCI combustion by delaying the initiation of a Diesel base fuel but due to its versatile aspect, it is difficult to control, for all the experimental conditions, its effect on the initiation and the HCCI combustion of a Diesel base fuel.</div>

<div class="section abstract"><div class="htmlview paragraph">Advanced combustion concepts that rely on the lean-burn approach are a proven solution for increasing the efficiency and reducing the harmful emissions of SI engines. The pre-chamber spark ignited (PCSI) engines utilize high ignition energy of the multiple jets penetrating from the pre-chamber, to enable fast and stable combustion of lean mixture in the main chamber. The combustion is still governed by the flame propagation, so the dilution level and efficiency benefits are highly restricted by strong decrease of laminar flame speeds. Homogeneous charge compression ignition (HCCI) combustion allows a higher dilution level due to rapid chemically driven combustion, however the inability to directly control the ignition timing has proven to be a major setback in HCCI deployment. The addition of the spark plug, to improve the controllability and widen the operating range, resulted in a combustion concept known as spark-assisted compression ignition (SACI), however spark ignition of lean mixtures is very difficult even at close to HCCI conditions. Since one of the features of active pre-chamber is the ability to control mixture dilution at the spark plug location, a combustion concept called pre-chamber induced HCCI combustion (PC-HCCI) that combines pre-chamber ignition and HCCI combustion is proposed. In this concept a near stoichiometric mixture in the pre-chamber is spark ignited and the combustion in pre-chamber triggers kinetically controlled combustion of lean main combustion chamber mixture. As a first step of the research, numerical modelling of the proposed combustion concept is made by employing 3D-CFD and 1D/0D simulation models. The main purpose of the modelling is to define main geometrical and operating parameters required for achieving a pre-chamber induced HCCI combustion and to predict possible benefits of employing such combustion concept.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The purpose of this paper is to find a way to extend the high load limit of homogeneous charge compression ignition (HCCI) combustion. This paper presents the effect of in-cylinder flow and stratified mixture on HCCI combustion by experiments and three-dimensional computer fluid dynamics coupled with a detailed chemical reaction calculation. The first study was conducted using a rapid compression and expansion machine (RCEM) equipped with a flow generation plate to create in-cylinder turbulent flow and with a control unit of in-cylinder wall temperature to create in-cylinder temperature distribution. The study assesses the effect of the turbulent flow and the temperature distribution on HCCI combustion. In the second study, the numerical simulation of HCCI combustion was conducted using large eddy simulation coupled with a detailed chemical reaction calculation. The study analyzes the interaction between in-cylinder turbulent flow and mixture distribution and HCCI combustion. The result shows that turbulent flow effects a change in the distribution of chemical species which is a trigger of combustion initiation.</div></div>

An ion current probe using a spark plug was applied to gasoline-fuelled homogeneous charge compression ignition (HCCI) combustion with hot residual gas in order to verify the possibility of using it as a combustion sensor. The ion current signal for single-cycle HCCI combustion had a simple profile and effectively one maximum value. There is a possibility that a similar ion current signal corresponding to the completed reaction can be obtained, depending on the location of the probe during HCCI combustion. The ionization reaction for HCCI combustion is affected by the chemical ionization reaction with heat release, and there is a possibility that the ion current can be used to detect heat release corresponding to the chemical ionization reaction. A strong correlation between the timing of the integrated ion current and the timing of the mass fraction burned is observed. The timing of the mass fraction burned is assumed from the timing of the integrated ion current, and the mass fraction burned (up to 70 per cent) can be determined, even if the engine driving condition changes within the scope of the test. There is a correlation between the timing of the maximum ion-current and the maximum rate of heat release, and there is a possibility that the maximum value of the rate of heat release can be inferred by detecting the timing of the maximum ion current. There is a correlation between the timing of the maximum ion current and the timing of the maximum pressure at each cycle. Therefore, it may be possible to monitor the variation of the HCCI combustion phasing.

Homogenous charge compression ignition (HCCI) combustion in diesel engines can provide for cleaner operation with ultra-low NOx and soot emissions. While HCCI combustion has generated significant attention in the last decade, however, to date, it has seen very limited application in production diesel engines. HCCI combustion is typically characterized by earlier than top-dead-center (pre-TDC) phasing, very high pressure rise rates, short combustion durations and minimal control over the timing of the combustion event. To offset the high reactivity of the diesel fuel, large amounts of EGR (30 to 60%) are usually applied to postpone the initiation of combustion, shift the combustion towards TDC and alleviate to some extent, the high pressure rise rates and the reduced energy efficiency. In this work, a detailed analysis of HCCI combustion has been carried out on a high-compression ratio, single-cylinder diesel engine. The effects of intake boost, EGR quantity/temperature, engine speed, injection scheduling and injection pressure on the operability limits have been empirically determined and correlated with the combustion stability, emissions and performance metrics. The empirical investigation is extended to assess the suitability of common alternate fuels (n-butanol, gasoline and ethanol) for HCCI combustion. On the basis of the analysis, the significant challenges affecting the real-world application of HCCI are identified, their effects on the engine performance quantified and possible solutions to overcome these challenges explored through both theoretical and empirical investigations. This paper intends to provide a comprehensive summary of the implementation issues affecting HCCI combustion in diesel engines.

Homogenous charge compression ignition (HCCI) combustion is a low temperature combustion process which combines high combustion efficiency with ultra-low [Formula: see text] raw emissions. Steep increases of the in-cylinder pressure and unstable combustion sequences at the limits of the operating range can damage the engine and limit the use of HCCI to part load operation. This can be done using closed loop combustion control based on combustion parameters like the indicated mean effective pressure and the combustion phasing. Since in-cylinder pressure sensors are expensive components and therefore not suitable for series application, ion current sensors can be used as an additional source of information about the combustion. Combustion analysis using methods similar to those used in pressure based measurements can be implemented using an online analysis of the ion current signal. In this study, the ion current sensor will be examined for its suitability for combustion control under HCCI conditions with lean air/fuel ratios and high compression ratios. Research has found that the ion current signal is strongly depended on the boundary conditions. Especially the air/fuel ratio which plays an important role for signal strength during the combustion process. When using valve timings with negative valve overlap in combination with a fuel pre-injection, a further peak of the ion current signal close to the gas exchange top dead center can be found in addition to the one during combustion. At the same time, it is hard to extract information from the cylinder pressure signal during NVO. Under lean conditions this peak even exceeds the signal during combustion. This study analyzes the ion current signal during NVO and its potential to be used for future combustion control concepts. The ion current signal shows potential to stabilize HCCI combustion at high loads. However, the prediction of late combustion cycles is still challenging.