Proceedings of National Symposium for Post Graduate Students (NSPGS 2010), 24 – 25 April, 2010: Vol 1 – Mechanical Sciences
HCCI COMBUSTION CONTROL STUDIES IN A C.I ENGINE Prabhu.S1, Ganesh.D. Internal Combustion Engineering Division, Department of Mechanical Engineering, College of Engineering, Anna University Chennai, Chennai – 600 025, India 1 Corresponding Author, Mob: 9940022039, e-mail:
[email protected]
Abstract Homogeneous charge compression ignition (HCCI) is a promising concept for achieving increased efficiency and low emissions. HCCI process operates on the principle of having a lean, premixed, homogeneous charge that reacts and burns volumetrically throughout the cylinder as it is compressed by the piston. One of the major problems in achieving stable HCCI combustion is to obtain direct control of the auto ignition process over the different engine operating conditions, especially when the operating range moves from the part load to high load operation and also in cold start conditions. Therefore, to manage HCCI combustion it is essential to identify most important control parameters, which influences the auto-ignition process. The present work concentrates on controlling the start of combustion, using parameters which are indirectly related. The only way to achieve the combustion control or combustion phasing is by developing a thermodynamic closed loop model, where the model is created based on the feedback system of the cycle. The model is based on single zone approach of incylinder process in order to identify the influence of main control parameters on HCCI auto ignition. The model is separated into two parts, steady state and dynamic state. The steady state analysis was done and compared with the experimental results. Keywords: HCCI, single zone model, HCCI auto ignition, closed loop model, steady state model.
I. Introduction The HCCI engine is an alternative to the conventional gasoline or diesel engines. HCCI could be regarded as a type of operating mode rather than a type of engine. The HCCI mode can be obtained in a conventional two or four-stroke gasoline or diesel engines to run in a full- time or part-time HCCI mode. HCCI is the auto-ignition of a homogeneous mixture by compression. HCCI works without using any external ignition source, unlike traditional SI (Spark Ignition) and diesel engines where ignition is started with either spark or injected fuel respectively. The major benefit of HCCI compared to CI is the low emissions of NOx and PM. The CI (Compression Ignition) engine normally has a trade-off between particulates and NOx. If the engine operates at conditions with higher in-cylinder peak temperature, the oxidization of soot will be good but the thermal production of NO will increase. If on the other hand the engine is operated with lower temperature NO can be suppressed but PM will be high due to bad oxidation. In the CI engine, NO is formed in the very hot zones with close to stoichiometric conditions and the soot is formed in the fuel rich spray core. The in-cylinder average air/fuel ratio is always lean but the combustion process is not. This means that we have a large potential to reduce emissions of NOx and PM by simply mixing fuel and air before combustion. The changeover from diffusion-controlled high-temperature combustion to homogenised low-temperature combustion is a great step in the row of improvements in combustion processes that equals a paradigm shift. The fact that with this step into the threshold of combustion the latter has to be governed by a control increases the problem's complexity. Despite this complexity, it is a worthwhile long-term goal to explore homogenised low-temperature combustion stabilised by a control for its potential, especially in the field of emissions.
II. Control issues in HCCI engine The practical application of HCCI in car engine faces fundamental control challenges. HCCI combustion control issues are mainly linked with strong variations in the air and burnt gas mass demands that are needed during transient operation. If the air path response is not optimised NOx, PM, soot and noise peaks may appear, especially during combustion mode switches. Moreover the fuel loop dynamics is faster than the air loop dynamic. Thus the engine control must take into account these two separate dynamics and avoid disturbances. HCCI combustion makes the control task much more difficult than in the case of conventional diesel combustion, due to higher burnt gas fraction variations at combustion mode transitions. This is enhanced by the poor distribution of burnt gas fractions in multi-cylinder engines that may increase emissions and lead to cylinder to cylinder combustion discrepancies. There is also a cycle-to-cycle coupling through the temperature of the reintroduced air/burnt gas mixture and to the engine block temperature. This coupling is stronger during mode transition, especially from high loads to part loads.
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Proceedings of National Symposium for Post Graduate Students (NSPGS 2010), 24 – 25 April, 2010: Vol 1 – Mechanical Sciences
III. Experimental Setup A single cylinder, four stroke CI engine was utilized in this study. The engine is coupled to an eddy current dynamometer for speed and torque measurement control. Engine tests were operated at a constant speed of 1500 rpm. A provision is made on the intake manifold to include the heating element which vapourises the fuel. The heating element was wound over the stainless steel pipe. The port fuel injector is mounted on the top of the fuel vapouriser to supply the correct quantity of fuel to the vapouriser. The port fuel injector is controlled by an electronic control unit. The ECU controls both the timing and quantity of the fuel. The fuel vapouriser is heated by a power supply. A temperature controller is used to maintain the temperature (360°C) for providing diesel vapour at all loads. To monitor and analyze data efficiently, the entire signal collected by transducers was managed by Engine combustion pressure (ECP) analysis software. The cylinder pressure was detected by piezoelectric cylinder pressure (Kistler model 601A). Coupling with crank angle position signal to trigger the record of cylinder pressure, 137-consecutive cycles during combustion period were sampled with resolution of 1deg CA. According to the collected cylinder pressure for each operating point, Heat release rate was calculated from the relation of pressure and crank position. A k-type thermocouple and a temperature indicator were used to measure the exhaust gas temperature. A k-type thermocouple and a temperature indicator were used to measure the exhaust gas temperature. An orifice meter attached with an anti-pulsating drums measure air consumption of an engine with the help of a U- tube manometer. The anti-pulsating drum fixed on the inlet side of the engine maintains a constant suction pressure, to facilitate a constant air flow through the orifice meter.
Figure.1 Experimental layout
IV. Modeling approaches A single-zone model is often used if there exists a need to have a fast and preliminary analysis of the engine performance. Single zone models assume that the cylinder charge is uniform in both composition and temperature, at all time during the cycle. This approach is often used when simulation is made of a gasoline engine due to the homogeneous combustion. To use a single-zone model in the diesel case the model must be based on empirical heat-release laws. This approach needs a wide identification analysis. A multi dimensional model, resolve the space of the cylinder on a fine grid, thus providing a great amount of special information. This approach has its downside in computational time.
A. Model Description An HCCI combustion model incorporating a combination of physical and empirical models to predict CA50 is proposed. CA50 is chosen because it is a good and robust indicator of HCCI combustion and CA50 can be reliably used for feedback control of HCCI combustion. The temperature and pressure variations during compression stroke between IVC and SOC could be modeled using polytropic process. To predict the HCCI operation a zero dimensional or single zone model is developed. The cylinder charge is assumed homogeneous in both temperature and composition. In HCCI engine there are usually large differences in effective burn rate due to temperature gradients. In this model Arrhenius equations is considered due its importance given to in-
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Proceedings of National Symposium for Post Graduate Students (NSPGS 2010), 24 – 25 April, 2010: Vol 1 – Mechanical Sciences
cylinder species concentration at the beginning of combustion. Peak combustion pressure predicted by single zone model is always over estimated. Hence the addition of double-Wiebe function model was accounted to separate the slower combustion that occurs in the cooler boundary region near the walls and the faster combustion in the hot core. Since gas mixtures at core always hotter and greater spread of auto ignition time than in the cooler. During combustion phase, HCCI model employs only the Wiebe function option since a turbulent flame model has no role in HCCI combustion. Engine Kinematics (1) Inlet valve close to start of combustion The pressure and temperature histories during compression are calculated using a polytropic compression from IVC to SOC. (2) (3) 1) Modeling Start of Combustion: The prediction of SOC is based on Arrhenius equation, SOC is a function of temperature, pressure, concentration of mixture, empirical co-efficient as shown below. (4) In this equation the combustion timing of an auto-ignition process can be described as an integral that sums up the reaction rate of the radicals until the concentration of radicals reaches a critical value. Integration from inlet valve close until the combustion is imitated corresponds to a critical value of the concentration of radicals. Since 1% of fuel is burnt which at point of initial of combustion. The chemical concentration of fuel and oxygen can be considered constant throughout the compression stroke before ignition and equal to the concentrations at inlet valve close. Start of combustion mainly depends on the equivalence ratio and temperature of the mixture. 2) Modeling of mass burn rate: The temperature rise due to combustion and temperature change depends on LHV of the fuel, A/F ratio, combustion efficiency and specific heat values. Tca50 = Tsoc + ∆T (5) (6)
Figure 2. Comparison of Single and Double-Wiebe functions Air and fuel mixture inside the combustion chamber will not burn at the same time and rate. This burn rate can be captured using Double-Wiebe function combustion model. (7)
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Proceedings of National Symposium for Post Graduate Students (NSPGS 2010), 24 – 25 April, 2010: Vol 1 – Mechanical Sciences
Where, w is derived from the Wiebe function between specific burn fractions of 0.1 and 0.9. (8) In the standard Single-Wiebe function the parameters that appear in the function vary with the operating conditions, which are clearly indicated in figure 2. i.e with the same parameter constants at different operating condition the mass burn rate is not complete. Double-Wiebe function includes parameters which captures the slower burning portion of HCCI combustion. By considering the shape factor (m) as 0.1, the ratio of slow burn duration to standard burn duration (K) considered as 10, the doube-Wiebe function is solved. A much more significant and extended late combustion period can now be represented as shown in figure 2. After the completion of combustion phase, polytropic expansion is performed to simulate the phenomena in expansion stroke. The steady state methodology is developed by incorporating suitable changes in Arrhenius equation and as well as in the double-Wiebe functions for HCCI combustion.
V. Steady State Analysis Results Mixture component and quantity were affected by the equivalence ratio. The model began at inlet valve close; thermal properties of mixture and thermal condition were given. Pressure increasing behaves polytropic until meet SOC requirement. Charge temperature and equivalence ratio were used as the index for determine SOC point. This is similar to the Arrhenius equation where combustion start after the accumulation of temperature while others variables are assumed to be constant. Equivalence ratio is considered like concentration of reactants. They increase the amount of accumulated radicals in the combustion chamber to meet auto-ignition criteria.
Figure 3. Predicted Temperature History using Steady State Analysis
Figure 4. Predicted Combustion pressure using Steady state Analysis
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Proceedings of National Symposium for Post Graduate Students (NSPGS 2010), 24 – 25 April, 2010: Vol 1 – Mechanical Sciences
Figure 5. Maximum Pressure at Various Equivalence Ratios Since HCCI engines always operates below an equivalence ratio of 0.5, so values less than 0.5 are considered as an input for the program. During combustion phase, the maximum pressure depended on amount of equivalence ratio and higher equivalence ratio cause increase in peak pressure. Maximum pressures were 62, 70, 75 and 78 bar at equivalence ratio of 0.31, 0.38, 0.44, and 0.52 respectively. Collected pressure history coupled with the relation of cylinder volume was also converted to temperature history using ideal gas law. Temperature history at equivalence ratio of 0.5 was shown in figure.3 as peak pressure and temperature were the same position. Combustion durations implied form Arrhenius equation. Pressure trace of 0.52 equivalence ratio showed the earliest in SOC and combustion duration was 8 degrees till CA90. At equivalence ratio of 0.31 expressed the longest burn duration as the most retard in SOC compare with others. According to SOC, peak pressure location shift to TDC as increasing in equivalence ration as shown in figure.4. These refer to shorter combustion duration and led to higher in Pressure Rise Rate. In the figure.5 the peak pressure predicted using single Wiebe function is found to be higher than the Double Wiebe function, this limits the over prediction of peak pressure when compared with the experimental values.
VI. Conclusion HCCI operation at equivalence ratio 0.31 to 0.52 have been run and investigated. Experiment results were analyzed and fitted for corrected the model. Fitting parameters of slower combustion, α of 10 and K of 0.25 were used in the model. Combustion phase coupled with double-Wiebe function helped model results close to experiment at 5% difference. The model calculated temperature and pressure trace during IVC and EVO interval. Single zone model with double-Wiebe function showed good accuracy of calculation in SOC and the highest pressure and temperature in combustion phase. The model could be possibly incorporated with control software to predict the occurrence of SOC in HCCI operation since the computation time is short and the required information such as intake charge temperature could be obtained on line. Future work will concentrate towards the dynamic controlling of the combustion process using EGR as a control parameter.
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