Optimization Towards Low-temperature Combustion in a HSDI Diesel Engine, Using Consecutive Screenings

<div class="htmlview paragraph">The production unit number of small diesel engine cars tends to decline except recreational vehicles in Japanese market in recent years, while the production unit number in Europe market keeps on increasing owing to the merits of the durability and the fuel consumption. The small diesel engines will have to be improved in the near future by solving major problems such as noise and vibration pollution, environmental pollution, improvement in performance of diesel engines, in order to expand the production of the engines.</div> <div class="htmlview paragraph">This paper refers to a basic study on the experimental and analytical methods for the reduction of resonant vibration in each vibration mode on some cylinder blocks of small high-speed diesel engines in rated engine speed range. Hammering test method, which is easy and useful for measuring frequency response functions, is carried out in the experiments. In this test, the cylinder blocks are directly excited with the impulse hammer, and more the main journals are excited via the newly-made force transmission parts in the next experiment. Two kinds of the engines which have been recently recognized as low noise and vibration engines are chosen in this study. The cylinder blocks of the engines are called of long skirt type and ladder frame type, respectively. The vibration characteristics of these cylinder blocks are obtained in the frequency range from 500 <i>Hz</i> to 2 <i>kHz</i> using the experimental modal analysis. This frequency range is chosen in consideration of the reliability of the vibration experiments. And the force transmission routes from the piston heads to the crank journals of each cylinder block are considered as mentioned in the latter experimental method. The experimental results are used to calculate the natural frequencies by the modal analysis and the calculated results are compared with the ones obtained from FEM analysis. Furthermore, the cylinder blocks are remodeled in order to reduce the vibrations of all modes using FEM analysis.</div> <div class="htmlview paragraph">Consequently, it has been shown that the vibration stress of the cylinder block can be reduced by higher rigidities and more lightening weights of the crankcase and the crank journals in the measured frequency range.</div> <div class="htmlview paragraph">The conclusions are described as follows;</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"> <span class="li-label">1</span> <div class="htmlview paragraph">The hammering test method using the force transmission parts is useful to measure the natural frequencies of a cylinder block. This method can be easy to get the equal results with the measured ones on running conditions without engine operation.</div> </li> <li class="list-item"> <span class="li-label">2</span> <div class="htmlview paragraph">The hammering test method using the force transmission parts can easily analyze the natural frequencies at all the vibration modes on equal conditions with running ones. Then the method is a useful tool for improving the vibration characteristics.</div> </li> <li class="list-item"> <span class="li-label">3</span> <div class="htmlview paragraph">It has been proved from the analytical viewpoints that the cylinder block of the ladder frame type can be adopted as low-noise, small diesel engines with little increase of the engine weight while maintaining higher rigidity.</div> </li> </ol> </div>

<div class="htmlview paragraph">Further restrictions on NO<sub>x</sub> emissions and the extension of current driving cycles for passenger car emission regulations to higher load operation in the near future (such as the US06 supplement to the FTP-75 driving cycle) requires attention to low emission combustion concepts in medium to high load regimes.</div> <div class="htmlview paragraph">One possibility to reduce NO<sub>x</sub> emissions is to increase the EGR rate. The combustion temperature-reducing effects of high EGR rates can significantly reduce NO formation, to the point where engine-out NO<sub>x</sub> emissions approach zero levels. However, engine-out soot emissions typically increase at high EGR levels, due to the reduced soot oxidation rates at reduced combustion temperatures and oxygen concentrations.</div> <div class="htmlview paragraph">This paper presents an analysis of the effects of varying injection timing, fuel mass distributions in split injections and fuel rail pressure on emissions, combustion noise and fuel consumption during the medium load operation (≥10 bar IMEP) at high EGR rates (41%) of a single-cylinder test engine, with the overall objective to optimize a triple injection strategy. The results of some of the test cases are compared with those obtained from modelling in KIVA-3V.</div> <div class="htmlview paragraph">Using an optimized triple injection strategy, soot and NO<sub>x</sub> emission levels from the test engine could be reduced to < 0.04 g/kWh and < 0.4 g/kWh, respectively, at the medium engine load of 10 bar IMEP.</div>

<div class="section abstract"><div class="htmlview paragraph">To investigate the effects of fuel volatility on combustion and emissions in a diesel engine, a high-volatility fuel of n-heptane was blended into diesel fuel with different volumetric fractions (0%, 40%, 70%, 100%). A wide range of EGR rates from 0% to 65% were investigated, which covered both the conventional diesel combustion and low temperature combustion. Experiments under two engine load conditions, ∼5.2 bar and ∼10.5 bar gross IMEP were performed at 1500 rpm. The injection timing was fixed at 8°CA BTDC for all test cases. Results show that even if the ignition delay and combustion duration are nearly the same for all tested fuels, the premixed combustion fractions are increased for higher volatility fuels due to the improvement on mixing process during the ignition delay period. The indicated specific fuel consumption is decreased as using high-volatility fuels. The effect of fuel volatility on soot emissions depends on engine loads. At lower load, more significant reduction in soot can be achieved compared to higher load as using high-volatility fuels. The high-volatility fuel has almost no effect on NOx emissions, while it results in higher THC and CO emissions at relatively high EGR rates. As toluene is added into pure n-heptane, soot emissions are similar to that of pure n-heptane. Therefore, the reduction on soot emissions as fueling n-heptane mainly causes by the higher fuel volatility, while the effect of aromatics on soot reduction is minor. The effect of injection pressures on soot emissions also depends on engine loads. At higher load, soot emissions decrease with the increase of injection pressure for both diesel and high-volatility fuels as EGR rate is lower than 40%. The higher injection pressure can endure higher EGR rates where soot emissions do not show a notable increase for both diesel and high-volatility fuel, but the injection pressure has relatively smaller effect on peak soot emission (∼50% EGR rate). At lower load, the higher injection pressure can eliminate soot-bump region for both diesel and high-volatility fuel. High-volatility fuel can eliminate the soot-bump and achieve low soot emissions at lower injection pressure.</div></div>

<div class="section abstract"><div class="htmlview paragraph">A gasoline compression-ignition combustion system is being developed for full-time operation over the speed-load map. Low-temperature combustion was achieved using multiple late injection (MLI), intake boost, and moderate EGR for high efficiency, low NOx, and low particulate emissions. The relatively long ignition delay and high volatility of RON 91 pump gasoline combined with an advanced injection system and variable valve actuation provided controlled mixture stratification for low combustion noise.</div><div class="htmlview paragraph">Tests were conducted on a single-cylinder research engine. Design of Experiments and response surface models were used to evaluate injection strategies, injector designs, and various valve lift profiles across the speed-load operating range. At light loads, an exhaust rebreathing strategy was used to promote autoignition and maintain exhaust temperatures. At medium loads, a triple injection strategy produced the best results with high thermal efficiency. Detailed heat release analysis indicated that heat losses were significantly reduced. At higher loads, a late-intake-valve-closing strategy was used to reduce the effective compression ratio. For all tests, intake air temperature was 50 C.</div><div class="htmlview paragraph">3D CFD simulations of fuel injection, mixing, and combustion were important to understand the emissions formation processes. With multiple late injections and low-to-moderate fuel pressure, spray penetration was low, mixing was fast, and wall wetting could be avoided. Fuel sprays were characterized in a spray chamber. Injection rate was measured using a rate tube.</div><div class="htmlview paragraph">Results showed that ISFC was very low. Minimum ISFC of 181 g/kWh was measured at 2000 rpm-11 bar IMEP. For IMEP from 2 to 18 bar, engine-out NOx and PM emissions were below targets of 0.2 g/kWh and 0.1 FSN, respectively, indicating that aftertreatment for these species may be reduced or eliminated. It was found that combustion noise levels, characterized by several noise metrics, could be effectively controlled by the injection process. Measurements of exhaust particulate size distribution indicated very low particle count, especially for a preferred injector with low levels of in-cylinder swirl.</div><div class="htmlview paragraph">Collectively, these results demonstrate the potential feasibility of full-time GDCI using RON 91 gasoline at low-to-moderate injection pressures with high fuel efficiency. While more development work is needed, there is good potential for a practical GDCI powertrain system based on these concepts.</div></div>

<div class="htmlview paragraph">In recent years, small diesel engines for industrial use are required to achieve still higher power output per engine size (high power density) and to have the performance to comply with stringent exhaust emissions standards as well. To meet these social demands and market requirements, there are various engineering tasks yet to accomplish. In this paper, two important technical issues have been focused, those are, combustion system improvement to enable lower exhaust emissions and higher power output than conventional models and new cooling system as a solution for increased heat load issue accompanying the progress of high power density.</div> <div class="htmlview paragraph">Firstly, for combustion system improvement, the swirl adjustment technique has been developed that will optimize the swirl ratio for each engine application with different load and speed condition. Intake airflow has been analyzed using Computational Fluid Dynamics (CFD) approach to enable adjustment of swirl ratio that plays an important role in creating air fuel mixture. The swirl was successfully controlled by providing the originally designed configuration on the wall, from cylinder block side, which introduces the intake air from valve throat area into cylinder. As a result, it is now possible to set optimum swirl ratio for each engine application while securing high volumetric efficiency.</div> <div class="htmlview paragraph">Next, a new cooling system has been introduced to cool down the neighborhood of top deck between cylinders where temperature becomes highest in cylinder block. The new coolant channels with built-in multiple water passages was incorporated by casting between cylinders. As a result, the temperatures between cylinders and in piston ring groove could be lowered remarkably.</div> <div class="htmlview paragraph">After overcoming these engineering challenges, the advanced high power density has successfully been materialized in small direct injection (DI) diesel engine for industrial use.</div>

<div class="section abstract"><div class="htmlview paragraph">An investigation of high speed direct injection (DI) compression ignition (CI) engine combustion fueled with gasoline (termed GDICI for Gasoline Direct-Injection Compression Ignition) in the low temperature combustion (LTC) regime is presented. As an aid to plan engine experiments at full load (16 bar IMEP, 2500 rev/min), exploration of operating conditions was first performed numerically employing a multi-dimensional CFD code, KIVA-ERC-Chemkin, that features improved sub-models and the Chemkin library. The oxidation chemistry of the fuel was calculated using a reduced mechanism for primary reference fuel combustion. Operation ranges of a light-duty diesel engine operating with GDICI combustion with constraints of combustion efficiency, noise level (pressure rise rate) and emissions were identified as functions of injection timings, exhaust gas recirculation rate and the fuel split ratio of double-pulse injections. Parametric variation of the operation ranges was also investigated with respect to initial gas temperature, boost pressure and injection pressure. Following the modeling, experiments were performed under the conditions suggested by the numerical results in order to confirm the feasibility of GDICI operation at full load, as well as to validate the numerical simulations.</div><div class="htmlview paragraph">The results showed good agreement between the experiments and the model predictions. Due to the high volatility and low cetane index of gasoline combined with reduction of combustion temperature through utilization of EGR, both PM and NOx emissions could be reduced to levels of about 0.1 g/kg-f while maintaining experimental gross ISFC at about 180 g/kw-hr. The numerical simulations helped to explain the in-cylinder spray combustion behavior and to identify characteristics of GDICI that differ from those of diesel-fueled operation. Maps of operable conditions were generated that allow extension of low-emission engine concepts to full load operation in high speed GDICI engine operation with fuel efficiencies comparable to those of corresponding diesel fuel operation while meeting emissions mandates in-cylinder.</div></div>

<div class="htmlview paragraph">The objective of this work is to develop a strategy to reduce the penalties in the diesel engine performance, fuel economy and HC and CO emissions, associated with the operation in the low temperature combustion regime. Experiments were conducted on a research high speed, single cylinder, 4-valve, small-bore direct injection diesel engine equipped with a common rail injection system under simulated turbocharged conditions, at IMEP = 3 bar and engine speed = 1500 rpm. EGR rates were varied over a wide range to cover engine operation from the conventional to the LTC regime, up to the misfiring point. The injection pressure was varied from 600 bar to 1200 bar. Injection timing was adjusted to cover three different LPPCs (Location of the Peak rate of heat release due to the Premixed Combustion fraction) at 10.5° aTDC, 5 aTDC and 2 aTDC. The swirl ratio was varied from 1.44 to 7.12.</div> <div class="htmlview paragraph">Four steps are taken to move from LTC to ALTC. The first is to advance LPPC, while keeping the other parameters constant. This was found to improve fuel economy and engine stability, but causes an increase in NO<sub>x</sub>. Since advancing LPPC allows the use of higher EGR rates, the second step is to increase EGR to reduce NO<sub>x</sub>, but this caused an incremental increase in soot emissions. To remedy this, the third step is to increase the injection pressure and/or swirl ratio. The fourth step is to optimize LPPC, injection pressure and swirl ratio. The effects of taking each of these steps on the combustion process and engine-out emissions are explained. Advancing the start of combustion and the associated strategy is referred to as (ALTC) that results in a more stable combustion process and engine operation, better fuel economy and reduced engine-out emissions.</div>

<div class="section abstract"><div class="htmlview paragraph">In-cylinder large scale and small scale structures are widely recognized to strongly influence the mixing process in HSDI Diesel engines, and therefore combustion and pollutant emissions. In particular, swirl motion intensity and temporal evolution during the intake and compression strokes must be correctly estimated to properly target the spray jets. The experimental characterization of the attitude of a valve/port assembly to promote swirl is traditionally limited to the steady flow bench, in which the analysis is carried out for fixed valve positions / fixed pressure drops and with no piston. Since flow bench analyses cannot reproduce the highly complex instantaneous flow conditions typical of actual engine operations, the use of fully-transient in-cylinder numerical simulations can become extremely useful to correctly address the engine ability to promote adequate flow structures and patterns.</div><div class="htmlview paragraph">CFD analyses of in-cylinder flow motion development and decay are usually carried out using relatively simple yet stable turbulence models, among which k-epsilon and RNG k-epsilon, in conjunction with near-wall algebraic functions, are probably the most popular ones. Nevertheless, it is widely recognized that such models show deficiencies in correctly capturing complex rotating flow structures such as those dominating in-cylinder flows of compression ignition engines. While waiting for the diffusion of more refined approaches such as Large Eddy Simulation (LES) or Detached Eddy Simulation (DES), the use of better performing RANS models such as Reynolds Stress and k-omega SST, in conjunction with adequate resolution of the near wall flow, would be recommendable.</div><div class="htmlview paragraph">The paper reports a numerical activity aiming at evaluating the influence of the turbulence model choice on the prediction of in-cylinder flow patterns in a HSDI Diesel engine for automotive applications. Three different engine operations are investigated, i.e. two part-load / low engine speeds ones and a full-load / peak power engine speed one. For each engine condition, both wall-function k-epsilon and low-Reynolds k-omega SST models are used, and for the last one two different near-wall grid refinements are adopted. In order to better assess the predictive capabilities of the models, analyses are preliminarily compared to experimental measurements for both a simplified engine-like geometry and flow structure available in literature and steady flow-bench operations of the actual engine head.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The use of piezoelectric cylinder pressure sensors is very popular during engine testing, but cylinder pressure information is becoming mandatory also in several on-board applications, where Low Temperature Combustion (LTC) approaches require a feedback control of combustion, due to poor combustion stability and the risk of knock or misfire.</div><div class="htmlview paragraph">Several manufacturers showed the capability to develop solutions for cylinder pressure sensing in on-board automotive and aeronautical applications, and some of them have been patented. The most straight-forward approach seems the application of a piezo-electric washer as a replacement of the original part equipping the spark plug; the injector could also be used to transfer the cylinder pressure information to the piezoelectric quartz, in diesel or Gasoline Direct Injections (GDI) engines.</div><div class="htmlview paragraph">The paper describes the features of signals acquired using piezoelectric washers, discussing possible applications, highlighting the factors which impact the sensors accuracy, and proposing algorithms to compensate potential errors in the evaluation of combustion metrics. The sensors have been first tested on a press, then in two different gasoline engines: a naturally aspirated V12 and a turbocharged 2 cylinders with Variable Valve Lift system (VVL). Signals have been compared to those obtained with lab-grade cylinder pressure sensors, with particular attention to peak pressure, combustion phase and knock intensity.</div><div class="htmlview paragraph">The main issue affecting the accuracy of cylinder pressure measurement using the piezoelectric spark plug washer is the effect of temperature variations both on the force transmitted by the thread to the washer and piezoelectricity properties.</div></div>

<div class="htmlview paragraph">In the last years the development of diesel particulate traps and trap regeneration systems has led to some very promising concepts. In parallel the development of diesel engine technology for passenger cars, as well as for light and heavy duty vehicles, has resulted in remarkable improvements especially regarding particulate and NO<sub>x</sub> emissions, engine performance and fuel economy.</div> <div class="htmlview paragraph">Unfortunately, for some aspects the development and application of particulate trap systems on the one hand and diesel engine technology on the other have led to conflicting solutions.</div> <div class="htmlview paragraph">For example, exhaust gas temperatures of at least 500 °C to 600 °C are necessary to burn off the soot that has been emitted by the engine and collected in a particulate trap. However, the increased fuel efficiency of modern TDI diesel engines and the trend to reduced average traffic speed very often cause exhaust temperatures below 200 °C at urban driving conditions. During high speed highway operation exhaust temperatures higher than 550 °C occur only seldom. Currently, there are many applications available for regeneration technology concerning passive diesel particulate trap systems. The wide range of engine operation and different compositions in exhaust, respectively, in some cases can limit fail- save operation for trap regeneration systems. Active regeneration systems (e.g. diesel fuel burners or electrical heater), that are able to cover almost the entire operating range can become very expensive. Due to the technical requirements trap regeneration systems operating in a wide range are not practical for vehicle particulate traps, either because of system costs being too high or there is limited package conditions in a vehicle.</div> <div class="htmlview paragraph">In this paper boundary conditions of different engines and different types of vehicles in varied operation modes are published. Furthermore, the potential of combining different regeneration systems will be presented and discussed.</div>

<div class="section abstract"><div class="htmlview paragraph">Future vehicles will increasingly be required to improve their efficiency, reduce both regulated and CO₂ emissions, and maintain acceptable levels of driving, safety, and noise performance. To achieve this high level of performance, they will be configured with more advanced hardware, sensors, and control technologies that will also enable their operation on a broader range of fuel properties. These capabilities offer the potential to design future vehicles to operate on the most widely available and GHG-reducing fuels.</div><div class="htmlview paragraph">In previous studies, fuel flexibility has been demonstrated on a compression ignition bench engine and vehicle equipped with an advanced engine management system, closed-loop combustion control, and air-path control strategies. An unresolved question is whether engines of this sort can operate routinely on market gasoline while achieving diesel-like efficiency and acceptable emissions and noise levels.</div><div class="htmlview paragraph">This paper describes initial engineering and experimental steps to assess this potential. Using an advanced diesel bench engine having a higher compression ratio, optimized valve timing, and flexible fuel injection, the engine could be operated on a European market gasoline over full to medium part loads. The combustion was found to be highly sensitive to EGR rates, however, and the simultaneous optimization of all regulated emissions and combustion noise was a considerable challenge. An advanced glow plug was tested to improve low load performance but did not extend the engine operating range as much as expected.</div></div>

<div class="htmlview paragraph">A particulate trap with additive supported regeneration is a very effective way of reducing soot emissions of diesel exhaust gas. Particulate traps presently available on the market clearly show that the regeneration process is the most important detail in particulate trap technology. In this specific case of particulate traps, active rare earth oxides are added into the fuel to produce an initial and almost continuous regeneration without external burners, resistance heating, etc., as is well known from other systems. It should not be forgotten that an externally initiated regeneration will always produce a temperature peak inside the soot collecting filter media which may destroy them. Such damage can be avoided by catalytically supported regeneration of particulate traps. In the presence of an active catalyst, an inorganic cerium compound, regeneration temperature will decrease from 550 to 600 deg. C normally to about 350 to 400 deg. C.</div> <div class="htmlview paragraph">Exhaust gas temperature of modern turbo-charged diesel engines is very often below 200 deg. C during urban traffic. Hence, the passive method with fuel additives is not sufficient to provide trap regeneration during such continuous low engine speed/load conditions. The active trap regeneration process (e. g. with diesel burner and electrical heating of exhaust gas) is very expensive and energy-consuming.</div> <div class="htmlview paragraph">This paper explains possibilities for the development of a particulate trap system for many applications, and shows the advantages of combining different regeneration methods for particulate trap systems.</div>

<div class="section abstract"><div class="htmlview paragraph">Pilot injections are normally used for the reduction of diesel engine emissions and combustion noise. Nonetheless, with a penalty on the indicated thermal efficiency. The cost is reduced by the minimization of the pilot mass, which on its counterpart increases the risk of pilot misfire. Pilot misfire can have a higher penalty on the indicated efficiency if it is not compensated adequately. This paper investigates how in-cycle closed-loop combustion control techniques can reduce the effects of pilot misfire events.</div><div class="htmlview paragraph">By closed-loop combustion control, pilot misfire can be detected and counteracted in-cycle. Two injection strategies are investigated. The first is the control of the main injection, the second includes an additional second pilot injection. Based on the in-cycle misfire diagnose, two architectures are investigated. The first uses a cycle-to-cycle controller to set the main injection under each scenario. The second is a fully in-cycle controller with feedback from predictive models. All the algorithms were tested experimentally in a Scania D13 engine.</div><div class="htmlview paragraph">The results confirmed that in-cycle closed-loop combustion control can effectively reduce the effects of pilot misfire. An error of +1.5CAD on the main SOC and −0.5bar IMEP on the engine load was reduced to 0±0.6CAD and 0±0.4bar IMEP using the cycle-to-cycle architecture. The predictive in-cycle control can further reduce the error of the main SOC to 0±0.4CAD and 0±0.2bar IMEP for the engine load. These two approaches had one degree of freedom, and therefore only one of the combustion timing parameters (SOC or CA50) was regulated successfully. With the additional degree of freedom of the second pilot injection, the misfire effects were not only reduced, but also fully counteracted.</div><div class="htmlview paragraph">The methods are limited by the time window where pilot misfire observability and controllability overlap. This is set by the injections’ separations and the respective ignition delays. The second pilot injection can be further improved by the regulation of its injection timing and duration. The results can enhance the nominal set-up optimization by including the in-cycle controllability and regulation performance in the constraints.</div></div>

<div class="htmlview paragraph">From september 1988 a fleet test started in Italy on urban buses equipped with a single ceramic monolith filter plus a by-pass and Diesel fuel burners for regeneration. The operation of the particulate trap, during the normal bus running in the city, has been assessed in three different ways:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"><span class="li-label">1.</span> <div class="htmlview paragraph">Continuous control, by means of a data acquisition system purposely designed, of the main parameters affecting regeneration (i.e. time between consecutive regenerations, temperature in three selected zones in the filter during regeneration,filter loading);</div> </li> <li class="list-item"> <span class="li-label">2.</span> <div class="htmlview paragraph">Periodic control of smoke opacity of the bus measured, with and without the filter, under free acceleration and when bus is accelerating from 50 to 65 km/h on proving ground;</div> </li> <li class="list-item"> <span class="li-label">3.</span> <div class="htmlview paragraph">Particulate collection efficiency of the trap by using a dilution tunnel with the engine operated at the test bench.</div> </li> </ol> </div> <div class="htmlview paragraph">Information carried out from the fleet tests and additional information deriving from laboratory results were used for the system improvement.</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>