J Benajes
email: jbenajes@mot.upv.es

S Molina

J Martín

R Novella

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine

Keywords: Diesel engines, Diesel combustion, Diesel pollutant emissions, variable valve actuation

An experimental investigation has been performed on the modification of in-cylinder gas thermodynamic conditions by advancing the intake valve closing angle in a HD Diesel engine. The consequences on the diffusion-controlled combustion process have been analysed in detail, including the evolution of exhaust emissions and engine efficiency. This research has been carried out at full load (100%) and low engine speed (1200 rpm) with the aim of generating a long and stable diffusion-controlled combustion process. The intake oxygen mass concentration was kept at 17.4% to obtain low NO x levels in all cases. The required flexibility on intake valve motion has been attained by means of an electro-hydraulic variable valve actuation system. The results obtained from advancing the intake valve closing angle (IVC) have shown an important reduction on in-cylinder gas pressure and density, whereas the gas temperature showed less sensitivity. Consequently, the diffusion-controlled combustion process is slowed down mainly due to the lower in-cylinder gas density and oxygen availability. Important effects of advancing IVC have also been observed on pollutant emissions and engine efficiency. Where NO x production decreases, soot emissions increase. Finally, the results of pollutant emissions and engine efficiency have been compared with those obtained retarding the start of injection.

First revision of paper draft:

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -2-

Introduction

In the last decade, the direct injection Diesel engine is growing in popularity mainly due to its high thermal efficiency. This competitive efficiency, together with its high reliability, make the direct injection Diesel engine particularly suitable for heavy-duty applications, like medium and heavy trucks or power generation engines.

However, the diffusion-controlled combustion characteristic of Diesel engines generates significant levels of nitrous oxides (NO x ) and soot emissions [START_REF] Heywood | Internal Combustion Engine Fundamentals[END_REF][START_REF] Flynn | Diesel combustion: An integrated view combining laser diagnostics, chemical kinetics, and empirical validation[END_REF]. One of the main challenges in the frame of Diesel engine combustion research is how to attain extremely low levels of NO x and soot emissions without worsening the competitive Diesel engine thermal efficiency.

With the aim of avoiding the emission drawbacks of Diesel engines, new advanced combustion processes based on replacing the diffusion-controlled combustion by a totally (or partially) premixed compression-ignition combustion process are being investigated, such as the HCCI [START_REF] Epping | The Potential of HCCI Combustion for High Efficiency and Low Emissions[END_REF] and the PCI [START_REF] Jacobs | The attainment of premixed compression ignition low-temperature combustion in a compression ignition direct injection engine[END_REF] concepts. Despite the interesting emission results, most of these advanced combustion processes can be only applied at low and medium engine load, switching to conventional diffusion-controlled combustion at high load operation conditions [START_REF] Stanglmaier | Homogeneous charge compression ignition (HCCI): Benefits, compromises, and future engine applications[END_REF]. Therefore, improving the characteristics of diffusion-controlled combustion in direct injection Diesel engines for reducing pollutant production is still required to fulfil the future emission standards, especially at high load conditions.

Keeping the injection settings constant, diffusion-controlled combustion basically depends on instantaneous in-cylinder gas chemical composition and thermodynamic

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -3-conditions. In-cylinder thermodynamic conditions during the closed cycle (compression, combustion and expansion) can be directly controlled by adjusting the intake valve closing angle (IVC), which defines the total intake mass flow rate and the effective compression ratio of the engine [START_REF] Millo | The Potential of Dual Stage Turbocharging and Miller Cycle for HD Diesel Engines[END_REF]. Interesting improvements in terms of NO x emissions and fuel consumption have been obtained with this strategy in spark ignition engines [START_REF] Tuttle | Controlling engine load by means of early intake-valve closing[END_REF][START_REF] Cleary | Unthrottled Engine Operation with Variable Intake Valve Lift, Duration, and Timing[END_REF]. The potential of advancing IVC, together with turbocharging, for NO x emissions control has also been explored in Diesel engines with success, achieving reductions in the range of 10-20% of this pollutant [START_REF] Millo | The Potential of Dual Stage Turbocharging and Miller Cycle for HD Diesel Engines[END_REF][START_REF] Steber | Reduction of NOx Emissions of D.I. Diesel Engines by Application of the Miller-System: An Experimental and Numerical Investigation[END_REF][START_REF] Wang | Experimental investigation of applying Miller cycle to reduce NOx emission from diesel engine[END_REF].

Thus, the main objective of this investigation has been to improve the knowledge of the influence of advancing IVC on in-cylinder conditions and how it affects the diffusion-controlled combustion, exhaust emissions and engine efficiency. In this research, the required intake valve motion flexibility has been attained by an electrohydraulic variable valve actuation system. Finally, pollutant emissions and engine efficiency results obtained from advancing IVC have been compared to those attained with a conventional injection strategy based on retarding the start of injection.

Experimental setup

The engine used in this work was a single-cylinder, four-stroke, direct-injection Diesel research engine with 1.8 litre displacement. Detailed specifications of the engine are given in Table 1. The engine was installed in a fully instrumented test cell, with all the auxiliary facilities required for its operation and control. Figure 1 shows a scheme of the installation.

First revision of paper draft:

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -4-When exhaust gas recirculation (EGR) needs to be produced, exhaust gas is taken from the exhaust settling chamber, cooled by a gas-water heat exchanger, heated inside the EGR settling chamber to the desired temperature, and finally introduced into the intake pipe, as shown in Figure 1. In this study, the intake temperature (after mixing fresh air and EGR) was kept at a constant value of 66º C in all the tests. The exhaust backpressure produced by the turbine in the real engine (0.04 MPa) was replicated by a valve placed in the exhaust system, controlling the pressure in the exhaust settling chamber. This backpressure also ensures enough EGR mass flow rate in all operating conditions.

The molar concentration of NO x , CO, HC, intake and exhaust CO 2 , and O 2 was measured with specific state-of-the-art analysers. Determining the EGR rate was carried out using experimental measurements of intake and exhaust CO 2 concentrations. Smoke emission was measured with a variable sampling smoke meter, providing results directly in FSN (Filter Smoke Number). These units were transformed into dry soot mass emissions by means of a suitable correlation [START_REF] Christian | A New Method for the Filter Smoke Number Measurement with Improved Sensitivity[END_REF].

At every engine operating point, the in-cylinder pressure traces from a piezoelectric transducer were recorded during 50 consecutive engine cycles in order to compensate for variability in engine operation. The actual fuel injection rate was measured during 50 consecutive engine cycles in a commercial test rig using the Bosch system [START_REF] Bosch | Fuel rate indicator is a new instrument for display of the characteristics of individual injection[END_REF].

The variable valve actuation in the engine was attained through an electro-hydraulic camless system patented by Sturman Industries [START_REF] Turner | Design[END_REF]. The system flexibility includes variable timing, variable duration and variable lift for each valve independently. If necessary, two intake and/or exhaust events per cycle can be easily carried out. The First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -5-system, adapted for a single cylinder engine configuration, requires a specific oil circuit with pumps driven by electrical motors, independent of the engine. At the moment, these pumps are oversized and their power consumption has not been considered in the estimations of engine efficiency. Additional information about this VVA system and its application on HD Diesel engines can be found in the work of Lombard et al. [14].

The values for IVC presented in this paper correspond to the target values introduced into the VVA control software. These are the angles for which the intake valves lift is around 1 mm, but the actual IVC occurs around 15 cad after this logical value because the VVA control software introduces a final closing ramp with low slope to avoid strong impacts between the valves and the cylinder head. In this investigation, the values of in-cylinder gas thermodynamic conditions at IVC have been calculated at the actual IVC.

Methodology and test conditions

The methodology followed in this investigation has been based on the analysis of the isolated effect of advancing IVC at three different SoI, whereas the other parameters were kept constant. Three values of IVC timing (including the base-line) and three start of injection angles (SoI) were tested independently. The intake oxygen concentration was changed First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -6-by recirculating the required amount of exhaust gases, starting from an initial 27% EGR rate. Advancing IVC reduces the intake fresh air mass flow and, as a consequence of the lower air/fuel ratio, the oxygen concentration in the exhaust gas also decreases.

Therefore, the EGR rate had to be reduced when operating with different IVC to keep the intake oxygen concentration constant. The objective with this procedure was to isolate the main effect of advancing IVC, which changes the in-cylinder gas thermodynamic conditions. All these tests were carried out with one conventional injection event close to TDC.

Other advanced injection strategies are out of the scope of this research at the present time. At each engine operating point, the filtered average pressure and injection rate signals were used as inputs for a combustion diagnostic model [15,[START_REF] Payri | Influence of measurement errors and estimated parameters on combustion diagnosis[END_REF].

The instantaneous flame temperature T f has been calculated assuming constant pressure at each calculation step, and that the absolute enthalpy of the reactants at the initial state equals the absolute enthalpy of the products at the final state. The piston movement has been taken into account to estimate the instantaneous reactants temperature during combustion. This temperature has been estimated assuming an adiabatic compression starting from the in-cylinder gas temperature at the start of combustion (SoC), so that the temperature of the reactants is not the mean in-cylinder gas temperature. The products composition has been determined following the scheme proposed by Wey [START_REF] Way | Methods for determination of composition and thermodynamic properties of combustion products for internal combustion engine calculations[END_REF]. Since T f and the composition of the final products are coupled, an iterative process is required to solve the problem.

First revision of paper draft:

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -7-

Results and discussion

The analysis of the results has been divided in three sections following a logical structure. The first section focuses on the effect of advancing IVC on in-cylinder conditions. The second section analyses the diffusion-controlled combustion process in detail. The third section discusses the trends followed by pollutant emissions and engine efficiency, relating these with the differences observed in combustion process.

In-cylinder conditions

Since the in-cylinder conditions are independent from the injection parameters, only the results for the most retarded SoI (+2 cad aTDC) will be discussed. The main reason is to keep a complete compression process up to TDC. Figure 2 shows the most important parameters related to the in-cylinder conditions at the three IVC tested.

Advancing IVC shortens the duration of the intake process, which reduces the total mass flow rate through the engine. Figure 2 corroborates this reduction in m IVC , and ρ TDC , but also in A/F. Thus, it is important to highlight that the lower A/F also implies less Y O2 in the exhaust gases, so the EGR rate should be reduced to keep the intake Y O2 constant.

Early IVC also decreases the effective compression ratio, CR eff . At the earliest IVC, CR eff was decreased from 16 to 14. The theoretical effect of CR eff on pressure and temperature at the end of the compression stroke (p TDC and T TDC ) can be estimated considering the compression process as a polytropic evolution with constant exponent k (typical k values are between 1.35 and 1.38).

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -8-

With this assumption, the relation between CR eff , p TDC and T TDC is given by the equations ( 1) and (2).

• • k k TDC IVC eff p V const p p CR = → = (1) 1 1 • • k k TDC IVC eff T V const T T CR - - = → = (2)
Considering these two equations, p TDC is proportional to

k eff
CR and T TDC is proportional to 1 k eff CR -, so p TDC is more sensitive to CR eff than T TDC . This explains theoretically why the reduction in p TDC at earlier IVC is much greater than that observed in T TDC .

The results shown in Figure 3 confirm how the effect of CR eff is more pronounced in p and ρ than in T. Comparing the earliest and the nominal IVC profiles; p TDC and ρ TDC are both reduced about 21.5%, whereas T TDC decreases only by 2.3%.

In addition, recovering m IVC by increasing p in at the earliest IVC will recover the nominal ρ TDC and increase p TDC until values close to the nominal case, but T TDC will be reduced in a similar proportion.

Diffusion-controlled combustion process

After understanding the effects of advancing IVC on the in-cylinder conditions, a complete analysis on how these changes affect the overall combustion process has been carried out.

In direct injection Diesel engines, the combustion process is divided in three stages:

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -9-auto-ignition delay time, premixed combustion stage and diffusion-controlled combustion stage. At full load, the two earlier stages are extremely short. Hence, this analysis has mainly focused on the diffusion-controlled combustion stage. But some comments on the behaviour of the auto-ignition delay time and the premixed combustion stage have been also introduced.

Figure 4 presents the actual injection rate and the RoHR, whereas Figure 5 shows the estimation for T f and also the evolution of the in-cylinder Y O2 . The auto-ignition delay time, τ, is defined in Figure 4 as the difference between the SoI and the SoC. Advancing IVC reduces p, ρ and T after the SoI, thus lengthening τ and retarding the SoC .

Nevertheless, all τ values are between 0.065 and 0.1 ms and, as Figure 4 confirms, these differences of τ do not introduce noticeable modifications of the RoHR.

The premixed combustion stage is closely related to τ . Keeping the same injection rate profile, the longer τ increases the quantity of fuel injected before the SoC and the mass of fuel burnt during the premixed stage [START_REF] Alkidas | On the Premixed Combustion in a Direct-injection Diesel Engine[END_REF]. But as τ is extremely short at high load, the premixed combustion stage is practically inexistent and the final pollutant emissions and engine efficiency depends on the diffusion-controlled combustion stage characteristics.

Analysing the effect of advancing IVC on diffusion-controlled combustion, Figure 4 reveals an appreciable reduction in the RoHR once the combustion reaches the steadystate. Thus, advancing IVC deteriorates the diffusion-controlled combustion process. This result is consistent with those obtained theoretically by Arrègle et al. [START_REF] Arrègle | Development of a zerodimensional Diesel combustion model. Part 1: Analysis of the quasi-steady diffusion combustion phase[END_REF][START_REF] Arrègle | Development of a zerodimensional Diesel combustion model: Part 2: Analysis of the transient initial and final diffusion combustion phases[END_REF].

Starting from the turbulent gas jet theory in steady state and adapting it to Diesel-like

ACCEPTED MANUSCRIPT

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -10-spray conditions, they found that in diffusion-controlled combustion conditions the RoHR is directly related to the instantaneous fuel injection velocity, ρ and in-cylinder Y O2 . It is also inversely dependent on nozzle hole diameter.

According to Figures 3 and5, advancing IVC slows the diffusion-controlled combustion process due to the lower instantaneous ρ and faster reduction of in-cylinder Y O2 . This decreases the air/fuel (and also oxygen/fuel) mixing process and finally explains the trends observed in the RoHR.

However, Figure 4 shows a slightly higher RoHR for the earliest IVC after the end of injection, during the late diffusion-controlled combustion phase. Advancing IVC results in a slower combustion process during the injection time interval, reducing the mass of fuel burnt before the end of injection. This increases the mass of fuel that should be burnt during the late diffusion-controlled combustion, when in-cylinder gas rapidly expands (reducing ρ) and combustion temperature sharply decreases. The higher quantity of fuel burnt in low flame temperature environment and extremely poor air/fuel mixing conditions should have consequences on final soot emissions.

Finally, the flame temperature during combustion, T f , is also an important parameter closely related to pollutant emissions, especially NO x formation. Figure 5 shows the estimated values for instantaneous T f . This figure shows how advancing IVC results in lower T f during the overall combustion process. T f mainly depends on the initial temperature of the reactants at the start of combustion and, in this case, this temperature is reduced by about 25K for the earliest IVC. Thus, slight decrements in reactant initial temperatures cause noticeable reductions in T f .

First revision of paper draft:

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -11- The apparent combustion speed has been calculated as the inverse of the apparent combustion time (ACT) [START_REF] Kitamura | Fundamental Investigation of NOx Formation in Diesel Combustion Under Supercharged and EGR Conditions[END_REF][START_REF] Ladommatos | Measurements and predictions of diesel soot oxidation rates[END_REF]. The injected fuel is sequentially divided in different chronological fuel parcels. After that, ACT is experimentally calculated for each package as the difference between its injection time and its combustion time. Thus, ACT is inversely related to air/fuel mixing speed.

Pollutant emissions and engine efficiency

In Figure 6, combustion progress has been normalised by the fraction of fuel burnt, rather than relating the process to the engine timescale. This particular representation is convenient for relating the trends observed in combustion process to those detected in pollutant emissions.

Figure 7 shows the pollutant emissions and engine efficiency results. According to this figure, advancing IVC reduces INO x emissions. Figure 6 confirms how the main reason is the lower T f during the combustion process, especially before the end of injection, where most of thermal NO x forms [START_REF] Kitamura | Fundamental Investigation of NOx Formation in Diesel Combustion Under Supercharged and EGR Conditions[END_REF].

On the negative side, Figure 7 shows that advancing IVC increases Isoot emissions. This trend can be explained also by analysing Figure 6. Due to the combustion deterioration discussed in the previous section, higher fraction of fuel should burn after

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -12-

the end of injection, during the late diffusion-controlled combustion. Focusing on this last part of combustion, Figure 6 shows how the apparent combustion speed, T f and incylinder Y O2 are clearly lower for earlier IVC. These worse conditions slow down the late soot oxidation process. On the light of these results, the main conclusion is that the increment in Isoot emissions is caused by the lower in-cylinder Y O2 and T f during the late diffusion-controlled combustion [START_REF] Ladommatos | Measurements and predictions of diesel soot oxidation rates[END_REF]. But it seems that the poor air/fuel mixing process and after the end of injection could also contribute to this increment in Isoot.

The same reasoning is also valid for ICO emissions. Figure 7 indicates an increment in this pollutant at earlier IVC. This is caused by combustion deterioration (lower ρ) and lack of oxygen (lower in-cylinder Y O2 and A/F). IHC emissions are slightly affected by advancing IVC. The extremely low IHC emissions observed in Figure 7 are consistent with that expected at high load, where spray/wall impingement is avoided and the premixed combustion phase is almost negligible.

Figure 7 shows how advancing IVC results in a significant reduction in η i , increasing isfc. The engine efficiency decreases basically due to the important changes in the p-V diagram introduced by the lower CR eff and deteriorated diffusion-controlled combustion.

Finally, the results of pollutant emissions and engine efficiency attained by advancing IVC have been compared to those obtained by retarding the SoI. This comparison has been based on the trade-off plots shown in Figure 8.

In terms of INO x emissions, similar results were achieved with both strategies. However, in low INO x conditions, advanced IVC strategies produced higher Isoot and ICO emission levels. The key difference between both strategies is ρ. Starting from the First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -13-same reference point, advancing the IVC ρ is lower during the overall combustion process than in the case of retarding SoI, but especially after the end of injection. This deteriorates the diffusion-controlled combustion and the final result is an important deterioration in CO and soot oxidation processes during the late diffusion-controlled combustion. Finally, both strategies have been observed to noticeably increase isfc.

The results obtained increasing EGR from the nominal conditions have been included in Figure 8. The trade-off confirms how increasing EGR is the best strategy for reducing INO x emissions up to very low levels without worsening engine performance, despite the higher Isoot and CO emissions.

Conclusions

A detailed investigation on the potential of advancing IVC for pollutant control at three different SoI has been carried out in a HD Diesel engine operating at high load and low engine speed. This work focused on in-cylinder gas thermodynamic conditions (including instantaneous oxygen mass concentration), injection-combustion processes, exhaust emissions and engine efficiency. The most relevant conclusions are:

•

The direct consequences of advancing IVC are the reduction in the effective compression ratio of the engine and also in the total intake mass flow rate. These two effects introduce important modifications in the thermodynamic properties of the in-cylinder gas during the closed cycle. Temperature, pressure and density decrease during the compression stroke and combustion takes place in different environmental conditions.

•

The in-cylinder gas pressure and density are more sensitive to the effective First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -14-compression ratio than temperature. At the end of compression stroke, density and pressure are both reduced about 21.5%. Whereas temperature decreased only by 2.3%, in the range analysed.

• Changes in the combustion process are basically caused by a large reduction in gas density, not by the lower gas temperature. At full load, the ignition delay and premixed combustion stages are always very short. The air/fuel mixing process deterioration during the diffusion-controlled combustion stage results in a slower combustion process.

• Earlier IVC reduces flame temperatures due to lower initial temperature before the start of injection. Thus, noticeable reduction in INO x emissions is attained by advancing IVC. However, the main drawback is an increment in Isoot and ICO emissions because of lower flame temperature and oxygen mass concentration during the late diffusion-controlled combustion (after the end of injection), which slows down both CO and soot oxidation processes.

• At high load, advancing IVC significantly reduces the engine efficiency. This is the result of lower effective compression ratio and longer diffusion-controlled combustion process, which modifies the overall p-V diagram. As a consequence, fuel consumption increases.

•

Increasing boost pressure until recovering the nominal A/F should avoid the most critical drawbacks observed when advancing IVC, which are the sharp increment in soot, CO emissions and fuel consumption. However, the settings of present tests already simulate the boost conditions attained in a two stage turbocharged First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -15-engine (4.6 bar). Thus, advancing IVC at full load without worsening the combustion process, exhaust emissions and engine performance requires a further development of the state-of-the-art turbocharging devices.

As an additional conclusion extracted from this research, it has been proved that the strategy based on retarding the start of injection provides better results than advancing IVC in terms of INO x -Isoot trade-off, whereas the results in fuel consumption are comparable. From these results, the best compromise between pollutant reduction and engine performance is attained by increasing EGR strategy. Nevertheless, advancing IVC has shown its potential for NOx emissions control.

Finally, authors want to mention that the conclusions of this investigation can be universalized if two conditions are accomplished: The boost pressure should be kept constant (isolated effect of advancing IVC) and the combustion process must be basically controlled by air/fuel mixing processes (conventional diffusion-controlled combustion). Due to these restrictions, the effect of advancing IVC at low engine loads has to be separately investigated. At low-load conditions the combustion process shifts towards a highly premixed combustion when advancing IVC. Thus, the effects on combustion process, exhaust emissions and engine performance could differ significantly from those explained in this paper. The CR eff has been calculated at the nominal IVC, when the intake valve lift is 1 mm.

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD

Figure 6

 6 Figure 6 presents the most relevant instantaneous parameters concerning the injection and combustion processes for the different IVC cases: the in-cylinder Y O2 ,

First revision of paper 20 -Figure 1 .Figure 2 .

 2012 Figure 1. Single cylinder research engine and complete test cell scheme

First

  revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine

Figure 3 .

 3 Figure 3. Effect of early IVC on in-cylinder pressure, temperature and density.

Figure 4 .Figure 5 .Figure 6 .

 456 Figure 4. Effect of early IVC and SoI on combustion process.

Figure 7 .

 7 Figure 7. Effect of early IVC and SoI on pollutant emissions and engine efficiency.

First

  revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine

Figure 8 .

 8 Figure 8. Comparison between advancing IVC, retarding SoI and increasing EGR strategies.

Table 2

 2

	contains the complete test plan designed for this experimental work. The
	basic engine operation conditions correspond to mode A100 of the European Steady
	Cycle (ESC) for heavy-duty engines, defined by 1200 rpm engine speed and a constant
	injected fuel mass, m f , of 232.3 mg per cycle.

Table 2 .

 2 Main test settings. † These are the values for the EGR ratio at nominal IVC.

	Speed	SoI	p inj	m f	p in	* 2 O Y	EGR IVC
	[rpm]	[cad aTDC]	[bar]	[mg]	[bar]	[%]	[%]	[cad]
							27.0 †	535
		-2					25.7	510
							23.1	485
							27.0 †	535
	1200	+0	1950 232.3 4.600 19.4	25.7	510
							23.1	485
							27.0 †	535
		+2					25.7	510
							23.1	485

*Oxygen concentration in the intake gas.

Main test settings

Acknowledgments

This investigation has been carried out within Integrated Project GREEN of the EU's 6 th Framework Programme for Research and Technological Development. First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -24-2350 2450 2550 2650

ACCEPTED MANUSCRIPT

First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -16-First revision of paper draft: Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD Diesel engine -17-and control of a two-stage electro-hydraulic valve actuation system, SAE Paper 2004-01-1265, 2004.

[14] B. Lombard and R. Le Forrestier, Advanced combustion and engine integration of a hydraulic valve actuation system (camless), SIA Variable Valve Actuation Conference Proceedings, 2006.