Investigation of In-Cylinder Steam Injection in a Turbocharged Diesel Engine for Waste Heat Recovery and NO x Emission Control

In this study, an in-cylinder steam injection method is introduced and applied to a turbocharged diesel engine for waste heat recovery and NOx emission reduction. In the method, cool water was first heated into superheated steam by exhaust. Then the superheated steam was directly injected into the cylinder during the compression stroke. The potential for fuel savings and NOx emission reduction obtained by this method was investigated. First, a two-zone combustion model for the baseline engine was established and calibrated with the experimental data. Based on the model, the effects of steam injection mass, temperature, and timing on engine performance and NOx emission were investigated. The results demonstrate that in-cylinder steam injection can improve engine performance and reduce NOx emissions significantly at all engine speeds. Optimal steam injection mass is obtained under full load at engine speed from 1000 rpm to 1900 rpm when the steam injection timing and temperature are −30◦ and 550 K, respectively. Under those conditions, engine torque is increased by 9.5–10.9%, brake-specific fuel consumption (BSFC) is reduced by 8.6–9.9%, and NOx emission is decreased by 83.4–91.8%. Steam injection mass and injection timing are the main parameters that significantly affect engine performance and NOx emission.


Introduction
At present and in the short to medium term, internal combustion engines are major power sources for various mechanical devices.As fuel consumption and emissions regulations are becoming more stringent, energy saving and emissions reduction have become the impetus for development and the goal of modern internal combustion engines [1,2].Various technologies have been proposed to achieve this goal, such as turbocompounding [3], Miller cycle [4], exhaust gas recirculation (EGR) [5], waste heat recovery [6,7], use of alternative fuel [8], and so on.Water injection, another useful technology to improve engine performance and reduce emissions, has also been widely researched.
Water injection was initially proposed to reduce emissions of nitrogen oxides (NO x ).Since the latent heat of water evaporation is large, the maximum in-cylinder temperature will be reduced remarkably with water injection, which is good for reducing NO x emissions.However, with water injection, the combustion in the cylinder will be greatly suppressed.The ignition delay will be prolonged and the maximum in-cylinder pressure will decrease significantly.As a result, the engine's thermal efficiency will drop remarkably.Tesfa et al. [9] investigated the influence of water injection at the intake manifold on the performance and emission characteristics of a compression ignition engine running with biodiesel.It was shown that up to 50% reduction of NO x emissions was obtained with In order to make effective use of waste heat to further reduce fuel consumption and NO x emissions, an in-cylinder steam injection method is proposed and applied to a turbocharged diesel engine in the present study.First, a zero-dimension (0-D) two-zone combustion model is established and calibrated with the experimental data.Then the influence of steam injection mass, temperature, and timing on engine performance and NO x emissions is discussed.Finally, the potential of in-cylinder steam injection for improving engine performance and NO x reduction is evaluated.

Simulation Model
The schematic of the turbocharged diesel engine with in-cylinder steam injection is shown in Figure 1a.First, room-temperature water was pressured by a pump.Then the water was heated into superheated steam in the heat exchanger by the exhaust.In the changing of liquid water into superheated steam, the enthalpy of the water greatly increased by absorbing lots of thermal energy from the exhaust.After that, the superheated steam was directly injected into the cylinder during the compression stroke.The crank angle range that the steam can be injected into the cylinder is shown in Figure 1b.The injection timing ranges from inlet valve closed (IVC) to top dead center (TDC).Since the in-cylinder pressure near TDC is ultra-high, it is not easy to inject steam into the cylinder under these conditions.Therefore, the latest injection timing is set before TDC.With steam injection, the thermodynamics cycle in the cylinder will change.Therefore, the engine performance and NO x emissions will be influenced by the in-cylinder steam injection method.In addition, the corrosion problem will be prevented by this method.
Energies 2018, 11, x 4 of 21 and calibrated with the experimental data.Then the influence of steam injection mass, temperature, and timing on engine performance and NOx emissions is discussed.Finally, the potential of in-cylinder steam injection for improving engine performance and NOx reduction is evaluated.

Simulation Model
The schematic of the turbocharged diesel engine with in-cylinder steam injection is shown in Figure 1a.First, room-temperature water was pressured by a pump.Then the water was heated into superheated steam in the heat exchanger by the exhaust.In the changing of liquid water into superheated steam, the enthalpy of the water greatly increased by absorbing lots of thermal energy from the exhaust.After that, the superheated steam was directly injected into the cylinder during the compression stroke.The crank angle range that the steam can be injected into the cylinder is shown in Figure 1b.The injection timing ranges from inlet valve closed (IVC) to top dead center (TDC).Since the in-cylinder pressure near TDC is ultra-high, it is not easy to inject steam into the cylinder under these conditions.Therefore, the latest injection timing is set before TDC.With steam injection, the thermodynamics cycle in the cylinder will change.Therefore, the engine performance and NOx emissions will be influenced by the in-cylinder steam injection method.In addition, the corrosion problem will be prevented by this method.

Engine Model Description
The impact of in-cylinder steam injection on turbocharged diesel engine performance and NOx emissions was evaluated by an engine simulation model.The simulation model was established by using GT-POWER commercial software.In the model, 1-D unsteady flows are used to describe the flows in the intake pipe and exhaust and the 0-D two-zone combustion model is adopted to describe the energy conversion in the cylinder.Based on the first law of thermodynamics, the energy conservation in the cylinder can be expressed as Equation (1) The internal energy  can be expressed as Equation (2) and the constant volume specific heat   is calculated using Equation (3):

Engine Model Description
The impact of in-cylinder steam injection on turbocharged diesel engine performance and NO x emissions was evaluated by an engine simulation model.The simulation model was established by using GT-POWER commercial software.In the model, 1-D unsteady flows are used to describe the flows in the intake pipe and exhaust and the 0-D two-zone combustion model is adopted to describe the energy conversion in the cylinder.Based on the first law of thermodynamics, the energy conservation in the cylinder can be expressed as Equation (1): The internal energy U can be expressed as Equation ( 2) and the constant volume specific heat c v is calculated using Equation (3): By substituting Equation (3) and the equation of state as shown in Equation (4) into Equation (2), Equation ( 5) can be obtained.By taking derivatives of Equation ( 5) with respect to crank angle, Equation ( 5) can be expressed in the form of Equation ( 6): Combining Equations ( 1) and ( 6), Equation ( 7) is obtained.From Equation ( 7), the in-cylinder pressure is related to the heat release rate of the fuel, heat transferred to the cylinder wall, stroke volume, and specific heat ratio.
To accurately predict combustion at the premixed stage, main stage, and tail stage, a three-term Wiebe function is adopted to describe the heat release rate in the cylinder.The Wiebe function is presented in Equation (8).The combustion duration ∆ϕ and shape factor s are the key factors affecting the shape of the curve of the heat release rate.
An assumption is made in the model that steam injection has no impact on the heat release rate.Since the combustion in the cylinder is complexly impacted by steam injection, it is hard to give the functional relationship between the heat release rate and the steam.Future work should be carried out to study the impact of steam injection on combustion in the cylinder.
Based on Fourier's law, the heat transferred to the cylinder wall can be calculated by Equation (9), and the coefficient of the heat transfer is estimated based on the Woschni model, as shown in Equation (10): The stroke volume depending on the crank angle can be expressed as Equation (11): As for the specific heat ratio, it is influenced by the components of the in-cylinder working fluids.Specific heat ratio will change remarkably after steam injection into the cylinder.The specific heat ratio of the mixture in the cylinder can be expressed as Equation (12), and the mole fraction of the steam can be obtained from Equation (13): x = m w /M w m a /M a + m w /M w (13) Energies 2018, 11, 936 According to Equations ( 7) and (12), because of the changed specific heat ratio, in-cylinder pressure will be significantly changed by steam injection.
The in-cylinder temperature will also be remarkably influenced by steam injection.Another assumption is made in the model that the steam instantaneously mixes with the air when it is injected into the cylinder.Thus, the in-cylinder temperature after mixing can be expressed by Equation ( 14): Therefore, because the in-cylinder pressure and temperature are notably affected, engine performance and NO x emissions will be influenced by in-cylinder steam injection.

NO x Formation Kinetic Model
NO x emissions produced in the cylinder are mainly thermal NO.The principal source of NO is oxidation of atmospheric nitrogen.As the consideration of chemical equilibrium cannot correctly predict NO concentration, a generally accepted kinetics formation scheme described by an extended Zeldovich model [36] is used in this study.The reactions of the extended Zeldovich model are shown in Reactions ( 15)-( 17): N Therefore, the NO formation rate can be expressed as Equation (18): where ] is the concentration of the component, and [] e is the equilibrium concentration of the component.According to Reactions ( 15)- (17), in-cylinder temperature and O 2 concentration are the major factors that influence the formation of NO x emissions, and both are impacted by steam injection.Apart from these factors, it should be noted that the NO x emission kinetics formation scheme may be changed by steam injection.Even though H 2 O as a combustion product already exists in the cylinder, the amount is small.However, there is a lot of H 2 O participating in the reactions of NO x emission formation when the steam injection is introduced.Due to the immaturity of investigations into NO x emission kinetics formation with H 2 O, it does not consider this impact in the model.It may make predictions of NO x emissions with in-cylinder steam injection drift from the actual results.Future research should be conducted on the influence of lots of H 2 O on the formation mechanism of NO x emissions.

In-Cylinder Steam Injection Subsystem
The heat exchange between exhaust and water directly affects the enthalpy of superheated steam.The process of generating superheated steam in the heat exchanger is given in Figure 2, using an enthalpy-entropy (T-S) diagram.Before the room-temperature water is heated in the heat exchanger, it is first pressured by pump from point 0w to point 1w.In this process, the enthalpy of the water increases, thus more power is consumed by the pump to compress the water.The power consumed by the pump can be calculated by Equation (19): According to the definition of enthalpy, it can be expressed as Equation (20): By substituting Equation (20) into Equation (19), Equation ( 21) is obtained.Since the temperature and volume of the water change only a little after compression, Equation ( 21) can be simplified as Equation (22): It can be seen from Equation ( 22) that as the water pressure increases, the power consumed by the pump also increases.However, in order to inject steam into the cylinder, the steam injection pressure needs to be higher than the in-cylinder pressure at the steam injection timing.Besides, according to the properties of water, the evaporating temperature will increase when the pressure increases.Therefore, the minimum temperature of steam injection will increase as the injection pressure increases.(The properties of water can be obtained by using REFPROP [37], developed by the US National Institute of Standards and Technology.) After the room-temperature (300 K) water is pressured by pump, it is heated into superheated steam from point 1w to point 4w by exhaust in the heat exchanger, as shown in Figure 1a.In this process, the enthalpy of water rises sharply.Based on the first law of thermodynamics, the heat exchange between water and exhaust can be expressed as Equation (23): It can be seen from Equation (23) that the heat transfer capacity of the heat exchanger is large when the water is heated into superheated steam.Therefore, to ensure the heat transfer capacity of the heat exchanger, pinch point analysis is adopted.The pinch point is the location where the minimum temperature difference between cooling and heating sides in the heat exchanger exists.For a given heat flux, a larger minimum temperature difference requires a smaller heat transfer area.Thus, the mass and volume of the heat exchanger can be smaller and the cost will be lower.In this study, the minimum temperature difference is set at 20 °C (or 20 K) to ensure the driving force of Before the room-temperature water is heated in the heat exchanger, it is first pressured by pump from point 0w to point 1w.In this process, the enthalpy of the water increases, thus more power is consumed by the pump to compress the water.The power consumed by the pump can be calculated by Equation (19): According to the definition of enthalpy, it can be expressed as Equation ( 20): By substituting Equation (20) into Equation (19), Equation ( 21) is obtained.Since the temperature and volume of the water change only a little after compression, Equation ( 21) can be simplified as Equation (22): It can be seen from Equation ( 22) that as the water pressure increases, the power consumed by the pump also increases.However, in order to inject steam into the cylinder, the steam injection pressure needs to be higher than the in-cylinder pressure at the steam injection timing.Besides, according to the properties of water, the evaporating temperature will increase when the pressure increases.Therefore, the minimum temperature of steam injection will increase as the injection pressure increases.(The properties of water can be obtained by using REFPROP [37], developed by the US National Institute of Standards and Technology.) After the room-temperature (300 K) water is pressured by pump, it is heated into superheated steam from point 1w to point 4w by exhaust in the heat exchanger, as shown in Figure 1a.In this process, the enthalpy of water rises sharply.Based on the first law of thermodynamics, the heat exchange between water and exhaust can be expressed as Equation (23) It can be seen from Equation (23) that the heat transfer capacity of the heat exchanger is large when the water is heated into superheated steam.Therefore, to ensure the heat transfer capacity of the heat exchanger, pinch point analysis is adopted.The pinch point is the location where the minimum temperature difference between cooling and heating sides in the heat exchanger exists.For a given heat flux, a larger minimum temperature difference requires a smaller heat transfer area.Thus, the mass and volume of the heat exchanger can be smaller and the cost will be lower.In this study, the minimum temperature difference is set at 20 • C (or 20 K) to ensure the driving force of heat transfer.The pinch point may occur at the point 1w, 2w, or 4w, as shown in Figure 1a.The minimum temperature difference can be expressed as Equation (24): The exhaust temperature at point 7m can be calculated by Equation (23).Then the temperature difference at point 1w can be obtained.To calculate the temperature difference at point 2w, the exhaust temperature at point 6m needs to be solved, which can be done by using Equation (25): . m w (h 4w − h 2w ) = . m e c p,e (T 5m − T 6m ) In addition to the restriction of the minimum temperature difference, the exhaust temperature at the heat exchanger outlet should be higher than the water condensation point, otherwise the corrosion problem will happen in the exhaust pipe system.The lowest exhaust temperature at the heat exchanger outlet is set at 120 • C (or 393.15 K) [35] to keep steam from condensing.

Engine Model Validation
The accuracy of the turbocharged diesel engine simulation model was calibrated with the experimental data.The engine performance experimental data were obtained from an 11 L turbocharged diesel engine, which is usually used to power heavy-duty trucks.The basic specifications of the turbocharged diesel engine are shown in Table 1.It is an in-line 6-cylinder Euro IV engine equipped with a common rail injection system.The compression ratio of the engine is 16.4.The engine gets its maximum power and torque at 1900 rpm and 1200 rpm, respectively.The experiments were carried out using the engine test bench.Engine torque and speed were measured by eddy current dynamometer.Static temperature and pressure at the inlet and outlet of the compressor and turbine (positions 1m and 5m in Figure 1a) were measured.Air intake flow rate was measured at the pipe upstream from the compressor, at position 1m as shown in Figure 1a.The NO x emissions without aftertreatment were measured by an exhaust gas analyzer.During the experiments, the in-cylinder pressure was also monitored and stored.An automatic data acquisition and processing system was developed in the engine test bench to attain steady-state average parameters.The engine operation conditions (speed and load) were controlled by an industrial personal computer, which was connected to electronic control units in the engine.
In order to validate the accuracy of the experiments, an error analysis is necessary.The measurement range and systematic errors of various parameters are given in Table 2.The uncertainty levels of the calculated parameters are with respect to the measured ones.That was performed using the differential method of propagating errors based on Taylor's theorem [10], giving an overall uncertainty for BSFC (g/kWh) of 0.57%.To accurately predict engine performance and NO x emissions, the engine model needs to be carefully calibrated.The procedure of engine model validation can be structured in several main steps.First, engine intake pressure and exhaust pressure are calibrated to be close to the experimental data.In such a condition, the engine pumping power will be near the measurement data.In addition, the predictions of boosting level and turbine expansion will also be close to the experimental data.Then, intake air mass flow rate and exhaust temperature are calibrated to ensure that the turbine power is right.Next, the coefficients of the combustion model are adjusted to fit the in-cylinder pressure profile.Under this condition, the indicated power is obtained.After that, the friction power coefficient is adjusted to make sure the total power output is close to the experimental value.Finally, the multipliers in the NO x emission formation kinetic model are calibrated to reduce the errors of NO x emission between experiments and predictions, including NO x calibration multiplier, N 2 oxidation rate multiplier, N 2 oxidation activation energy multiplier, etc.Among them, the NO x calibration multiplier plays an important role in NO x emission validation.If the prediction of NO x emission is higher than the experimental results, the NO x calibration multiplier needs to be decreased.If the reverse, it should be increased.In fact, the calibration procedure is more complicated than the descriptions above.Repeated calibrations are required to obtain good agreement between the predictions and the experimental results.
Comparisons of the in-cylinder pressure profile between the experiment and the simulation at 1900 rpm and 1200 rpm operating conditions are shown in Figure 3.It can be seen that the in-cylinder pressure profiles from the simulation are well fitted with the experimental profiles.Therefore, the simulation model can precisely predict in-cylinder pressure.To accurately predict engine performance and NOx emissions, the engine model needs to be carefully calibrated.The procedure of engine model validation can be structured in several main steps.First, engine intake pressure and exhaust pressure are calibrated to be close to the experimental data.In such a condition, the engine pumping power will be near the measurement data.In addition, the predictions of boosting level and turbine expansion will also be close to the experimental data.Then, intake air mass flow rate and exhaust temperature are calibrated to ensure that the turbine power is right.Next, the coefficients of the combustion model are adjusted to fit the in-cylinder pressure profile.Under this condition, the indicated power is obtained.After that, the friction power coefficient is adjusted to make sure the total power output is close to the experimental value.Finally, the multipliers in the NOx emission formation kinetic model are calibrated to reduce the errors of NOx emission between experiments and predictions, including NOx calibration multiplier, N2 oxidation rate multiplier, N2 oxidation activation energy multiplier, etc.Among them, the NOx calibration multiplier plays an important role in NOx emission validation.If the prediction of NOx emission is higher than the experimental results, the NOx calibration multiplier needs to be decreased.If the reverse, it should be increased.In fact, the calibration procedure is more complicated than the descriptions above.Repeated calibrations are required to obtain good agreement between the predictions and the experimental results.
Comparisons of the in-cylinder pressure profile between the experiment and the simulation at 1900 rpm and 1200 rpm operating conditions are shown in Figure 3.It can be seen that the in-cylinder pressure profiles from the simulation are well fitted with the experimental profiles.Therefore, the simulation model can precisely predict in-cylinder pressure.Comparisons of the simulation results and the experimental data under full load at engine speeds from 1000 rpm to 1900 rpm are shown in Figure 4.It can be seen that the average errors of BSFC and engine torque are 1.67% and 1.63%, respectively.The maximum errors of BSFC and engine torque are both within 4%.Thus, the engine simulation model can successfully predict the main performance of the turbocharged diesel engine.NOx emissions are among the major emissions of diesel engines, and must be reduced to satisfy stricter emissions regulations [38].The maximum and average errors of NOx emission are 1.40% and 0.92%, respectively.Thus, the simulation model can accurately predict NOx emissions of the engine.Exhaust temperature (position 5m in Figure 1a) Comparisons of the simulation results and the experimental data under full load at engine speeds from 1000 rpm to 1900 rpm are shown in Figure 4.It can be seen that the average errors of BSFC and engine torque are 1.67% and 1.63%, respectively.The maximum errors of BSFC and engine torque are both within 4%.Thus, the engine simulation model can successfully predict the main performance of the turbocharged diesel engine.NO x emissions are among the major emissions of diesel engines, and must be reduced to satisfy stricter emissions regulations [38].The maximum and average errors of NO x emission are 1.40% and 0.92%, respectively.Thus, the simulation model can accurately predict NO x emissions of the engine.Exhaust temperature (position 5 m in Figure 1a) is an important parameter in the present study.It determines the available thermal energy of exhaust transferred to water.The maximum and average errors of exhaust temperature are 2.05% and 1.12%, respectively.Therefore, the deviations of exhaust temperature between the simulation and the experiment are within a reasonable range.

Results and Discussion
In this section, based on the turbocharged diesel engine simulation model, the effects of steam injection mass, temperature, and timing on engine performance and NOx emissions under full load at various engine speeds are evaluated.After that, the potential of in-cylinder steam injection to improve engine performance and reduce NOx emissions is presented.

The Limits of Maximum Steam Mass and Temperature
From the analysis of the in-cylinder steam injection subsystem in Section 2.3, it can be seen that the minimum temperature difference ∆ min and the exhaust temperature at the heat exchanger outlet  7m will decrease as steam injection mass and temperature increase.According to the pinch point analysis, the maximum steam injection mass and temperature are restricted by ∆ min .In addition, temperature  7m should be larger than the condensation temperature, which also limits the maximum steam mass and temperature.In this subsection, the impact of steam injection mass and temperature on ∆ min and  7m will be analyzed.
The analysis is carried out under the condition that the steam injection timing is −90° and the injection pressure is 8 bar.The selection of injection pressure is based on the principle that it must be higher than the in-cylinder pressure.In addition, the minimum value of the steam injection temperature range is selected on the principle that it must be larger than the evaporating temperature.Figure 5 shows the influence of steam injection mass and temperature on ∆ min and  7m under full load at an engine speed of 1900 rpm.It is found that the location of ∆ min is highly affected by steam injection mass and temperature.As shown in Figure 1a, the location of ∆ min

Results and Discussion
In this section, based on the turbocharged diesel engine simulation model, the effects of steam injection mass, temperature, and timing on engine performance and NO x emissions under full load at various engine speeds are evaluated.After that, the potential of in-cylinder steam injection to improve engine performance and reduce NO x emissions is presented.

The Limits of Maximum Steam Mass and Temperature
From the analysis of the in-cylinder steam injection subsystem in Section 2.3, it can be seen that the minimum temperature difference ∆T min and the exhaust temperature at the heat exchanger outlet T 7m will decrease as steam injection mass and temperature increase.According to the pinch point analysis, the maximum steam injection mass and temperature are restricted by ∆T min .In addition, temperature T 7m should be larger than the condensation temperature, which also limits the maximum steam mass and temperature.In this subsection, the impact of steam injection mass and temperature on ∆T min and T 7m will be analyzed.The analysis is carried out under the condition that the steam injection timing is −90 • and the injection pressure is 8 bar.The selection of injection pressure is based on the principle that it must be higher than the in-cylinder pressure.In addition, the minimum value of the steam injection temperature range is selected on the principle that it must be larger than the evaporating temperature.Figure 5 shows the influence of steam injection mass and temperature on ∆T min and T 7m under full load at an engine speed of 1900 rpm.It is found that the location of ∆T min is highly affected by steam injection mass and temperature.As shown in Figure 1a, the location of ∆T min occurs at point 4w when the steam injection mass is small and its temperature is high.However, ∆T min occurs at point 2w under the condition that the steam injection mass is large and its temperature is low.
Energies 2018, 11, x 11 of 21 occurs at point 4w when the steam injection mass is small and its temperature is high.However, ∆ min occurs at point 2w under the condition that the steam injection mass is large and its temperature is low.As shown in Figure 5a, ∆ min decreases sharply as the steam injection mass increases.The value of ∆ min is lower than 20 °C (or 20 K) when the steam injection mass reaches 900 mg per cylinder per cycle.The impact of steam injection temperature on ∆ min is remarkable when the steam injection mass is small.However, as steam injection mass increases up to 500 mg per cylinder per cycle, the steam temperature affects ∆ min only slightly.From Figure 5b, it can be seen that  7m is significantly influenced by the steam injection mass, but it is slightly affected by the steam temperature.The exhaust temperature at the heat exchanger exit  7m will be lower than the limit of 393.15K when the steam injection mass reaches 900 mg per cylinder per cycle.

The Impact of Steam Injection Mass
In this subsection, the impact of steam injection mass on BSFC, torque, and NOx emissions of the turbocharged engine are discussed.Figure 6 shows the influence of steam injection mass on in-cylinder pressure and temperature with respect to crank angle under full load at an engine speed of 1900 rpm.The results are obtained under the conditions of −90° steam injection timing, 8 bar steam injection pressure, and 500 K steam injection temperature.Since the working fluids in the cylinder increase after the steam is directly injected, the in-cylinder pressure increases significantly.As shown in Figure 6a, the maximum in-cylinder pressure increases by 24.9 bar (15.6%) compared with the baseline engine when the steam injection mass is 600 mg per cylinder per cycle.Besides, the in-cylinder peak pressure will be larger when the steam injection mass increases.It can be seen in Figure 6b that the in-cylinder temperature is significantly reduced when steam is injected into the cylinder.This is because the specific heat of steam is high.As a result, the specific heat of the mixture in the cylinder increases remarkably after the steam is injected.Compared with the baseline engine, the peak in-cylinder temperature decreases by 153.4 K (9.4%) with a steam injection mass of 600 mg per cylinder per cycle.
The influence of steam injection mass on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm is shown in Figure 7. Since the peak in-cylinder pressure increases with steam injection, the piston work during the power stroke increases.As a result, engine power improves.Therefore, engine torque also increases.As shown in Figure 7, engine torque increases as the steam injection mass increases.Besides, engine torque improves more significantly at low/medium engine speed.Compared with the baseline engine, engine torque increases by 1.0-2.5% at different engine speeds when the steam injection mass is 600 mg per cylinder per cycle.The largest increase of engine torque is 2.5%, which occurs at 1000 rpm with 600 mg per cylinder per cycle.As shown in Figure 5a, ∆T min decreases sharply as the steam injection mass increases.The value of ∆T min is lower than 20 • C (or 20 K) when the steam injection mass reaches 900 mg per cylinder per cycle.The impact of steam injection temperature on ∆T min is remarkable when the steam injection mass is small.However, as steam injection mass increases up to 500 mg per cylinder per cycle, the steam temperature affects ∆T min only slightly.From Figure 5b, it can be seen that T 7m is significantly influenced by the steam injection mass, but it is slightly affected by the steam temperature.The exhaust temperature at the heat exchanger exit T 7m will be lower than the limit of 393.15K when the steam injection mass reaches 900 mg per cylinder per cycle.

The Impact of Steam Injection Mass
In this subsection, the impact of steam injection mass on BSFC, torque, and NO x emissions of the turbocharged engine are discussed.Figure 6 shows the influence of steam injection mass on in-cylinder pressure and temperature with respect to crank angle under full load at an engine speed of 1900 rpm.The results are obtained under the conditions of −90 • steam injection timing, 8 bar steam injection pressure, and 500 K steam injection temperature.Since the working fluids in the cylinder increase after the steam is directly injected, the in-cylinder pressure increases significantly.As shown in Figure 6a, the maximum in-cylinder pressure increases by 24.9 bar (15.6%) compared with the baseline engine when the steam injection mass is 600 mg per cylinder per cycle.Besides, the in-cylinder peak pressure will be larger when the steam injection mass increases.It can be seen in Figure 6b that the in-cylinder temperature is significantly reduced when steam is injected into the cylinder.This is because the specific heat of steam is high.As a result, the specific heat of the mixture in the cylinder increases remarkably after the steam is injected.Compared with the baseline engine, the peak in-cylinder temperature decreases by 153.4 K (9.4%) with a steam injection mass of 600 mg per cylinder per cycle.Figure 8 illustrates the effects of steam injection mass on BSFC with respect to engine speed under full load condition.Because of the increased engine power, BSFC decreases with steam injection.It can be seen from Figure 8 that more steam injection mass results in a greater reduction of BSFC.Also, BSFC improves more remarkably at low/medium engine speed.BSFC is reduced by 1.0-2.5% at different engine speeds compared with the baseline when the steam injection mass is 600 mg per cylinder per cycle.According to the generation mechanism of NOx emission described by the extended Zeldovich model, NOx emission is mainly affected by the peak in-cylinder temperature and the O2 The influence of steam injection mass on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm is shown in Figure 7. Since the peak in-cylinder pressure increases with steam injection, the piston work during the power stroke increases.As a result, engine power improves.Therefore, engine torque also increases.As shown in Figure 7, engine torque increases as the steam injection mass increases.Besides, engine torque improves more significantly at low/medium engine speed.Compared with the baseline engine, engine torque increases by 1.0-2.5% at different engine speeds when the steam injection mass is 600 mg per cylinder per cycle.The largest increase of engine torque is 2.5%, which occurs at 1000 rpm with 600 mg per cylinder per cycle.Figure 8 illustrates the effects of steam injection mass on BSFC with respect to engine speed under full load condition.Because of the increased engine power, BSFC decreases with steam injection.It can be seen from Figure 8 that more steam injection mass results in a greater reduction of BSFC.Also, BSFC improves more remarkably at low/medium engine speed.BSFC is reduced by 1.0-2.5% at different engine speeds compared with the baseline when the steam injection mass is 600 mg per cylinder per cycle.Figure 8 illustrates the effects of steam injection mass on BSFC with respect to engine speed under full load condition.Because of the increased engine power, BSFC decreases with steam injection.It can be seen from Figure 8 that more steam injection mass results in a greater reduction of BSFC.Also, BSFC improves more remarkably at low/medium engine speed.BSFC is reduced by 1.0-2.5% at different engine speeds compared with the baseline when the steam injection mass is 600 mg per cylinder per cycle.
Figure 8 illustrates the effects of steam injection mass on BSFC with respect to engine speed under full load condition.Because of the increased engine power, BSFC decreases with steam injection.It can be seen from Figure 8 that more steam injection mass results in a greater reduction of BSFC.Also, BSFC improves more remarkably at low/medium engine speed.BSFC is reduced by 1.0-2.5% at different engine speeds compared with the baseline when the steam injection mass is 600 mg per cylinder per cycle.According to the generation mechanism of NOx emission described by the extended Zeldovich model, NOx emission is mainly affected by the peak in-cylinder temperature and the O2 According to the generation mechanism of NO x emission described by the extended Zeldovich model, NO x emission is mainly affected by the peak in-cylinder temperature and the O 2 concentration in the cylinder.The impact of steam injection mass on the reduction of NO x emissions under full load at various engine speeds is given in Figure 9. Since the peak in-cylinder temperature and the O 2 concentration both decrease, NO x emission is sharply reduced by steam injection.When steam injection mass is 600 mg per cylinder per cycle, NO x emissions are reduced by 70.7-76.2%across all engine speeds.In addition, with larger steam injection mass, the NO x emission is further reduced.When steam injection mass increases from 200 mg per cylinder per cycle to 600 mg per cylinder per cycle, NO x emissions are further reduced by 59.8%.

The Impact of Steam Injection Temperature
The effects of steam injection temperature on in-cylinder pressure and temperature under full load at 1900 rpm are shown in Figure 10.The results were obtained in the situation of steam injection mass of 400 mg per cylinder per cycle, steam injection timing of −90°, and steam injection pressure of 8 bar.It can be seen that steam injection temperature has relatively little impact on in-cylinder pressure and temperature.This is because the enthalpy of the steam increases only slightly as the steam injection temperature increases from 450 K to 600 K. Therefore, in-cylinder pressure and temperature increase weakly as steam injection temperature increases.The maximum in-cylinder pressure and temperature increase by 3.4% and 1.5%, respectively, when the steam injection temperature increases from 450 K to 600 K.

The Impact of Steam Injection Temperature
The effects of steam injection temperature on in-cylinder pressure and temperature under full load at 1900 rpm are shown in Figure 10.The results were obtained in the situation of steam injection mass of 400 mg per cylinder per cycle, steam injection timing of −90 • , and steam injection pressure of 8 bar.It can be seen that steam injection temperature has relatively little impact on in-cylinder pressure and temperature.This is because the enthalpy of the steam increases only slightly as the steam injection temperature increases from 450 K to 600 K. Therefore, in-cylinder pressure and temperature increase weakly as steam injection temperature increases.The maximum in-cylinder pressure and temperature increase by 3.4% and 1.5%, respectively, when the steam injection temperature increases from 450 K to 600 K.
pressure of 8 bar.It can be seen that steam injection temperature has relatively little impact on in-cylinder pressure and temperature.This is because the enthalpy of the steam increases only slightly as the steam injection temperature increases from 450 K to 600 K. Therefore, in-cylinder pressure and temperature increase weakly as steam injection temperature increases.The maximum in-cylinder pressure and temperature increase by 3.4% and 1.5%, respectively, when the steam injection temperature increases from 450 K to 600 K.The impact of steam injection temperature on engine torque, BSFC, and NOx emissions under full load at 1900 rpm is shown in Figure 11.It can be seen that engine torque is slightly reduced and BSFC is slightly increased as steam injection temperature increases.This is because the steam The impact of steam injection temperature on engine torque, BSFC, and NO x emissions under full load at 1900 rpm is shown in Figure 11.It can be seen that engine torque is slightly reduced and BSFC is slightly increased as steam injection temperature increases.This is because the steam injection temperature has only a little impact on in-cylinder pressure.Although the increased maximum in-cylinder pressure benefits the improvement in engine power, the negative compression work is also increased.Therefore, the engine power is slightly reduced as the steam injection temperature increases.The reduction of engine torque and the increase of BSFC are both within 0.4% when the steam injection temperature increases from 450 K to 600 K.As for NO x emission, it is sensitive to the change of in-cylinder temperature.Therefore, the impact of steam injection temperature on NO x emission is remarkable.The in-cylinder temperature increases as steam injection temperature increases, which favors the generation of NO x emissions.When the steam injection temperature increases from 450 K to 600 K, NO x emissions increase by 17.3%.
Energies 2018, 11, x 14 of 21 injection temperature has only a little impact on in-cylinder pressure.Although the increased maximum in-cylinder pressure benefits the improvement in engine power, the negative compression work is also increased.Therefore, the engine power is slightly reduced as the steam injection temperature increases.The reduction of engine torque and the increase of BSFC are both within 0.4% when the steam injection temperature increases from 450 K to 600 K.As for NOx emission, it is sensitive to the change of in-cylinder temperature.Therefore, the impact of steam injection temperature on NOx emission is remarkable.The in-cylinder temperature increases as steam injection temperature increases, which favors the generation of NOx emissions.When the steam injection temperature increases from 450 K to 600 K, NOx emissions increase by 17.3%.In summary, although the enthalpy of steam increases slightly as steam injection temperature increases, it does not benefit the improvement of engine performance and reduction of NOx emissions.Besides, for a given steam injection mass, a larger heat transfer area is required to increase the steam injection temperature.Therefore, a small superheated degree should be adopted when the steam is injected into the cylinder.

The Impact of Steam Injection Timing
Before discussing the results of the impact of steam injection timing on engine performance and NOx emissions, the minimum steam injection pressure with respect to steam injection timing and engine speed is introduced, as shown in Figure 12.Since the in-cylinder pressure increases as the crank angle increases, the minimum steam injection pressure must also increase to ensure that In summary, although the enthalpy of steam increases slightly as steam injection temperature increases, it does not benefit the improvement of engine performance and reduction of NO x emissions.Besides, for a given steam injection mass, a larger heat transfer area is required to increase the steam injection temperature.Therefore, a small superheated degree should be adopted when the steam is injected into the cylinder.

The Impact of Steam Injection Timing
Before discussing the results of the impact of steam injection timing on engine performance and NO x emissions, the minimum steam injection pressure with respect to steam injection timing and engine speed is introduced, as shown in Figure 12.Since the in-cylinder pressure increases as the crank angle increases, the minimum steam injection pressure must also increase to ensure that the steam can be injected into the cylinder.Besides, the engine speed slightly affects the minimum steam injection pressure.To ensure that the steam can be efficiently injected into the cylinder, all the steam injection pressures are set at 50 bar in the following calculations.In addition, the steam injection temperature is set at 550 K, which is slightly higher than the evaporating temperature.The negative compression work is reduced as the steam injection timing slows down, which benefits the increase of engine power.Figure 14 illustrates the effects of steam injection timing on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm.It can be seen that engine torque increases as the steam injection timing slows down.Engine torque increases by 5.9-7.8% at different engine speeds as the steam injection timing slows down from −120° to −30°. Figure 13 presents the influence of steam injection timing on in-cylinder pressure and temperature under full load at an engine speed of 1900 rpm.From Figure 13, it can be seen that the in-cylinder pressure and temperature both reduce as the steam injection timing slows down.This is because the negative compression work is transferred into the internal energy of the working fluids during the compression stroke.When the steam injection timing slows down, the negative compression work decreases significantly.Therefore, in-cylinder pressure and temperature are both reduced.When the steam injection timing slows down from −120 • to −30 • , the peak in-cylinder temperature and maximum in-cylinder temperature decrease by 4.8% and 2.9%, respectively.
The negative compression work is reduced as the steam injection timing slows down, which benefits the increase of engine power.Figure 14 illustrates the effects of steam injection timing on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm.It can be seen that engine torque increases as the steam injection timing slows down.Engine torque increases by 5.9-7.8% at different engine speeds as the steam injection timing slows down from −120 • to −30 • .The negative compression work is reduced as the steam injection timing slows down, which benefits the increase of engine power.Figure 14 illustrates the effects of steam injection timing on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm.It can be seen that engine torque increases as the steam injection timing slows down.Engine torque increases by 5.9-7.8% at different engine speeds as the steam injection timing slows down from −120° to −30°.The negative compression work is reduced as the steam injection timing slows down, which benefits the increase of engine power.Figure 14 illustrates the effects of steam injection timing on engine torque under full load at engine speeds from 1000 rpm to 1900 rpm.It can be seen that engine torque increases as the steam injection timing slows down.Engine torque increases by 5.9-7.8% at different engine speeds as the steam injection timing slows down from −120° to −30°.The impact of steam injection timing on BSFC under full load at various engine speeds is shown in Figure 15.Since engine power increases as steam injection timing slows down, BSFC is reduced.When steam injection timing slows down from −120 • to −30 The impact of steam injection timing on BSFC under full load at various engine speeds is shown in Figure 15.Since engine power increases as steam injection timing slows down, BSFC is reduced.When steam injection timing slows down from −120° to −30°, BSFC decreases by 5.6-7.3% at different engine speeds.Due to the reduction of peak in-cylinder temperature when the steam is injected at the late stage of the compression stroke, the generation of NOx emissions is suppressed.The influence of steam injection timing on reduction of NOx emissions is shown in Figure 16.It can be seen that at different engine speeds, NOx emission is reduced by 21.6-26.7%as the steam injection timing slows down from −120° to −30°.Due to the reduction of peak in-cylinder temperature when the steam is injected at the late stage of the compression stroke, the generation of NO x emissions is suppressed.The influence of steam injection timing on reduction of NO x emissions is shown in Figure 16.It can be seen that at different engine speeds, NO x emission is reduced by 21.6-26.7%as the steam injection timing slows down from −120 • to −30 • .Due to the reduction of peak in-cylinder temperature when the steam is injected at the late stage of the compression stroke, the generation of NOx emissions is suppressed.The influence of steam injection timing on reduction of NOx emissions is shown in Figure 16.It can be seen that at different engine speeds, NOx emission is reduced by 21.6-26.7%as the steam injection timing slows down from −120° to −30°.

Optimal Steam Injection Mass at Different Engine Speeds
The impact of steam injection mass, temperature, and timing on engine performance and NOx emissions is discussed above.It can be concluded that steam injection mass and timing are the major parameters that affect engine performance and NOx emissions.The engine BSFC and torque performance is improved and NOx emissions are reduced significantly as the steam injection mass increases and injection timing slows down.However, steam injection mass is restricted by ∆ min and  7m , and injection timing is restricted by the in-cylinder pressure.It is difficult to inject steam near TDC when the in-cylinder pressure is ultra-high.
In this subsection, the steam injection timing is set at −30°.The injection temperature and pressure are 550 K and 50 bar, respectively.Under these conditions, the optimal steam injection mass and changes of maximum in-cylinder pressure and temperature are obtained, as shown in Figure 17.It can be seen that the optimal steam injection mass increases and then decreases as engine speed

Optimal Steam Injection Mass at Different Engine Speeds
The impact of steam injection mass, temperature, and timing on engine performance and NO x emissions is discussed above.It can be concluded that steam injection mass and timing are the major parameters that affect engine performance and NO x emissions.The engine BSFC and torque performance is improved and NO x emissions are reduced significantly as the steam injection mass increases and injection timing slows down.However, steam injection mass is restricted by ∆T min and T 7m , and injection timing is restricted by the in-cylinder pressure.It is difficult to inject steam near TDC when the in-cylinder pressure is ultra-high.
In this subsection, the steam injection timing is set at −30 • .The injection temperature and pressure are 550 K and 50 bar, respectively.Under these conditions, the optimal steam injection mass and changes of maximum in-cylinder pressure and temperature are obtained, as shown in Figure 17.It can be seen that the optimal steam injection mass increases and then decreases as engine speed increases.The largest optimal steam injection mass value is 850 mg per cylinder per cycle, which occurs at 1400 rpm.At all engine speeds, the maximum in-cylinder pressure increases by 9.3-12.7%and the maximum in-cylinder temperature reduces by 11.2-13.9%at different engine speeds.
With the optimal steam injection mass, the largest improvement of engine performance and the maximum reduction of NO x emission under full load at different engine speeds conditions are presented in Figure 18.It is found that with the optimal steam injection mass value, over all engine speeds, the engine torque is increased by 9.5-10.9%,BSFC is decreased by 8.7-9.9% and NO x emission are reduced by 83.5-91.8%.In summary, in-cylinder steam injection can remarkably improve the engine performance and reduce NO x emission at all engine speed conditions.
increases.The largest optimal steam injection mass value is 850 mg per cylinder per cycle, which occurs at 1400 rpm.At all engine speeds, the maximum in-cylinder pressure increases by 9.3-12.7%and the maximum in-cylinder temperature reduces by 11.2-13.9%at different engine speeds.With the optimal steam injection mass, the largest improvement of engine performance and the maximum reduction of NOx emission under full load at different engine speeds conditions are presented in Figure 18.It is found that with the optimal steam injection mass value, over all engine speeds, the engine torque is increased by 9.5-10.9%,BSFC is decreased by 8.7-9.9% and NOx emission are reduced by 83.5-91.8%.In summary, in-cylinder steam injection can remarkably improve the engine performance and reduce NOx emission at all engine speed conditions.

Conclusions
An in-cylinder steam injection method for waste heat recovery and NOx emission reduction has been introduced.In the study, 300 K water was heated into superheated steam by exhaust.Then the superheated steam was directly injected into the cylinder during the compression stroke to increase engine power and reduce NOx emissions.The impact of steam injection mass, temperature, and timing on the performance and NOx emissions of a turbocharged diesel engine was investigated using a numerical method.In addition, the optimal steam injection mass, the largest improvement of engine performance, and the maximum reduction of NOx emissions under full load at engine speeds from 1000 rpm to 1900 rpm were obtained.Based on the above analysis, the following conclusions can be drawn.With the optimal steam injection mass, the largest improvement of engine performance and the maximum reduction of NOx emission under full load at different engine speeds conditions are presented in Figure 18.It is found that with the optimal steam injection mass value, over all engine speeds, the engine torque is increased by 9.5-10.9%,BSFC is decreased by 8.7-9.9% and NOx emission are reduced by 83.5-91.8%.In summary, in-cylinder steam injection can remarkably improve the engine performance and reduce NOx emission at all engine speed conditions.

Conclusions
An in-cylinder steam injection method for waste heat recovery and NOx emission reduction has been introduced.In the study, 300 K water was heated into superheated steam by exhaust.Then the superheated steam was directly injected into the cylinder during the compression stroke to increase engine power and reduce NOx emissions.The impact of steam injection mass, temperature, and timing on the performance and NOx emissions of a turbocharged diesel engine was investigated using a numerical method.In addition, the optimal steam injection mass, the largest improvement of engine performance, and the maximum reduction of NOx emissions under full load at engine speeds from 1000 rpm to 1900 rpm were obtained.Based on the above analysis, the following conclusions can be drawn.

Conclusions
An in-cylinder steam injection method for waste heat recovery and NO x emission reduction has been introduced.In the study, 300 K water was heated into superheated steam by exhaust.Then the superheated steam was directly injected into the cylinder during the compression stroke to increase engine power and reduce NO x emissions.The impact of steam injection mass, temperature, and timing on the performance and NO x emissions of a turbocharged diesel engine was investigated using a numerical method.In addition, the optimal steam injection mass, the largest improvement of engine performance, and the maximum reduction of NO x emissions under full load at engine speeds from 1000 rpm to 1900 rpm were obtained.Based on the above analysis, the following conclusions can be drawn.
(1) The in-cylinder steam injection method can improve engine performance significantly at all engine speeds.This is because maximum in-cylinder pressure increases significantly as the working fluids in the cylinder increase after steam injection.Therefore, engine torque increases and BSFC decreases.When the steam injection timing is −30 • , steam injection temperature is 550 K and steam injection pressure is 50 bar; with optimal steam injection mass, the maximum in-cylinder pressure increases by 9.3-12.7%across all engine speeds.As a result, engine torque increases by 9.5-10.9%and BSFC decreases by 8.6~9.9%.(2) With optimal steam injection mass, 83.4-91.8%reductions of NO x emissions are obtained at different engine speeds.The remarkable reduction is due to the fact that steam injection decreases maximum in-cylinder temperature and O 2 concentration.The peak in-cylinder temperatures at different engine speeds are reduced by 11.2-13.9%.(3) Steam injection mass and injection timing are the key parameters that greatly influence engine performance and NO x emissions.As steam injection mass increases and injection timing slows down, engine performance is further improved and NO x emissions are lower.However, the steam injection mass is restricted by the minimum temperature difference ∆T min and the exhaust temperature at the heat exchanger outlet T 7m .Besides, the injection timing is restricted by the in-cylinder pressure.(4) Steam injection temperature weakly affects engine performance.This is because the enthalpy of steam changes slightly as the steam injection temperature increases.Therefore, in-cylinder pressure and temperature are slightly impacted by the steam injection temperature.The changes of engine torque and BSFC are both within 0.4% when steam injection temperature increases from 450 K to 600 K.However, NO x emission is very sensitive to the change of in-cylinder temperature.NO x emissions increase by 17.3%, when the steam injection temperature increases from 450 K to 600 K.
In general, according to the simulation results, in-cylinder steam injection may be a good solution to both improve engine performance and reduce NO x emissions, but the data must be further confirmed by experimental tests.However, to carry out the experiment successfully, modification of the engine is complex.There are still some difficulties.Unlike water injection, which can use the fuel injector, in-cylinder steam injection is more difficult due to its compressibility.A steam injector with high response and large flux is required for such a study.However, a mature product in the market that can fulfill the requirements cannot be found.Manufacturing such an injector is not easy.Therefore, future work should be carried out to first develop an appropriate steam injector.It may take a long time to successfully apply the technology to the engine.

Figure 1 .
Figure 1.(a) Schematic of the turbocharged diesel engine with in-cylinder steam injection; (b) steam injection timing range.

Figure 1 .
Figure 1.(a) Schematic of the turbocharged diesel engine with in-cylinder steam injection; (b) steam injection timing range.

Figure 2 .
Figure 2. Enthalpy-entropy (T-S) diagram of the process of generating superheated steam.

Figure 2 .
Figure 2. Enthalpy-entropy (T-S) diagram of the process of generating superheated steam.

Figure 3 .
Figure 3.Comparison of in-cylinder pressure profiles at 1900 rpm and 1200 rpm under full load conditions: (a) 1900 rpm; (b) 1200 rpm.

Figure 3 .
Figure 3.Comparison of in-cylinder pressure profiles at 1900 rpm and 1200 rpm under full load conditions: (a) 1900 rpm; (b) 1200 rpm.

Figure 5 .
Figure 5. Impact of steam injection mass and temperature on ∆ min and  7m at engine speed 1900 rpm (steam injection timing = −90°, steam injection pressure = 8 bar).(a) Minimum temperature difference ∆ min ; (b) exhaust temperature at heat exchange outlet  7m .

Figure 5 .
Figure 5. Impact of steam injection mass and temperature on ∆T min and T 7m at engine speed 1900 rpm (steam injection timing = −90 • , steam injection pressure = 8 bar).(a) Minimum temperature difference ∆T min ; (b) exhaust temperature at heat exchange outlet T 7m .

Figure 6 .
Figure 6.The influence of steam injection mass on in-cylinder pressure and temperature at engine speed 1900 rpm (steam injection timing = −90 • , steam injection pressure = 8 bar, steam injection temperature = 500 K).(a) In-cylinder pressure; (b) in-cylinder temperature.

Figure 7 .
Figure 7.The influence of steam injection mass on engine torque at different engine speeds (steam injection timing = −90 • , steam injection pressure = 8 bar, steam injection temperature = 500 K).

Energies 2018 ,Figure 9 .
Figure 9.The influence of steam injection mass on reduction of NOx emissions at different engine speeds (steam injection timing = −90°, steam injection pressure = 8 bar, steam injection temperature = 500 K).

Figure 9 .
Figure 9.The influence of steam injection mass on reduction of NO x emissions at different engine speeds (steam injection timing = −90 • , steam injection pressure = 8 bar, steam injection temperature = 500 K).

Figure 10 .
Figure 10.The influence of steam injection temperature on in-cylinder pressure and temperature at engine speed 1900 rpm (steam injection mass = 400 mg, steam injection timing = −90°, steam injection pressure = 8 bar).(a) In-cylinder pressure; (b) in-cylinder temperature.

Figure 10 .
Figure 10.The influence of steam injection temperature on in-cylinder pressure and temperature at engine speed 1900 rpm (steam injection mass = 400 mg, steam injection timing = −90 • , steam injection pressure = 8 bar).(a) In-cylinder pressure; (b) in-cylinder temperature.

Figure 12 .
Figure 12.Minimum steam injection pressure with different steam injection timings at various engine speeds.

Figure 12 .Figure 13 .
Figure 12.Minimum steam injection pressure with different steam injection timings at various engine speeds.

Figure 14 .
Figure 14.The influence of steam injection timing on engine torque at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 13 .Figure 13 .
Figure 13.The influence of steam injection timing on in-cylinder pressure and temperature at engine speed 1900 rpm (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).(a) In-cylinder pressure; (b) in-cylinder temperature.

Figure 14 .
Figure 14.The influence of steam injection timing on engine torque at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 14 .
Figure 14.The influence of steam injection timing on engine torque at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 15 .
Figure 15.The influence of steam injection timing on BSFC at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 15 .
Figure 15.The influence of steam injection timing on BSFC at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 15 .
Figure 15.The influence of steam injection timing on BSFC at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 16 .
Figure 16.The influence of steam injection timing on reduction of NOx emissions at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 16 .
Figure 16.The influence of steam injection timing on reduction of NO x emissions at different engine speeds (steam injection mass = 400 mg, steam injection pressure = 50 bar, steam injection temperature = 550 K).

Figure 17 .
Figure 17.The optimal values of steam injection mass and the changings of the maximum in-cylinder pressure and temperature (steam injection timing = −30°, steam injection temperature = 550 K, steam injection pressure = 50 bar).

Figure 18 .
Figure 18.The largest improvements of engine torque and BSFC and maximum reductions of NOx emissions with optimal steam injection mass at different engine speeds (steam injection timing = −30°, steam injection temperature = 550 K, steam injection pressure = 50 bar).

Figure 17 .
Figure 17.The optimal values of steam injection mass and the changings of the maximum in-cylinder pressure and temperature (steam injection timing = −30 • , steam injection temperature = 550 K, steam injection pressure = 50 bar).

Figure 17 .
Figure 17.The optimal values of steam injection mass and the changings of the maximum in-cylinder pressure and temperature (steam injection timing = −30°, steam injection temperature = 550 K, steam injection pressure = 50 bar).

Figure 18 .
Figure 18.The largest improvements of engine torque and BSFC and maximum reductions of NOx emissions with optimal steam injection mass at different engine speeds (steam injection timing = −30°, steam injection temperature = 550 K, steam injection pressure = 50 bar).

Figure 18 .
Figure 18.The largest improvements of engine torque and BSFC and maximum reductions of NO x emissions with optimal steam injection mass at different engine speeds (steam injection timing = −30 • , steam injection temperature = 550 K, steam injection pressure = 50 bar).

Table 1 .
Specifications of the turbocharged diesel engine.

Table 2 .
Errors of measured parameters.

Table 2 .
Errors of measured parameters.
Energies 2018, 11, x 10 of 21 is an important parameter in the present study.It determines the available thermal energy of exhaust transferred to water.The maximum and average errors of exhaust temperature are 2.05% and 1.12%, respectively.Therefore, the deviations of exhaust temperature between the simulation and the experiment are within a reasonable range.