Energy efficiency and environmental impact have become dominant topics in internal combustion engines development. Among many strategies to improve power and emissions outputs from diesel engines is the partial mix of hydrogen and air as fresh charge components to form extremely lean and homogenous mixture, which resist the spontaneous combustion, while diesel fuel is injected directly inside combustion chamber using the conventional fuel injection systems. This contribution presents an analytical and experimental investigation for the effects of adding hydrogen on diesel engines power output and the reduction of emissions. Parametric analysis is used based on lamped parameters modeling of intake manifold to estimate in cylinder trapped charge. The fuel energy flow to engine cylinders is compared for a range of loads and concentrations to simulate relevant case studies. Diesel fuel reduction for significant range of part-load operation can be achieved by introducing hydrogen, along with power improvement emission reductions are affected positively as well. This is achievable without compromising the engine maximum efficiency, given that most engines are operated at small and part-load during normal driving conditions, which allow for introducing more hydrogen instead of large quantities of excess air during such operation conditions that also can be further improved by charge boosting.
Currently, thermal systems are required to become energy efficient and produce less emission. Diesel engines are one of these systems that need to be considered for improvement in this respect. Hydrogen also is expected to become one of the most important fuels for reducing greenhouse gas emission problem in the near future. Based on the characteristics of both fuels, this contribution provides an insight into the possibility of improving diesel engines power and emissions outputs by hydrogen addition. The effectiveness of the proposed technique will be demonstrated analytically and experimentally.
Diesel engines are characterized by their carbon oxides, nitrogen oxides, soot, and particulate emissions. Related literature shows that introducing hydrogen to partially substitute diesel fuel charge will enhance engine performance [
In the present contribution, the performance of diesel engine is considered using addition of hydrogen and based on mean value engine modeling oriented for power and emissions outputs improvement. Hydrogen gas is introduced into the intake manifold using the available gas fuelling techniques. The concept is based on continuous injection inside intake manifold where a central metering valve and distributor deliver vaporized hydrogen into the intake runners. For this purpose, engine coolant heat is used for hydrogen evaporation and to control its temperature. Allowing relatively low-temperature hydrogen gas into the intake manifold helps to cool the fresh charge, which increases the charge density and helps to reduce engine emissions. Lean mixture prevents backfiring into the intake ducts. Diesel fuel is injected directly inside the combustion chamber using the current fuelling technologies with some minor modifications to reduce diesel fuel flow, which will be replaced by hydrogen fuel flow [
For the purpose of this contribution, a lamped parameter based model for lean air-hydrogen mixture formation inside the intake manifold is developed. The presented modeling allows also for considering the possibility of exhaust gas recirculation. As for starting, it is enough accurate to consider that intake manifold as single capacity connected to the boosting system whenever there is one on the input side along with the EGR system. It is connected from the other side as output to the engine cylinders. This capacity exchanges heat to the surroundings to allow for isothermal conditions, which allows for the assumption that single pressure and single temperature values can be used to characterize the whole capacity state changes. Closed volume around the intake manifold to account for mass flow to the engine cylinders is used. Therefore, the rate of mass change inside this capacity can be given by the continuity equation as following:
where mm is the intake manifold mass content,
Assuming that all intake manifold air, hydrogen, re-circulated exhaust gas are perfect gases, the mass in this case can be expressed Equation (2).
where Pm is the pressure, Tm is the temperature, and Rm is the gas constant of the manifold mixture, while Vm is the manifold volume. Accordingly, the pressure rate of change inside the intake manifold is given by
Rm and Tm of the mixture at the engine inlet valve can be found by Equation (4) and Equation (5), respectively, using the mixture corresponding variables of the air, hydrogen, and exhaust gas. The two equations are based on the implicit assumption that perfect and adiabatic mixing takes place in the intake manifold.
where
where
where
where
For compression ignition engines and gasoline direct injection engines the volumetric efficiency,
where
Rearranging Equation (13), the following quadratic relationship can be obtained:
For the purpose of this study, it is assumed that there is no exhaust re-circulation,
where
For physical parameters, the expression of the air mass flow can be given by Equation (16), if the fuel’s temperature, specific heat, and gas constant are known.
To account for air mass trapped inside engine cylinder, which is the available portion for the combustion of the injected diesel fuel. Therefore, it is possible to assume that all introduced hydrogen will be burned according to stoichiometric conditions. Based on that, all the rest air trapped could be used for diesel combustion as follows:
To account for diesel fuel flow, which is injected inside cylinders, the following relation can be obtained by rearranging Equation (17).
where
Equation (19) helps to give insight into the available energy for producing useful power from introducing hydrogen and reducing diesel, though increasing this amount will increase the efficiency. Of course, maintaining or increasing this amount by replacing diesel by hydrogen will satisfy the sought target.
In this section, the developed modeling will be used to analysis the effects of partially adding hydrogen in the induction system on the charge concentrations and the total fuel energy flow to the diesel engine. At first, the effect on charge trapped quantity is considered at various hydrogen concentrations to give insight on the effect of its gaseous state on the fresh charge composition. Then the effect of varying relative air-fuel ratio for both fuels on diesel mass flow is considered to allow for relating the three working fluids at various engine loads. After that, fuel energy flow to engine cylinders at various loads in the form of excess air factor is considered, followed by considering the effect of manifold pressure variation to give insight into possibility of increasing hydrogen flow.
For the purpose of this study, engine speed is kept constant as its main effect is on volumetric efficiency and for not complicating results analysis.
Therefore, specific diesel fuel consumption decreases with load as the percentage of hydrogen partial mixing increases at constant engine speed. Due to the mixing of hydrogen with air, more efficient burning process of fuel mixture is achievable, which could increases engine power, given the brake mean effective pressure increases with load when the percentage of hydrogen mixing increases.
Parameter | Symbol | Value |
---|---|---|
Engine | Vd (cm3) | 2151 |
N (rpm) | 2000 | |
rc | 19:1 | |
Diesel Fuel | AFRs (kg/kg) | 14.5 |
C10.8H18.7 | ||
LHV (MJ/kg) | 42.5 | |
Hydrogen Fuel | AFRs (kg/kg) | 34.3 |
Cp (kJ/kg×K) | 14.21 | |
LHV (MJ/kg) | 120 |
mixing, which gives better combustion characteristics; therefore, it becomes possible to use lower excess air factor for diesel. This allows for higher hydrogen induction rate for various loading of diesel engine running at constant speed. For duel fuel system, the equivalence ratio decreased as hydrogen induction rate increased for a given load and speed.
The general trend is consistent with that observed in
The experimental procedure is designed to validate that using hydrogen gas a part-load could improve diesel engine power with less emissions. To achieve hydrogen this goal, hydrogen was introduced at constant engine speed and constant diesel flow. By increasing the load using the dynamometer, the difference of extra power generated from hydrogen addition is then measured. The experimental results for different tests, which are presented in
To increase cylinder energy content from fuel without increasing diesel fuel flow, it is possible to achieve that in two ways. The first one is to lower the excess air factor at part-load operating conditions allowing for more hydrogen mass flow.
Speed (rpm) | Diesel Only | Diesel and Hydrogen Mixture | ||||
---|---|---|---|---|---|---|
Torque (N×m) | Power (kW) | Hydrogen (cm3/s) | Torque (N×m) | Power (kW) | ||
950 | 20 | 1.98 | 2.1 | 24 | 2.38 | |
5.25 | 28 | 2.78 | ||||
7.36 | 33 | 3.28 | ||||
1200 | 34 | 4.27 | 2.1 | 39 | 4.9 | |
5.25 | 41 | 45 | ||||
7.36 | 58 | 9.71 | ||||
1600 | 45 | 7.53 | 2.1 | 49 | 8.21 | |
5.25 | 56 | 9.38 | ||||
7.36 | 58 | 9.71 | ||||
are kept constant. This indicates that more power could be achieved at part-load condition by increasing hydrogen flow. Of course, this is very positive issue as most of engines installed onboard are operated within its part-load range. To have more insight into the effect of increasing hydrogen flow at part-load conditions, adiabatic combustion is considered. It is used to evaluate the adiabatic flame temperature Tc, given it is a direct indicator of the engine indicated power; a higher obtained temperature means a higher indicated power output. Based on this fact, constant pressure system is considered as given by
Equation (20) can be rearranged to obtain Equation (21), which is more suitable for using JANAF thermodynamic properties tables.
The chemical reaction is given by the flowing equation, which is based on complete combustion of the introduced fuel (Diesel and Hydrogen) in the presence of excess air.
From the equation above, it is very clear that excess air adds many moles of diatomic molecules (O2 and N2) into the products that does not contribute to heat release just soaks it up. Therefore; the above discussed case is also considered for evaluating adiabatic flame temperature. Look up enthalpy values from JANAF tables, and iteration gives for λd = 4, the obtained Tc = 1920 K, but when excess air is reduced to λd = 3.5, Tc = 1980 K. It is obvious that allowing for more hydrogen flow at part-load helps to increase power output and reduce carbon emissions for certain engine load and speed. For evaluating T1, isentropic compression is assumed for calculations simplicity.
The second way is to increase manifold pressure while keeping diesel flow constant, this allows to increase hydrogen flow as can be seen in
Experimental results shows obvious increase in power output with introduction of hydrogen, which means that diesel consumption rate could be decreased with increase in hydrogen flow. This leads to decrease in carbon oxides emission by a very significant percentage. Further, to reduce the risk of hydrogen mixture auto-ignition, it is possible to benefit from the introduction of hydrogen at low temperature in the intake manifold; this could easily lower the mixture temperature before the compression process
The presented analytical and experimental investigation for the effects of adding hydrogen on diesel engines power output and the reduction of emissions shows obvious positive signs to follow-up the obtained results. Engine power to weight ratio can be improved at optimized hydrogen-diesel operating conditions. Carbon oxides emissions can be reduced along with the increasing of hydrogen percentage. Increasing the intake manifold pressure can improve the engine filling, however, knock limit remains an important concern. This is achievable without compromising the engine maximum efficiency, given that most engines are operated at small and part-load during normal driving conditions, which allow for introducing more hydrogen instead of the large quantity of excess air during such operation conditions that also further are improved by charge boosting. In view of the above observations, experimental investigations are further needed to be carried out to revile all technical issues and demonstrate practical solutions viability.
Acknowledgement is given to the technicians and automotive engineering students from Mechanical Engineering Department in Palestine Polytechnic University, for technical support and help in conducting the experimentation, respectively.
MomenSughayyer, (2016) Effects of Hydrogen Addition on Power and Emissions Outputs from Diesel Engines. Journal of Power and Energy Engineering,04,47-56. doi: 10.4236/jpee.2016.41003