USE OF HYDROGEN FUEL ON VEHICLES

THE CAR AS A VEHICLE APPEARED ABOUT 100 YEARS AGO AND TODAY IT IS EXTREMELY WIDESPREAD AND PERFORMS IMPORTANT FUNCTIONS IN THE ECONOMY OF ALL COUNTRIES. THERE ARE MORE THAN 300 MILLION CARS NOW AND THEIR WORLD PRODUCTION INCREASES EVERY YEAR.

 The vehicle transport is a major consumer of petroleum fuels (it accounts for about one-eighth of production) and is one of the main sources of environmental pollution. The share of harmful emissions with exhaust gases of automobile engines amounts to 39-63 % of the total pollution of the environment. The experts estimate that world oil reserves amount to 100 billion tones, i.e.  we have oil for 15 years at present rates of consumption.

Currently energy and environmental problems are of prime importance. The solution to energy and environmental problems (to a greater or lesser extent) can be provided by the following activities:

  • the development of more sophisticated power installation of a new type;
  • the improvement of the workflow of a conventional internal combustion engine and application of exhaust gas aftertreatment systems;
  • the use of a conventional internal combustion engine with new types of fuel.

 At first glance, in terms of solving problems, the electric power installations using electrochemical energy sources — accumulator batteries and electrochemical generators are becoming of great interest.

Despite of some significant advantages (high adaptability to the intermittent mode of urban traffic, high durability, easy maintenance and environmental friendliness), practical application of electric vehicles remains problematic for two main reasons. First, there are no reliable, light and, most importantly, adequately energy efficient electrochemical power sources for these vehicles. The power-to-weight ratio and power capacity of accumulator batteries and fuel cells are about next lower order than a modern internal combustion engine. Secondly, the transition of the entire automobile fleet on electrochemical batteries will cause the expenditure of large amounts of electric power for charging of the accumulators. In industrialized countries, the total capacity of car engines is several times higher than the capacity of all power plants. Besides that, the vast majority of electricity is produced by burning of fossil fuels, so the energy and environmental problem would be passed from the automotive sphere to the sphere of thermal power plants.

The development and application of new types of engines such as external combustion engines (steam engines andStirlingengines) for vehicles allow to achieve low polluting emissions with combustion products and to provide promising tight toxicity standards. However, in this case the problem of shortage of fuel resources is not solved. The practical application of new schemes of engines for road transport requires the solution of several complicated technical problems, especially with regard toStirlingengine. In addition, restructuring of the automotive industry would require huge investments. Therefore, the possibility of the widespread introduction of these engines is postponed for quite a while.

The solution of energy and environment problem of road transport by improving the design of existing internal combustion engines focuses on the improvement of the working process of the power supply subsystems to ensure maximum combustion efficiency in all modes of engine operation as well as on application of various devices for treatment of the exhaust gas through afterburning, catalytic and liquid neutralization, filtration, etc. Unfortunately, the application of these rather complicated and expensive devices is ineffective and associated with significant costs. 

MOTOR PROPERTIES OF HYDROGEN AND PREREQUISITES OF ITS USE AS FUEL FOR INTERNAL COMBUSTION ENGINES

 The idea of using hydrogen as a fuel for piston internal combustion engines is not new. The development of hydrogen propulsion engine for aerostats and submarines was started inGermanyandEnglandin the late 20's, early 30-ies of this century. However, these works were suspended due to technical difficulties of the organization, which were caused by the motor properties of hydrogen and the lack of effective and safe methods of its accumulation.

In the early 70s the return to hydrogen as a fuel, which is environmentally friendly and has unlimited raw material base, is justified due to the worsening energy situation. 

MOTOR PROPERTIES OF HYDROGEN

 The suitability of any fuel for the transport internal combustion engines is determined by its motor properties. Hydrogen as a motor fuel has several features that distinguish it from other fuels. The use of hydrogen allows a new approach to organization of work process of internal combustion engines, to significantly improve their fuel efficiency and reduce polluting emissions with exhaust gases. Hydrogen is one of the most energy intensive fuels, its lowest calorific value is almost three times higher than the one of petroleum motor fuels and is 120 X 103 kJ/kg. However, due to the low stoichiometric ratio of hydrogen - air (the combustion of 1 mole of hydrogen requires 2.38 moles of air, at the same time 1 mole of petroleum motor fuel requires about 50 moles) and low-density of hydrogen, the heating value of hydrogen and air mixture of stoichiometric mixture will be lower than fuel and air mixture of traditional fuels, which would lead to the reduction of the piston engine power, when converted to hydrogen.

The comparative values of energy density of a charge for gasoline and hydrogen engines as well as the characteristics of the fuels are given in table. 1. The data show that fuel - air volume ratio of hydrogen engine in a stoichiometric mixture amounts to 0,42, at the same time this ratio of gasoline engine is only 0.02. The large volume fraction of hydrogen in the fuel and air mixture cause a significant reduction of energy intensity of the charge at external mixture formation, despite the very high calorific value of hydrogen.

Table 1. The power capacity of the charge piston engine on gasoline and hydrogen. 

Fuel

kJ/t

unreadable

unreadable

unreadable

un

readable

unreadable

1,0

1,1

1,2

1,3

2,0

2,1

3,0

4,0

5,0

gasoline

44,0

10,06

4,37

0,020

un

2,370

3,44

3,16

2,53

-

-

-

-

-

hydrogen

120

2,38

0,09

0,42

readable

3,19

4,53

2,98

4,12

2,80

3,78

2,36

3,02

1,87

2,27

1,55

1,81

1,32

1,51

1,02

1,13

0,53

0,9

 

In case of the stoichiometric composition of mixture, the energy intensity of the hydrogen engine charge with external carburation is 15% lower, than the gasoline engine. In case of internal carburation, on the contrary, the energy intensity of the hydrogen engine charge is 12% higher that allows to reach quite high values of the mean effective pressure (up to 0.85 MPa). However, based on the available data we cannot still conclude of the possible maximum power-per-liter of the hydrogen engine. Its value will largely depend on applicability of the area of the stoichiometric composition of mixtures in connection with tendency to self-ignite at intake, tendency to detonate and high emission of nitrogen oxides. Therefore, the composition of the hydrogen-air mixture at full power can be limited by the air-fuel ratio 1.5.

The fuel properties determining the carburation quality have great impact on the engine cycle. When using hydrogen as fuel for internal combustion engines (ICE), several carburation principles can be applied: for spark ignition engines - external and internal (hydrogen supply both during intake and to compression lines); for self-ignition engines - external and internal (hydrogen supply to compression lines and ignition by injection of ignition dose of the liquid hydrocarbon fuel, as well as hydrogen supply at the end of  compression cycle according to particular law together with ignition dose of the liquid hydrocarbon fuel); for gas turbines - internal with continuous feed of hydrogen to the combustion zone.

In case of external carburation, the mixture homogeneity is determined by such fuel properties as boiling point and diffusion capacity. In this regard, the hydrogen has perfect properties: boiling point is253°Cthat under any conditions of engine operation excludes the liquid phase of hydrogen in the mixture; the diffusion coefficient of hydrogen in the air under normal conditions amounts to 0.63 cm2/sec, that is eight times higher than the diffusion coefficient of hydrocarbon fuels in the air.

The specified properties of hydrogen provide formation of high-homogeneous mixture and exclude formation of fluid film on the surfaces of the intake path due to mixture overcooling during carburation and its separation under acceleration in bends of the intake path and flow pulsations at the intake.

In case of internal carburation with fuel supply to the compression lines, the requirements for fuels by speed of the homogeneous mixture formation are more stringent, as the time for mixture formation, in this case, is several times less, than in case of external carburation. The specified properties of hydrogen meet these requirements better than any of the hydrocarbon fuels, both fluid and gaseous.

The stringent requirements for fuels on the homogeneous mixture formation disappear in case of internal carburation with fuel supply at the end of compression, as it is burned during supply to the cylinder. At the same time, the fuel should have ability to form the combustible mixture for very short period of time (about 1 ms). Hydrogen, having high diffusion rate, in this regard is a fine fuel. However, as this carburation principle can be realized in combination with forced ignition, there can be certain difficulties in clear balancing the ignition point and moment of hydrogen supply. In addition, there can be certain problems related to the equipment of high-pressure hydrogen injection due to its low density and compressibility.

In the gas-turbine engines due to high fuel consumption, the combustion of hydrogen should occur continuously under any conditions with premixed flames. When using hydrogen with much greater diffusion capacity, as well as when using hydrocarbon fuels, the internal carburation ensuring the fast and high homogenization of the mixture in the combustion zone is reasonable.

The cycle features of the hydrogen-fueled engines are mainly determined by the properties of the hydrogen-air mixture, namely: ignition limits, ignition temperature and energy, flame front spread rate, flame chilling distance. All these properties of hydrogen are much better, than of the hydrocarbon fuels.

Ignition limits. The change limits of fuel-air mixture compositions, at which their ignition and combustion is possible, are called ignition limits and estimated by volume ratios of fuel content in the mixture or air-fuel ratio. The ignition limits are determined by experiment and their values depend on the method of determination and conditions of experiment.

The volume ratio of the hydrogen-air mixture lower limit under normal conditions amounts to 0.04-0.1, upper — 0.7-0.8, for gasoline respectively – 0.014-0.024 and 0.04-0.08, for methane –0.05- 0.06 and 0.127-0.150.

In terms of the motor properties of fuel, the lower ignition limit is of the great interest, as it allows to estimate the extent of the effective leaning of the fuel-air mixture and determines the engine control method. It is several times higher for hydrogen than for hydrocarbon fuels. Even at low temperatures, it is possible to qualitatively adjust the engine power that allows to obtain high fuel efficiency in comparison with gasoline engine in the wide range of loads and rotational rates.

  Fig. 1. Dependence between temperature and limit of hydrogen ignition in the air. 1 - Upper limit, 2 - Lower limit.

  Fig. 2. Ignition point according to Prettr.

 Ignition temperature. The ignition temperature means the temperature at which after certain exposure the mixture ignites and continues to burn. The most accurate data on ignition temperatures of the hydrogen-air mixtures can be found at V. Jost.

The ignition temperature values for the hydrogen-air mixtures near the stoichiometric composition vary widely depending on the research method and conditions. The maximum deviation of results exceeds500°C, therefore, it is very difficult to specify the ignition temperature limits corresponding to the ignition conditions in the ICEs. Probably, the closest temperatures should be considered the temperatures obtained byDixonand Croft by the adiabatic compression method. They showed that with reduction of the hydrogen concentration in the air, the ignition temperature decreases. This is confirmed also by the results of Prettr's research (Fig. 2).

 Fig. 3. Dependence of hydrogen ignition temperature in the mixture with dry air on the pressure (figure on the curves - induction period in seconds).

 

The pressure, at which the temperature exists, have particular impact on the ignition temperature of the hydrogen-air mixture. The dependence of the ignition temperature on the pressure (Fig. 3), which was obtained byDixonby the method of mixing the preheated gases in the concentric tubes, shows that when the pressure drops below 0.1 MPa, the ignition temperature drops dramatically, and if the pressure is below 0.01 MPa, the ignition is impossible at all.

According to V. Jost, the long periods of induction, which were obtained byDixon, are determined not so much by chemical processes, as by carburation quality. This can be confirmed by data of Prettr and some other authors, who during very detailed researches of the hydrogen-air mixtures failed to detect the considerable periods of induction.

Based on the above-mentioned analysis, it is impossible to precisely determine the ignition temperature of the hydrogen-air mixtures, but it is possible to determine the limits of its change. When the pressure is р: = 0.1 MPa, the temperature range of the hydrogen ignition in the air is within 530-630°Cthat is slightly higher, than that of gasoline.

BACKGROUND OF USE OF HYDROGEN AS A FUEL FOR VEHICLE ENGINES

 Motor properties of hydrogen allow to make some assumptions about the possibility and feasibility of using hydrogen as a fuel for modern car engines.

The wide concentrated limits of hydrogen combustion in the air (a = 0,15 - 10) provide an opportunity to move over to high-quality regulation of engines working on the Otto cycle. The use of quality control significantly reduces pumping losses, which, combined with other factors (improved combustion efficiency, excellent mixture formation and stability of the mixture content in the cylinders) can significantly affect the increase of the productive efficiency of the engine.

It is known that the degree of sophistication of any engine is determined by how its real cycle corresponds to the theoretical one.

For internal combustion engine with spark ignition, operating on a cycle with a supply of heat at a constant volume, this correspondence is determined by the combustion rate as theoretical cycle assumes instantaneous heat supply, i.e. infinite combustion rate. In this respect, the real cycle of the engine when operating on hydrogen is much closer to the theoretical one than when using any hydrocarbon fuel.

The wide concentrated limits and the high rate of hydrogen combustion in the air make it possible to organize quality control of the working process of the engine, at this even at full load, the fuel-air ratio of below one is impractical for use. At comparing of the efficiency of gasoline engine for which an optimum fuel-air ratio is 0,85-0,9, with the hydrogen engine, it can be noted that theoretically the efficiency of the latter should be higher by 10-15%. At partial load in the engine with quantitative regulation a significant impact on reducing of the efficiency is made by part- throttling, it can be avoided in the hydrogen engine at the qualitative regulation.

In addition to this a certain positive influence on the efficiency of the hydrogen engine can have a smaller heat transfer in the combustion chamber wall due to the lower radiative power of hydrogen flame as compared with the hydrocarbon flame.

High-speed combustion of hydrogen-air mixture in a wide range of fuel-air ratios give the guarantee of stable flow of the work process on all modes of engine operation, however at the combustion of mixtures, which are close to the stoichiometric in composition, due to the very high speed of combustion it is possible the sharp increase in the growth rate of pressure in the cylinder as compared on cycle with gasoline. This in turn implies a higher maximum temperature of the cycle of hydrogen engine.

Higher temperatures of the cycle and the presence of free oxygen in the combustion chamber (a = 1.0 - 1.15) at full load modes of the hydrogen engine should contribute to a more intense formation of nitrogen oxides than it is in the gasoline engine. However, at partial load due to quality control (a> 1.5) a sharp reduce of emissions of nitrogen oxides to a negligible level is possible. The presence of any other toxic substances in the exhaust gas in the hydrogen engine is practically eliminated. This includes the ability to create cleaner car engine.

Considering the wide concentrated limits and a high speed of hydrogen combustion and its high diffusion coefficient, it can be used as an additive, initiating the process of combustion of poor hydrocarbon-air mixtures.

The use of hydrogen as a supplemental fuel for gasoline car engines opens up the possibility of the essentially new approach to organization of the working process. With minimal modification of modern gasoline engine, relating mainly to the supply system, you can significantly improve its fuel efficiency and sharply reduce the toxic level of exhaust gas.

 

Масса – weight

Объем – volume

 Fig. 4. Comparison of methods of storing hydrogen and gasoline 1 - gasoline tank; 2 - the cryogenic tank for LH2; 3 – hydride accumulator for FeTiHx; 4 - High pressure bottle (40 MPa); 5 - tank for H2 under normal conditions.

Discussing the possibility of the use of hydrogen for car engines, we cannot ignore such an important issue, as the storage of hydrogen on board of the vehicle.

Unlike stationary energy consumers and electric transport, connected to the power line, the majority of vehicles requires storage and transportation of a significant amount of energy (required for their movement) on board. Today, the task of energy accumulation is achieved mainly due to transportation of liquid hydrocarbon fuels on board.

Hydrogen, unlike hydrocarbon fuels, has a very low density and at atmospheric conditions can exist only in the gaseous phase. This raises the problem of its compact storage on the vehicle, in particular in the car.

ENERGY AND ECOLOGICAL INDICATORS OF HYDROGEN INTERNAL COMBUSTION ENGINE (ICE)

Energy and ecological indicators of ICE essentially depend on the fuel used. These indicators can be objectively evaluated by analyzing the thermodynamic parameters of the working medium at different stages of the workflow. With this purpose, the calculations of the theoretical cycle of internal combustion engine were performed considering the equilibrium composition of the combustion products on hydrogen, a mixture of gasoline and hydrogen in various proportions and on gasoline in a wide range of fuel - air ratio and compression ratios.

COMPOSITION OF COMBUSTION PRODUCTS

 In the process of combustion at high temperatures there is the thermal dissociation (decay) of complex molecules into simpler molecules, radicals and atoms. At this dissociation is accompanied by energy consumption to break molecular bonds and increasing of energy of decay products, resulting in a lower maximum temperature and useful heat efficiency in the cycles of internal combustion engine.

Due to dissociation the amount of light mono- and biatomic gases increases in the composition of the working medium, causing its molecular weight decrease. Therefore, in a case of isochoric combustion process, the dissociation leads to increase of pressure of the combustion products. In turn, the pressure rise during combustion suppresses all reactions occurring with an increase in number of moles, and leads to the recombination i.e. to a compound of previously dissociated atoms and molecules.

Thus, the dissociation level of the combustion products is determined by two main factors - the maximum temperature and pressure increase ratio. Of course, the determining factor is temperature. The composition of the combustion products in general, is caused mainly by chemical composition of fuel and composition of the fuel-air mixture, i.e. excess fuel ratio. At dissociation, it gets under a significant influence of temperature and pressure of combustion process.

 Fig. 5 shows the dependence of the equilibrium composition of the combustion products on the fuel-air ratio and for gasoline, hydrogen and air mixtures. The field, which is limited by the curve of concentrations of component, for example NO, corresponds to its concentration in the combustion products of gasoline, hydrogen and air mixtures of different composition. The calculations were performed with the compression ratio e = 8,5, and the initial parameters of the working medium T - 320 K and P - 0.08 MPa. The equilibrium composition of the combustion products of gasoline, hydrogen and air contains a minimum number of components.

 

 

Мольные доли – Mole fractions;

Компоненты – Components.

 

Fig. 5. Equilibrium composition of combustion products of gasoline, hydrogen and air mixture (full line —100 % hydrogen; the dashed line —100% gasoline). Fig. 6. The content of radicals in the combustion products of gasoline, hydrogen and air mixture (full line —100 % hydrogen; the dashed line —100 % gasoline).

The increase of hydrogen fraction in the composition of conventional fuel definitely leads to the increase of content of nitrogen oxides in combustion products that results in isoconcentration NO levels shift to the poorer mixtures with decrease of С/Н. However, taking into account the possibility of significant extension of combustion limits of fuel and air mixtures, which are enriched with hydrogen, to the poor compositions, it is possible to achieve very low levels or completely eliminate nitrogen oxides from exhaust gas.

 Download full version of book (format: DJVU 1.7 Mb) Author Mishchenko A. I., Kyiv – 1984

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