IMPROVEMENT of ENGINES EFFICIENCY COMBUSTION
Theoretical modeling of heat engines and in fact the apparition of thermodynamics as a branch of physics has experienced a breakthrough after publication by Sadi Carnot of his study entitled 'Reflexions sur la puissance du feu driving propres et sur les machines à developper cette puissance" (in 1824).
It is accepted that in a monothermal cyclic process, only mechanical work can be converted into heat, but not vice versa. In order to obtain the conversion of heat into mechanical work it is necessary that heat machine operates with two heat sources at different temperatures .
Carnot envisioned the working principle for an ideal bithermal reversible machines whose performances has shown to upper limit the yield for any real thermal machines which operate between the same temperature extremes as the Carnot cycle .
Real thermal machines differs from the ideal one because they have moving parts with friction and heat loss and these facts change the energy balance.
In fig. 11 is presented the ideal cycle of a thermal machine often called as ,,the Carnot cycle”. . This cycle consists of four processes. Between states A and B there is a expansion of isothermal gas, the cylinder being in thermal contact with the hot source at temperature T1. During this isothermal expansion, the gas receives a quantity of heat Q1 from the hot source.
Then, the contact with hot source is broken and gas undergoes an adiabatic expansion between states B and C. In state C, the gas temperature reaches temperature T2 equal with the temperature of cold source.
In the course of the A - B and B - C transformations, gas molecules produces mechanical work on piston ( pushes it ) and this make the flywheel rotate.
Once reaching the temperature T2, the gas is placed in thermal contact with the second tank heat named cold source. Because of inertia, the flywheel rotates further, while piston is engaged and compress isothermally (at T2) gas between C and D. During this isothermal compression, gas transfers to the heat source an amount of heat Q2. In state 4, the thermal contact with the cold source is interrupted and adiabatic piston compresses the gas to the initial state when the temperature returns to T1.
When the cycle is performed in clockwise direction, this is called direct Carnot cycle. The total work supplied (L > 0), in terms of coordinates p-V in Figure 11, will be the area contained within the cycle, positive by convention.
As you can see, the aria is described by two isothermal curves on sections AB and CD and two adiabatic curves in portions BC and DA. These transformations are considered in ideal conditions and so a engine runnig after this heat cycle would ideally yield :
η = (T1 – T2) / T1
T1 – temperature of warm source
T2 - temperature of cold source
To achieve this performances in practice, the transition from T1 to T2 should be done abruptly, so that vapors do not meet on the road intermediate temperatures and conversions have to be perfectly reversible.
In reality, this efficiency is never attainted and sometimes the yield is quite far from it. As a general idea of heat engine efficiency varies quite broad, as follows:
• Simple steam = 2 %
• Steam refined = 20 %
• Steam turbines = 25 %
• carburettor = 35 %
• Diesel engine = 45 %
How to improve the efficiency of the combustion process ?
This material is a extension of a previous texts and clarifies important aspects of how to run basically a combustion process to achieve maximum work. The main item we want to clarify are the use of water injected direct into cylinder or used as emulsion or solution to increase the mechanical output.
Can deionized water emulsioned with normal fuel or fed directly into the combustion cylinder to improve the engine output ?
As described earlier, an internal combustion engine generates mechanical work due to a pressure gradient generated by the fuel burned in the cylinder. In general, combustion engines are running on natural gas (pressurized GPL), gasoline, diesel and other petroleum distillation fractions .
I saw on the internet a few patents that propose to add water in varying proportions from 10 to 40 % to liquid fuels in order to increase engine efficiency. Until few years ago, I would have thought that these patents are absurd, or so it seems from the point of view of a classical physics. It is not permissible for a certain fuel that has a calorific value, to be diluted with a non combustible substance that reduces its calorific course and you claim that at the end you get a higher yield of engine operation. Now, when anyone carefully analyzes the phenomena in terms of a new theory of thermodynamics, this appears to be the simplest and most obvious way to convert heat (or a large portion of this heat ) directly into mechanical work .
Although the actual thermodynamic did not pay attention to these patents, a careful study of combustion can prove that these patents have a very simple and intuitive scientific explanation.
Let us consider again the diesel engine and do a comparison between the operation of such a motor with octane and then fed with a mixture 45% octane, 50 % water and 5% emulsifier. Of course we are interested in engine efficiency, expressed by the ability to produce mechanical work, for these two combustible substances .
For simplicity, let us again assume that the injector is commanded to receive each time a fuel volume of 0.1 ml during the ignition phase and the volume of the combustion cylinder is 0.7 liter.
Using normal fuel (pure octane for simplicity), in the normal operating cycle of the engine after combustion we have the following volumes :
for we enthalpy of reaction there is ΔH = -5460 kJ/mol.
One mole of liquid octane occupy a volume of about 0.2 L while a mole of gas (oxygen, water or carbon dioxide) occupies a volume of 22.4 L. From the stoichiometry of the equation we have that 12.5 moles of oxygen that means 280 liters are consumed during combustion of a mole octane. After burning, due to the rhe reaction conditions, all the resulted products are in gaseous form, and such products will occupy 381 L.
If the injector takes only 0.1 ml of octane to the cylinder, it means that the during admission it will be required a minimum volume of 0.14 l oxygen and that means a minimum air volume of 0.7 L. The resulting volumes calculations give that 0.0896 L CO2 and 0.1008 L of H2O are produced. The analysis of engine is made between two successive phases of the operating cycle: inferior dead point when the cylinder is full of air and the second phase after combustion, when after combustion piston rotates and moves toward inferior dead point again. The amount of nitrogen, ie 0.56 l N2 ( 0.7 minus 0.14 ) from cylinder is only heated and not participate in chemical processes.
If per mole of liquid octane (200 ml) an amount of -5460 kJ heat is generated, then 0.1 ml of it will produce -2.73 kJ yields. Again, for simplicity, let us consider that the whole heat of reaction only contributes to increasing gas temperature in the cylinder.
Without going into details, let us approximate the specific heat of the gas mixture from cylinder with that of nitrogen Cv = 0.8 kJ / kg K. For a detailed calculation it should be considered specific heat for each, specific heat variation with temperature and percentage composition of the final gas, but as far this variation is not so large we can make this apporximation for a reasonable characterization of the phenomenon.
We need to know the mass of gases generated during burning inside the cylinder and this can be obtained from well known relations used in chemistry:
For nitrogen :
where M = 28, v = 0.56 l and VM = 22.4
consequently the mass of nitrogen in the cylinder is
similar mass of carbon dioxide is
and the water vapors
The total mass of gases after combustion in the cylinder is about 0.957 g and the mixture gets a heat -2.73 kJ .
From the definition of the heat capacity :
With these approximations and idealizations, in the final phase of relaxation, when combustion gases are expelled into the atmosphere their temperature is about 3500 °C, and of course the heat is lost.
In practice, this temperature is not reached because a cooling circuit that takes some of the heat output, motor body is not insulated and lose other direct heat transfer etc. It is believed that the maximum temperature obtained during the combustion of the gas mixture is between 2000 and 2500 °C, and the temperature of the exhaust gases in the cylinder is between 600 and 900 °C.
Useful work is given by the expansion of gases ( heated nitrogen, carbon dioxide and water) in the engine cylinder.
Certainly our example is purely theoretically but it can can be easily adapted to a real modeling.
Let's see what happens if the fuel contains 50 % water and for the simplification the emulsifinat has the same caloric power as octane.
In this case, although the injector take the same volume of liquid toward cylinder (0.1 ml), only 0.05 ml is octane and the rest of 0.05 ml is water. Of course experiment takes place without changing the technical characteristics of the engine ( the intake stage takes the same amount of air and so on).
Redo the same calculation above we have that 0.05 ml octane will produce half the heat before, will consume half of the oxygen in the cylinder combustion will generate half the carbon dioxide and water produced previously.
If you take 0.05 ml of octane injector to the cylinder, and the intake of air is 0,7 l, there will be an excess of unreacted oxygen, because during combustion process only 0.07 l is consumed. From reaction in this case we will get 0.0448L CO2 and 0.0504 L of H2O.
The amount of nitrogen and excess oxygen, ie 0.63 l ( 0.7-0.07 ), taken inside cylinder is only heated and not participate in chemical processes; the burning will release in this case about 1365 kJ. Again, for simplicity, let us consider that the whole heat of reaction contributes only to the vaporization of water added in the fuel and also increase the cylinder temperature.
In order to turn 0.05 ml liquid water into steam about are necessary.
These results are amazing. Although we introduced 50% water in the fuel, and the water got steamed into the cylinder, and of course this steam can create a bigger pressure gradient which can be further converted into useful mechanical work, the temperature of exhaust gases is still big enough to need a secondary cooling device for the engine as a whole. I am sure that someone will be able to generate a mechanical work using even 20% fuel and 80% water and in this case the yield of such engine already overpass the Carrnot limits.
Coming back to our example, the heat to be removed from the system will be 1.23 kJ.
If we repeat the above calculations in order to get the temperature of the exhaust gases from the cylinder in the combustion considering ideal conditions, it can be seen that it is about 1500 °C.
gas mass in the cylinder after firing :
For nitrogen :
where M = 28, v = 0.56 l and VM = 22.4
consequently the mass of nitrogen in the cylinder is 0,7 g.
similar mass of carbon dioxide is
and the water vapor
From the definition of molar heat :
With these approximations and idealizations, in the final phase of relaxation, when combustion gases are expelled into the atmosphere their temperature is about 1500 °C, and of course this secondary heat is lost.
The experiment shows us that although a little temperature gradient is lost, is can be obtained a substantial increase in useful work due to the vaporization of the water in the cylinder which generates a gradient of pressure.
The book will make a more detailed calculation of the yield of useful work for ideal and real cases.
Although this calculation was done for an ideal motor, the modelling is very easy to be adjusted to real engines in such a way that a rate of up to 75 % or even 80 % water can be emulsionated in the fuel. Of course, an increase in the percentage of water in the fuel raises issues relating to burning, corrosion and so on, but these are secondary issues that can be overcome easily.
What is not understood in the current thermodynamic and of a lot of theoreticians is the simple fact that, in principle, an engine can operate without the need for two sources of heat. The need fo a heat transfer toward a cold source is necessary only from practical conditions becasuse it dammage the engine, but it is not a ,,principial” necessity.
Such a motor unfortunately can not be further classified as ,,internal combustion engine" He is actually a steam-engine combustion combined with direct steam generation in cylinder combustion engine.
How can be improved the production of electricity in the plantpower?
Approximately 80% of the world's electricity is based on burning fuels (coal, oil, biomass, etc). Unprecedented growth of electricity consumption raises serious environmental pollution, climate change and generates a lot of other damage that adversely affects the entire planet.
Certainly, there is a tendency to limit the effects of human industrial activity through a series of regulations, but major playerss are still reluctant to introduce restrictive rules. Their reluctance is due to the fact that the adoption of these rules are necessary investments to limit pollutants.
Of course, sooner or later it will be necessary to have a clean energy production without secondary emissions, but this research addresses the current situation and shows that with no regulation the energy production can be at least doubled, without increasing fuel consumption. That means we can double the energy production, maintaining the current limits of pollutants emitted, which is already a huge step .
Conventional thermal production is by the Rankine cycle Fig. 1.
Figure 1 . Simplified Rankine Cycle producing the mechanical work L
In this cycle a volume of water undergoes physical changes ( evaporation, condensation, temperature increase or decrease) as a result of contact with two heat tanks at different temperatures. Water is evaporated in the boiler and then is transferred to vapor turbine which performs mechanical work rotating the turbine and this further generates electricity. The vapor is then passed to a condenser and then to the heat pump and placed back in the heating cycle.
Currently, fuel combustion in power plants takes place at constant pressure ( atmospheric pressure ) and this leads to loss of yield, since the gradient of pressure that is generated during the combustion of liquid fuels can not be used.
In order to improve the yield of entire process for the combustion of fuel must be done in a large turbo engines, and in this case the gradient of pressure is recovered first as mecanical power, and then the gradient of temperature is transfered to a Rankine cycle tobe further converted into mechanical power as in fig. 2. .
Figure 2 . Improved version of burning liquid fuel
Under these conditions only improved combustion gas turbine mechanical work recovered from (L1) is greater than the work product L classic Rankine cycle (Fig. 1). Using further as agent of vaporization/condensation an organic compound or a mixture of organic compounds that make better use of lower temperatures for this cycle, the mechanical work recovered can be double of the work obtained for classical Rankine cycle.
Of course for new designed powerplants, the Rankine cycle can be completely dismissed and the maxi turbo engines has to be designed to use either a water-fuel emmulsion or to inject directly water into the burning chamber and recover as much mechanical power as possiblle.