Maximum achievable fuel combustion

For conventional gasoline, the stoichiometric ratio is approximately 14.7. Its precise value varies slightly with the composition of the gasoline. Maximum power is achieved with a slightly rich air/fuel ratio— say, 12.5. Maximum efficiency is achieved with a slightly lean mixture—say, 16—although this best-economy mixture ratio is somewhat dependent on combustion quality. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Figure 20.4 Point of absolute combustion in terms of maximum achievable combustion of fuel.

Variations in turbojet engine altitude, flight speed, and thrust require combustion to be maintained over a wide range of inlet air and fuel environment. Furthermore, the combustion must be nearly complete in order to achieve maximum aircraft range. Combustion limits and efficiency are, then, two major performance criteria of the turbojet combustion system. Under conditions very favorable to combustion, the influence of operating, design, and fuel variables are minimized, and only in adverse combustion environment is their influence measurable. 

An automobile engine requires a carburetor or fuel injector to mix the fuel with air. The fuel and air must be mixed in a particular ratio to achieve maximum efficiency in the combustion of the fuel. What might happen if the fuel and air mixture is too rich (if there is too much fuel) ... 

PEM-type fuel cells have the advantage of operating at pressures from 101 to 810 kPa. In addition, they work well at lower temperatures (a feature not present in other types of fuel cells). These fuel cells can operate at 55% efficiency versus a maximum of 30% for internal combustion engines [1], In order to achieve the necessary energy to power the motors for propulsion, fuel cells are manufactured in a stack arrangement. In these arrangements, other components are needed (such as humidifiers to keep the cell moist, air compressors, and gas filters) to make up the fuel-cell propulsion unit. Each stack has a flat cathode sheet and a flat anode sheet, which are stacked in order to meet the particular vehicle s power requirements [1], Current vehicle testing... 

Another very important quality characteristic is the combustion efficiency (fuel efficiency) of radiant tubes. The efficiency strongly depends on the flue gas outlet temperature of radiant tube systems. In order to achieve maximum efficiency, air preheating systems use the enthalpy of the flue gas for preheating the combustion air simultaneously reducing the flue gas outlet temperature of such systems. High efficiencies with less flue gas losses result in lower fuel consumptions and lead to a reduction of the CO2 emission per ton of product. 

Current state-of-the-art in the understanding of these phenomena, as well as progress made in achieving empirical and quantitative descriptions of these combustion processes, are reviewed. The specific topics discussed are i) the maximum attainable turbulent flame speed in an obstacle array, ii) computer simulation of turbulent flame accelerations, iii) correlation between the detonation cell size and the dynamic parameters of fuel-air detonations, and iv) the transition from deflagration to detonation. Future directions in the investigation of these problems are also discussed. 

An Opel Zanfira powered by a 30-kW fuel cell, which was supplied with hydrogen from a methanol fuel processor, was developed by General Motors [582]. The maximum efficiency of this system amounted to 30%, which considerably exceeded the efficiency of internal combustion engines. One of the development goals was to achieve a gasoline consumption equivalent of 3 L per 100 km [583]. 

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