Spark-ignition engines

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When oxidized in a fuel cell, the only significant emission is water vapor. When combusted in an internal-combustion engine (spark-ignition or diesel) some oxides of nitrogen and peroxides may be produced, depending on the calibration of the fuel system and configuration of the engine. None of the toxic emissions typical of petroleum fuels are present [1.35]. 

Internal combustion engines, Spark ignition—Alternate fuels. 

A mixture of air and a fine spray of gasoline at ambient (outside air) temperature is fed to a set of piston-fitted cylinders in an automobile engine. Sparks ignite the combustible mixtures in one cylinder after another, and the consequent rapid increase in temperature in the cylinders causes the combustion products to expand and drive the pistons. The back-and-forth motion of the pistons is converted to rotary motion of a crank shaft, motion that in turn is transmitted through a system of shafts and gears to propel the car. 

Dispersingagents, such as polyethylene polyamide succinimides or methacrylate-type copolymers, are added to motor oils to disperse low-temperature sludge formed in spark-ignition engines. 

Power output is controlled, not by adjusting the quantity of fuel/air mixture as in the case of induced spark ignition engines, but in changing the flow of diesel fuel introduced in a fixed volume of air. The work required to aspirate the air is therefore considerably reduced which contributes still more to improve the efficiency at low loads. 

A minimum volatihty is frequently specified to assure adequate vaporization under low temperature conditions. It can be defined either by a vapor pressure measurement or by initial distillation temperature limits. Vaporization promotes engine start-up. Fuel vapor pressure assumes an important role particularly at low temperature. For example, if fuel has cooled to —40°C, as at arctic bases, the amount of vapor produced is well below the lean flammabihty limit. In this case a spark igniter must vaporize enough fuel droplets to initiate combustion. Start-up under the extreme temperature conditions of the arctic is a major constraint in converting the Air Force from volatile JP-4 to kerosene-type JP-8, the military counterpart of commercial Jet Al. 

There are several important Hquid fuels, ranging from volatile fuels for internal combustion engines to heavy hydrocarbon fractions, sold commercially as fuel oils. The technology for the combustion of Hquid fuels for spark-ignition and compression-ignition internal combustion engines is not described here. 

A. A. Quader, Why Intake Charge Dilution Decreases Nitric Oxide Emissionsfrom Spark Ignited Engines, SAE 710009, Society of Automotive Engineers, Warrendale, Pa., 1971. 

Figure 16-2. Idealized P-V diagram for a 4-cycle, spark-ignited engine.

Inlet port injection is used with liquid fuel, spark ignition engines only. Fuel is injected into the inlet port and mixed with the inlet iii The injection process may be either timed or continuous. 

Unlike carbon dioxide and water that are the inevitable by products of complete combustion of hydrocarbons, species such as carbon monoxide, ethene, toluene, and formaldehyde can be emitted because combustion has been interrupted before completion. Many factors lead to emissions from incomplete combustion. Emitted unburned hydrocarbons and carbon monoxide are regulated pollutants that must be eliminated. In automobiles with spark ignited engines, these emissions are almost entirely removed by the catalytic converter. 

Because many practical flames are turbulent (spark ignited engine flames, nil field flares), an understanding of the interaction between the complex fluid dynamics of turbulence and the combustion processes is necessary to develop predictive computer models. Once these predictive models are developed, they arc repeatedly compared with measurements of species, temperatures, and flow in actual flames for iterative refinement. If the model is deficient, it is changed and again compared with experiment. The process is repeated until a satisfactory predictive model is obtained. 

Carbon monoxide (CO) is a colurless, odorless, and tasteless gas. Inhalation of as little as 0.3 percent by volume can cause death within thirty minutes. The exhaust gas from spark ignition engines at an idle speed has a high CO content. For this reason NEVER allow an engine to run in an enclosed space such as a closed garage. 

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