Use of a Diagnostic Methodology for Spark Ignited Engines to Investigate the Effect of AFR on the Performance and Combustion Characteristics of a Reciprocating Aircraft Engine

Abstract

Reciprocating engines are still frequently used in aviation especially in applications such as recreation planes, taxi-planes, fire extinguishing aircraft and generally applications that do not require a high power density. For such applications they have a significant advantage against turbine engines as far as purchase and maintenance cost is concerned. The proper and efficient operation of these engines in aviation applications is critical and therefore techniques that are used to determine engine condition and to detect potential faults are extremely important. The performance of these engines depends strongly on the condition of the ignition system and the quality of the supplied mixture. For this reason in the present work it is examined the effect of mixture AFR on the combustion mechanism and engine performance using an existing diagnostic methodology for spark ignited engines developed by the present research group. The investigation is conducted on a radial, spark-ignited reciprocating engine used on the CL-215 fire extinguishing aircraft. The diagnostic technique is used to investigate the effect of AFR on the main combustion and performance characteristics of the engine and specifically brake power output, rate of heat release, cumulative heat release, peak firing pressure, ignition and injection timing and duration of combustion. Furthermore the diagnostic technique is used to derive information for spark advance, spark duration, compression condition etc. The diagnostic technique is based on a thermodynamic two-zone combustion model for spark ignited engines. To examine the effect of AFR on the combustion mechanism a detailed experimental investigation was conducted on an engine (radial, supercharged, air-cooled, eighteen-cylinders) mounted on a test bench. The measurement procedure involved measurements at various operating conditions (load and speed) and various AFR values. During the experimental investigation beyond the conventional test bench measurements, measurements were taken using a fast data acquisition system of cylinder pressure and the electric signal of both spark plugs. Engine diagnosis is established by processing of these measured data. From the results of the diagnosis procedure it is revealed that the diagnosis method provides detailed information for the operating condition of the engine and the values of parameters that cannot be measured on the field. The diagnosis results reveal that the proposed technique can determine the effect of AFR ratio on the combustion mechanism adequately and thus it can be used during engine testing to determine the optimum AFR ratio in combination with the remaining engine settings and mainly spark advance. The results obtained are positive and reveal that the proposed diagnostic technique can be easily applied on any type of spark-ignited engine and especially on aircraft piston engines (i.e. aviation applications), where the accurate estimation of the engine condition and settings is extremely important.

Application of a diagnostic technique for evaluating the quality of the air–fuel mixture and the ignition quality of a spark-ignition reciprocating aircraft piston engine

“…The motivation of the present study was to investigate the possibility of prediction the impact of the operating condition having two critical engine subsystems (i.e. the fuel injection system and the spark ignition system) in the operational behavior of an aircraft piston engine, through the use of an existing diagnostic technique, developed by the authors, [7][8][9][10][11][12][13][14][15] which can be applied on the field (i.e. when the engine is mounted on the aircraft) and it does not require specialized technical equipment.…”

“…As known, 15 the use of cylinder pressure provides significant information concerning the combustion process inside the cylinder. The specific information is extremely important, especially for aircraft piston engines, where the accurate estimation of the engine operating condition and the proper engine settings are two points of vital importance.…”

“…The proposed diagnostic technique is based on the recording and the processing of the experimental mean cylinder pressure diagram, to estimate various engine parameters, such as the top dead center position (TDC), the inlet valve opening (IVO), the exhaust valve opening (EVO), the duration of combustion, the duration of ignition, etc. [15][16][17][18][19][20] As known, [10][11][12][13][14] the accurate estimation of the TDC position is of vital importance for the diagnosis, since it provides the ability to generate graphs for various engine parameters on a crank angle basis. As reported in various studies, [12][13][14][15][21][22][23] the proposed diagnostic technique does not require any measurement of the TDC position.…”

“…[15][16][17][18][19][20] As known, [10][11][12][13][14] the accurate estimation of the TDC position is of vital importance for the diagnosis, since it provides the ability to generate graphs for various engine parameters on a crank angle basis. As reported in various studies, [12][13][14][15][21][22][23] the proposed diagnostic technique does not require any measurement of the TDC position. This is a very important advantage of the proposed technique since it is extremely difficult to install and use a TDC sensor on such an engine.…”

“…Moreover, the specific technique makes use of simple electrical signals acquired from the ignition system (high voltage) providing important information for its conditions and tuning. 15 The last is achieved by appropriate processing of the waveforms corresponding to the electrical signals measured directly from the ignition system. 15 Furthermore, it must be stated here that the ability of the proposed technique, both in determining the operational state and in detecting possible malfunctions of a reciprocating piston engine, has been extensively evaluated in the past and results of this evaluation have been published in various studies.…”