Industry has developed various techniques using ionization signals for detecting abnormal combustion conditions such as misfire, knock, and approximate air/fuel ratio for stochiometric engines. Free ions present in the combustion gases are electrically conductive and are measurable by applying a voltage across an ionization probe. Alternatively, the voltage is applied across the electrodes of a spark plug after the spark plug has ignited the combustion mixture. The applied voltage induces a current in the ionized gases which is measured to provide the ionization signal. The ionization signal is used as a control parameter in the control of the engine. For example, in U.S. Pat. No. 6,029,627, ionization signals and a single O2 sensor in the exhaust are used to control the air/fuel ratio in engines to achieve stoichiometric operation. This technique uses the O2 sensor to achieve stoichiometry of the overall stoichiometric mixture of the engine and then equalizes the amplitude or location of the first local peak of the ionization signal in each individual cylinder. Another technique disclosed in U.S. Pat. No. 5,992,386 performs a frequency analysis of the ionization signal to detect abnormal combustion conditions such as knock. These systems integrate the ionization signal and compare the magnitude of the integrated signal to the magnitude of the integrated signal of a normal combustion event. The abnormal combustion condition is detected if the magnitude of the integrated signal is above a threshold level, which is set above the magnitude of the integrated signal of a normal combustion event.
One of the drawbacks of stochiometric engines is the emission of pollutants. With fixed engine timing and load, the NOx emissions level of a typical gas engine is dependent upon the air/fuel ratio. Near a chemically correct (i.e., stoichiometric) ratio, the NOx emissions peak and then drop significantly as the amount of excess air is increased. Maintaining a stable combustion process with a high air/fuel ratio is difficult to manage. As a result, conventional spark-ignited gas engines typically operate near the stoichiometric air/fuel ratio and depend upon exhaust after treatment with catalytic converters to reduce the NOx emissions.
Government agencies and industry standard setting groups are reducing the amount of allowed emissions in an effort to reduce pollutants. As a result, industry is moving towards using lean burning engines to reduce emissions despite the difficulty of maintaining a stable combustion process in lean burning engines. By using more air during combustion, turbocharged lean-burn engines can enhance fuel efficiency without sacrificing power and produce less nitrous oxide pollutants than conventional stoichiometric engines.
Another method to reduce emissions is to add exhaust gas to the combustion mixture instead of more air to reduce emissions (i.e., exhaust gas recirculation). Exhaust gases have already combusted, so they do not burn again when they are recirculated. Since the exhaust gas does not burn, it lowers the peak combustion temperature. At lower combustion temperatures, nitrogen cannot form compounds with oxygen and is carried out of the system with the exhaust gas. NOx control requirements vary on various engines and so there are various control systems to provide these functions. The most commonly used in most modern day cars is the vacuum-operated EGR valve. There are also other controls that relate directly to the EGR systems and which complete the same function within the system like the thermal vacuum switch, variable valve timing, ported vacuum switch, venturi vacuum amplifier, EGR delay timer control, back pressure transducer, etc. The amount of EGR gas flow for each engine has to be calibrated as too much or too little can hamper performance by changing the engine breathing characteristics and may cause misfire or knock. Too much exhaust gas flow will retard engine performance and may cause misfire. Too little exhaust gas flow will increase NOx and may cause engine knock.
Ionization sensing has not been utilized to any significant extent in these EGR controlled engines. Because of the nature of the mixture, the ionized species concentration, including NOx, is much less than at stoichiometric conditions. As a result, the ionization signal is of very low intensity and has great variability. The techniques developed using ionization signals for stochiometric operation are unsuitable for high EGR operation. For example, the ionization signals of some high EGR operated engines are sufficiently variable and are low enough in magnitude that integrating the signal can not be done reliably due to a number of factors. These factors include higher levels of noise relative to the ionization signal magnitude, the variability of the ionization signal, and the low magnitudes of the resultant integrated signal.
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