Alternative Methods of Engine Balance Based on In-Cylinder Determination of Air/Fuel Ratio
Engines in the natural gas pipeline industry operate under even more stringent emissions restrictions. Some of these engines, particularly in the Northeast, have been subject to regulations requiring NOx emissions reductions down to ~3 g/BHP-HR or less. Balance is a key to achieving and maintaining these low emissions. However, recent research has shown the traditional methods of balance, based on peak combustion pressure, may not result in optimal NOx emissions. Rather, optimum balance for emissions occurs when each cylinder operates at the same air/fuel ration. Variations in cylinder charging and scavenging can result in differences in the air trapped in each cylinder. Consequently, even when fuel is precisely metered by modern electronic fuel injection systems, the air/fuel ratio will not necessarily be the same. Peak pressure balance then results in some cylinders operating leaner than optimal while others operate richer than optimal. Recognizing these limitations, AETC has been investigating air/fuel ratio based balance methods for over 5 years, funded in part by the Pipeline Research Council International (PRCI). This effort has resulted in two methods of determining and balancing air/fuel ratio in typical lean burn pipeline engines. The first method, pressure ratio, is based on work in the automotive industry from the 1990s. The technique exploits low cost/low performance methods of measuring in-cylinder pressure by calculating the pressure ratio and then using this as a surrogate for Mass Fraction Burned (MFB). The MFB directly correlates with air/fuel ratio, as does pressure ratio when properly calculated. The second method, again based on automotive technology, utilizes post ignition ionization measurements from a standard spark plug. This data rich signal contains information on air/fuel ratio in addition to misfire, detonation, peak pressure, location of peak pressure and IMEP. AETC has used both methods to determine air/fuel balance ratio from typical p
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