Methods of Pegging Cylinder Pressure to Maximize Data Quality
To be published on April 2, 2019 by SAE International in United States
Engine cylinder pressure measured with piezo-electric pressure transducers must be referenced or pegged to a known pressure at some point in the engine cycle. Traditionally, the pressure has been pegged to the pressure in the intake manifold plenum at Bottom Dead Center (BDC) at the end of the intake stroke. However, an error in pegging induces an error in the cylinder pressure trace, which has an adverse effect on the entire combustion analysis. This research is focused on assessing the pegging error for several pegging methods across a wide range of engine operating conditions, and ultimately determining best practices to minimize error in pegging and its propagation to calculated combustion metrics. The study was conducted through 1D simulations run in the commercially available GT-Power. The points studied included variations of speed, load, intake runner length and intake valve timing (including Late Intake Valve Closing (LIVC) and Early Intake Valve Closing (EIVC)). Five different pegging locations were compared (intake manifold plenum, intake port, intake valve, exhaust valve, and exhaust manifold runner). For each of the five locations, the optimal location in the cycle (in terms of Crank Angle) to peg was identified. In addition to pegging to a measured pressure, pegging to an assumed Polytropic Compression Coefficient was included as an alternate method. Analysis included an examination of error in terms of pressure, but also, an examination of how that error in pressure impacted combustion metrics including burn rate, Mass Fraction Burned, and Polytropic Compression / Expansion Coefficients. The results showed that when operating in an Early Intake Valve Closing (EIVC) mode, pegging should not be done with a reference pressure measured on the intake side of the engine. Pegging to a reference pressure measured on the exhaust side should be used when operating in EIVC. Excluding EIVC, all pegging locations under consideration resulted in similar error, and did not make a strong case for a given location. The results showed that pegging to a fixed polytropic compression index produces the least pegging error of all methods investigated.