Methods of Pegging Cylinder Pressure to Maximize Data Quality 2019-01-0721

Engine cylinder pressure is traditionally measured with a piezo-electric pressure transducer, and as such, must be referenced or pegged to a known value. Frequently, the cylinder pressure is pegged to the pressure in the intake manifold plenum whereby the manifold absolute pressure (MAP) at the end of the intake stroke is measured and the cylinder pressure trace for the entire cycle is adjusted such that the cylinder pressure is set equal to the manifold pressure at the end of the intake stroke. However, any 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 the calculated combustion metrics. The study was conducted through 1D simulations using the commercially available GT-Power. The test matrix included variations of speed, load, intake runner length and intake valve timing. Five different pegging locations were compared, and for each location the optimal crank angle to peg was identified. Pegging to an assumed Polytropic Compression Coefficient was included as an alternate method. Results 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, Polytropic Compression Coefficient (PolyC) and Polytropic Expansion Coefficient (PolyE). The results showed that pegging to a fixed polytropic compression index is the best method of pegging. If pegging to a reference pressure, from the five locations analyzed, the intake plenum (traditional pegging location) is the best pegging location, except during EIVC operation when it is better to peg the cylinder pressure during the exhaust stroke to the exhaust pressure.