A Piezoelectric Sensor Concept for Measuring Piston Wetting in DISI Engines

Abstract

A piezoelectric sensor to measure the mass of fuel that impacts the piston top during injection in a direct injection spark ignition (DISI) engine was developed. The sensor used a 3.18 cm (1.25-inch) long, 0.318 cm (0.125-inch) wide piezo bending motor. The principle of operation is based on the change in natural vibration frequency that occurs to the cantilever piezo beam due to a change in its mass caused by the presence of liquid fuel on its surface. An electrical impulse is used to set the piezo element in vibration after which the natural vibrational frequency is measured using a FFT analyzer. The concept was evaluated outside the engine and calibrated for the frequency shift as a function of the weight of liquid on the bending element. The change in the frequency was found to be approximately proportional to the liquid mass on the sensor. The piston top of the engine was modified to accommodate the sensor on its surface. Due to temperature limitations, all fuel film mass measurements were taken under motored conditions. A wide range of equivalence ratios and injection timings were considered in the experiments. The measurements showed that the fuel film mass deposited on the sensor surface was small, typically less than 2 mg, which corresponds to a film height of 32 μm. The fuel film mass followed a general trend increasing in value and reaching a maximum for injection timings when the piston surface approached TDC. The maximum measured mass for stoichiometric operation was 2.3 mg at TDC and corresponds to 6.3% of the fuel injected. The average evaporation rate of the fuel film mass was 3.3 kg/m 2 -s, and was nearly the same for all injection timings. The average measurement uncertainty was calculated as ±0.56 mg for stoichiometric operation.