Computer-Aided Data Acquisition

Abstract

Combustion diagnostic research on reciprocating engines often requires the observation and recording of rapidly changing variables such as in-cylinder pressure, diesel fuel injection pressure, diesel injector needle position, ignition current, etc. The magnitude of these variables often changes significantly in milliseconds. Typically, data needs to be recorded on a crank angle basis in engine cycle-resolved measurements, which requires a high-speed data acquisition system. The present chapter describes the general principles of data acquisition system along with the functions of its elements. Generally, in any data acquisition system, the essential sequence of operation includes input signal generation by transducers, signal conditioning, multiplexing, analog-to-digital conversion (sampling of data), and data processing. All these operations involved in data acquisition are described with a specific focus on engine combustion measurement. Typical analog and digital signals involved in engine combustion measurement along with their signal conditioning are also presented. Data sampling needs to be carried out properly to circumvent the loss of signal that can lead to misinterpretation of actual phenomena during the combustion process. This chapter describes the issues involved in the sampling of data and sampling rate requirement for different engine combustion modes and operating conditions. Additionally, offline and online processing of measured combustion data is also briefly discussed.

Discussion/Investigation Questions

• 1. Discuss the requirement of data acquisition systems for engine combustion analysis of reciprocating engines. List the signals which necessarily require high-speed data acquisition. Additionally, list down the signal that can be acquired with low-speed data acquisition systems.

• 2. Describe the constituents of a typical data acquisition systems used for cylinder pressure measurement in reciprocating engines.

• 3. Discuss the purpose of multiplexing in the data acquisition system. Explain why typically it is used in data acquisition of experiment test bench for engine research.

• 4. How you will differentiate between analog and digital signal. Describes the characteristics used to define analog and digital signal.

• 5. Define the static and dynamic measurand. Discuss the different types of dynamic signals. Write few examples of dynamic signals during engine combustion measurements.

• 6. Discuss why signal conditioning is required for various transducers. Write the typical process performed during the signal conditioning process. Describe typical sensors along with their signal conditioning required during engine combustion measurements.

• 7. Describe the important factors that need to be considered for selecting the appropriate data acquisition system for engine combustion measurements. Discuss the variable which is important for considering the data acquisition for typical CI or SI engines. Explain the parameter needs to be taken care for selecting the data acquisition for cylinder pressure measurement from the engine used for racing vehicles.

• 8. Define the sampling rate of the data acquisition system. Describe how sampling rate is provided in typical cylinder pressure measurement. How can you vary the sampling rate for the different portions of an engine cycle; for example, gas exchange and combustion period can have different sampling rates.

• 9. Assume a minimum 20 kHz sampling rate is required during knocking conditions of engines. Calculate the minimum resolution of crank angle encoder is required for engine operation at 1500 and 3000 rpm. Plot a graph of minimum resolution requirement as a function of engine speed in the range of 600–5000 rpm.

• 10. Define the resolution of A/D conversion of the signal. Write the typical resolution of a data acquisition system that is used for cylinder pressure measurement. Assuming pressure signal voltage is in the range of 0–10 V, calculate the minimum detectable voltage by data acquisition system.

• 11. Discuss how engine cycle reference is generated during cylinder pressure measurement. Describe the clock used for the measurement, and how you will calculate the sampling rate at a particular engine speed?

• 12. Define aliasing of the measured signal and describe when it can happen. Discuss the engine operating conditions at which a higher sampling rate is required and it is a critical issue. For a fixed resolution of crank angle encoder, what is the major factor governing the sampling rate during cylinder pressure measurement on crank angle basis?

• 13. Calculate the sampling rate during cylinder pressure measurement at 1000 rpm with crank angle encoder of resolution six pulse per degree. Assuming only rising edge of “A” pulse of crank angle encoder is used, what will be the sampling rate when both (A and B) pulses of the encoder are used during data acquisition that is performed at rising as well as falling edge. Discuss the engine operating conditions, when you would like to use both pulses (A and B) of crank angle encoder.

• 14. Define the sampling rate and resolution of data acquisition card/system. How will you choose the data acquisition card for in-cylinder pressure measurement for a spark ignition engine? Discuss all the factors that need to be considered and equations involved in deciding the data acquisition card.