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Session 2633

Dynamic Otto Cycle Analysis Joseph P. Callinan Loyola Marymount University

Abstract

Engineering students encounter the Otto cycle in their first course in thermodynamics (usually during the sophomore year). This cycle is the theoretical basis for the spark ignition (SI) internal combustion engine (ICE). The traditional analysis (the air-standard analysis) of the Otto cycle is a static thermodynamic analysis that cannot be used to predict the dynamic performance of a SI ICE. Given sufficient information, the work per cycle for a particular engine can be computed. However, by making three simple modifications, the air-standard analysis can be extended to include a computation of the dynamic performance of a SI ICE. The first of these modifications is the selection of representative values of specific heats and specific heat ratios for the working fluid during each process. This improves the accuracy of the analysis. The second is an equation relating the heat release during combustion to pertinent engine parameters (the fuel-air ratio and the compression ratio). The third is the inclusion of an equation for the volumetric efficiency of the engine as a function of engine speed. This incorporates into the analysis the single most significant loss and results in performance that is dependent on engine speed. The resulting analysis predicts the dynamic performance (power and torque as a function of engine speed) of contemporary SI ICE engines with reasonable accuracy. Most importantly, this analysis can be easily understood and conducted by engineering students in their first thermodynamics course. Students have used this analysis, with excellent results, to analyze typical engines for a variety of applications (various types of passenger cars, pick-up trucks, SUV’s, Formula 1 vehicles and, even, “monster” trucks).

Background

The engine used for most contemporary motor vehicles is the four-stroke spark-ignition (SI) internal combustion engine (ICE). The engine typically has 4, 6 or 8 cylinders. The SI ICE combines non-flow and semiflow thermodynamic processes. The four strokes, which occur for each cylinder over two revolutions of the engine’s crankshaft, are the intake stroke, the compression stroke, the power (expansion) stroke and the exhaust stroke. Combustion of fuel and air occurs as the compression stroke ends and the power stroke begins. These processes and their thermodynamic modeling are discussed in detail in books on thermodynamics1 and internal combustion engines2.

The theoretical thermodynamic model for the SI ICE is the Otto cycle. The Otto cycle is shown on pressure-volume coordinates in Figure 1. It is a stationary, closed thermodynamic cycle consisting of the following four internally reversible processes: isentropic compression (1-2), constant volume heat addition (2-3), isentropic expansion (3-4) and constant volume heat rejection (4-1). The idealized Otto cycle includes the following five assumptions (referred to

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Callinan, J. P. (2000, June), Dynamic Otto Cycle Analysis Paper presented at 2000 Annual Conference, St. Louis, Missouri. 10.18260/1-2--8319