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Research Papers: Internal Combustion Engines

An Analytical Approach for the Evaluation of the Optimal Combustion Phase in Spark Ignition Engines

[+] Author and Article Information
A. Beccari, S. Beccari

Department of Mechanics, University of Palermo, Palermo 90128, Italy

E. Pipitone1

Department of Mechanics, University of Palermo, Palermo 90128, Italypipitone@dima.unipa.it

1

Corresponding author.

J. Eng. Gas Turbines Power 132(3), 032802 (Nov 24, 2009) (11 pages) doi:10.1115/1.3155395 History: Received September 11, 2008; Revised May 21, 2009; Published November 24, 2009; Online November 24, 2009

It is well known that the spark advance is one of the most important parameters influencing the efficiency of a spark ignition engine. A change in this parameter causes a shift in the combustion phase, whose optimal position, with respect to the piston motion, implies the maximum brake mean effective pressure for given operative conditions. The best spark timing is usually estimated by means of experimental trials on the engine test bed or by means of thermodynamic simulations of the engine cycle. In this work, instead, the authors developed, under some simplifying hypothesis, an original theoretical formulation for the estimation of the optimal combustion phase. The most significant parameters involved with the combustion phase are taken into consideration; in particular, the influence of the combustion duration, of the heat release law, of the heat transfer to the combustion chamber walls, and of the mechanical friction losses is evaluated. The theoretical conclusion, experimentally proven by many authors, is that the central point of the combustion phase (known as the location of the 50% of mass fraction burnt, here called MFB50) must be delayed with respect to the top dead center as a consequence of both heat exchange between gas and chamber walls and friction losses.

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Figures

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Figure 1

Thermodynamic cycle with noninstantaneous combustion (AB, CD, and EFG have the same duration)

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Figure 2

The Wiebe function and its derivative, which is not a symmetric curve

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Figure 3

The function 14 and its derivative

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Figure 4

Progress of ζ=TVk−1 as a function of the crank position ϑ for three combustions with different phase and same heat release law (see Eq. 3)

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Figure 5

Progress of ζ=TVk−1 as a function of the crank position ϑ for three instantaneous combustions with different phase

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Figure 6

Effect of instantaneous combustion phase changes on the Otto cycle

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