Internal Combustion Engines. Allan T. Kirkpatrick
Читать онлайн книгу.
pressure follows the constant pressure line from point 6 to point 1. For early inlet valve closing, the inlet valve is closed at point 1 and the cylinder pressure decreases during the expansion to point 7. As the piston moves upward on the compression stroke, the cylinder pressure retraces the path from point 7 through point 1 to point 2. The net work done along the two paths 1‐7 and 7‐1 cancel, so that the effective compression ratio
Figure 2.7 The Miller cycle.
For late inlet valve closing, a portion of the intake air is pushed back into the intake manifold before the intake valve closes at point 1. Once the inlet valve closes, there is less mixture to compress in the cylinder, and thus less compression work.
Performing a first‐law analysis of the Miller cycle, we first define the parameter,
(2.34)
The energy rejection has two components
(2.35)
As detailed in Example 2.2 below, the thermal efficiency is
(2.36)
Equation (2.36) reduces to the Otto cycle thermal efficiency as
(2.37)
The thermal efficiency of the Miller cycle is not only a function of the compression ratio and specific heat ratio but also a function of the expansion ratio and the load
Figure 2.8 Ratio of Miller to Otto cycle thermal efficiency with same compression ratio,
The ratio of the Miller/Otto cycle imep is plotted as a function of
Figure 2.9 Ratio of Miller to Otto cycle imep with same compression ratio,
A related cycle, the Atkinson cycle, is one in which the expansion stroke continues until the cylinder pressure at point 4 decreases to atmospheric pressure. This cycle is named after James Atkinson (1846–1914), an English engineer, who invented and built an engine he named the
Example 2.2 Miller Cycle Analysis
Derive the equations for the Miller cycle efficiency, Equation (2.36), and the Miller cycle imep, Equation (2.37).
Solution
We need to write
1 Miller cycle efficiency derivation:Solving for efficiency:
2 Miller cycle imep derivation:
2.7 Ideal Four‐Stroke Process and Residual Fraction
The simple gas cycle models assume that the heat rejection process occurs at constant volume,