Page 389 - Advanced thermodynamics for engineers

378 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

P16.10 Use SPIE to evaluate the optimum equivalence ratio (l) for the engine defined in basic.data,
with the following operating conditions:

N ¼ 4000 rev/min; Compression ratio ¼ 10.0:1; a ig ¼ 695 (25 btdc); flame speed factor,

f f ¼ 5.0; and residual fraction ¼ 0.050.
How did you define this optimum value? Did you use maximum power, or maximum effi-
ciency?
P16.11 Use SPIE to evaluate the optimum values for the data in P16.10, but with a lower
compression ratio of 8:1.
Comment on how the results have changed.

P16.12 Discuss the results of P16.10 and P16.11. Consider the pressure–crank angle diagrams, the
temperature diagrams, the power outputs and the thermal efficiencies in your discussion.
P16.13 Vary the flame speed factor over the following range: 3.0 < f f < 7.0. Discuss the effects.
Describe which types of combustion chamber will produce the low values of f f , and which
will produce the high ones.
P16.14 Investigate the effect of varying the ‘spark timing’ over the range 685 < a ig < 710 .

Describe the influence of the spark timing on the engine performance. What is the limitation
to having very early ignition? Basic engine data: N ¼ 4000 rev/min; Compression
ratio ¼ 9.0:1; flame speed factor, f f ¼ 5.0; and residual fraction ¼ 0.050.
P16.15 The Honda VTEC engine can operate at values of l ¼ 0.67 (ε ¼ 22:1 for petrol/gasoline)
over certain parts of its operating range – see Fig. P16.15, based on Horie and Nishizawa
(1992). Using SPIE, and limiting the fuel to octane, investigate what has to be done to
achieve viable cycles at these very weak mixtures (Note that the Honda engine reduces the
area of the inlet valve at low load, but this will not be done in this question. This will have a

One-intake valve operation Two-intake valve operation
1.0
λ = 1.18
Intake manifold pressure / bar 0.8 λ = 0.67 λ = 1.10

0.6

0.4
0 1000 2000 3000 4000 5000 6000
Engine speed / rev/min
FIGURE P16.15
Based on operating region of Honda VTEC engine Horie and Nishinaga (1992).