As the intake pressure is throttled from 101.3 to 50 kPa, what is the change in the volumetric
efficiency, imep, and residual fraction of a four-stroke gasoline fuel--air Otto cycle engine?
Assume Ti = 300 K, φ = 0.9, r = 11, and Pe = 105 kPa.
 
 

Use the  Steps to answer:

Given:

Required:

Illustration:

Assumption:

Formula:

Solution:

The efficiency and imep are given by We.v. m3 n = = m a We.v.[1+(1f)FA] (1-f) FA, a Wev.1+(1f)FA] imep= (4.83) (4.84) Results obtained using the above modeling for different compression and equivalence ratios are given in Figures 4.14 and 4.15. Important conclusions are 1. The efficiency decreases with increased equivalence ratio. This is consistent with the Otto cycle fuel-air model, where the efficiency is maximum for lean mixtures. 2. The imep increases with equivalence ratio. This is also consistent with the Otto cycle fuel-air model, where the imep is maximum for slightly rich mixtures. Since the com- bustion is heterogeneous, the engine can be controlled without throttling the air. 3. The efficiency and imep are a weaker function of compression ratio relative to an Otto cycle. This is due to the constraint on peak pressure. 4. Both efficiency and imep increase with increasing limit pressure. 5. Even in the absence of heat and mass loss, the ratio n/notto may be as low as 0.85.