Question

A reputable engine company has been making carbureted spark-ignition engines that operate with an equivalence ratio = 1.0 and a compression ratio of 9:1 for years. In order to control emissions, the engines operate with 20% exhaust gas recirculation (EGR) and a three-way catalytic converter. Since the company’s market share is steadily dropping, they consider hiring a MEC graduate to rescue them. The ambitious young engineer proposes a port fuel-injected (PFI) design, which operates lean at 0.7 with a high compression ratio (15:1) and vigorous inlet swirl. The new engine has the spark-plug mounted at the center of the combustion chamber, while the production engine has the spark-plug at the side.

(a) Will the flame development period of the new chamber be shorter, longer, or about the same as the current design?

(b) Will the rate of the combustion of the new chamber be faster, slower, or about the same as the current design?

(c) Sketch the mass fraction burned vs. crank-angle curves for these two combustion chambers when spark-timing is set for maximum brake torque. Clearly label the sparktiming, ignition delay, and overall burn duration for each case.

(d) The engineer claims that the proposed lean-burn engine will have the same octane requirement as the production SI engine. Could this claim be true?

(e) Will the proposed lean-burn engine clearly develop greater power at full load than the current SI engine?

(f) Would there be any additional advantages or disadvantages if the proposed engine were direct-injected, stratified charge, instead of homogeneous, operating at the same overall equivalence ratio?

Answer 1