Engine Performance & Tuning Flashcards
7 cards from real CPT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Engine Performance & Tuning flashcards as text
What is the stoichiometric air-fuel ratio for a gasoline engine, and why is it significant?
Answer: 14.7:1 — ideal ratio for complete combustion and emissions control
14.7:1 is the chemically ideal ratio where all fuel and air are consumed, enabling the three-way catalytic converter to function most efficiently.
In a turbocharged engine, what is intercooling primarily designed to accomplish?
Answer: Cool the compressed intake charge to increase air density and reduce knock risk
Compression heats the air charge; the intercooler removes that heat to increase air density (more oxygen per volume) and reduce the risk of detonation.
A scan tool shows engine timing is advancing significantly less than the commanded advance during acceleration. The most likely cause is:
Answer: Knock sensor activity causing the PCM to retard timing
When the knock sensor detects detonation, the PCM retards timing as a protective measure, resulting in less actual advance than commanded.
What effect does increasing an engine's compression ratio have on thermal efficiency?
Answer: Increases thermal efficiency by extracting more work from each combustion event
Higher compression ratios allow the expanding gases to do more work on the piston before the exhaust valve opens, increasing the proportion of heat converted to mechanical work.
A carbon-fouled spark plug with a dry, black, sooty deposit typically indicates:
Answer: Prolonged rich running or excessive idle time
Dry black carbon deposits form when unburned fuel carbon accumulates on the plug due to rich mixture operation or prolonged low-load running.
Which statement best describes the role of the PCM's open-loop mode?
Answer: The PCM uses pre-programmed fuel maps without O2 sensor feedback
In open-loop mode, the PCM ignores O2 sensor feedback and delivers fuel based solely on calibrated lookup tables — typical during cold start or WOT.
What does a wide-band (wideband) oxygen sensor offer over a conventional narrow-band O2 sensor?
Answer: Linear lambda measurement across a wide range of air-fuel ratios rather than a simple rich/lean toggle
Wideband sensors produce a continuous, linear output proportional to lambda across a broad range (0.7–∞), enabling precise fuel control beyond the ±0.1 lambda narrow-band window.