Engine torque characteristic — rise to a peak then fall In real spark-ignition and compression-ignition engines, the torque typically increases with engine speed up to a peak value and then decreases at higher speeds.

Introduction / Context:
Vehicle performance depends strongly on the torque curve. Understanding why torque rises to a maximum and then declines helps explain gear ratios, shift points, and drivability. This concept is universal across naturally aspirated and turbocharged engines, though peak locations differ.

Given Data / Assumptions:

• Healthy production engine with proper fueling and ignition or injection timing.
• No unusual restrictions or control limits beyond typical OEM strategies.
• Consider steady-state dynamometer measurements.

Concept / Approach:
At low speed, volumetric efficiency and charge motion are weak, so torque is modest. As speed increases, gas exchange improves and torque rises. Beyond a certain speed, valve timing, intake and exhaust tuning, friction, and time available for combustion reduce efficiency. Volumetric efficiency falls and frictional losses rise approximately with speed, so net torque declines.

Step-by-Step Solution:
At low rpm: limited air mass per second and weak scavenging produce small torque.Midrange: tuned lengths and cam timing improve cylinder filling, increasing torque.High rpm: flow separation, valve overlap losses, pumping losses, and reduced combustion duration effectiveness cause torque to decrease.

Verification / Alternative check:
Dyno sheets show bell-shaped torque curves. Power, proportional to torque * speed, peaks at a higher rpm than torque because speed continues to rise while torque has begun to fall.

Why Other Options Are Wrong:
Claiming a monotonic increase or perfectly flat torque ignores real-world gas dynamics and friction trends, and it does not match measured data.

Common Pitfalls:
Confusing torque with power; assuming turbocharged engines avoid the decline entirely. Even with boost control, choking and thermal limits eventually reduce torque.

Final Answer:
Correct