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Difficulty: HardCoasting and Engine Braking

A driver travelling at 60 mph on a dual carriageway disengages the clutch and selects neutral well before braking for a deceleration lane. Which TWO of the following are direct consequences of this action?

  1. The vehicle loses engine braking resistance, increasing the workload on the footbrakeAnswer
  2. Fuel consumption increases slightly compared to remaining in gear, as the engine must inject fuel to maintain idleAnswer
  3. C
    The anti-lock braking system (ABS) is deactivated due to transmission disengagement
  4. D
    The driver's thinking distance increases because the vehicle travels faster without gear resistance

Answer

Selecting neutral removes the vehicle's engine braking assistance—putting greater strain on the footbrake—and actually increases fuel consumption relative to overrun deceleration because modern engine control units must supply fuel to keep the engine idling.
Coasting disconnects the engine from the driveline, eliminating engine braking and placing the entire deceleration burden onto the footbrake. Furthermore, modern fuel-injected engines cut off fuel flow when decelerating with a gear engaged, meaning coasting in neutral actually uses more fuel to keep the engine idling.

Step-by-Step Solution

1
Analyse the kinetic and mechanical effects of transmission disengagement at high speed.
With the clutch disengaged or neutral selected, the engine is disconnected from the driven wheels, eliminating engine compression resistance (engine braking).
This places sole deceleration responsibility on the friction brakes, increasing brake wear and operating temperatures.
2
Evaluate modern vehicle fuel management systems during coasting versus deceleration in gear.
Modern fuel-injected engines utilize deceleration fuel shut-off when the accelerator is released in gear, consuming zero fuel. In contrast, coasting in neutral forces the ECU to inject fuel to sustain idle RPM.
Coasting fails to save fuel and marginally increases consumption compared to properly utilizing overrun.
3
Evaluate electronic and safety system dependencies.
Safety systems like ABS and human reaction parameters (thinking distance) remain unaffected by gear selection.
Thinking distance is purely a function of human reaction speed and initial velocity, while ABS is independent of gearbox engagement.

Key Concept

Mechanics of Coasting and Engine Braking
Estimated Time:1m 30s
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Topics

Coasting and Engine Braking

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