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ASE Alternate Fuels

AF1 Alternate Fuels — CNG — practice test

Studying for AF1 (Alternate Fuels — CNG)? Overhaul Prep has 171 verified AF1 questions written to the current task list — in the same formats the real exam uses (direct, Technician A/B, EXCEPT and most-likely-cause). Every answer comes with a written explanation, so you learn why instead of memorising a letter.

171AF1 questions
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Sample AF1 questions

Straight from the bank — answers highlighted, with the explanation underneath.

The standard settled service pressure for most modern CNG fuel storage cylinders is:

  1. 900 psi
  2. 1,500 psi
  3. 3,600 psi at 70°F
  4. 10,000 psi
WhyModern CNG systems use a 3,600 psi (24.8 MPa) settled service pressure referenced to 70°F (21°C); older systems used 3,000 psi. Pressure is always stated at a settled temperature because gas pressure varies with temperature.

A CNG cylinder is being filled at a fast-fill station. Technician A says the cylinder can momentarily read above rated service pressure during the fill because of heat of compression. Technician B says once the gas settles to 70°F the pressure should equal the rated service pressure. Who is correct?

  1. Technician A only
  2. Technician B only
  3. Both Technicians A and B
  4. Neither Technician
WhyFast filling heats the gas, so pressure temporarily reads high; as it cools to the reference 70°F, settled pressure equals the rated service pressure. Both statements are correct.

A CNG vehicle sits for a week and its pressure reading falls steadily. With the manual cylinder valve open and the lock-off solenoid de-energized (closed), a gauge shows the high-pressure section losing roughly 300 psi per day. The MOST likely cause is:

  1. A leaking gaseous injector
  2. A leak in the high-pressure section between the cylinder valve and the lock-off solenoid
  3. Normal cool-down of the gas after fueling
  4. A faulty pressure transducer
WhyClosing the lock-off isolates the low-pressure side, so the loss must be upstream of it: the cylinder valve, fill line, filter or the high-pressure fittings. A day-over-day loss of this size is far more than temperature-related settling.

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