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CDL · Air Brake Parts

What is the purpose of the alcohol evaporator in some air brake systems?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

To put alcohol into the air system in winter, lowering the freezing point of any water in the lines

  1. A To clean the air filter
  2. B To evaporate water from the engine coolant
  3. C To put alcohol into the air system in winter, lowering the freezing point of any water in the lines
  4. D To add fuel to the engine in cold weather

Why this is the answer

Some air brake systems include an alcohol evaporator, especially in cold climates. The evaporator slowly introduces methyl alcohol into the air supply during winter operation. The alcohol mixes with any moisture in the system and lowers its freezing point, preventing ice from blocking lines or valves in freezing weather. The driver must check the alcohol level regularly during cold weather and refill as needed. Modern trucks rely more on air dryers (which remove moisture from compressed air before it reaches the tanks) than on alcohol evaporators, but both still exist on the road. Either way, daily draining of the air tanks remains essential.
Source: FMCSA CDL Manual 5.1.6 Alcohol Evaporator

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About this topic: Air Brake Parts

Knowing the parts of the air brake system — compressor, governor, supply tank, dryer, slack adjusters, spring brakes — is required so you can recognize defects during inspection. This is one of 22 Air Brakes questions on air brake parts, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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