🚗 Engines and Combustion

Part 1 of 7 — Auto Information on the ASVAB

Topic Overview

What Auto Information (AI) Is

Auto Information is the 7th subtest on the CAT-ASVAB, right after Electronics Information. You get 10 scored questions in 7 minutes, about 42 seconds each, with 4 answer choices per question. It is followed by Shop Information (10 questions in 6 minutes).

You do not need to have worked on a car. AI asks about a fairly small list of parts: what each one does, what happens when it fails, and what symptom a driver would notice. This course teaches every part from zero, in exactly that order.

Key Fact: AI is not part of your AFQT. It is combined with Shop Information into the AS (Auto & Shop) score, which feeds mechanical and maintenance line scores: for example the Air Force Mechanical score (M = AR + MC + AS + 2·VE), the Marine Corps Mechanical Maintenance score (MM = AR + MC + EI + AS), and the Navy Engineering (ENG = MK + AS) and Mechanical (MEC = AR + MC + AS) scores. In the Army, AS is the biggest single factor in the Mechanical Maintenance score and weighs heavily in General Maintenance and Operators and Food. Ask your recruiter which line scores your target job uses.

On the paper version of the test, Auto and Shop are a single combined subtest (AS: 25 questions in 11 minutes). The high-school CEP computer version has 11 AI questions.

🚗 Engines and Combustion

Part 2 of 7 — Engine Parts: Block, Pistons, Crankshaft and Valves

How an Engine Makes Turning Power

A car engine is an internal combustion engine: fuel burns inside sealed cylinders. The burning gas expands and shoves a piston down. A connecting rod passes that push to the crankshaft, which turns the up-and-down motion of the pistons into rotation, the same way your legs turn a bicycle's pedals.

Follow one push from top to bottom in Figure 2 (next screen):

1. A spark lights the fuel-air mixture in the combustion chamber.
2. The expanding gas pushes the piston down the cylinder.
3. The connecting rod carries the push to the crankshaft.
4. The crankshaft spins, and the spinning power goes on to the transmission and wheels.

🚗 Engines and Combustion

Part 3 of 7 — The Four-Stroke Cycle

Suck, Squeeze, Bang, Blow

Almost every car engine runs on the four-stroke cycle. A stroke is one trip of the piston, either down or up. Four strokes make one complete cycle:

Then the cycle repeats.

Key Fact: Only the power stroke produces power. The other three strokes are kept going by the momentum of the crankshaft and flywheel (and by the other cylinders firing).

Counting revolutions

• Each stroke is half a turn of the crankshaft (down = ½ turn, up = ½ turn).
• One full cycle = 4 strokes = 2 crankshaft revolutions (720°).
• Each valve opens once per cycle, so the camshaft turns at half the speed of the crankshaft. That is why the cam gear is twice the size of the crank gear on the timing belt or chain.

🚗 Engines and Combustion

Part 4 of 7 — Ignition and Combustion

The Ignition System's Job

A gasoline engine needs a strong spark in each cylinder at exactly the right moment near the end of the compression stroke. The car's battery supplies only about 12 volts, but jumping the spark plug gap under compression takes tens of thousands of volts.

The order in which the cylinders fire, such as 1-3-4-2 on many 4-cylinder engines, is the firing order.

🚗 Engines and Combustion

Part 5 of 7 — Lubrication and Engine Diagnosis

Why Oil Matters

Inside a running engine, metal parts slide and spin against each other thousands of times a minute. Without oil they would weld themselves together in minutes. Engine oil has four jobs:

1. Lubricate: a thin film keeps metal from touching metal and reduces friction
2. Cool: carries heat away from parts the coolant cannot reach, like the bottom of the pistons
3. Clean: picks up dirt and combustion byproducts and carries them to the filter
4. Seal: helps the piston rings seal against the cylinder walls

The oil's path (Figure 9)

The oil pan (sump) at the bottom of the engine holds the oil. The oil pump, driven by the engine, draws oil up through a pickup screen, pushes it through the oil filter, and forces it through drilled passages (oil galleries) to the crankshaft and connecting-rod bearings, the camshaft, the valve train and the cylinder walls. Then it drains back into the pan.

Key Fact: The oil pressure warning light means stop the engine now. Running without oil pressure can destroy the bearings in minutes.

🚗 Engines and Combustion

Part 6 of 7 — Engine Test Strategy and Common Traps

Your Time Budget

AI questions are short and mostly recall: you either know what a camshaft does or you don't. Read the question, decide your answer before looking at the choices, then find it. If you don't know, eliminate what you can and pick the best remaining choice; don't sink a minute into one question.

CAT reminders: no going back, you must answer to move on, and any question might be an unscored tryout, so treat each one as scored. If time is short, keep answering for real; a burst of random wrong answers at the end is penalized heavily.

Three questions to ask yourself

1. Which system? Is this about fuel, spark, oil, cooling or the engine's mechanical parts? Choices from the wrong system are usually wrong.
2. What does it do? Say the part's job in your own words.
3. What would fail look like? Match the symptom to the part whose job just stopped happening.

🚗 Engines and Combustion

Part 7 of 7 — Mixed Mastery Set

Ten questions covering the whole topic, at real AI difficulty. Give yourself about 40 seconds each.