Engines and Combustion

🚗 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.

The Car from Zero

A car turns the chemical energy in fuel into motion. Every system you will study exists to make that happen or to keep it running safely.

The question pattern

Most AI questions fit one of three molds:

1. What does it do? "The purpose of the camshaft is to ..."
2. Which system is it part of? "The thermostat is part of the ... system."
3. What causes this symptom? "Blue smoke from the tailpipe usually means ..."

Warm-up: use the map. 🎯 Don't worry if these are new; Parts 2–5 explain every one.

Key Takeaways — Part 1

• AI is the 7th CAT-ASVAB subtest: 10 questions in 7 minutes, about 42 seconds each
• AI + Shop form the AS score, used in mechanical and maintenance line scores; it is not in the AFQT
• Learn every part as what it does → what fails → what symptom you notice
• The engine burns fuel and air to spin the crankshaft; every other system feeds it, cools it, cleans up after it, or carries its power to the wheels

Up Next: Part 2 — the parts inside an engine and what each one does.

🚗 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.

Find each part as you read the table on the next screen. The shaded space is where the fuel actually burns.

The Parts, One by One

Key Fact: Combustion happens in the combustion chamber, the space between the top of the piston and the cylinder head. Not in the carburetor, the manifold or the crankcase.

Name the numbered parts. 🎯 Use Figure 3.

Engine Layouts and Size

Layouts. Cylinders can sit in a single row (inline, such as an inline-4), in two rows forming a V (V6, V8), or lying flat and opposed (flat or "boxer").

Displacement is the total volume all the pistons sweep through, in liters (L) or cubic inches. More displacement usually means more power and more fuel used. A 4-cylinder engine whose cylinders each sweep 0.5 L is a 2.0-L engine.

Compression ratio compares the cylinder's volume with the piston at the bottom of its stroke to its volume with the piston at the top. If the space shrinks from 500 cc to 50 cc, the compression ratio is 500 ÷ 50 = 10 : 1. Higher compression squeezes the mixture harder and gets more power from each drop of fuel, but needs higher-octane fuel to avoid knocking (Part 4).

Top dead center (TDC) is the piston's highest point; bottom dead center (BDC) is its lowest.

Match the part to its job. 🔽

Engine numbers. ✍️ Numbers only.

1) A 6-cylinder engine has cylinders that each sweep 0.5 L. What is its displacement, in liters?

2) With the piston at the bottom, a cylinder holds 450 cc; at the top, 50 cc. What is the compression ratio? (Give the first number of the ratio, as in "__ : 1".)

Key Takeaways — Part 2

• Burning gas pushes the piston; the connecting rod passes the push to the crankshaft, which turns it into rotation
• Combustion happens in the combustion chamber, between the piston top and the cylinder head
• Piston rings seal compression and control oil; worn rings → oil burning, blue smoke, low compression
• The camshaft opens the valves; the timing belt/chain keeps it in step with the crankshaft
• Head gasket seals head to block; flywheel smooths power and is turned by the starter
• Displacement = total swept volume; compression ratio = bottom volume ÷ top volume

Up Next: Part 3 — the four-stroke cycle: intake, compression, power, exhaust.

🚗 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.

Figure 4. The four strokes. Watch the valves: only one stroke has the intake valve open, only one has the exhaust valve open, and both are closed for compression and power.

Read the cylinder. 🎯

Two Other Engine Types

Two-stroke engines

Lawn equipment, chainsaws, some outboard motors and small motorcycles often use a two-stroke engine. It completes the whole cycle in two strokes (one crankshaft revolution), so it gets a power stroke every revolution. It usually has ports in the cylinder wall instead of valves, and its oil is mixed into the fuel (that is why you add oil to the gas can for a chainsaw). It is light and simple but less efficient and dirtier.

Diesel engines

A diesel is also usually a four-stroke engine, but it ignites its fuel differently:

Diesels are common in trucks, military vehicles, buses and heavy equipment because they give strong pulling power (torque) and good fuel economy.

Check the cycle. 🎯

Fill in the cycle. 🔽

Key Takeaways — Part 3

• Intake (down, intake valve open) → compression (up, both closed) → power (down, both closed, spark) → exhaust (up, exhaust valve open)
• Only the power stroke makes power
• One cycle = 2 crankshaft revolutions; the camshaft turns at half crankshaft speed
• Two-stroke: power every revolution, oil mixed with fuel
• Diesel: air only on intake, compression ignition, no spark plugs, glow plugs for cold starts, higher compression ratio

Up Next: Part 4 — the ignition system and what makes combustion go right or wrong.

🚗 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.

The Spark Plug

The high voltage arrives at the terminal, travels down the center electrode inside the ceramic insulator, and jumps the gap to the ground electrode, which is attached to the metal shell. The gap is set with a feeler or wire gauge:

• Gap too wide: the spark may not jump, causing a misfire.
• Gap too narrow: a weak, small spark that may not light the mixture well.

Heat range

A spark plug's tip has to run hot enough to burn off carbon deposits, but not so hot that it becomes a glowing hot spot. Heat range describes how fast the plug passes heat from its tip to the cylinder head, and it is set mainly by the length of the insulator nose (Figure 8, coming up):

Heat range. 🎯

Timing and Abnormal Combustion

Ignition timing

The mixture takes a moment to burn, so the spark fires slightly before the piston reaches top dead center on the compression stroke. That lets the pressure peak just as the piston starts down. Firing earlier is called advancing the timing; later is retarding it. Engines advance the timing as RPM rises, because there is less time for the burn.

When combustion goes wrong

Octane

The octane rating on the pump measures a fuel's resistance to knock, not how much energy it holds. Higher-compression and turbocharged engines need higher octane; putting premium in an engine designed for regular adds nothing.

Check ignition and combustion. 🎯

Diagnose it. 🔽

Key Takeaways — Part 4

• Ignition path: battery → ignition coil (steps 12 V up to tens of thousands of volts) → distributor or computer → spark plugs, in the firing order
• Spark fires slightly before top dead center on the compression stroke (timing advance)
• Plug gap too wide → misfire. Hot plug = long insulator nose, resists fouling; cold plug = short nose, for hot-running engines
• Detonation/knock/ping: low octane, over-advanced timing, overheating, carbon. Preignition: hot spot fires the mixture before the spark. Misfire: a cylinder doesn't fire
• Octane = knock resistance, not extra energy
• Diesels have no spark plugs (compression ignition)

Up Next: Part 5 — oil, lubrication, and reading engine symptoms.

🚗 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.

Oil Grades

Viscosity is how thick an oil is (how easily it flows). Oil thins as it heats up. A multigrade oil such as 10W-30 behaves like a thin 10-weight oil when cold, so it flows quickly at startup (the W stands for winter), and like a thicker 30-weight oil at operating temperature.

• Lower first number: flows better in the cold
• Higher second number: stays thicker when hot

Use the grade the owner's manual specifies.

Reading Engine Symptoms

Many AI questions describe a symptom and ask for the cause. The exhaust smoke color is the classic one.

Blow-by is combustion gas that leaks past the piston rings into the crankcase. Worn rings mean more blow-by.

Diagnose it. 🎯

Match the symptom to the cause. 🔽

Key Takeaways — Part 5

• Oil lubricates, cools, cleans and seals
• Path: oil pan → pickup → pump → filter → galleries → bearings and moving parts → back to the pan
• Oil pressure light on = stop the engine
• 10W-30: W = winter; first number = cold flow, second = hot thickness
• Smoke: blue = oil (rings, valve seals); black = rich mixture (too much fuel); thick white, sweet = coolant (head gasket); milky oil = coolant in the oil
• Deep knock rising with speed = worn bearings; low compression = rings, valves or head gasket

Up Next: Part 6 — how to work engine questions fast and avoid the traps.

🚗 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.

The Traps

Words that point to answers

• "Ping," "knock" when accelerating → detonation (octane, timing, overheating, carbon)
• "Uses oil," "blue smoke" → rings or valve seals
• "Milky oil," "sweet white smoke" → head gasket
• "Rough idle," "shakes," "one cylinder" → misfire (plug, wire, coil, injector, compression)
• "Oil light" → low oil pressure: stop the engine

Spot the trap. 🎯

Which system? 🔽 Choose the system each part belongs to.

Key Takeaways — Part 6

• CAT AI: 10 questions, 7 minutes, about 42 s each; on paper, AS is 25 questions in 11 minutes
• Ask: which system → what does it do → what would failure look like?
• Watch the swaps: crankshaft/camshaft, blue/black smoke, hot/cold plugs, octane/energy, gas/diesel ignition
• Symptom words point to causes: ping → detonation; blue smoke → rings; milky oil → head gasket; rough idle → misfire

Up Next: Part 7 — a mixed mastery set.

🚗 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.

Question 1. 🎯

Questions 2–5. 🎯

Questions 6–10. 🎯

Key Takeaways — Engines and Combustion

• Piston → connecting rod → crankshaft (rotation); camshaft opens valves at half crank speed; timing belt/chain keeps them in step
• Combustion chamber = where fuel burns; piston rings seal compression and control oil
• Four strokes: intake, compression, power, exhaust; 2 crank turns per cycle; only power makes power
• Ignition: coil steps 12 V up; plug gap; hot plug = long nose, cold plug for hot-running engines; knock = detonation (octane, timing, heat, carbon)
• Oil: lubricate, cool, clean, seal; oil light = stop; blue smoke = oil, black = rich, sweet white = coolant

Next topic: Drivetrain, Brakes, Steering and Suspension.