Summary of "자동차정비 자격증 취득 - 자동차공학 엔진 이론 2"

Overview

The lecture covers piston-engine fundamentals: how two-stroke and four-stroke engines operate, their advantages and disadvantages, lubrication and causes of exhaust smoke, a brief note on forced induction (blowers/turbos), and ignition/cycle classifications (Otto, Diesel, Sabathe) with their PV-diagram characteristics — material highlighted as important for exams.

Two-stroke engine — structure and operation

Key concepts

Common in small motorcycles.
Completes one thermodynamic cycle in one crankshaft revolution (two piston strokes: down and up).
Uses the piston itself to open/close intake/scavenge and exhaust ports (no poppet valves).
Crankcase is used for pre-compression/inlet charging (so no separate oil sump in that area).

Typical one-revolution sequence (down + up)

At TDC the mixture above the piston is compressed and ignition (spark) occurs.
Explosion forces the piston downward (power stroke). As it descends it first uncovers the exhaust port and expanding gases exit.
Further descent uncovers the scavenge/intake port(s); the compressed fresh mixture from the crankcase is forced into the cylinder, pushing residual exhaust gases out (scavenging) while exhaust continues.
Piston reaches BDC; on the rise it first closes the scavenge port, then the exhaust port. Once the exhaust port is closed the trapped charge above the piston is compressed as the piston continues upward, and the cycle repeats.

Scavenging and the deflector piston

Without flow guidance, fresh charge can short-circuit out the exhaust.
A deflector (a raised/recessed feature on the piston crown) directs incoming flow upward to sweep residual exhaust toward the exhaust port, improving scavenging.

Lubrication and consequences

Oil must be mixed into the fuel because the crankcase handles the charge — this produces characteristic smoky exhaust.
Piston-as-valve design causes faster or uneven wear of piston and rings.
Burning oil in the mixture leads to visible white/blue smoke (normal for two-strokes).

Four-stroke engine — main points

Operation

Four distinct strokes: intake → compression → power (combustion) → exhaust.
One complete cycle requires two crankshaft revolutions.
Uses a valve-train (valves, camshaft, rocker arms) to control intake and exhaust.

Advantages

Distinct separation of strokes improves control over intake, compression and exhaust.
Smoother operation and a wider usable RPM range (better drivability).
Better volumetric control and less short-circuiting of fresh charge.
Easier starting and lower per-cycle oil burning when healthy.

Disadvantages

More complex valvetrain → increased parts, weight, manufacturing and maintenance cost.
More mechanical noise and additional potential failure points.
Small single-cylinder four-strokes can stall or run roughly at low speed.
Valve overlap can allow some fresh mixture to escape and ignite in the exhaust (backfire/afterfire).
Higher cost/weight per horsepower in simple small-engine designs.

Forced induction (blower / turbo) — brief explanation

Forcing air (supercharging/superchargers or turbocharging) increases the mass of air entering the cylinder above what suction alone admits.
Higher intake charge mass allows more fuel to be burned, increasing power and improving charging efficiency.
Types mentioned: loop blowers and centrifugal blowers (general categories used to increase charging efficiency and output).

Lubrication, oil consumption and exhaust smoke (diagnostics)

Smoke color and typical meanings

Two-stroke: oil mixed with fuel → normal white/blue (bluish) smoke.
Four-stroke:
- White/blue smoke = burning engine oil (not normal). Common causes: worn piston rings or damaged/hard valve-guide seals allowing oil into the combustion chamber.
- Black smoke = incomplete combustion or fuel-rich conditions (combustion problem).

Common causes and classifications of oil consumption

Combustion (oil burned in the cylinders).
Leakage (external leaks).
Valve-guide seals/guides failure: hardened or damaged seals let oil run down valve stems into the chamber — often a more likely cause of oil-burning smoke than piston-ring wear.

Comparative summary — two-stroke vs four-stroke

Two-stroke

Simpler (no valvetrain), lighter, cheaper.
Higher power-per-weight for small engines.
Requires oil mixed with fuel → smoky exhaust.
More mechanical wear due to piston functioning as valve.
Narrow but high rev range; scavenging and charge short-circuiting are design challenges.

Four-stroke

More complex valvetrain, heavier, more parts.
Smoother and broader speed range with better control of intake/exhaust.
Generally less per-cycle oil burning (if engine healthy).
Higher manufacturing/maintenance cost and more potential failure points.
Possible backfire during valve overlap; oil burning often linked to valve-guide issues.

Ignition types and mechanical/thermodynamic cycle classifications

Ignition types

Spark ignition (SI) — typical for gasoline/LPG engines.
Compression ignition (CI) — diesel engines.

Key thermodynamic cycles (frequently tested)

Otto cycle (constant-volume combustion)
- Idealized gasoline engine model.
- PV-diagram: combustion approximated as a vertical pressure rise at nearly constant volume.
Diesel cycle (constant-pressure combustion)
- Combustion occurs at (near) constant pressure while the piston moves; PV shape differs from Otto.
Sabathe (Seiliger / combined) cycle
- Combination of constant-volume and constant-pressure combustion steps; models some diesel behaviors, especially in high-speed automotive diesels.

PV-diagram notes

Learn the shapes and sequences for Otto, Diesel and Sabathe cycles.
Be able to identify which portions correspond to intake, compression, combustion (constant-volume vs constant-pressure), expansion and exhaust.
For technical/industrial-engineer exams, memorize PV-process sequences and the thermal efficiency formulas where required.

Memorize the PV-diagram shapes for Otto, Diesel and combined cycles and understand which parts represent intake, compression, combustion and expansion — this is commonly required on tests.

Practical / exam tips emphasized by the lecturer

Know the operational sequence of two-stroke scavenging and how piston/deflector geometry affects it.
Recognize and memorize PV-diagram shapes for Otto, Diesel and Sabathe cycles; be prepared to recall thermal efficiency formulas for exam-level engineering questions.
Recall causes of oil consumption (combustion vs leakage) and diagnostic signs: black smoke = incomplete combustion; white/blue smoke = oil burning.

Speakers / sources

Primary speaker: unnamed instructor/lecturer referencing standard engine theory and course textbook material.
Implicit sources: course textbook(s) and standard references on Otto/Diesel/Sabathe cycles and common motorcycle/automotive mechanical practice.