Small Engine Mechanics Honor

Vocational Activities

Requirements

Describe the design and operation of the 2-stroke engine and the 4-stroke engine.
Answer: 2-stroke engine: completes 1 cycle every 2 movements of the piston (1 turn of the crankshaft) — in one movement intake and compression occur, in the other combustion and exhaust. It uses ports (windows) in the cylinder instead of valves, and the oil is mixed with the fuel. 4-stroke engine: completes 1 cycle every 4 movements of the piston (2 turns of the crankshaft), in separate phases — intake (the air/fuel mixture enters), compression (the piston compresses the mixture), combustion/explosion (the spark plug fires and pushes the piston), and exhaust (the burned gases exit). It uses intake and exhaust valves and has separate lubrication by the sump. — The 4-stroke engine was created by Nikolaus Otto in 1876 (the Otto cycle), and the principle of the 2-stroke engine with crankcase compression is credited to Dugald Clerk (1878-1881). The 2-stroke is simpler and lighter, but less efficient and more polluting (it burns oil along with the fuel), which is why it was banned from new urban motorcycles in Brazil from the 2000s onward for environmental reasons.
Name the parts of the 2-stroke engine and state what each part is for.
Answer: Cylinder (the chamber where the piston moves and the explosion occurs); piston (transforms pressure into linear motion); connecting rod (links the piston to the crankshaft); crankshaft (transforms linear motion into rotary motion); spark plug (generates a spark to burn the mixture); carburetor (mixes air and fuel). — In the 2-stroke engine, the crankcase functions as a 'second compression chamber,' unlike the 4-stroke — when the piston rises, it creates a vacuum in the crankcase and draws in the air/fuel mixture, a process essential to the short cycle.
List the types of fuel used in small engines and explain how they work.
Answer: Common fuels: pure gasoline (4-stroke engines), gasoline+oil 2-stroke mixture (ratio 25:1 to 50:1 for 2-stroke engines), diesel (generators and large brush cutters), LPG (liquefied petroleum gas), and ethanol. They work by internal combustion: the fuel mixes with air in the carburetor, is compressed in the cylinder, and burned by a spark (gasoline/alcohol) or by compression (diesel). — The 50:1 ratio (50 parts gasoline to 1 part 2-stroke oil) is the modern standard in Stihl and Husqvarna chainsaws, while older engines required 25:1 — the difference is due to the refinement of modern synthetic oils with detergent additives.
Show the safety precautions for handling and storing fuel.
Answer: Precautions: use approved jerrycans in a ventilated place away from sources of heat/sparks; never smoke nearby; refuel with the engine off and cold; use a funnel to avoid spillage; keep a Class B dry chemical fire extinguisher available; clean up leaks immediately; do not store in PET bottles (the fuel dissolves the plastic); keep children and animals away. — Gasoline has a flash point of -43°C and its vapors are heavier than air, accumulating on the ground — which is why closed garages with engines or small spills are so dangerous, a principle required by the NR-20 standard of the Ministry of Labor.
Describe 2 types of ignition system.
Answer: Two types of ignition system: 1) Magneto (magnetic flywheel) — generates the spark by means of a coil and rotating magnets fixed to the engine's flywheel; the movement of the magnets induces current in the coil, which sends high voltage to the spark plug. It needs no battery and is common in 2-stroke chainsaws and lawn mowers. 2) CDI (Capacitive Discharge Ignition) — an electronic system that stores energy in a capacitor and releases it all at once to the coil, generating a strong spark at the right moment. It has no moving parts or contacts that wear out, is more reliable, and replaced the old mechanical points system. — The CDI replaced the mechanical breaker points (the old system) in the 1980s because the points wore out after 5,000-10,000 km and required adjustment — the digital CDI has no moving parts and lasts the entire service life of the engine.
List 2 acceptable cleaning fluids for small engines. (Why is gasoline an unsuitable cleaning fluid?).
Answer: Two acceptable cleaning fluids: 1) Kerosene — a petroleum-derived solvent that dissolves grease and dirt with good efficiency and low volatility; 2) Diesel — a similar solvent, cheaper and less flammable. Specific spray carburetor cleaners also work. GASOLINE is unsuitable as a cleaning fluid because it is highly volatile and flammable, evaporating and forming vapors that catch fire from any spark (a serious fire/explosion risk on the workbench); in addition, it is toxic to the skin and respiratory tract, attacks the engine's rubber and seals, and leaves residue. For these reasons, using it to clean parts is dangerous and not advised. — The risk of gasoline as a solvent is confirmed by NR-20 and ISO 4406 — vapors accumulate in low areas and can be ignited by a simple electrostatic discharge, causing explosions with as little as 1 mm³ of concentrated vapor.
List and tell how 3 basic lubrication systems operate.
Answer: Three lubrication systems: (1) Mixture — in 2-stroke engines, the oil is mixed with the fuel and burns along with it, lubricating the parts as the mixture passes through; it is simple but polluting. (2) Wet sump — in 4-stroke engines, the oil is stored in the sump and a pump circulates it through the moving parts; it is the standard in cars and motorcycles. (3) Dry sump — the oil is kept in a separate reservoir and pumps deliver the oil to the parts and return it to the reservoir; it ensures lubrication even in inclined positions, being used in high-performance engines and in applications that operate at extreme angles. — The dry sump system was developed for World War II aircraft (1940s) — in combat maneuvers the oil from a wet sump would leak out through the breather, and the separate reservoir ensured continuous lubrication even during loops.
List, in order, the proper procedures for troubleshooting problems in general.
Answer: Sequence: (1) identify the symptom (won't start, misfires, loses power); (2) check the fuel (quality, correct mixture for 2-stroke); (3) test the spark plug and ignition system; (4) inspect the carburetor (clogs, adjustment); (5) check the cylinder compression; (6) evaluate the mechanical parts (air filter, exhaust); (7) correct the defect and test operation. — The 'from simplest to most complex' rule is the key principle of engine diagnosis — 80% of problems are solved in steps 1-3 (fuel, spark plug, carburetor), avoiding the unnecessary disassembly of the entire engine.
Demonstrate how to properly overhaul, clean, inspect, assemble, and adjust any small engine.
Answer: Stages: (1) Overhaul — turn off and cool the engine, drain the fuel and oil. (2) Clean — disassemble with the correct wrenches, clean the parts with kerosene, dry them. (3) Inspect — check the wear of the piston, rings, connecting rod, spark plug. (4) Assemble — follow the manual with the correct torque on the bolts and use new gaskets. (5) Adjust — regulate the carburetor, ignition (spark plug gap 0.6 mm) and test at idle before applying load. — The correct bolt torque is critical in small engines — over-tightening can strip the aluminum thread of the block, and under-tightening causes leaks; always use a calibrated torque wrench as per the manufacturer's manual.
Demonstrate how to test and clean spark plugs and glow plugs.
Answer: To test: remove the spark plug with the appropriate wrench, connect it to the ignition cable, hold the metal body against the engine block and start the engine — there should be a bluish spark visible at the electrode. To clean: use a fine steel brush on the electrodes to remove soot; fine sandpaper (P400) if there is oxidation. — The standard 0.7 mm gap of the NGK BPR6ES spark plug is universal in 2-stroke chainsaws and lawn mowers — a larger gap disperses the spark (engine misfires) and a smaller one causes pre-ignition (the engine pre-detonates with a risk of breakage).