Engine Theory and Cycles
What is the four‑stroke cycle (Otto cycle)?
intake, compression, combustion (power), and exhaust
This part moves up and down inside the cylinder
piston
in older cars, this mixes air and fuel in proper proportion before entering the cylinder
carburetor
This part produces the spark to ignite the air-fuel mix.
spark plug
Before servicing, always do this to prevent accidental start
disconnect the negative battery cable
The ratio comparing the volume of the cylinder and combustion chamber when piston is at bottom vs top
compression ratio
Connects the piston to the crankshaft
connecting rod (con rod)
This intake system component - when clogged - can restrict airflow and make the engine run rich.
air filter
This gap between the spark plug electrodes is critical for proper spark
spark plug gap
This should be changed or topped up regularly to maintain lubrication
engine oil
The term for how many times per minute the engine’s crankshaft rotates.
RPM (revolutions per minute)
This rotates and converts reciprocating piston motion into rotational motion
crankshaft
When there’s too much fuel relative to air, this condition exists
rich fuel mixture
This system removes excess heat from the engine to prevent overheating
cooling system
If an engine won’t start, check these three essentials: spark, fuel, and this
air/compression
The process by which chemical energy of fuel is converted into mechanical energy by burning inside the cylinder.
internal combustion
this part stores rotational energy and smooths out power pulses
flywheel/flexplate
This device sprays fuel under pressure (in EFI systems) instead of mixing via carb
fuel injector
This system ensures moving parts are protected and reduces friction
lubrication system / engine oil
Black smoke or soot in exhaust often means this issue
too much fuel/rich fuel mixture
In a two-stroke engine, these two events happen in one revolution instead of four strokes.
Combustion and Exhaust;
Intake and Compression
This part opens or closes the path for intake air/fuel or exhaust gases
valve
This adjusts the fuel flow (size of opening) in the carburetor
main jet/pilot jet
This generates electricity for the ignition system (or charging) in small engines.
magneto
An engine that “seizes” usually suffers from this
lack of lubrication/ oil starvation