Engine Design Classifications

Understanding engine types, designs, and classifications is essential for troubleshooting and repair.
Familiarity with engine terminology is crucial for effective communication and work in engine service.

Cylinder arrangement refers to the position of cylinders relative to the crankshaft.
Four basic cylinder arrangements:
- Inline: Cylinders lined up in a single row, parallel to the crankshaft. Common in 4-, 5-, and 6-cylinder engines.
- V-type: Two banks of cylinders at an angle to each other, reducing engine length and height.
- W-type: Two cylinder banks forming a "V," with cylinders in each bank offset to form a second narrow "V" ( $V + V = W$ ), allowing increased displacement without increasing engine size.
- Opposed: Cylinders lie flat on either side of the crankshaft (also known as "pancake engine" or "boxer engine"), minimizing frictional horsepower loss at the main bearings.

Pistons move right and left against each other, preventing a downward thrust on the main bearings.
Lower center of gravity, improving cornering ability in sports or racing cars.

Car and truck engines typically have 4, 6, or 8 cylinders, with some having 3, 5, 10, 12, or 16 cylinders.
More cylinders generally increase engine smoothness and power.
An 8-cylinder engine produces twice as many power strokes per crank revolution as a 4-cylinder engine, reducing power pulsations and roughness at idle.
Typical arrangements:
- Four-cylinder: inline, slant, or opposed.
- Six-cylinder: inline, slant, or V-type.
- Five-cylinder: inline.
- Eight-, 10-, and 12-cylinder: V-type.
Engine manufacturers number cylinders to aid technicians in repairs; cylinder numbering can vary between manufacturers.

Two types of cooling systems:
- Liquid cooling: most common, uses coolant (water and antifreeze) to carry combustion heat away from the cylinder head and engine block.
- Air cooling: circulates air over cooling fins on the cylinders to remove heat; less common in passenger cars but used in motorcycles, lawnmowers, and some high-performance cars.

Engines are classified by the fuel they burn:
- Gasoline engines burn gasoline.
- Diesel engines burn diesel fuel.
- Other fuels include liquefied petroleum gas (LPG), gasohol (10% alcohol, 90% gasoline), and pure alcohol, but these are used in limited quantities.

Two basic methods to ignite fuel in the combustion chamber:
- Spark ignition: uses an electric arc at the spark plug to ignite the fuel (gasoline engines).
- Compression ignition: compresses air until it's hot enough to ignite the fuel (diesel engines), without spark plugs.

Engine classification based on valve location:
- L-head engine (flat head): both intake and exhaust valves in the block; cylinder head is a simple cover; camshaft in the block pushes upward to open valves; automotive engines aren't L-head types anymore ( lawnmower engines can be ).
- I-head engine (overhead valve, OHV): both valves in the cylinder head; numerous variations in use.

Two basic camshaft locations: in the block and in the cylinder head.
- Cam-in-block engine: uses push rods to transfer motion to rocker arms and valves; also called overhead valve (OHV) engine.
- Overhead cam (OHC) engine: camshaft located in the cylinder head; no push rods needed; refinement of the overhead valve engine.

Single overhead cam (SOHC): one camshaft per cylinder head; cam acts directly on valves or uses rocker arms.
Dual overhead cam (DOHC): two camshafts per cylinder head; one operates intake valves, the other operates exhaust valves; frequently used in engines with four-valve combustion chambers.

Four basic combustion chamber shapes for gasoline engines:
- Pancake (bathtub chamber): valve heads almost parallel to the top of the piston; chamber forms a flat pocket over the piston head.
- Wedge (wedge head): shaped like a triangle or wedge; valves side-by-side, spark plug next to valves; may include a squish area where the piston squeezes the air-fuel mixture at TDC to improve mixing at low speeds.
- Hemispherical (hemi-head): shaped like a dome; valves are canted on each side, spark plug near the center; very efficient with minimal hidden pockets and small surface area, reducing heat loss and incomplete combustion.