Engine Construction and Working Principles
This domain tests the parts of a reciprocating engine and the sequence of events in the four-stroke petrol (Otto) and diesel (compression-ignition) cycles, plus the two-stroke cycle.
Main components
Cylinder block and head, water jacket, liner, piston with compression rings and oil control ring, gudgeon (wrist) pin, connecting rod, crankshaft with counterweights, flywheel, camshaft, valves and springs, pushrods and rockers (in overhead-valve engines), timing gears, chain or belt, and the sump.
The four-stroke cycle
Suction: piston moves down, inlet valve open, fresh charge drawn in. Compression: both valves closed, charge compressed, about 180 degrees of crank rotation per stroke. Power: spark at the plug (petrol) or fuel injection near the end of compression (diesel) ignites the charge and drives the piston down. Exhaust: exhaust valve opens, burnt gas is pushed out. Four strokes take 720 degrees of crankshaft rotation, two revolutions. In a two-stroke engine, transfer and exhaust ports are uncovered by the piston, giving one power stroke per revolution, with crankcase scavenging and, in small engines, oil mixed in the petrol.
Useful terms and data
TDC and BDC are the extreme piston positions. Swept volume is the volume displaced by one piston stroke. Compression ratio equals (swept volume + clearance volume) divided by clearance volume. Typical petrol engines run about 8:1 to 10:1 and diesels about 16:1 to 22:1. Common four-cylinder firing orders are 1-3-4-2 and 1-2-4-3.
Worked example
A single-cylinder engine has bore 80 mm, stroke 90 mm and clearance volume 50 cm³. Swept volume = (pi/4) x 8² x 9 = 0.7854 x 64 x 9 = 452.4 cm³. Compression ratio = (452.4 + 50) / 50 = 10.05, that is about 10:1, a normal petrol-engine figure.
Traps that cost marks
Saying one stroke equals 90 degrees (it is 180); saying a diesel engine has a spark plug; forgetting that a two-stroke fires once per revolution; confusing swept volume with total cylinder volume.