Two-Wheeler Engine Construction, Operation, Lubrication and Cooling
What the exam tests
Engine construction, the four-stroke cycle, valve timing, compression, the lubrication circuit, and cooling of air-, oil- and liquid-cooled two-wheelers.
Construction and operation
- Capacity = 0.7854 x bore squared x stroke. Oversquare engines (bore greater than stroke) rev higher; undersquare engines give more torque.
- Four strokes: intake, compression, power, exhaust, in two crankshaft revolutions. Valve clearance is set at TDC on the compression stroke, when both rockers are free.
- Valve timing: the intake opens before TDC and closes after BDC; the exhaust opens before BDC and closes after TDC. Overlap helps scavenging at high speed but upsets idle.
- Compression ratio = (swept volume + clearance volume) / clearance volume. Carbon raises it and causes detonation; a blown head gasket lowers it and causes hard starting.
Lubrication
Wet-sump circuit: strainer, trochoidal or gear pump, filter (centrifugal or paper), crankshaft and camshaft galleries, then back to the sump; small engines combine pressure and splash feed. Oil also cools, seals, cleans and protects against corrosion. Use the specified grade (for example 10W-30 SJ) and never overfill, because aerated oil is forced out of the breather.
Cooling
Air-cooled: fins must be clean, straight and fitted with their cowl; blocked fins lead to overheating and piston seizure. Oil-cooled engines add a small cooler and pump. Liquid-cooled engines use a radiator, thermostat, water pump and fan; check the coolant at the radiator cap only when cold.
Worked example
Bore 57 mm, stroke 48.8 mm: swept volume = 0.7854 x 3249 x 48.8 = 124,500 mm3, about 124.6 cc. For a 9.9:1 ratio, clearance volume = 124.6 / 8.9 = 14.0 cc. If carbon deposits reduce that to 12.5 cc, the new ratio is 137.1 / 12.5 = 10.97:1, so the engine now needs higher-octane fuel and may knock.
Traps
Setting clearance at TDC on the exhaust stroke, measuring a bore at one height only instead of checking taper, ignoring a blocked breather, and using car oil with friction modifiers in a wet clutch.