Catégorie
📚
ÉducationTranscription
00:00In a 2-stroke petrol engine, the oil and petrol are usually mixed before they enter the engine.
00:08Different engines use different ratios, as given in manufacturer's specifications.
00:14Oil, air and petrol enter the crankcase.
00:18The high temperatures vaporise the petrol and the oil lubricates the moving parts.
00:24A 4-stroke petrol engine crankshaft is basically the same as that in a 2-stroke.
00:32At first, the shape may be a surprise.
00:36Why won't a simpler shape do the job?
00:38It's all to do with balancing forces.
00:42With its weight evenly balanced, this top spins properly.
00:48By adding a magnet, it now becomes unbalanced.
00:51Without removing this magnet, how can the top be made to spin properly again?
00:58Put an identical magnet on the other side.
01:01Then the weight is balanced again.
01:04The second magnet acts as a counterweight.
01:09Counterweights appear in unusual places.
01:12They're essential for some systems to work at all.
01:15Crankshaft counterweights help keep the rotating components in balance.
01:24They help keep the crankshaft turning as smoothly as possible.
01:29The crankshaft turns because of the forces transmitted through the connecting rods.
01:34At the same time, it must be held in place.
01:37That's done by bearings.
01:38The crankshaft is held in place in the engine block by main bearings at points called journals.
01:51Different bearings do different jobs.
01:55These bearings support the crankshaft in place, but let it turn freely.
02:00The crankshaft also needs to be located to stop lateral movement.
02:07This is done by using flanges.
02:11Between the connecting rod and the crankshaft are connecting rod bearings.
02:16They protect the spinning crankshaft at points also called journals.
02:21On the rear of the crankshaft is a heavy flywheel.
02:26It stores up energy from the power stroke and helps keep the crankshaft turning.
02:32In a four-stroke engine, only one of the four strokes in a cycle delivers power.
02:39In one cycle, the crankshaft makes two revolutions.
02:44In a two-stroke, every second stroke is a power stroke.
02:47Without the flywheel to store energy from the power stroke, the crankshaft will slow down and stop.