During the 'running-in' period of any new vehicle, be it a car or a bike, extra caution must be taken while driving. We should allow the various mechanical parts of the vehicle to 'bed in' properly before they can be used up to their limits. This applies even more strictly to the engine.
The owner's manual of any vehicle will carry information that resembles something like this: "vary the engine speed regularly for better cooling of the engine parts". What do they mean by this? And how will the cooling be better if the engine speed is varied? This article contains simple answers to these questions.
When the engine speed (RPM) is constant, the volume of the air-fuel mixture (charge) sucked into the engine is also constant (provided the air density doesn't change). The charge is combusted and the resulting combustion products are sent out into the atmosphere. However, it must be noted that not all the combustion products leave the engine--some portion remains inside the engine and they're referred to as the 'residual exhaust'.
The amount of residual exhaust left over from the previous cycle plays an important role in deciding the volume of fresh charge that goes into the engine when the intake stroke of the next cycle starts. If the amount of residual exhaust is constant, the amount of fresh charge sucked in remains constant too. The residual exhaust volume will remain constant when the engine speed is constant. So, we have a situation wherein, when the engine speed remains constant, the 'cooling effect' provided by the fresh charge also remains constant.
However, the temperature rise in the combustion chamber walls need not necessarily remain constant--with prolonged operation, the temperature might continue to rise. A 'non-sync' is created between the amount of cooling that the engine gets through the intake of fresh charge, and the temperature rise.
In order to combat this situation, the amount of cooling needs to increase to match or exceed the temperature rise. To increase the cooling, the amount of charge intake should be increased. And for this, the engine speed should be increased.
That's why manufacturers recommend 'varying engine speed during prolonged engine operation' specifically during running-in periods.
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