So if an internal combustion engine is idling, when we add more gas/air by opening the throttle why does the engine accelerate? Do the explosions in the cylinders increase in velocity? I would have assumed that the explosions occur at the same speed regardless of the amount of fuel/air in the cylinder. So I don’t understand why the engine rpm increases.
thanks
In: Engineering
To start, the flame front (it doesn’t explode, it burns) itself doesn’t go any faster to within rounding error, you’re correct there.
Piston engines both provide rotation, the thing that spins the wheels, but the pistons also move back and forth, so there’s energy required to keep that piston moving back and forth, starting and stopping it about ten times per second at idle, and this is why cars consume fuel at idle: to keep those pistons moving back and forth (as well as some minor frictional losses in the rotating parts).
By adding more fuel and air, the velocity of the burning doesn’t change, but the amount of force it generates does. This pushes the piston down faster, causing the RPM to increase. In engines built for race applications, they intentionally make the pistons as light as possible in order to make it easier for the flame to move the piston faster. The speed of the burning also puts a hard limit on exactly how fast you can make an engine spin – look at F1 cars for pushing that to the absolute limit, over 15,000 RPM.
A great way to see this is to look at a steam locomotive, where you can actually see the piston and driving rods at work. As more steam is added to the cylinder, it applies more force to the piston, despite remaining at the same pressure and therefore speed – this then needs to be stopped at the end of the piston travel and turned around, requiring more force to do so.
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