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
> I would have assumed that the explosions occur at the same speed regardless of the amount of fuel/air in the cylinder.
You would assume incorrectly. More fuel burning creates greater force from the expanding hot exhaust in the cylinder. That pushes the piston down harder and faster.
More fuel in the cylinder equates to a larger explosion, which provides more force, pushing the piston back harder/faster.
When the piston travels down on the intake stroke, it is sucking air and fuel into the engine cylinder by creating a low pressure zone. It only happens for a short time, however. The intake throttle butterfly valve will change how much air passes through into the engine during that stroke. By opening more, it allows more air to enter the engine. More air/fuel mix creates a more forceful combustion in the cylinder, which accelerates the piston down during the power strike more forcefully.
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.
An engine burns a certain amount of fuel per second. There’s two ways that can be controlled: Fuel allowed per intake, OR the rate of intakes per second.
So, one way is to relax on the fuel line and let more fuel per intake.
The more fuel during intake, the bigger the explosions. The bigger the explosions, the more force. The more force, the more inertia is overcome. The more inertia overcome, the quicker the shaft moves. The quicker the shaft moves, the faster the engine runs.
The engine will have more strokes the faster it gets, which is more fuel volume per second into the engine as mentioned above, which makes the engine run even faster.
This runaway effect is basically only a problem for diesel engines because compressing diesel and air together is all you need for a boom, which can happen in a few ways outside of your control, so they need extra safety measures like blocking the air intake to shut the engine off, or at least making sure the air intake doesn’t take in diesel fumes.
The explosions absolutely change in strength and in timing.
Accelerating the engine, you want the spark plug to fire earlier in the rotation of the engine, because a fire does take a specific time go get to maximum compression, and that time doesnt change, but if the engine is spinning faster, the spark has to happen earlier in the crankshafts rotation in order to be making maximum compression when the piston starts its downtravel on the ‘power’ stroke.
And an explosion isnt equal at idle and wide open throttle. You get a lot more air with the throttle blades open, which requires a lot more fuel to burn at optimum mixture (stoichiometric ratio if you’re interested) which is about 14.7:1, air (in weight) to fuel. Adding 1cc of air and the appropriate amount of fuel is not the same explosion as 1000cc of air and the proper amount of fuel
More air and fuel means bigger explosions, which cause the pistons to physically move faster. You’re increasing the amount of energy added to a place the same volume, so you’re going to make more pressure, which means more force and acceleration of the pistons.
If I remember right, adding fuel just increases the strength of the explosions. The explosion rate stays the same
It is not an explosion, it is deflagration.
Explosions (aka detonations) are supersonic. Deflagration is subsonic.
Just by these definitions you can see that the flame front can have different speeds.
When you step on the gas, you increase the frequency of the explosions in the cylinder. Its similar to biking, by pedaling harder. You’re increasing the number of pedal cycles per unit time. That is what leads to the bike or car moving faster. It’s measured as RPM. Power from the engine is amplified by the transmission then to the wheels. The transmission is a slider that gives more torque at lower gear. And more travel at higher gear. Bike gearing analogy still holds.
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