How does an engine know to increases its RPMs when the throttle opens rather than just trying to force more air and fuel into the cylinder?

Posted by nycengineer111@reddit | askcarguys | View on Reddit | 13 comments

My understanding of a throttle is that it limits the amount of fuel and air going into an engine. So my question is, why does increasing this make the engine cycle faster? Why is the result not just that the cylinder gets more fuel and air in but cycles at the same speed while making more power in the same way an electric motor does when it gets more load (eg how an elevator rises at the same speed regardless of load or how an electric pump spins at the same speed whether or not it is moving water or dead-headed) or the way a cyclist has to push harder uphill in the same gear to maintain the same speed on a flat but the RPM of the crank is the same? If the answer is “a fancy computer controls stuff” how did this work 100 years ago?

Reply to Post

13 Comments

[-]

simpfortrading@reddit

Can i increse my rpm on bajaj pulsar rs 200 bs 6

[-]

realitysvt@reddit

its the advancement in timing, paired with more fuel and air to make more pressure.

[-]

nycengineer111@reddit (OP)

What controls the timing though?

[-]

AntiqueInspector4394@reddit

I was going to say the cam-shaft, but I don't know enough to know if that's a correct answer 😆

[-]

trump_the_dog@reddit

I think you're confused with how an engine works. The rpm increasing is a direct result of fuel and air getting shot into the cylinders. With all that extra fuel the engine starts spinning faster and faster. And that spinning force goes from the engine, to your transmission, and all the way to your diff when the rotation is directed to your tires. Unless I misunderstood what you're asking here

[-]

Mikasa96i@reddit

The question is how rpm increases when shot of fuel and air increase

[-]

wrober9@reddit

No one tackled the last question from op yet? I’m stoked. I daily driver a 54 year old mustang. Work on classics. Distributors! Carbs! Hundred years ago? You can manipulate and “test and tune” your cars. Today, no. A “tune” today is like updating your pc. Back then, you twist this part or turn that doodad. Vacuum gages. Timing lights or guns. Etc. Even just using your ears and getting that feel for things.

[-]

nycengineer111@reddit (OP)

Right but how does your old mustang know to change its timing when you open the throttle? Like what is the process?

[-]

wrober9@reddit

Modulator valves, vacuum pressure, etc. Timing curves, initial set timing vs advanced total timing. I loosen a screw, measure the timing of the engine based upon marks on the balancer, hold the distributor in place then watch the markings as I bring the rpm up to 2k-3k. Math, adjust from there.

[-]

nycengineer111@reddit (OP)

I understand there are things that can be adjusted, but what I’m going for here is kind of a step by step process of what happens to make the engined cycle faster when you open the throttle. Like: 1. You step on the accelerator and the throttle opens. 2. Increases airflow pulls in more fuel. But then what? What controls the actual RPMs and how does whatever that is know how to do it’s thing base on what is happening in the carburetor?

[-]

wrober9@reddit

The simple answer is the throttle blades open and allow more air in, but there's a lot more to it than that. You have to realize up front that an internal combustion engine (ICE) is nothing more than a big air pump. Air goes in, mixes with fuel, gets compressed by the piston, the spark plug lights the mixture forcing the piston down, which rotates the crank, which creates power. The burnt air/fuel (now exhaust) is routed out of the engine and through the exhaust. This is called the Otto Cycle and is the basis of all four stroke engines. When the foot steps on the pedal, one of three things can happen, depending on what type of engine management system is in place. If drive by cable (DBC) with fuel injection, the foot steps on the pedal and the pedal moves a cable, which directly moves the throttle plates inside the throttle body. A mechanism called a throttle position sensor (TPS), which then tells the computer how far the throttle plates are open, which it then commands the injectors to fire a predetermined amount of fuel into the engine, which mixes with the incoming air. Then the spark plug goes off at the right time and things start to happen. If drive by cable (DBC) with a carburetor, the cable opens the throttle plates, which allows more air to enter the engine. The incoming air which rushes through the venturis of the carburetor which allows air/fuel to mix, which enters into the engine. Then the spark plug goes off at the right time and things start to happen. If drive by wire (DBW), the foot steps on the pedal and the pedal reacts with a rheostat, which tells the computer the amount of movement of your foot. Then the computer commands the throttle body to open the amount required to match the command. A motor in the throttle body then rotates the throttle plates which allows the amount of air into the engine to do what is required. The computer also commands the injectors to fire a certain amount of fuel into the cylinders, which mixes with the air. Then the spark plug goes off at the right time and things start to happen. (NOTE: There is no carbureted version of DBW. This is fuel injected only.)

[-]

Miserable-Hamster-10@reddit

That's a solid explanation dude!!! Made everything crystal clear

[-]

CrazyParticular956@reddit

More air and fuel simply means harder explosion, which means piston gets pushed harder and if the load/resistance is kept same, then a harder push translates to a faster ( more accelerated ) push, and since while driving a car u mostly keep the gear ( resistance/load ) same, your rev increases, conversely if u downshift on same throttle, your power will translate to torque ( acceleration ) while your revs drop. The key I believe - is to develop intution on the load.