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General question about controlling car engine ??? Answered

Hi ...

I would like to ask, how do call the "thing" that control RPM of your engine ??? What I mean ... there is something - kind of small motor, that controls the RPM. When you start your engine and you don't have any gear it holds the RPM on cca 1000 RPM. But in winter (when the engine is cold) it brings the RPM to 1500 ... and then, when the engine becomes warmer, it decreases the RPM.

1. How do you call the part, that controls these RPM ???
2. Is there any restriction that says what is the maximum RPM that this "small motor" can provide... like it will not make more then 2000RPM ... or you could set it up up to 7000RPM as well ???

Thank you in advance...



Best Answer 8 years ago

To answer your question -- the engine outputs in mechanical force what it has input from energy (fuel), less friction/inefficiency. There is no motor attached to the engine that makes it turn -- the engine IS the motor. As the others reply, the engine computer, or electronic control unit (ecu) controls the throttle, sparks, injection, and air intake. The computer knows how fast the engine needs to go to idle, provide power, warm up, etc -- and it makes the conditions for the engine to do that speed. http://www.youtube.com/watch?v=41kEbvZjtso rich editor wont work for me right now, you'll have to copy paste the link. As you ask -- can you just 'change' the idle? Yes. 2 ways: 1, convince the computer that it should idle higher, or 2, convince the computer that you are pressing the gas when you are not. Ambulances (and other emergency vehicles/tow trucks) have this feature. It's a switch on the dashboard, or sometimes when the ebrake is set, the engine goes into 'high idle' to ensure the alternator can deliver enough power to the lights and other equipment/power take off/hydraulics, etc. More gas = more power to engine = higher rpm's at a given load.

Thanks for your reply Frollard. The video was quite interesting :) Now in your answer, you described how it all works. Probably I made a mistake again, by not asking clearly for what I wanted to know = as for your 2nd way of how to run the engine on higher RPM. So far as I know, when I press the pedal - the force/puling if transferred by a cable. And the cable is mounted to the throttle - right ??? But when you are not pressing the pedal, the engine is still running at some RPM. I know, it's the computer that decides what RPM it should be ... but what I wanted to know what is the name of the PART, that transfers the computer orders to throttle. What regulates the RPM physically ??? Is there a kind of servo/motor that is pulling the cable - as I would press the pedal ... or there is additional control, independent from the cable that can control the RPM ???

The part that connects the throttle pedal to the engine/computer is (in internet terms ironically) called the TPS switch -- the Throttle Position Switch. It encodes the position of the pedal to the computer so the computer has a heads up that you are pressing the pedal. Some engines have a parallel connection -- as in the throttle is physically connected to a sort of valve on the engine via cable, and the computer still reads this value, but has less effect on the fuel supply -- just the rest of the controls. Again, on fuel injected cars, the 'throttle' is simply how long the computer opens the fuel injector ports. There is always pressure on the supply fuel side, so you MUST interface with the computer to tell it to add more fuel. The only easy way to do it is tricking it into thinking the throttle pedal is pressed further. Each model will be different on exactly how that happens.

WHat you're talking about is a mechanical throttle. Most cars don't have one for safety reasons but stationary engines like on pumps, generators etc. have them

My car is Suzuki Swift 1998. Why is it called MECHANICAL ??? I thought, that it has something to do with electronics - because there are wires (I think 4) going in. I'll have a better look tomorrow.

Never mind, I thought you were talking about something else entirely.

.  On modern engines, throttle position determines how much fuel is introduced into the combustion chamber by the injectors via commands from the computer. The computer also controls spark timing.
.  In most ICEs, the maximum engine speed is determined by the valve train or injectors. The valves will begin to "float" at higher RPM or the injectors won't flow enough fuel. As a very rough rule of thumb, the larger the displacement of a "factory" engine, the lower the max RPM.

Would you classify engine from Suzuki Swift 1998 as modern engine ??? Do I understand it right - that it has kind of mechanical restriction, right ??? Although you would sent through computer signal for more RMP it wouldn't be possible because of the valve or injector restriction. Right ???

.  Yes, I would classify a 1998 as modern.
.  Oops. I forgot to mention that throttle position regulates air intake via the throttle plate, also. On most cars I've seen, it is a direct, mechanical connection between the throttle pedal and the throttle plate. Everything else goes through the computer (there is a Throttle Position Sensor (TPS) that tells the computer how hard you are stepping on the gas).