Why rpm doesnt stay up in neutral/clutch engaged

TheCleaner

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ok i know the overall basics of how an engine works and some things in more detail, but theres a few things ive played with in my mind, Ill keep it as short and sweet as possible (although i will end up ranting, haha)

- How does the engine increase rpm in the first place? sound mad? yes i know more fuel in the mix, and im guessing the ignition tinming will change, but im wondering how this works in sequence
as surely just making the cylinder that is first to recieve the higher fuel mix spark sooner will cause ignition during the compression stroke and send the piston back down before the camshaft turns over, which would cause the engine to try and go back on itself (yikes)

- When in neutral, or even less resistant, when the clutch is engaged, and the engine is just spinning its fly wheel, why is it that the engine will not remain at a relatively high rpm after really revving it and then coming off teh pedal.. im guessing again its to do with ignition timing, as fuel mix amount surely doesnt play much of a factor when its just the flywheel spinning.
I would expect that if the clutch is engaged and ignition timing were kept at the high rpm constant it would still spin at the high rpm even if getting a low fuel mix as the 'work done' is just the engine/flywheel, alot less that turning wheels..

which hopefully brings me to my last point, if that above statement is true, why not (maybe they do already) make it possible so theers no need for double clutching, so the car car detect that when the clutch is engaged, and the throttle not, keep rpm at a max (or the rpm the driver last had it when the clutch was first engaged.. thye rpm they would set in double clutching)

maybe im just underestimating the force required to keep just an engine spinning compared to when the engine is engaged with the wheels, especially where turbos and superchargers are concerned

Thanks
 

fbc

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Re: Why rpm doesnt stay up in neutral/clutch engaged

TheCleaner said:
- How does the engine increase rpm in the first place? sound mad? yes i know more fuel in the mix, and im guessing the ignition tinming will change, but im wondering how this works in sequence
as surely just making the cylinder that is first to recieve the higher fuel mix spark sooner will cause ignition during the compression stroke and send the piston back down before the camshaft turns over, which would cause the engine to try and go back on itself (yikes)

- When in neutral, or even less resistant, when the clutch is engaged, and the engine is just spinning its fly wheel, why is it that the engine will not remain at a relatively high rpm after really revving it and then coming off teh pedal.. im guessing again its to do with ignition timing, as fuel mix amount surely doesnt play much of a factor when its just the flywheel spinning.
I would expect that if the clutch is engaged and ignition timing were kept at the high rpm constant it would still spin at the high rpm even if getting a low fuel mix as the 'work done' is just the engine/flywheel, alot less that turning wheels..

The key here is the one thing you've not mentioned - air. The bigger the throttle opening, the more air flows into the engine (and thus more fuel), that's what determines what rpm the motor runs at (essentially).

As for how fast the engine loses revs in neutral - that depends on the mass being turned (clutch, flywheel, piston, conrods, etc) - the heavier these items are the slower the engine will lose revs (it will also not rev-up as fast), and likewise a motor with a small, light flywheel, and light pistons and conrods will lose revs very quickly - like a racing motor for instance.

The ignition timing will remain the same regardless. Of course modern engines have ECU's that alter the timing based on engine load, revs, fuel etc, but that's about making the engine run as efficiently as possible.
 

TheCleaner

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still what i dont understand,a nd maybe it will take some hardcore physics to explain, is that why stuffing more air/fuel into a piston will make it spin faster rather than just the same speed with more torque (actually, thinking about it, that does happen).. I realise energy needs to be transfered somehow, and it does, which is why the engine spins faster and its louder and more hot.. but what im wondering is how the engine instigates this?

Does the carborator just shot more fuel/air in the piston, and it just starts going faster... if so then ok, that will just have to do

But surely ignition time doesnt stay the same? an engine spinning at 2000rpm would only fire half as much as the engine going twice the speed at 4000rpm? and thats just in a regular car
 

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TheCleaner said:
But surely ignition time doesnt stay the same? an engine spinning at 2000rpm would only fire half as much as the engine going twice the speed at 4000rpm? and thats just in a regular car

Ah, see I thought you meant Ignition Timing in the normal sense - ie. the timing of the spark plug firing relative to the position of the piston.

So yes, an engine running at 4000rpm will have twice as many ignition firings as one running at 2000rpm, but the timing of those firings wouldn't necessarily be any different.

As for the rest - thinking very very slow-motion here, say the engine's running at 2000rpm and you open the throttle, more air is sucked in through the now larger throttle opening, and so more fuel is added to that air, on the next revolution the engine effectively takes a bigger gulp of air & fuel so you get a larger bang and more force pushing the piston down and increasing the piston speed - and the engine speed follows. That process continues until the motor has reached a speed that is in balance with the amount of air it can suck in. However I'm not really a techie / physics junkie, so maybe someone else can explain it better / correct me.
 

Viper007Bond

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Yep, as said, RPMs are controlled by the air and fuel, not by the spark plugs firing. The spark plugs just fire when they should no matter the RPM.
 

Blythy

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still what i dont understand,a nd maybe it will take some hardcore physics to explain, is that why stuffing more air/fuel into a piston will make it spin faster rather than just the same speed with more torque (actually, thinking about it, that does happen).. I realise energy needs to be transfered somehow, and it does, which is why the engine spins faster and its louder and more hot.. but what im wondering is how the engine instigates this?

Ok, you're stook behind some bastard who's going slow, you go into the next lane and you floor it.

when you floor it, you increase the size of the valve opening that lets air into the engine

now because you've got to have a preset mix of fuel and air (y'know like on art attack, 2 parts water, 1 part pva) so more fuel is shoved in

because more fuel is going into the cylinder, you're gonna get a bigger bang

and cos you've got a bigger bang, and mr newton's little holy grail of an equation F=ma your force is increasing, the mass of the cylinder head is staying the same, so the acceleration of the cylinder head has to increase to keep newton happy


as for iginiton timing and what not

fuel/air and exhaust gases are predetermined by the valves of the cylinder, open a valve, fuel air goes in (the movement of the cylinder head controls this movement of gas - basically high to low pressure movement) open a different valve later in the cycle, exhaust gas pushed out,


(It might be best to explain the four stroke cycle here, just in case.

first stroke is induction, the cylinder head is moving down, the fuel valve(s) are open, and fuel/air is being drawn in.
next is compression, the fuel valves are closed, the fuel air mixture is being compressed
at the top of this cycle is the ignition
then the third stroke occurs - torque, all valves closed, the force of the explosion pushes down the cylkinder
finally exhaustion - exhaust valves are open, gases are pushed out by the cylinder)

in a "normal" engine the valves are controled by cams (little tear drop shaped things) and they are geard to the speed of the engine, so no matter how fast the engine moves, the cams will be moving at the speed required for that speed of the engine

variable valve timing comes from some thing wierd where the camshaft moves depending on what speed the engine is turning at, and slightly different cam positions are there so that the fuel valve might be open a slight fraction of a second later than before or so - all in the interests of extra torque.



anyways in answer to your question, it's F=ma , increase the force, keep the mass of the object the same, and the acceleration will increase too.
 

TheCleaner

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f=ma.. why didnt that occur to me!?!?! :bangin:

i should be ashamed to say i did a-level physics haha

thanks guy, that pretty much wraps it up
 

sandor_

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one other point - you ask why the RPMs dont stay high when you are in neutral or disengage the clutch - two words - kinetic friction.

you have hundreds of metal bits rubbing against each other inside an engine, and their natural tendency is to decrease movement to zero, unless a force greater than the force of friction is applied.
 

Blythy

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and the main source of it - a vacuum that develops somewhere and that acts against the engine at all times - its where most power is wasted and what causes the engine braking effect

which is the same force that slows you down most when you take your foot off the accelerator at high engine speeds and when you engage the clutch

(its why in performance driving you change gear so that you use the maximum force of the engine to help slow the car, and why you keep the engine in a low gear going down a hill as well as up it)
 

bone

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sandor_ said:
one other point - you ask why the RPMs dont stay high when you are in neutral or disengage the clutch - two words - kinetic friction.

euhm, true, but kinetic friction has nothing to do with

When in neutral, or even less resistant, when the clutch is engaged, and the engine is just spinning its fly wheel, why is it that the engine will not remain at a relatively high rpm after really revving it and then coming off teh pedal..

you said it yourself, coming off the pedal
that will close the throttle body, making it impossible for air to enter the engine, so if the engine would want to stay at the same revs, he would have to pull a vaccuum, instead of taking in mixture during intake stroke, which he can't, resulting in rev dropping

an engine will always try to make as much RPM as possible, if you woud install no throttle body, headers, .... (but still would be able to inject some fuel) the engine would rise in revs instantly and explode. so what you actually try to do, is keep the engine as low in revs as possible

the best thing you can do to understand sth like this, is look into carbs, when you understand what a carb does, and how it works, your questions will be answered[/img]
 

sandor_

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bone said:
sandor_ said:
one other point - you ask why the RPMs dont stay high when you are in neutral or disengage the clutch - two words - kinetic friction.

euhm, true, but kinetic friction has nothing to do with

When in neutral, or even less resistant, when the clutch is engaged, and the engine is just spinning its fly wheel, why is it that the engine will not remain at a relatively high rpm after really revving it and then coming off teh pedal..

you said it yourself, coming off the pedal
that will close the throttle body, making it impossible for air to enter the engine, so if the engine would want to stay at the same revs, he would have to pull a vaccuum, instead of taking in mixture during intake stroke, which he can't, resulting in rev dropping

an engine will always try to make as much RPM as possible, if you woud install no throttle body, headers, .... (but still would be able to inject some fuel) the engine would rise in revs instantly and explode. so what you actually try to do, is keep the engine as low in revs as possible

the best thing you can do to understand sth like this, is look into carbs, when you understand what a carb does, and how it works, your questions will be answered[/img]

true true. it was the "when in neutral (or) when the clutch is engaged" that threw me off....


i hope the original poster understands now -

increased air/fuel intake into the cylinder causes increased RPMs,
throttle controls air/fuel mixture flow, therefore with a closed throttle the engine falls back to a preset air/fuel flow (idle)


i have to say rebuilding a carburetor really helps this all fall in place...
 

Peter3hg

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I understand all of this but just as a quick question. Will a rotary engine take longer to come down of the revs(comared to an engine with the same weight flywheel etc.) because the movement is a continous one not a constantly reversing movement.
 

TheCleaner

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very interesting reading guys and if id bothered to glance at my old physics notes (its been a good while now) alot of this would fall obvious :oops:

thanks for clearin alot up.. ah good ol wiki
 

bone

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peter3hg2 said:
I understand all of this but just as a quick question. Will a rotary engine take longer to come down of the revs(comared to an engine with the same weight flywheel etc.) because the movement is a continous one not a constantly reversing movement.

don't think so

it's not the inversing movement which slows you down (well it does a little bit) the vaccuum that is created does, and a rotary creates as much vaccuum as a piston engine does

(not entirely sure though)

and you had another interesting question i missed
still what i dont understand,a nd maybe it will take some hardcore physics to explain, is that why stuffing more air/fuel into a piston will make it spin faster rather than just the same speed with more torque

you have to be carefull what you write in this case, stuffing more air in the cylinder WILL create more torque, that's what happens with a turbocharged engine
it's the ability of the air to flow through the engine that will increase your revs, that's why headers and exhaust collectors are so important for performance
the max amount of revs an engine can make will be determined by the amounth of air (cubic, unregarded of density) you can get through the system

the same headers and collectors put on a 2.0l will make it rev twice as high as you would put them on a 4.0l (since it's twice the size, it'll only need half the amount of cycles to pass all the air through)
 
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