How do the accelerator pedal and rpm interact in an internal combustion engine?

[Sailor]

Hello everyone,

this is my first contribution in this forum,

I'm aware that this question had been asked many times either in this forum or elsewhere and I have indeed read at least one thread with a similar topic, however, unfortunately, I have yet to find specific answers that nail the core fundamentals in a solid means, that's why I preferred to post this thread separately rather than bumping up an older thread in hope it will be more focused on the subject this time.

How does the action of the accelerator pedal changes the rpm of the internal combustion engine ?

And which quantities are or should be* constant and which are variable through the whole rpm range (e.g from idle to red line) as the engine accelerates when pressing the pedal ?
1- Compression ratio
2- Air to fuel ratio
3- Ignition rate
4- Air flow rate through intake manifoldPlease note that :
- This subject is beyond the basic fundamentals that describe the primary functions and terminology of internal combustion engine and their theoretical cycles.
- *: In general I'm not referring to an absolute constant quantity, so if there would be any variation that is not controllable or not meant to regulate the engine speed (rpm) then this should not be considered in the explanation, such variations are the resultant of thermal deficiencies or mechanical friction.

 

[PhysicoRaj]

Hi and welcome to the forum. What do you think the answer is? Which of them is constant and which are variable according to you?

 

[PhysicoRaj]

If you have a good idea of what those four things are (and how they work), it is easy to figure this out.

 

[Baluncore]

Fascinating. I have read carefully through the post and have even examined the title.
I cannot find a question mark anywhere. If there is no question, then we have no destination.

So first I think we must determine …

We don't know where we are going, so why must we determine anything ?

 

[PhysicoRaj]

Fascinating. I have read carefully through the post and have even examined the title.
I cannot find a question mark anywhere. If there is no question, then we have no destination. We don't know where we are going, so why must we determine anything ?

 

[PhysicoRaj]

Atleast the OP used a 'wh' word..

So first I think we must determine which quantities are or should be constant...

OP can be forgiven for that

 

[Ranger Mike]

Welcome sailor,

A very thought out question!
Made me stop and think about it..

Compression ratio- of all 4 items listed, the compression ratio is going to be that which will change the least. More specially the CR is as the engine is built. There are small variances in dynamic compression ratio when things start spinning up to 7000 RPM. But Dynamic CR will always be lower than static CR and Dynamic CR will not change over the operation of the engine rpm.

every other item listed will vary. If you have fixed fuel air mixture at idle and no provision to make it richer as RPM increases , the mixture will lean out. Ignition rate may appear to be fixed but this too will vary as rpm increase and the initiation of the ignition must occur earlier as rpm increase so we have a mechanism to advance the spark. As a matter of physics as the rpm increases so does the air flow through the intake manifold.

 

[Sailor]

If you have a good idea of what those four things are (and how they work), it is easy to figure this out.

If you have a good idea of what those four things are (and how they work), it is easy to figure this out.

Hi PhysicoRaj and thanks for the welcoming.

I would say that both the compression ratio and the air to fuel ratio are (should be) constant, while the firing rate and the air flow rate are variable, therefor, theoretically it's these two that should control the engine speed.
And more specifically, since the the accelerator pedal is linked to the butterfly which in essence control the air flow through the manifold, it should be the primary influential factor in the process, because the other variable which is the firing rate is dependent on the resultant rpm through either the distributor in old vehicles or the ECU input and crankcase positioning sensor in modern ones.

That's what I think is the case, theoretically at least, so what are your thoughts about it.

 

[Sailor]

Fascinating. I have read carefully through the post and have even examined the title.
I cannot find a question mark anywhere. If there is no question, then we have no destination.


We don't know where we are going, so why must we determine anything ?

sure if you are not interested in the subject, then you mustn't do anything about it

at least you could make an objection and try to define the relation between the accelerator pedal and the change in rpm in a better way

duh..., again, in case you're interested in the first place

Atleast the OP used a 'wh' word..
OP can be forgiven for that

It isn't a new question that's why I supposed the relation in the title is self explanatory

and further more I hinted about the lack of solid answers to it

I have yet to find specific answers that nail the core fundamentals in a solid means.

 

[PhysicoRaj]

Hi PhysicoRaj and thanks for the welcoming.

I would say that both the compression ratio and the air to fuel ratio are (should be) constant, while the firing rate and the air flow rate are variable, therefor, theoretically it's these two that should control the engine speed.
And more specifically, since the the accelerator pedal is linked to the butterfly which in essence control the air flow through the manifold, it should be the primary influential factor in the process, because the other variable which is the firing rate is dependent on the resultant rpm through either the distributor in old vehicles or the ECU input and crankcase positioning sensor in modern ones.

That's what I think is the case, theoretically at least, so what are your thoughts about it.

You are correct about the compression ratio. But regarding air fuel ratio, you are wrong.
Ranger Mike has already provided an answer, a detailed explanation too. I hope you find that post informative enough for you. If you still find any difficulties, ask it over!

 

[Sailor]

Welcome sailor,

A very thought out question!
Made me stop and think about it..

Compression ratio- of all 4 items listed, the compression ratio is going to be that which will change the least. More specially the CR is as the engine is built. There are small variances in dynamic compression ratio when things start spinning up to 7000 RPM. But Dynamic CR will always be lower than static CR and Dynamic CR will not change over the operation of the engine rpm.

every other item listed will vary. If you have fixed fuel air mixture at idle and no provision to make it richer as RPM increases , the mixture will lean out. Ignition rate may appear to be fixed but this too will vary as rpm increase and the initiation of the ignition must occur earlier as rpm increase so we have a mechanism to advance the spark. As a matter of physics as the rpm increases so does the air flow through the intake manifold.

Hi Ranger Mike, thank you for welcoming and the elaborated input,

I agree that the CR should almost held constant theoretically or at least through some rpm range, but what if we already had rich air-fuel mixture at idle ? then we can assume it is almost constant ...

I have assumed the ignition rate to be variable, and it actually happens as a resultant from the incrementation of the rpm and not beforehand, this is especially true in the case of the distributor and the timing belt,

 

[Baluncore]

Ideally, they are all variable.
I have a governor that adjusts the fuel to maintain RPM at that directly indicated by the actual position of the accelerator pedal. My air/fuel ratio is highly variable, from lean as hell when idling to the edge of blowing black smoke when working hard. The compression ratio can also be arbitrarily increased through the use of water injection.

 

[Sailor]

You are correct about the compression ratio. But regarding air fuel ratio, you are wrong.
Ranger Mike has already provided an answer, a detailed explanation too. I hope you find that post informative enough for you. If you still find any difficulties, ask it over!

So my question here is what happens after the idle rpm, say from 2000 to 4000 rpm ?
shouldn't the air to fuel be at the range of 14.7 theoretically ? could there be such miniature incrementation in the air to fuel ratio that leads to the upshift in rpm ?

And what happens if we had a constant 14.7 ration from idle with a semi-closed butterfly ?

 

[PhysicoRaj]

Ideally, they are all variable. The compression ratio can also be arbitrarily increased through the use of water injection.

Agreed. But this isn't used in domestic automotive engines. I know about jet engines and gas turbines in powerplants using water injection, or even alcohol injection sometimes.

 

[Sailor]

Ideally, they are all variable.
I have a governor that adjusts the fuel to maintain RPM at that directly indicated by the actual position of the accelerator pedal. My air/fuel ratio is highly variable, from lean as hell when idling to the edge of blowing black smoke when working hard. The compression ratio can also be arbitrarily increased through the use of water injection.

Do they vary due to friction and out of control deficiencies or they are meant to be variable ?
and what would happens if we made the first two constant ?

 

[PhysicoRaj]

So my question here is what happens after the idle rpm, say from 2000 to 4000 rpm ?
shouldn't the air to fuel be at the range of 14.7 ? could there be such miniature incrementation that leads to the upshift in rpm ?

And what happens if we had a constant 14.7 ration from idle with a semi-closed butterfly ?

Isn't air-fuel ratio the major factor in increasing rev's? The ignition rate automatically increases if it's a magneto, or, if it's a digitized ignition, it is made to increase. It's a different case in diesel engines, where direct fuel injection and auto ignition takes place.

As for the air flow through the intake manifold: Higher rpm→increased strokes of piston per second→increased amount of air sucked in per second→→increased air flow through intake manifold.

 

[Baluncore]

Do they vary due to friction and out of control deficiencies or they are meant to be variable ?
and what would happens if we made the first two constant ?

They are deliberately designed to be continuously adjusted to improve fuel economy.
If you make the first two constant then you reduce the fuel economy by about 50%.

 

[PhysicoRaj]

Do they vary due to friction and out of control deficiencies or they are meant to be variable ?
and what would happens if we made the first two constant?

Friction b/n what?
Constant compression ratio will never affect change in rpm. Actually in all domestic and general use automotive engines, CR is constant. An engine with variable CR will be (with the present technology) bulky, complicated and expensive.
With a constant fuel-air ratio.. I don't think you can even make the slightest deflection in your tachometer needle! (with compression ratio constant).

 

[Baluncore]

An engine with variable CR will be (with the present technology) bulky, complicated and expensive.

With the dominant present technology, yes. But with much older, now largely forgotten technologies, no.

 

[PhysicoRaj]

With the dominant present technology, yes. But with much older, now largely forgotten technologies, no.

Sorry, but I didn't get what you were referring to..? Which tech. ?

 

[Sailor]

Isn't air-fuel ratio the major factor in increasing rev's? The ignition rate automatically increases if it's a magneto, or, if it's a digitized ignition, it is made to increase. It's a different case in diesel engines, where direct fuel injection and auto ignition takes place.

As for the air flow through the intake manifold: Higher rpm→increased strokes of piston per second→increased amount of air sucked in per second→→increased air flow through intake manifold.

That's what I've read in other forums but, I don't think it makes any sens at all, because there is an optimum air to fuel ratio that should be reached all the time, I remember I've once read that at idle it's something like 12:1 but that could be for an economic reason rather than to lower rpm

the increase in ignition rate happens as a resultant of the increased speed, each time the piston approaches TDC the ECU in modern cars will send a pulse to the spark plugs to ignite and any alteration in the timing of the spark ignition would serve other tasks than controlling the absolute rpm of the engine, some of these tasks are to control detonation

The air flow will always be restricted partially by the butterfly, your assumption indicates an unrestricted manifold (e.g wide open throttle)

 

[Sailor]

They are deliberately designed to be continuously adjusted to improve fuel economy.
If you make the first two constant then you reduce the fuel economy by about 50%.

that's true, but fuel economy has nothing to do with the incrementation in rpm

 

[Sailor]

Friction b/n what?
Constant compression ratio will never affect change in rpm. Actually in all domestic and general use automotive engines, CR is constant. An engine with variable CR will be (with the present technology) bulky, complicated and expensive.
With a constant fuel-air ratio.. I don't think you can even make the slightest deflection in your tachometer needle! (with compression ratio constant).

The friction could be either between solid parts or simply of the boundary between the sucked air and the manifold housing/butterfly, all that can affects the amount of air the inters the cylinder

If the rate of filling the cylinder is variable, then injecting the consequent amount of fuel in it should have no influence at the rpm. It is a fact that all engine makers tries to hold the optimum stoichiometric ratio of 14.7 through all the rpm range

 

[Baluncore]

that's true, but fuel economy has nothing to do with the incrementation in rpm

What is “the incrementation in rpm”?
What has it got to do with CR or air/fuel ratio, the first two in your list?

 

[PhysicoRaj]

That's what I've read in other forums but, I don't think it makes any sens at all, because there is an optimum air to fuel ratio that should be reached all the time, I remember I've once read that at idle it's something like 12:1 but that could be for an economic reason rather than to lower rpm

the increase in ignition rate happens as a resultant of the increased speed, each time the piston approaches TDC the ECU in modern cars will send a pulse to the spark plugs to ignite and any alteration in the timing of the spark ignition would serve other tasks than controlling the absolute rpm of the engine, some of these tasks are to control detonation

The air flow will always be restricted partially by the butterfly, your assumption indicates an unrestricted manifold (e.g wide open throttle)

That's what.. all of the four can be used to change rpm. If you talk about a butterfly throttle, then you will have observe some modification in the air-fuel ratio. Coordination of these two is most important, especially at huge rpm's.

Your question in the first post seemed that you were asking "which of these four remains constant/variable on rpm increment", rather than "which of these must remain constant/variable in order to increase rpm."
For the first question, another question as answer: by what means is the rpm increased?(Any of the four? other?)
For second question: All of them can change the rpm, provided you design the engine that way.

 

[PhysicoRaj]

If the rate of filling the cylinder is variable, then injecting the consequent amount of fuel in it should have no influence at the rpm. It is a fact that all engine makers tries to hold the optimum stoichiometric ratio of 14.7 through all the rpm range

Forgot about your butterfly already?

 

[Sailor]

What is “the incrementation in rpm”?
What has it got to do with CR or air/fuel ratio, the first two in your list?

I meant the increase in rpm

please note what I wrote in the beginning :

- *: In general I'm not referring to an absolute constant quantity, so if there would be any variation that is not controllable or not meant to regulate the engine speed (rpm) then this should not be considered in the explanation, such variations are the resultant of thermal deficiencies or mechanical friction.

 

[Baluncore]

Sorry, but I didn't get what you were referring to..? Which tech. ?

With a cup shaped piston, water injection parially fills the cup in the piston. If the compression is too high, some water boils off. So injecting water keeps the compression optimum and cools the piston.
http://en.wikipedia.org/wiki/Water_injection_(engine)

 

[PhysicoRaj]

With a cup shaped piston, water injection parially fills the cup in the piston. If the compression is too high, some water boils off. So injecting water keeps the compression optimum and cools the piston.
http://en.wikipedia.org/wiki/Water_injection_(engine)

Hmm.. old is gold after all. But I think this can pose some problem in small size, lower CR engines..(?)

 

[Ranger Mike]

congratulations fellows..you have just entered the world of hot rodding. Since the first IC came off Henry Ford’s production line the first guy to tinker with it wanted to go faster than the other guy. You have asked the one big question- How to make the IC go faster.

As one old hot rodder told me when I was learning to trouble shoot the IC engine.
you need fuel, you need air and you need a spark...AND... the spark at the proper time.

Nothing happens until we enter a fuel/air mix into the engine. It must be ignited at the proper time to make power.
The stoichiometric air-fuel ratio (14.7:1) that is the ideal ratio for lowest emissions, but this isn't the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1. This is roughly 13 parts of air to one part fuel. It's what combustion engineers call an excess fuel ratio and is intended to ensure that all the air is used to support the combustion process. This is because air is the oxidizer in combustion. Too many racers think that adding additional fuel beyond the ideal to create a richer mixture will make more power. This doesn't work because you can only burn the fuel when you have enough air to support combustion. That's why engines make more power when you add a supercharger or nitrous--you're shoving more air in the cylinder so that you can burn more fuel. Regardless of the amount of air in the cylinder, it still requires a given ratio of fuel to burn. Add too much extra fuel, and power will decrease.

The intake manifold is a big pipe and you can only fit so much fuel/air thru it..period. Typical normally aspirated internal combustion engine. So we start using superchargers to cram in more mix.
regarding the ignition. Todays advanced CD ignitions actually provide multiple sparks at idle and low RPM and provide a long spark at high RPM ( long being in crankshaft degrees). It does this to make more efficient burning of the fuel air mix at idle and higher RPM. We went from 25000 volts at the spark plug and a small gap of .025” to 50,000 plus volts over a 0.060 to .080 gap to again, more completely burn the mix.

one huge aspect you have not addressed is that you need to remove the spent fuel / air mix form the combustion chamber efficiently. This directly impacts on the intake manifold side of the equation.

bottom line is compression ratio is a constant but all other factors will vary all over the place depending upon the RPM.

 

[Sailor]

That's what.. all of the four can be used to change rpm. If you talk about a butterfly throttle, then you will have observe some modification in the air-fuel ratio. Coordination of these two is most important, especially at huge rpm's.

Your question in the first post seemed that you were asking "which of these four remains constant/variable on rpm increment", rather than "which of these must remain constant/variable in order to increase rpm."
For the first question, another question as answer: by what means is the rpm increased?(Any of the four? other?)
For second question: All of them can change the rpm, provided you design the engine that way.

That is not true, you do need a constant 14.7:1 fuel to air ratio for the optimum performance of your engine, to make it simpler, forget about idle for a moment, at 3000 rpm you should and in reality would get that ratio and at 6000 rpm again this is exactly what you will get

what happens when you floor the throttle especially in old cars is you deliberately inject more fuel THAT WILL NEVER BURN and therefor will not contribute in the whole process and again this is the kind of deficiencies I'm not talking about

My question again is :

which quantities are or should be* constant and which are variable

Besides this note :

- *: In general I'm not referring to an absolute constant quantity, so if there would be any variation that is not controllable or not meant to regulate the engine speed (rpm) then this should not be considered in the explanation, such variations are the resultant of thermal deficiencies or mechanical friction.

 

[PhysicoRaj]

That is not true, you do need a constant 14.7:1 fuel to air ratio for the optimum performance of your engine, to make it simpler, forget about idle for a moment, at 3000 rpm you should and in reality would get that ratio and at 6000 rpm again this is exactly what you will get

what happens when you floor the throttle especially in old cars is you deliberately inject more fuel THAT WILL NEVER BURN and therefor will not contribute in the whole process and again this is the kind of deficiencies I'm not talking about

As far as I know, that optimum ratio is used at or near top rpm. At the idle, I don't think all engines make use of the optimum ratio.. it is lower than that, i.e., lesser fuel than in the optimum ratio. In other words, a leaner mixture. A richer mixture than the optimum does not provide power, I agree. But a leaner mixture than the optimum obviously produces less power>>lower rpm!

 

[Sailor]

Forgot about your butterfly already?

I didn't meant Skepticism by saying if

it is true that the rate of filling the cylinders does vary, but the fuel to air ratio is not supposed to, even if it does for some extent, I don't think it's intended as a means to vary the rpm, but rather to enhance economy as is the case with new Toyota and Honda engines or simply due to some out of control defeciency

 

[Baluncore]

So first I think we must determine which quantities are or should be* constant and which are variable through the whole rpm range (e.g from idle to red line) :
1- Compression ratio
2- Air to fuel ratio
3- Ignition rate
4- Air flow rate through intake manifold

Compression ratio decides pre-ignition and is RPM dependent. Power at low RPM requires lower CR to prevent pre-ignition. At high RPMs, a higher CR improves fuel economy, since there is insufficient time to pre-ignite. CR should not be held constant.
With direct fuel injection, the AFR needs to be varied over the whole range. It is only when high octane fuel and concern over NOx emission is concerned that AFR must be carefully adjusted to remain stoichiometric.

… then this should not be considered in the explanation, such variations are the resultant of thermal deficiencies or mechanical friction.

That gross generalisation is a false assumption.

 

[PhysicoRaj]

it is true that the rate of filling the cylinders does vary, but the fuel to air ratio is not supposed to, even if it does for some extent, I don't think it's intended as a means to vary the rpm, but rather to enhance economy as is the case with new Toyota and Honda engines or simply due to some out of control defeciency

The answer:

With direct fuel injection, the AFR needs to be varied over the whole range. It is only when high octane fuel and concern over NOx emission is concerned that AFR must be carefully adjusted to remain stoichiometric.