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Old 1st March 2011, 14:22   #31
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Default re: RPM shoots up when changing gears

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Originally Posted by oxyzen View Post
Why this can not be the same rate at which fueling is increased? Manual throttles do it. Dont they? What is the advantage of increasing fuel gradually?


How can the engine stall when you are around 2000RPM? And if the engine reaches idle RPM (In case you take years to shift) optimum A/F ratio for idle will be maintained. Thus no stalling. (Manual Throttle body)
A1. I ll rephrase. The fuelling can increase with the same rate. It is just not let to increase with the same rate. This is because of at least the following reasons and there could be more - a. Air fuel ratio must be kept constant and within a narrow band b. the torque produced by the engine will increase causing a jerk. this is also called a surge. so in the ECU, there will be a correction map for reducing the surge. so the rate of change of velocity (acceleration) and rate of change of acceleration (jerk) are controlled. bosch call it 'surge damper'
in a carbed engine, you open the throttle to 100% but the fuel which goes in is governed by the mass of air flowing into the engine and the venturi depression created causing the fuel to be sucked from the jet(s) into the air stream. thus, here also the fuel does not increase with the same rate as that of the throttle.
so i would say, the fuelling can be increased only as fast as the rate of air mass going into the engine changes. and its usually slower to make the ride smooth. you can have aggressive fuelling for racing or different fuelling maps for different gears. there exist several possibilities really.

A2. The engine will stall if the fuelling is reduced at the same rate as the throttle closing and the clutch is depressed at the same time the foot slides off the throttle. This is because of the friction and accessories load still exist on the otherwise free revving engine. If it does not stall, it will be very jerky. If you leave the clutch depressed for a long time, the low idle governing function in the ECU will take over but not while changing gears during normal driving.
In a non ecu engine, the fuelling again is function of the mass flow and the spark is available always above a certain engine rpm (something like 200 odd..) so it would not stall.
basically, its an ECU and doesnt really have the fuzzy logic which we as humans have. it has to be told what to do for every condition.

yet, i am not sure if i explain the scond point to an acceptable satisfaction. let me know if i havent.

nitro
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Old 2nd March 2011, 20:37   #32
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Default re: RPM shoots up when changing gears

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Originally Posted by nitrogary View Post
A1. I ll rephrase. The fuelling can increase with the same rate. It is just not let to increase with the same rate. This is because of at least the following reasons and there could be more - a. Air fuel ratio must be kept constant and within a narrow band b. the torque produced by the engine will increase causing a jerk. this is also called a surge. so in the ECU, there will be a correction map for reducing the surge. so the rate of change of velocity (acceleration) and rate of change of acceleration (jerk) are controlled. bosch call it 'surge damper'
in a carbed engine, you open the throttle to 100% but the fuel which goes in is governed by the mass of air flowing into the engine and the venturi depression created causing the fuel to be sucked from the jet(s) into the air stream. thus, here also the fuel does not increase with the same rate as that of the throttle.
so i would say, the fuelling can be increased only as fast as the rate of air mass going into the engine changes. and its usually slower to make the ride smooth. you can have aggressive fuelling for racing or different fuelling maps for different gears. there exist several possibilities really.

A2. The engine will stall if the fuelling is reduced at the same rate as the throttle closing and the clutch is depressed at the same time the foot slides off the throttle. This is because of the friction and accessories load still exist on the otherwise free revving engine. If it does not stall, it will be very jerky. If you leave the clutch depressed for a long time, the low idle governing function in the ECU will take over but not while changing gears during normal driving.
In a non ecu engine, the fuelling again is function of the mass flow and the spark is available always above a certain engine rpm (something like 200 odd..) so it would not stall.
basically, its an ECU and doesnt really have the fuzzy logic which we as humans have. it has to be told what to do for every condition.

yet, i am not sure if i explain the scond point to an acceptable satisfaction. let me know if i havent.

nitro
I understand increasing fuel gradually is to blunt the response. Make it feel more linear. I would personally not prefer that.
Regarding second point, releasing the throttle with clutch depressed you mean to say it will provide rich mixture. (Air reduced but not fuel). Are you sure, I think it will affect engine braking as well as FE.
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Old 3rd March 2011, 20:36   #33
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Default re: RPM shoots up when changing gears

Hi,
@ Nitrogary, oxyzen, and others. Interesting discussion.

All discussions with respect to typical 4 stroke gasoline engines, conservatively tuned and used in a mass market family car.

Supposing we have an engine in which the throttle plate is mechanically connected to the acc pedal. ie. its position depends on the whim of the driver. It can be opened or closed gently, feathered, banged open, snapped shut etc.

In each of these cases what should be the ideal fuel delivery, and why? Then we can work out which systems allow us to meet these ideals, or find out what the deviations are. And then we can discuss the consequences of the deviations.

Regards
Sutripta
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Old 8th March 2011, 14:30   #34
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Default re: RPM shoots up when changing gears

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Originally Posted by Sutripta View Post
Hi,
@ Nitrogary, oxyzen, and others. Interesting discussion.

All discussions with respect to typical 4 stroke gasoline engines, conservatively tuned and used in a mass market family car.

Supposing we have an engine in which the throttle plate is mechanically connected to the acc pedal. ie. its position depends on the whim of the driver. It can be opened or closed gently, feathered, banged open, snapped shut etc.

In each of these cases what should be the ideal fuel delivery, and why? Then we can work out which systems allow us to meet these ideals, or find out what the deviations are. And then we can discuss the consequences of the deviations.

Regards
Sutripta
The case you mention would certainly be true for a carbed engine (???) in this case the fuelling is from the jet(s) and a function of the air stream.

And there do exist cases where the throttle is mechanical and electronic fuel injection (throttle body-whether it is single or multiple, it may not matter), the fuelling will be strictly a function of the amount of air flow. So its not very different from the carb. never the less, the ecu will get an input from the MAP (manifold absolute pressure) sensor and this would be its reference on how much fuel to inject. examples: early Bharat Stage 1 / BS2 compliant vehicles (maruti, hyundai, etc.)

you will not see an engine behavior which is being discussed (rpm shooting up while changing) in such engines. they have their own charm in driving as in this case, you can do a heel toe shifting and there would be a real difference in how the car moves.

i do not (cannot ??) use all three pedals at the same time when i drive a
new(er) generation common rail engine or a new advanced electronic control petrol engine to keep the engine 'ready' for a downshift.

a lot of the fuelling and fuel maps are driven be emissions control strategies these days. hence there may not be 'one ideal' fuel amount to be delivered. there are multiple corrections online which can change the fuelling.

coming to the second point about types of throttle closures. the fuelling quantities will certainly be different in each of the cases you mentioned and i am sure there are more possible scenarios here. but in the ECU, these cases are not segregated. but if the throttle is snapped shut / open, the air fuel ratio changes and is not stoichiometric until balance is restored. it depends on a few factors which include clutch status, gear status, vehicle speed, brake status, etc.

if the clutch is engaged, and you snap onen or shut the throttle, the inertia of the moving vehicle may prevent sudden changes in the engine speed. the torque will reduce / increase depending on the rate of change of fuelling which will be governed by the ECU and the surge damper along with several control loops. but yes, there will be a surge causing vehicle acceleration / deceleration. the extreme case is observed in racing cars where the fuelling is aggressive. so if you snap the throttle which often happens before taking a corner while braking, you can observe that the engine misfires because of excess fuel (rich mixture) and the fire may travel out of the exhaust pipe also which gives a marvellous glow. lovely to watch notice it in the motogp season opener at qatar which is a night race.

gradually modulating the throttle will smoothen the power delivery.

honestly, i have no work experience in gasoline engine calibration. i ve done only diesel. so my answer is an extrapolations of my theories and understanding. would love to hear from someone else as well.

cheers!

nitro
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Old 8th March 2011, 21:52   #35
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Default re: RPM shoots up when changing gears

^^^
Hi,
I think you have misunderstood my post. It has nothing to do with ECUs or carbs. It has to do with understanding the basics. Let me repeat:- If the airflow is controlled by the whims of man (and machine), and the fuel flow by God, what would God do, and why.

It is generally better to start from the simplest.

Employed at MM, I presume?

Regards
Sutripta
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Old 9th March 2011, 00:15   #36
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Default re: RPM shoots up when changing gears

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^^^
Employed at MM, I presume?
Sounds like Spike, huh? .. LOL!!.

Yes, its better to start from scratch.

Hopefully we have a very informative discussion here.
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Old 9th March 2011, 15:05   #37
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Default re: RPM shoots up when changing gears

Quote:
Originally Posted by Sutripta View Post
^^^
Hi,
I think you have misunderstood my post. It has nothing to do with ECUs or carbs. It has to do with understanding the basics. Let me repeat:- If the airflow is controlled by the whims of man (and machine), and the fuel flow by God, what would God do, and why.

It is generally better to start from the simplest.

Employed at MM, I presume?

Regards
Sutripta
to put it very simply, god would have to comply.

you cant really afford to deviate too much on the air-fuel ratio. but having said this, the throttle can be controlled by our whims. even so, the rate at which the throttle opens, may not be the same rate at which air going into the engine changes. that would depend on the engine breathing.

now to sum it up, i d say the amount of air going in will decide the amount of fuel going in as well. hows that?

ps: yes, i was with m&m for 2.5 years in engine development. now i am in pune and work on diesel engine turbochargers. this also reminds me to change my location

thanks & regards
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Old 9th March 2011, 15:08   #38
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Default re: RPM shoots up when changing gears

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Sounds like Spike, huh? .. LOL!!.

Yes, its better to start from scratch.

Hopefully we have a very informative discussion here.


spike and i are friends and we share a few common interests including the rx. but i did not get to work with him on the same project.. that would have been fun!
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Old 9th March 2011, 15:10   #39
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Default re: RPM shoots up when changing gears

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now to sum it up, i d say the amount of air going in will decide the amount of fuel going in as well. hows that?

thanks & regards
Hi,
Almost tautological!

What about on the overrun?

Regards
Sutripta
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Old 9th March 2011, 19:18   #40
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Default re: RPM shoots up when changing gears

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Hi,
Almost tautological!

What about on the overrun?

Regards
Sutripta
when you say overrun, i conclude that the vehicle is driving the engine and essentially it is on the fall side of the power curve. correct me if thats not what you mean...
in this region, the volumetric efficiency of the engine is very low. the speed at which the valves open and close is high. this doesnt permit much flow of air even if the throttle is open. so not much fuel goes in either.
there are provisions to cut the spark if the engine goes beyond its rev limit.

i have a wierd feeling i may be missing a point here. sutripta, can you clarify?
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Old 9th March 2011, 20:08   #41
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Default re: RPM shoots up when changing gears

^^^
Hi,
Throttle closed. Vehicle driving engine. RPM >> idling. Don't worry about cats and emissions.

Regards
Sutripta
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Old 9th March 2011, 20:51   #42
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Default re: RPM shoots up when changing gears

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Originally Posted by Sutripta View Post
^^^
Hi,
Throttle closed. Vehicle driving engine. RPM >> idling. Don't worry about cats and emissions.

Regards
Sutripta
not really sure how the vehicle will drive the engine in idle..

but here goes

for electronic engines, low idle governing is a function which takes care of this. the air and fuelling will change according to the load (of the accessories) and the effort necessary to keep the engine rotating (friction) to keep the engine alive until a limit is reached.

for carbed engines, they will stall if you dont open the throttle...

here, the mixture rich. somewhere around 11.5:1 to 13:1.

Last edited by nitrogary : 9th March 2011 at 20:53. Reason: addition of last line.
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Old 9th March 2011, 21:27   #43
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Default re: RPM shoots up when changing gears

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not really sure how the vehicle will drive the engine in idle..
Hi,
You are cruising. Then you take your foot off the throttle. Now you are coasting down in gear. Throttle is closed. Rpm is way above idle. What fuel should be supplied? What happened in the case of carbs?

Regards
Sutripta

PS. Your bike is a RD or RX? Prateesh's is a RX, if I'm not mistaken. If a RD, you should be a hands on expert on points, plugs, and timing.
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Old 9th March 2011, 21:58   #44
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Default re: RPM shoots up when changing gears

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Originally Posted by Sutripta View Post
Hi,
You are cruising. Then you take your foot off the throttle. Now you are coasting down in gear. Throttle is closed. Rpm is way above idle. What fuel should be supplied? What happened in the case of carbs?

Regards
Sutripta

PS. Your bike is a RD or RX? Prateesh's is a RX, if I'm not mistaken. If a RD, you should be a hands on expert on points, plugs, and timing.
first things first. i have both the bikes an 84 HT and a 5 speeder. am not a huge fan of the rx 100 (ready to get bashed here for this) but i love revvy two strokes.
points and plugs are my weakest link on the bike.

interesting case you mention here. when the carb is closed during coasting, main and/or pilot jet would be closed. so logically only the idle jet is active.
the higher suction pressure from the 'higher' revving engine will draw more fuel from the idle jet until its flow saturates. so the mixture will be rich.
thats the best answer i can come up with

ps: for electronic engines, the fuelling will stop. so the engine just pumps air and thats what gives you the engine braking.
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Old 9th March 2011, 23:01   #45
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Default re: RPM shoots up when changing gears

^^^
Hi,
Actually wanted to bring home the point that in the real world, things are far too complex to have simple answers.

In this day and age, who bothers about carbs. Sadly, a lost art.

In a normal carb, on the overrun, normally the mixture leans out. And so one last question, and we'll close. On the overrun, what will one observe if one supplies rich/ just right/ too lean a mixture?

Regards
Sutripta
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