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What accelerates an engine?

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Old 01-25-2006, 10:29 PM   #1
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Default What accelerates an engine?

I mean, when you push the gas pedal further down the TB plate is the first thing to sense that something has changed, right? What is the next piece/part of the whole system to do something?

Is it the MAF telling the fuel system to spray more because the plate has just moved open more, or does the MAF itself tell the PCM to spray more fuel because it senses increased airflow?

And how does the engine know to speed up at the second you hit the pedal, why doesn't it just bog down from the additional fuel being sprayed into the cylinders, because it hasen't started spinning faster yet.

Maybe a newb question, but I've always wondered this.

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Old 01-25-2006, 10:51 PM   #2
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The maf does sense the extra airflow. But also, the manifold pressure sensor senses the increased pressure almost instantaneously and through speed density calculations figures out the increased airflow. The maf and speed density calcs are combined through a calculation to get an airmass value for each cylinder. The pcm figures out injector pulsewidth based on that airmass value. Keep in mind the maf has a little inherent lag because it's based on heat transfer, but the map can respond pretty quickly since the only lag has to do with the speed of sound for the pressure wave moving from the tb to the sensor on the back of the intake.
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Old 01-25-2006, 11:38 PM   #3
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I think P Mack pretty much handled that one!
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Old 01-25-2006, 11:52 PM   #4
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Exactly, as soon as you increase manifold pressure you also instantly increase cylinder pressure and of course torque so you feel in in the back right away and theh car moves out!
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Old 01-26-2006, 08:09 AM   #5
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A low-tech explantion:

Think of it as a "demand system". The dumb pistons are moving up and down trying to pump air, but the throttle plate(s) are closed restricting the amount of air those hungry cylinders are demanding. As you let in a little air, and then the smart systems allow the appropriate amount of fuel in, the cylinders are happy to burn it and do some work. If the load on the engine doesn't hold it back (dyno step test), it accelerates and takes anything attached along with it.

Whether it's a mechanical carburetor and a mechanical distributor, or FI and computer controls, those smart systems are just there to satisfy the air pump's demands and yours as you control the throttle. IOW, it always wants to accelerate, we just control (throttle) it. It's a lot like a spirited horse.
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Old 01-26-2006, 11:22 AM   #6
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Wouldnt' it be a spirited pony?^ (for the fbody)
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Old 01-26-2006, 12:40 PM   #7
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Quote:
Originally Posted by gun5l1ng3r
Wouldnt' it be a spirited pony?^ (for the fbody)
That might be a M**tang, but I don't use that M-word.
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Old 01-26-2006, 01:35 PM   #8
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Just an FYI, the "GAS PEDAL" is somewhat of a misnomer..

if we were being technical, it should be called an "AIR pedal" The user controls the amount of air and the computer controls how much fuel... (and when to ignite it for that matter)

Fuel = ENERGY However you need the right mixture of Air with the Fuel to get the most energy extracted into usable form (heat/pressure) and convert it from stored chemical potential energy into kinetic mechanical energy
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Old 01-26-2006, 01:50 PM   #9
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One more element: When you move the pedal, the TPS senses how far and how fast, so that the computer can increase the injector pulse width in anticipation of the greater matching fuel requirement, just like the accelerator pump on a carburetor.
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Old 01-26-2006, 04:10 PM   #10
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Quote:
Originally Posted by H8 LUZN
Just an FYI, the "GAS PEDAL" is somewhat of a misnomer..

if we were being technical, it should be called an "AIR pedal" The user controls the amount of air and the computer controls how much fuel... (and when to ignite it for that matter)
Personally I like "Foot Feed". "Power Pedal" would be my second choice.

"Accelerator Pedal" used to be OEM-speak.

Perhaps "Throttle Pedal" is an accurate description...or "Unthrottle Pedal".

Question: With a diesel, isn't it closer to a "Fuel Pedal" than an "Air Pedal"?
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Old 01-26-2006, 04:21 PM   #11
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Quote:
"Accelerator Pedal" used to be OEM-speak.
Still is in the GM parts catalogs.
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Old 01-26-2006, 06:11 PM   #12
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Thanks for the replies. So in short:

pedal goes down
TB plate opens
sensors feel pressure change
PCM instructs fuel system to increase
larger explosions from the already anxious pistons trying to suck more air
rpm's increae

Got it
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Old 01-26-2006, 06:19 PM   #13
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Quote:
Originally Posted by H8 LUZN
Just an FYI, the "GAS PEDAL" is somewhat of a misnomer..

if we were being technical, it should be called an "AIR pedal" The user controls the amount of air and the computer controls how much fuel... (and when to ignite it for that matter)
Well on a carb motor too, it would considered a gas pedal, because you automatically let in gas(and air) when you hit the pedal...no computer firing it from injectors. Plus when the term "gas pedal" was coined there probably wasnt too many FI motors around.
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Old 01-26-2006, 09:18 PM   #14
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Quicken,
I think you understand now!!!! That was pretty quick for a learning curve. Good explanations everybody.

Just 1 thing to add.
Air is not sucked into an engine, it is pushed!!!
An engine is mearly a pressure drop, therefore, the pressure on the front of the throttle blade is greater than that behind it.
The pressure wants to be equal so the high pressure on the front of the throttle body wants to move in and take over the low pressure on the engine side of the throttle body.

I hope this helps evaluate it a little better for you. Or it may confuse the heck out of you, either way, its all about learning.
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Old 01-26-2006, 09:30 PM   #15
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I guess technically nothing is sucked in any situation, doesn't just apply to air and engines.
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Old 01-26-2006, 11:30 PM   #16
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Quote:
Originally Posted by UMIRacing
Quicken,
I think you understand now!!!! That was pretty quick for a learning curve. Good explanations everybody.

Just 1 thing to add.
Air is not sucked into an engine, it is pushed!!!
An engine is mearly a pressure drop, therefore, the pressure on the front of the throttle blade is greater than that behind it.
The pressure wants to be equal so the high pressure on the front of the throttle body wants to move in and take over the low pressure on the engine side of the throttle body.

I hope this helps evaluate it a little better for you. Or it may confuse the heck out of you, either way, its all about learning.
Not to be picky but...

Isn't the "lower pressure" on the inside of the throttle blade caused by the partial vaccuum created inside the cylinder when the piston moves down and the inlet valve is open? The piston kind-of sucks the air in creating the lower preessure in the inlet manifold?

Just my perspective...
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Old 01-27-2006, 07:53 AM   #17
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It's all in how you understand it but the higher pressure air goes towards the lower pressure region no matter what. If you want to call that sucking that's fine as long as you know what's really going on.
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Old 01-27-2006, 08:58 AM   #18
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The reason I say it is being pushed is because for advanced engine development, this is how you approach things. Yes, there is a "sucking" effect taking place. But when analyzing for fuel drop out and true airflow, it has to be thought of as being pushed.
Does this make sense?
The reason it is "sucking" is because there is no air to be pushed into the cylinder.
The pushing is how you can obtain over 100% VE!!!
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Old 01-27-2006, 09:07 AM   #19
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So what you're telling us is that LS1's really suck?
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Old 01-27-2006, 09:14 AM   #20
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Quote:
Originally Posted by UMIRacing
The reason I say it is being pushed is because for advanced engine development, this is how you approach things. Yes, there is a "sucking" effect taking place. But when analyzing for fuel drop out and true airflow, it has to be thought of as being pushed.
Does this make sense?
The reason it is "sucking" is because there is no air to be pushed into the cylinder.
The pushing is how you can obtain over 100% VE!!!
Well inertia is how you achieve over 100%... "an object in motion will stay in motion.. etc" So once the pressure is equalized the air is still moving, the energy it takes to slow down the air is put into pressure and achieves a higher density in the cylinder.

You are correct though that air is pushed into the cylinder. because the net force on the air molecules is "pushing" them to the lower pressure.
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Old 01-27-2006, 09:14 AM
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