What accelerates an engine?
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
With typical engine components and geometry, gasoline won't sustain a reliable spark-initiated flame in uniform mixtures leaner than perhaps 17-18:1, the "lean limit" of flammability. Even a slight misfire greatly impacts emissions and fuel economy. "Stratified Charge" engines provide a richer mixture near the spark plug and the resulting large flame front can propagate into much leaner regions of mixture, but the high combustion temperatures create a lot of oxides of nitrogen, which are very hard to 'clean up' in the exhaust. Hence, this technology is virtually unused in the USA, which has the most stringent standards in the world (especially CA)
Ok, did a little research on it, called a very reliable source. And he said the same thing that MadBill did. The fuel mixture would not be consitant across the volume of the chamber therefore causing a misfire.
He also said there would probably be pre-ignition even before you could reach the lean limit of the fuel. Although he did say that moving the timing back towards top dead center may delay this, but probably not by much.
Good job MadBill.
He also said there would probably be pre-ignition even before you could reach the lean limit of the fuel. Although he did say that moving the timing back towards top dead center may delay this, but probably not by much.
Good job MadBill.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by bdc95ta
Interesting discussion. But what about formula one engines, no throttle blades!
-b
-b
On The Tree
Joined: Aug 2005
Posts: 133
Likes: 0
From: TX
This has been a concept in the thread that has bugged me for a few months. I haven't been able to put it straight in my head.
Hit the gas and more air/fuel are recorded/metered and the engine burns more a/f creating more bmep. So is it the addition of bmep that makes the motor spin up higher? For instance if I'm at 1500 rpms and making 100 bmep and I open the throttle and make 110 bmep, is the motor going to spin up? How do we know how much? By 10 rpms? 100 rpms? 1000 rpms? Is it as simple as a bmep increase making the motor spin faster by pushing the pistons down harder?
Hit the gas and more air/fuel are recorded/metered and the engine burns more a/f creating more bmep. So is it the addition of bmep that makes the motor spin up higher? For instance if I'm at 1500 rpms and making 100 bmep and I open the throttle and make 110 bmep, is the motor going to spin up? How do we know how much? By 10 rpms? 100 rpms? 1000 rpms? Is it as simple as a bmep increase making the motor spin faster by pushing the pistons down harder?
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by Adnectere
This has been a concept in the thread that has bugged me for a few months. I haven't been able to put it straight in my head.
Hit the gas and more air/fuel are recorded/metered and the engine burns more a/f creating more bmep. So is it the addition of bmep that makes the motor spin up higher? For instance if I'm at 1500 rpms and making 100 bmep and I open the throttle and make 110 bmep, is the motor going to spin up? How do we know how much? By 10 rpms? 100 rpms? 1000 rpms? Is it as simple as a bmep increase making the motor spin faster by pushing the pistons down harder?
Hit the gas and more air/fuel are recorded/metered and the engine burns more a/f creating more bmep. So is it the addition of bmep that makes the motor spin up higher? For instance if I'm at 1500 rpms and making 100 bmep and I open the throttle and make 110 bmep, is the motor going to spin up? How do we know how much? By 10 rpms? 100 rpms? 1000 rpms? Is it as simple as a bmep increase making the motor spin faster by pushing the pistons down harder?
BMEP is Brake Mean Effective Pressure, which means the engine is actually driving something other than itself. If the engine is free reving in neutral or with the clutch disengaged, it is only producing enough torque and power to overcome it's own friction and that of the trans in neutral or to spin the converter and trans pump. It's not trying to move the vehicle or load the dyno so there's no Brake torque/power an no BMEP. There is however Indicated MEP or what the engine is producng internally. The engine friction is the FMEP and as you know, BMEP = IMEP- FMEP. Now in this case FMEP = IMEP so there is 0 BMEP. All the torque/power (or MEP) being produced is consumed overcoming internal losses.
Because IMEP (or BMEP) is directly related to torque/cubic inch (or cc), it might help to think of torque or power being produced instead. FWIW, a V8 free reving to 1500 may be producing something like 25 lb-ft (7.1 hp) to ovecome it's own friction. On a 350 thats about 11 psi IMEP.
If you let more air/fuel in, the engine will free rev until the torque/power generated equals the internal friction at a higher rpm. You suggested a 10% increase, so I'd guess just about a 10% increase in free rpm because friction torque/hp is almost linear at low rpm.
Clear as mud?
On The Tree
Joined: Aug 2005
Posts: 133
Likes: 0
From: TX
Ok then ignore the above and go with IMEP, it's what I mean at the root anyways. I'm imagining pressure on the cylinders sans losses of any type so IMEP is what I mean and should have said.
So it's as simple as the motor will have a certain IMEP needed to overcome its losses at various rpms and these losses will increase (vague statement) as the rpms increase so to rev the motor you need more force in terms of IMEP.
Let's say we have a throttle at 25%. And another at 100%. I would assume the latter to make 4x the IMEP of the first, ignoring losses that complicate. Is this true?
So it's as simple as the motor will have a certain IMEP needed to overcome its losses at various rpms and these losses will increase (vague statement) as the rpms increase so to rev the motor you need more force in terms of IMEP.
Let's say we have a throttle at 25%. And another at 100%. I would assume the latter to make 4x the IMEP of the first, ignoring losses that complicate. Is this true?
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by Adnectere
Ok then ignore the above and go with IMEP, it's what I mean at the root anyways. I'm imagining pressure on the cylinders sans losses of any type so IMEP is what I mean and should have said.
So it's as simple as the motor will have a certain IMEP needed to overcome its losses at various rpms and these losses will increase (vague statement) as the rpms increase so to rev the motor you need more force in terms of IMEP.
Let's say we have a throttle at 25%. And another at 100%. I would assume the latter to make 4x the IMEP of the first, ignoring losses that complicate. Is this true?
So it's as simple as the motor will have a certain IMEP needed to overcome its losses at various rpms and these losses will increase (vague statement) as the rpms increase so to rev the motor you need more force in terms of IMEP.
Let's say we have a throttle at 25%. And another at 100%. I would assume the latter to make 4x the IMEP of the first, ignoring losses that complicate. Is this true?
At power peak, say 6000 rpm, 100% vs 25% opening would make a lot of difference. The 25% would act like a very small restrictor, so you might see a 40% or so loss of power from 100% opening. IOW, the 100% opening might show 50% or so more BMEP and torque/power than the 25% opening. You could simulate this nicely on something like Engine Analyzer PRO if you want to see realistic numbers.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo On The Tree
Joined: Aug 2005
Posts: 133
Likes: 0
From: TX
So basically an easy way for me to think of this is...maybe you need 10 psi to power the motor at idle, maybe 25 psi to run it at 2500 rpms, maybe 50 psi to run it at 6000 rpms. All speculative pressure numbers, but is that the theory? Are we basically saying that as the piston gets forced down harder it swings back faster than the next time and so the engine climbs rpm?
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by Adnectere
So basically an easy way for me to think of this is...maybe you need 10 psi to power the motor at idle, maybe 25 psi to run it at 2500 rpms, maybe 50 psi to run it at 6000 rpms. All speculative pressure numbers, but is that the theory? Are we basically saying that as the piston gets forced down harder it swings back faster than the next time and so the engine climbs rpm?
The same thing happens when you are cruising on the freeway. You control the amount of air/fuel into the engine so it produces just enough power to overcome the friction and drag of the entire vehicle. Your foot, or your cruise control, is the feedback device that makes the minute adjustment of the amount of air/fuel the engine receives.
Don't overthink this.
Thread Starter
TECH Veteran
Joined: Dec 2001
Posts: 4,117
Likes: 0
From: Ft. Lauderdale, FL, USA
Look what I've started, and I still don't have a clear answer.
The way I see it so far is: As you sit idling at a light, the pistons are struggling and wanting more air, but each cylinder is being deprived that air because the throttle plate is not open. So, the second you push the pedal down and open that plate you just allowed those pistons to have what they're craving, so on each ensuing compression stroke for each piston that just got that extra gulp of air, along with the fuel injectors following suit immediately with more fuel........PRESTO.....the crank is turned faster than it was 1 second ago.
Can it be that simple of an explanation?
.
The way I see it so far is: As you sit idling at a light, the pistons are struggling and wanting more air, but each cylinder is being deprived that air because the throttle plate is not open. So, the second you push the pedal down and open that plate you just allowed those pistons to have what they're craving, so on each ensuing compression stroke for each piston that just got that extra gulp of air, along with the fuel injectors following suit immediately with more fuel........PRESTO.....the crank is turned faster than it was 1 second ago.
Can it be that simple of an explanation?
.
Staging Lane
Joined: Mar 2006
Posts: 65
Likes: 0
From: upstate NY
The sucking/pressure thing may be seen a little more clearly by considering- the airflow through an opening is only controlled by the size of the opening and the pressure of ambient air. Doesn't matter if it is a Hoover vacuum cleaner or a Hemi 426 doing the drawing (sucking), what determines how much air goes through that opening is the size of the opening and the atmospheric pressure- period.
That is why the 'old-school' first mod was more carburetor- more barrels, more carbs = more opening. Now days, bored throttle bodies. Engines are just big air pumps that happen to burn fuel, but, to get more power you have to pump more air. Better flow along the whole chain; heads, exhaust, intake-
That is why the 'old-school' first mod was more carburetor- more barrels, more carbs = more opening. Now days, bored throttle bodies. Engines are just big air pumps that happen to burn fuel, but, to get more power you have to pump more air. Better flow along the whole chain; heads, exhaust, intake-
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
acceleration is caused by force. This force is caused by the air fuel mixture exploding in the trapped space above the piston on the power stroke. when the throttle is opened you fill the cylinder with more air and cleaner air and fuel is added and the cylinder pressure increases and so does the force on top of the pistons. When the force on top of the piston pushing down on them increases so does the engine rpm and so does the vehicle as well if it's in gear and the clutch is out!
