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2010 2011 BMW 5 Series Forum F10 BMW 5-Series (F10) Forums General 5-Series Sedan and Wagon (F10 / F11) Forum RPM Question
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      05-08-2015, 07:30 PM   #23
Noggie
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Let go all the way back to basics.

How a 4 stroke combustion engine works.

Let's start with piston on top
1. Piston moves down and sucks inn air and fuel mix 1/2 crank revolution.
2. Piston moves up and compresses the air fuel mix. 1. Crank revolution completed.
3. Ignition and the explosion forces the piston to moe down 1,5 crank revolution.
4. Piston moves up and pushes the exhaust gasses out. 2 crank revolutions.

So as people say a cylinder ignites every second crank revolution.
Rpm is crankshaft revolutions.
At 5000 rpm each cylinder fires 2500 times a minute.

As the Pistoons move down they also move the "dead" cylinders through their inactive stroke.
For 4 cylinder car, the crank sees a firing cylinder ever half turn every 180 degrees, hence the shaking nature of the engine.
On a 6 cylinder engine the crank sees 3 cylinders firing each revolution, every 60 degrees and is much smoother.
A v8 sees 4 cylinders firing every revolution, every 90 degrees, and shakes less than a 4, but more than a 6.
A V12 sees 6 firing cylinders, every 30 degrees, and you can balance a coin on it.

Theoretically if you had 720 cylinders you could have one fire every single degree.
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      05-08-2015, 08:07 PM   #24
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If he manages to digest this much we should get into exhaust pulses, positive and negative pressure waves, and the giant leap forward provided by twin scroll turbos (and the associated manifold designs); which are ingenious, and one of those things that engineers look at and say, why didn't I think of that in the first place!
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      05-09-2015, 05:12 AM   #25
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Quote:
Originally Posted by Noggie
Let go all the way back to basics.

How a 4 stroke combustion engine works.

Let's start with piston on top
1. Piston moves down and sucks inn air and fuel mix 1/2 crank revolution.
2. Piston moves up and compresses the air fuel mix. 1. Crank revolution completed.
3. Ignition and the explosion forces the piston to moe down 1,5 crank revolution.
4. Piston moves up and pushes the exhaust gasses out. 2 crank revolutions.

So as people say a cylinder ignites every second crank revolution.
Rpm is crankshaft revolutions.
At 5000 rpm each cylinder fires 2500 times a minute.

As the Pistoons move down they also move the "dead" cylinders through their inactive stroke.
For 4 cylinder car, the crank sees a firing cylinder ever half turn every 180 degrees, hence the shaking nature of the engine.
On a 6 cylinder engine the crank sees 3 cylinders firing each revolution, every 60 degrees and is much smoother.
A v8 sees 4 cylinders firing every revolution, every 90 degrees, and shakes less than a 4, but more than a 6.
A V12 sees 6 firing cylinders, every 30 degrees, and you can balance a coin on it.

Theoretically if you had 720 cylinders you could have one fire every single degree.
Great. Now, I get it:

1. Two revolutions of the crankshaft = complete cycle
2. No matter how many cylinders, 1/2 will provide power on the first revolution and the other 1/2 will provide power on the second crankshaft revolution.

This explains why adding more cylinders will always net more power. A theoretical 20 cylinder engine will have 10 cylinders fire power within 1st crankshaft revolution and the remaining 10 cylinders would fire within the second crankshaft revolution.

I am sure there are exceptions, right?

Now, tell me me how my friend's 20 year old Geo Metro three cylinder car would work?

Let me guess: 1. All three cylinders fire within 1 revolution? Lol!

Thanks for all explanations!
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      05-09-2015, 05:19 AM   #26
BMWrules7
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Quote:
Originally Posted by AlanM
If he manages to digest this much we should get into exhaust pulses, positive and negative pressure waves, and the giant leap forward provided by twin scroll turbos (and the associated manifold designs); which are ingenious, and one of those things that engineers look at and say, why didn't I think of that in the first place!
When I look at an antique car engine from 1939, it looks almost the same as today's engines.

Doesn't pulsing the crankshaft seen antiquated? Everything's serialized. Why can't they design an engine to fire Pistons simultaneously to get more work.

Yes, the connection to the crankshaft would need to be completely reworked, but there has to be a way to get past serial pulses!
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      05-09-2015, 06:13 AM   #27
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Quote:
Originally Posted by BMWrules7 View Post
Great. Now, I get it:

1. Two revolutions of the crankshaft = complete cycle
2. No matter how many cylinders, 1/2 will provide power on the first revolution and the other 1/2 will provide power on the second crankshaft revolution.

This explains why adding more cylinders will always net more power. A theoretical 20 cylinder engine will have 10 cylinders fire power within 1st crankshaft revolution and the remaining 10 cylinders would fire within the second crankshaft revolution.

I am sure there are exceptions, right?

Now, tell me me how my friend's 20 year old Geo Metro three cylinder car would work?

Let me guess: 1. All three cylinders fire within 1 revolution? Lol!

Thanks for all explanations!
The 3 cylinder engine works just the same, but you have 3 pulses on two revolutions so 720/3 = one firing every 240 degree.

Every 4-stroke engine works like this regardless of number of cylinders.

A 2-stroke engine fires every single crankshaft revolution.
1. piston on top. ignition and the piston is forced down while going down is sucks inn new air/fuel from the top 1/2 revolution
2. when on bottom, the exhaust vent is opened (just a hole in the bottom of the cylinder) and the exhaust gasses are vented out by the pressure of the incomming air/fuel. the piston moves up, which closes the exhaust port, compresses the air/fuel mix and once on top it ignites, 1 crank revolution.

The problem here is that not all the exhaust is removed every time, so you have a mix of new air/fuel and some old exhaust in every combustion, so this engine is not as effective as a 4-stroke engine.
you'll find these in lawnmowers, mopeds, chainsaws ect.

Quote:
Originally Posted by BMWrules7 View Post
When I look at an antique car engine from 1939, it looks almost the same as today's engines.

Doesn't pulsing the crankshaft seen antiquated? Everything's serialized. Why can't they design an engine to fire Pistons simultaneously to get more work.

Yes, the connection to the crankshaft would need to be completely reworked, but there has to be a way to get past serial pulses!
No. remember, if you fire everything at once you get one pulse of power and then nothing for 2 crank revolutions. then a new pulse of power.
By timing this as is done you get the power pulses spread out through the revolutions and power delivery is smooth, but not as powerful.

Very little has changed in the design of the combustion engine since it was invented. on old engines you had pushrods instead of crankshafts to open the valves, a design that was left if the 50's, but taken up by Chevy in the 90's on their Corvette engines. to make the engine lower they ditched the overhead cam and returned to a pushrod type design to be able to get a low sleek hood on the Corvette.
Many motorcycles, especially choppers still use pushrods and not overhead camshafts.

The Japanese are the only ones building rotary engines, or Wankel engines in their RX models.
This is basically a triangle shaped block of metal that rotates inside a slightly ofset cylinder. this engine has no strokes as it does everything in one revolution of the triangle, you might call it a one stroke engine.
it has no cylinders either, but the 3 sides of the triangle can in a way be seen as 3 different "cylinders".

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      05-09-2015, 09:09 AM   #28
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Quote:
Originally Posted by BMWrules7 View Post
Doesn't pulsing the crankshaft seen antiquated? Everything's serialized. Why can't they design an engine to fire Pistons simultaneously to get more work.
We could use a turbine.

With the conventional internal combustion engine we really want more smaller pulses for smoothness and help with reducing vibrations which get transmitted into the drivetrain. Current down sizing (cylinder count) with greater torque outputs, creates its own set of problems.

The lower the cylinder count the more important is the Dual Mass Flywheel (DMF) with torsional damping. We have this need, perhaps more so in Europe with all the 4-cylinder diesel engines, many with manual transmission. The higher the torque at low rpm, the more critical is control of torque pulses into the clutch and gearbox. Even our current autos have torque converters with similar torsional damping to the DMF, both for running lower rpm at higher torque values, and to allow more TC lockup without torque pulse vibrations being transmitted.

HighlandPete
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      05-09-2015, 10:43 AM   #29
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Quote:
Originally Posted by HighlandPete View Post
We could use a turbine.
I can tell you how that works too
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      05-09-2015, 10:51 AM   #30
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One fallacy is that more cylinders = more power, and that's not necessarily true at all. I would rather think of it as (all else being equal) more displacement equals more power. If you have an 8 liter 4 cylinder engine, it should have more power than a 4 liter v8. If each cylinder is the same size, then adding more of them adds more displacement and therefore more power for more cylinders is correct. Think of a harley 2 cylinder, at 110cubic inch, that's 1.8 liters. Each cylinder is enormous but with only 2 cylinders and 1 power stroke per revolution, that's why it vibrates so much.

Now let's say you have a 4 liter 4 cylinder vs a 4 liter v12, since the v12 has so many more cylinders and all still firing every 2 revolutions, if the firing sequence is spaced out well, it will be a lot smoother. As far as power (before I said all else being equal but in reality it's not) usually when you have more cylinders for the same displacement the stroke of the piston is shorter, since more piston travel puts more stress on the connecting rod, less travel can allow higher RPM redlines. Since power = torque x rpm / 5252, if you can generate a higher redline by increasing cylinders and reducing piston travel, you can generate more power at the same level of torque...
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      05-09-2015, 12:36 PM   #31
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Quote:
Originally Posted by HighlandPete
Quote:
Originally Posted by BMWrules7 View Post
Doesn't pulsing the crankshaft seen antiquated? Everything's serialized. Why can't they design an engine to fire Pistons simultaneously to get more work.
We could use a turbine.

With the conventional internal combustion engine we really want more smaller pulses for smoothness and help with reducing vibrations which get transmitted into the drivetrain. Current down sizing (cylinder count) with greater torque outputs, creates its own set of problems.

The lower the cylinder count the more important is the Dual Mass Flywheel (DMF) with torsional damping. We have this need, perhaps more so in Europe with all the 4-cylinder diesel engines, many with manual transmission. The higher the torque at low rpm, the more critical is control of torque pulses into the clutch and gearbox. Even our current autos have torque converters with similar torsional damping to the DMF, both for running lower rpm at higher torque values, and to allow more TC lockup without torque pulse vibrations being transmitted.

HighlandPete
Aha! These last few responses are finally sinking in!

Now, Pete's (and the others here) explanation shows why the cylinder size of the 760 (12 cylinder) is smaller than 750..

That makes the engine smoother, right!

But, Genieman explains that the 760 is more powerful because the displacement is greater than the 750.

It's amazing how I could drive for 30 years and not really understand this until now. (Well, I understand a little anyway)

I guess that proves I am just a pozer. I still like the tech of BMW.
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