Noobish question on # of pistons

[S14Zilvia]

Sorry, I'm really not a newb to engines, but just answer me this.

Let's say you had a 3 liter nissan 4 cylinder motor and a 3 cylinder nissan 6 cylinder or 8 cylinder...

Why would more pistons = more power? (assuming everything else is basically the same)

It just seems like if you have the same displacement... and the 4 cylinder has larger pistons... sorry, it's just one of those things that I never really thought about.

[Anand]

First off... what the hell are you asking!?!?!?

[Get240DiZzY]

:withstup

[WhtTrshPunk]

Maybe he's asking if more cylinders mean more power, given equal displacement. Maybe?

[S14Zilvia]

Haha, sorry, let me try again.

If you had, let's say, a vg30de with 4 valves per cylinder, what if you made a comparable 4 cylinder with 4 valves per cylinder and the same 3 liter displacement?

If you have the same displacement, how does the # of pistons affect power?

[Former_240_s14]

Okay I think he is asking why ... car companies use more pistons with the same displacement that other car companies use but with less cylinders "ie. the spec v = 2.5liter, 4cyl. and the ford cougar = 2.5 liter, 6 cyl.".

I don't know the answer to this, but I think I like bigger displacement with fewer cylinders.

[S14Zilvia]

Maybe he's asking if more cylinders mean more power, given equal displacement. Maybe?

Yes!

Thank you, just had trouble phrasing that one.

[Tino]

like 4 big pistons vs 8 smaller ones

-for the kids

[EZcheese15]

Given all things are equal, the more number of pistons you have, the more power the engine makes. It also runs smoother because you have more combustion per revolution of the crank.

More pistons makes more power, because the distance the flame front has to travel accross the piston is less (pistons are smaller in diameter). This means the flame is more evenly distributed on the top of the piston.

Also, with smaller pistons, the combustion is moving less mass (smaller piston), and is more efficient by doing that. Efficiency = Power. By moving a bunch of small things, it is more efficient than moving a couple large things, thermally speaking

[Former_240_s14]

Also, with smaller pistons, the combustion is moving less mass (smaller piston), and is more efficient by doing that. Efficiency = Power. By moving a bunch of small things, it is more efficient than moving a couple large things, thermally speaking

Not disagreeing with you, you know a lot more than I, BUT, when you have more smaller things "emphasize on MORE", don't you have a higher chance of breaking things?

[S14Zilvia]

I see, so it's all about the area and mass, and how easy it is to move...

Does extra displacement cancel this out? (well, I'm sure it does, otherwise you wouldn't have different sized 4 cylinders) Just wondering if there is some magic number for area and weight vs. displacement.

One more question, for each rotation of the crank, are there the same number of power strokes taking place no matter how many pistons?

Now I'm just wondering why v8's don't rev as high as 4 cylinders... (usually) since they actually have less mass moving for each stroke. Or is it that the combined weight of all the pistons, even though they're smaller individually, outweighs the larger pistons of the smaller motor?

[S14Zilvia]

Not disagreeing with you, you know a lot more than I, BUT, when you have more smaller things "emphasize on MORE", don't you have a higher chance of breaking things?

That's what I always thought about... if you have 10 cylinders I'd be a lot more worried than 4. Seems like you're more likely to have bad compression if you have that many more cylinders, and that many more things to go wrong.

I'm just talking out my *** here though.

[EZcheese15]

Not disagreeing with you, you know a lot more than I, BUT, when you have more smaller things "emphasize on MORE", don't you have a higher chance of breaking things?

I suppose technically, yes, however since we are comparing pistons here (and all things connected, such as rods and the crank), they are *still* strong enough not to break.

Also, now that i think about it, a smaller piston (smaller diameter) has more strength when combusted, because the explosion is more evenly distributed on the piston top. Picture a piston of say 2" diameter. Big explosion happens on top, and pushes it down. That explosion was almost uniform anywhere on the top of that piston.

Now imaging a piston of 8" diameter. Same explosion on top. A lot of stress is in the center of the piston, where the main part of the explosion occured. As you go out towards the edge of the piston, it felt less of the explosion, and therefore less stress. Different intensities of stress throughout the piston will cause it to weaken. It's almost like twisting the metal, on a microscopic level.

[Former_240_s14]

Okay, but wouldn't the combustion on top of the cylinder be uniform on each side of the center of pistion "like a pebble droped in water and the ripple of water is even all around", and so since it is uniform and equal, not much stress would be created??

I am not questioning your knowledge, just questions of an ametuaer. : )

[S14Zilvia]

Okay, but wouldn't the combustion on top of the cylinder be uniform on each side of the center of pistion "like a pebble droped in water and the ripple is even all around", and so since it is uniform and equal, not much stress would be created??

I am not questioning your knowledge, just questions of an ametuaer. : )

Ah, but when you drop that pebble, where is the most effect created on the water?

If you have a really wide piston and the stress initializes in the center, it's probably worse than having that same force on a piston where the distance from side to side is not as great, as the top of the piston is supported by the sides.

(***-talk again)

[EZcheese15]

Okay, but wouldn't the combustion on top of the cylinder be uniform on each side of the center of pistion "like a pebble droped in water and the ripple is even all around", and so since it is uniform and equal, not much stress would be created??

I am not questioning your knowledge, just questions of an ametuaer. : )

Well, uniform in all directions, yes....

But *lots* of stress in the center, and little around the edge. Therefore, the difference in stress could weaken it.

Somewhere here I'm gonna dig up some formulas to show how power increases with # of pistons.....I got to find it here though.

[EZcheese15]

Ah, but when you drop that pebble, where is the most effect created on the water?

If you have a really wide piston and the stress initializes in the center, it's probably worse than having that same force on a piston where the distance from side to side is not as great, as the top of the piston is supported by the sides.

(***-talk again)

Exactly....that's what I'm trying to say

[Former_240_s14]

Ah, but when you drop that pebble, where is the most effect created on the water?

If you have a really wide piston and the stress initializes in the center, it's probably worse than having that same force on a piston where the distance from side to side is not as great, as the top of the piston is supported by the sides.

(***-talk again)

Yes, but if you have a larger area to realease the combustion, the combustion "should" be weaker in the center, causing less stress on the whole piston. not too sure on this, but sounds good, lol

[Former_240_s14]

Exactly....that's what I'm trying to say

Okay, then I will shut up now, sorry.

[S14Zilvia]

Yes, but if you have a larger area to realease the combustion, the combustion "should" be weaker in the center, causing less stress on the whole piston. not too sure on this, but sounds good, lol

The combustion still initializes in the same place. It hits the piston hardest right in the center. All of the force will eventually travel through to the sides of the piston, but the damage may already be done. The gases don't explode in the top of the chamber and then wait to press on the piston until the energy is evenly distributed. The energy initiates from the spark, and travels from there. It will hit the piston hardest directly under the spark I'm assuming.

[EZcheese15]

Ok, here, I dug up this formula:

Indicated Horsepower = Indicated mean effective pressure * stroke * piston area * #of power strokes per revolution * #of cylinders

For units, if you use IMEP in psi, stroke in ft, area in in^2, and divide the right hand side of the equation by 33000, you get HP on the left.

IHP is the power made per revolution of the crank.IMEP is the pressure in the combustion chamber during combustion.A 4-cycle engine makes half as many power strokes as it has cylinders.

So....

Lets say your IMEP is 600psi:

2 scenarios:

(Scenario 1)You have two engines, both 6 cylinders. One has a piston diameter of 2", a stroke of 8". The other has a piston diameter of 8", stroke of 1/2". Their displacement is equal.

If you plug in the formula:

600psi * 2/3 ft * pi * 3 * 6 = 22608 (engine 1)

Divided by 33000, you get .69 HP.

or:

600psi * 1/24 ft * 16pi * 3 * 6 = 22608 (engine 2)

Divided by 33000, you get .69 HP.

Therfore 2 engines with the same displacement and same # of cylinders will always equal the same HP, regardless of stroke/bore. (Given everything else equal)

(Scenario 2)

You have 2 engines:

Engine1: 6 cylinder, 8" diameter pistons, 1/2" strokeEngine2: 12 cylinder, 5.66" diameter pistons, 1/2" stroke

The displacment is the same for both engines. The only difference is one has twice as many cylinders, but 1/2 the cylinder area.

Engine 1:600psi * 1/24 ft * 16pi * 3 * 6 = 22608

22608 / 33000 = .69 HP

Engine 2:600psi * 1/24 ft * 8pi * 6 * 12 = 45216

45216 / 33000 = 1.37 HP

Therefore, give everything equal but the # of cylinders (and bore, respecitvely), you make more power with more cylinders.

So yeah....if you followed all that, then good

[specialkonrd]

like 4 big pistons vs 8 smaller ones

-for the kids

you LIKE big pistons dont u tino :P

[Tino]

Ezcheese I'm not even going to attempt to understand that, nod and smile... nod and smile...

[AZhitman]

Way to go Cheezy!!

Let's move this one over to the Infiniti or Nissan Technical Talk.

Sometimes the 240 guys don't get to access the incredible wealth of knowledge provided by our very own Q45tech (Dennis). He loves this stuff... Enjoy the lesson, kids!

[MrFox]

Ok, here, I dug up this formula:

Indicated Horsepower = Indicated mean effective pressure * stroke * piston area * #of power strokes per revolution * #of cylinders

I'd have to point out an error within your equation here...

The "# of power strokes" for this equation is per cylinder - i.e. is this a 2 stroke or 4 stroke. Another form of this equation is as follows:

Horsepower = P x L x A x N/33,000

P stands for the mep in pounds per square inch or psi; L for the length of the stroke in feet; A for the top surface area of one piston in square inches; and N for the number of power strokes per minute.

and so with all those ##s being the same in all the examples, example 4's result ends up being the same as the others.

given that the same amount of charge fills up an equal amount of volume/displacement and combusts equally well, it will result in the same amount of power output... power outputs differ because of mechanical and flow constrains.

[Chally]

Hey Lord Cheeze,

Good on ya! Without going to class on this subject, it's a bit hard to explain.

Don't forget, that when a fuel charge is ignited in the cylinder, the fuel doesn't burn all at once. This is why we have Ignition BEFORE TDC. The fuel takes time to burn & it has to be ignited at a specific time to give the best amount of power.

Igniting the flame too early, will result in the explosion trying to push the piston back down the cylinder as it's still coming up. This causes pinging & it's like hitting the piston with a hammer.

Igniting it too late, will result in the explosion having an air swing. (like hitting a ball on the string as it passes you, you swing like crazy & only just touch it, so the speed of the ball doesn't increase much) This will give you Low power & bad economy.

So, the size of the piston is not the main problem, it's getting the charge to cover as much of the piston as possible as it enters the chamber, ie: being able to Breathe Better. This is why our Q's, & other 4 vlave heads, can get the generous amount of revs we do, while other brands, of the same size, can't!

Also, the larger the weight of the Piston/con-rod combination, the lower the revs.Even though the engine displacement is equal between 2 engines, the one with the Shorter Stroke will be able to rev higher. This is because the Piston is made from lighter materials than the Con-rod, & the shorter the stroke, the shorter the rod & also the less strength that will have to be built into the rod.

One more thing, the More power strokes per revolution, the more power you will get (as that formula proved)& the smoother the engine will run.A good example of this is on a 500cc mtotr bike engine. The Single Piston model really thumps, where-as the 4 cylinder ones are really smooth.

I think I'm rambling... Better go befoe I'm kicked out.

[Q45tech]

There is an optimum area of piston vs the sealing ring circumference [ring friction]!The rule of thumb is 500 cc per cylinder: thus 2 liter 4s, 3 liter 6s, 4 liter V8s............this can be bent to 550-560cc IF special low friction rings and molybenum coats are used on piston skirts.

Nissan new 3.5 V6 [583 cc per cylinder] is on the ragged edge of being too large [a bore] for a smooth long lasting [300,000 miles] [Efficient] engine even though the stroke is small [3.2"] which allows safe 7,000 rpm operation [for a little while].

Ring friction is the largest single source of losses in an engine!Old style American engines used larger bores to improve low- medium speed torque at the expense of high rpm horsepower.

Big bores and big 2 valves mean the spark plug cannot be centered thus the assymetrical flame front and uneven pressure across the area of the piston which can lead to accelerated ring wear.

You have to design things so that the peak BMEP [pressure] occurs at 16-17 degrees after TDC, when the rod angularity is just right to achieve maximum downward pressure.

The new Nissan 5.6 liter V8 [for trucks] [375 lb/ft] is more American like..........low rpm design.

Because of tax structures [Japan and Europe] [taxed by displacement] have always placed a premium on engine efficiency......most power per cubic inch...........average Joe can only afford an under 2.0 [1.99999999] liter engine because jumping to a 2.01 liter costs another $3,000 in taxes!

In Europe you see loads of large MB limos with 2.8 liter 6 cylinders because of the $10,000 fee to jump over 2.9999 liters, Japan did the same with the Nissan 2.998 liter engines being the workhorse of the rich.

The Q and M45 engine are 4.494 to avoid a serious tax bill for 5.000 liters. Even th Lexus V12 is only 4.994 liters.

Since the US has never taxed by displacement [except in a round about way] by CAFE fines there has been little reason to push efficiency !

[MrFox]

good point...

what about rotaries? imagine a rx-7 and a march being taxed the same (1300cc)

[S14Zilvia]

I'd have to point out an error within your equation here...

The "# of power strokes" for this equation is per cylinder - i.e. is this a 2 stroke or 4 stroke. Another form of this equation is as follows:

Horsepower = P x L x A x N/33,000

P stands for the mep in pounds per square inch or psi; L for the length of the stroke in feet; A for the top surface area of one piston in square inches; and N for the number of power strokes per minute.

and so with all those ##s being the same in all the examples, example 4's result ends up being the same as the others.

given that the same amount of charge fills up an equal amount of volume/displacement and combusts equally well, it will result in the same amount of power output... power outputs differ because of mechanical and flow constrains.

Does everyone agree with this last paragraph? It doesn't seem right. What, then, would be the advantage of 3 liter 6's? Why not just make them 4's?

Or is this where that bore/ring friction ratio comes in. The reason for more pistons is to up displacement without causing excess ring friction.

Let me get this straight as well: how many power strokes does a 4 cycle engine make for each revolution of the crank? Is this dependent on # of pistons?

It seems that to keep the motors with more cylinders in time according to the 4 strokes there would have to be more power strokes for each rpm relative to the smaller motor.

For example, take a v4. One piston fires, and another piston is firing for each of the remaining three strokes of the 1st piston. Perfect timing.

Now what if you have a v8? The numbers don't work correctly... or you'd have extra strokes.

[reggiegsd]

"Ring friction is the largest single source of losses in an engine!"

This was Porsche's stated reason for the big 4 cylinder in the 944. They claimed a dramatic increase in efficiency compared to an equivalent 6. Of course it required balance shafts to keep it from shaking to death.