Noobish question on # of pistons

[Q45tech]

DUH! you always have half the number of cylinders firing per rpm in a 4 stroke.

A V8 has 4 power peaks every 90 degrees vs 3 for a 6 and 2 for a 4 cylinder and 6 for a V12........these peaks add algebraically and vectorally. The flywheel smooths the pulses and creates the average.

HP/Torque numbers are averages not the individual peaks"^v^v^v^v^v^v^v^v^v^v^v^v^v^=--------------------------really sine wave gaussian curves types.

The parametric equation for relative effect in changes has the bore with a 1.65 exponent and the stroke with a 0.55 so bore is 3 times more important for torque production than stroke. ..... keeping the displacement the same.

http://www.engr.colostate.edu/....htmlh ... u/....html

[S14Zilvia]

So that might be one of the reasons for more cylinders providing more power?

[Q45tech]

More cylinders don't provide more power just less friction if the total displacements are equal..............obviously there is a limit in that anything beyond an inline six or a V12 has a crank that is too long - requires too many bearing to support rigidly.

The term power is so wishy washy .........torque is the correct measurement term...........and its mathematical manipulation cousin Horsepower.

More cylinders are just smoother at low rpms where the where the time between dips are less in milliseconds.

Hopefully you realize that the flywheel power output is zero at idle otherwise the idle speed would increase, yet all the fuel is burned to just equal internal friction........called IMEP.

[miteymax86]

The Q45Tech is my God now. I wish to know everything he knows and one day in the very distant future be able to dispense such wisdom with as much ease as he does.

[Chally]

4 stroke/cycle means 4 movements of the piston in the bore to complete a full cycle.

1. Downstroke sucking in the air/ fuel charge.2. Compression stroke, where the piston goes back up & compresses the charge.3. Power stroke where the charge is burnt & produces the pushing power.4. Exhaust Stroke where the piston goes up & pushes the burnt gasses out the exhaust.

Therefore, there is a complete 2 revolutions of an engine for a full cycle to be completed.

On a 2 stroke/cycle, there is a powerstoke every time the piston comes down.

Go & do some searching on the web & check out the differences. It's very hard to explain without showing you diagrams etc. Like giving you a haircut over the phone...

[180fan]

Wouldn't you lose alot of power with the more amount of pistons you use given equal displacement? Since you've got to move the mass of the pistons, crank and everything else that is driven by the combustion of the gas, and the friction between the pistons and the walls of the cylinders, you loose power from those factors. The bigger the displacement, wouldnt you get potentially more torque vs. having more cylinders with less displacement per cylinder? Just a few thoughts.

[S14Zilvia]

Chally- I know the differences between a 4 and 2 stroke, I just wasn't sure on some of the differences between different motors... i.e. at what degree of rotation is each cylinder firing. It's been cleared up now though.

180fan- It probably doesn't work out to that much mass, as even though you're adding pistons and rods, they are each smaller due to the greater denominator. The most friction occurs between the rings and the block, and rings with less surface area = less friction.

[Chally]

Also with more power strokes per revolution, there is just that, more power strokes.

Friction has not much to do with it, as if you look at Subaru, with the pistons lying flat, they don't seem to wear any quicker than ones up straight.To actually get a large piston & a small piston engine with the same cubic inches, it would be very difficult to make the combustion areas the same to compare the different power outputs. As the piston size increases, so does the combustion style change. eg. a larger area takes longer to burn than a smaller area.

[S14Zilvia]

That's what I thought about more power strokes... but no one seemed to agree with that... I figured 4 power strokes would be better than 2.

[Chally]

Just look at a Ford 4 litre 6 cyl motor & then a Lamborghini 4Litre V12. Which one has the most power? Yes, the Lambo motor is built & the Ford put together, but if Lambo could get the same power from fewer pistons, you'd think they'd use it.

[Q45tech]

You keep equating the imaginary HP [a useless term designed to manipulate magazine readers/car buyers who don't understand Physics] to POWER...........

Look at peak TORQUE in lb/ft and then the torque at what ever rpm -- this is the TRUE measure of power!

In general there is a limit and that is 75-80 lb/ft per liter at the shaft for street cars. Pure race cars can get to 90-94 lb/ft on methanol with 8-12 throttle bodies] but the engines only last 501 miles.

BMW Z4 3.0i..........214/2.879= 74.33 lb/ft per literBMW 745...............330/4.398= 75.04 lb/ft per literFerrari 575M..........434/5.748= 75.50 lb/ft per literMaserati GT...........330/4.215= 78.29Q45 ......................333/4.494= 74.09 lb/ft per liter350z......................274/3.498= 78.33 lb/ft per liter [???]bet it's closer to the 262-265-270lb/ft that they speced on G35 but who knows Nissan enginers are as good a Maserati for sure!

All the performance V6 or V8 or V12 make the same power per liter of displacement.

[NWilner]

The question of number of cyls vs displacement has been asked and answered many times in the past 100 years of engine development. The ultimate issue is the size of the combustion chamber. Too big and you don't complete combustion (which is a burning not an explosion, and does not load the "center" but fills the chamber with expanding gasses). Too small and you have too much surface area for the volume enclosed, with consequent losses from unburned gasses in contact with the cooler surfaces (and greater amounts of cyl wall to oil, etc.) Successful engines have been made, however, at either extreme.

Aircraft engines have very large pistons; a 4 cyl Lycoming displaces 360 ci (but only makes 200 hp or so, at 2700 RPM). Typically they need two sparkplugs per cylinder to get complete combustion even at those low engine speeds.

Marine diesels can have pistons six feet in diameter. But they turn maybe 500 RPM.

Smaller, more numerous cyls add complexity and cost. However they result in lower inertial forces (but, the bearing area per cyl is also lower).

In the end it is a compromise with the basic physics not pointing overwhelmingly in either direction.

[Q45tech]

Again you are using HP a meaningless term! 200HP @2700 rpm is 389 lb/ft or torque at the same 2700 rpm......this 5.86 liter 4 cylinder airplane engine is what I call a truck engine 66 lb/ft per liter. Compromised [detuned] for reliability not stressed for peak power which would be 434 lb/ft

Safe rpm is a function of piston speed which is a function of stroke.

Just remember engines are at best 28% thermally efficient and internal ring friction is usually around 10% of peak torque ------the rest is wasted heating the coolant and blown out the exhaust.

http://www.engr.colostate.edu/....html

http://www.engr.colostate.edu/....html

If you take the time to use this 2nd Java applet you can calculate the friction losses for any design - on a Q the piston and rings run about 76 KPA FMEP out of a total of 224 KPA.....varies from 33% at 6,000 up to 39% at 2,000 cruise.......interesting that the valve train vs piston losses are equal at 2,000 rpm cruise.The total friction only doubles from 2,000 to 6,000 rpm and the largest contributor ratiowise are exhaust valve pumping loses at very high rpm. Crank and rod bearing losses are very small...........the applet even includes losses from front and rear engine seals............so I'll bet it is very accurate!

[NWilner]

The information on ring friction losses is very interesting.

More pistons per unit displacement would increase total ring length, since as diameter increases circumference increases slower than area.

You are exactly right about the detuned design. Though torque is an important measurement, I would not go so far as to call power irrelevant.

"Limits" on air engines is a subject that has been discussed a great deal, especially since manual turbocharger wastegates permit an operator to dial in more boost than then engine can stand. The limits are not clearcut but involve all of these factors:

(a) inertial stress, scaling as rpm^2(b) gas pressure (BMEP or peak?) stress, scaling as manifold pressure linearly, but only below detonation thresholds, after which they increase unpredictably(c) torque limits, scaling linerly with MP(d) cooling, proportional to power (HP) to something like the 1.5 power, but this may differ with different designs

[Datsun411]

I'm no engineer, but here's my observation:

1) fewer moving parts = greater reliability/reduced likelihood of failure. Smaller mass will also serve this same end.2) structurally-speaking, yes, a larger piston will have more mass by definition and by requirement for strength/durability.3) smaller individual components (eg, the 2 litre 8-cylinder) will allow a more compact design and fitment, as well as lower mass within the design to move/overcome.4) more cylinders/equal displacement also makes for shorter throws and more power-strokes per revolution, which will translate into greater torque (possible mis-application of terminology?) and power applied per revolution.

Perhaps like comparing one big burly 30 y/o man on a bicycle versus two wirey 15 y/o's on a tandem bike. Which one will win in the short and long runs?

Then again, I could be way off-base on this.FWIW.

[lessthanjakejohn]

Howstuffworkds.com had a pretty good explanation once on how a eight cyl would be dif from a 1 cyl:

In a eight cyl power would be delivered like this: 1-1-1-1-1-1-1-1At the same rpm a 1cyl would deliver like this : 8-0-0-0-0-0-0-0