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i am currently a student in high school and i am doing my project
on the Z. i am starting to make the engine in Inventor. i just need
to know a few things, so any engine goo-roos please add
anything you wish. anything is stated in here is by NO MEANS
perfectly correct. if u read this thread, please do not take any of
my advice. i am a engine noob when it comes to this stuff. if you
ANYthing in here wrong please tell me. i want my project to be as
real life as possible.
1) i have heard people say that the vq35 is a 60 degree V-6. i
understand that 60 degrees is optimum for a v6 b/c it majorly
reduces vibrations. is the 60 degrees measuring the distance of
the actual cylinders? i have tried to collect a few diagrams to
maybe clarify what i am asking.
in this picture, i noticed that the cylinders would come up in 3's. i
started thinking about this and i dont think it is possible. the cycle
of a piston from spark to spark goes like this:
from this i gathered that the engine has to turn ALL pistons twice
to complete one "cycle" of the piston. the way it looks in the
diagram, every piston 3 times before it fires again. this can not be
correct, atleast i dont think so. anyways, back to my original
topic, does the 60 degrees measure the angle from the center of
the pistons, if you were looking at the front of the engine? if so,
than how would the crankshaft work? (which brings me to my
2) To what degree does each section on the crankshaft need to
be set at? i did alot of doodling and brainstorming Friday during
class. i tried to set the correct degrees for each section so that
each piston would have 2 turns before it fired again. its kinda
simple math. 360 degrees / 6 pistons = 60 degrees for each
piston. but, if you think about this, that would only make 1
rotation of the piston until it came back to fire. since each piston
has to have 2 rotations for a complete cycle, as stated above, i
decided that the only way to do this was to set each piston 120
degrees from each other. just. this is with the number 1 piston up
in the compression stroke ready for the spark. with piston 1 fully
extended from the crankshaft, this puts it at -60 degrees from
straight up 0 degrees. next, piston 2 is at +60 degrees from 0.
and after that, for every piston, add another 120 degrees from
the previous piston. this gives you piston positions at 300, 60,
and 180 degrees on the crankshaft. here's the part where piston
firing order comes into question, which i will address later. so,
with this set up, you would have pistons 1 and 4 at 300 degrees,
2 and 5 at 60 degrees, and 3 and 6 at 180 degrees. the key to
this is while pistons 1 and 4 are up, only piston 1 is getting ready
to fire. piston 4 is on its exhaust/intake stroke. this applies to the
other 2 sets. this solves the problem of piston firing and cycles,
but creates another problem. this configuration will create a 120
degree motor. that would be great if i was running a 120 degree
v6, but im not. since the vq35 is a "60 degree" engine, i dont see
how this would ever work.
3) What is the firing order of the pistons? from what i have
learned so far, i think this is how it goes but someone please
clarify this and correct anything wrong.
well, i think thats all i can ask right now with the amount of
uncertanties i have. if you took you're time to read this entire
thread, thank you so much. i appreciate any help/info that anyone
A V6 has two banks, each with three cylinders. A 60 degree V has 60 degrees between the two banks, and a 90 degree V has 90 degrees between the two banks. An inline 6 is only one bank, but would be like having the two banks on top of each other, or zero degrees between them. A flat six has 180 degrees between the banks (911, Boxster Engine).
In all these cases, you will have six cylinders that need to fire evenly in two crankshaft revolutions (the four stroke engine, as you noticed, fires every other stroke, so it takes two revolutions for a full cycle). Therefore, in general, the angle of the V should have nothing to do with how the cylinders are oriented. As the crankshaft rotates, one of the cylinders will reach TDC every 720/6 or 120 degrees.
I think you are confusing the angle between the "V" banks with the angle between the cyliners on the crankshaft. These are definitely two different things.
Let me just say that I have never personally designed an engine, so hopefully we'll get some corroborating evidence from others here, but what I've written above seems to make sense, and it matches what you figured out.
Also, your firing order is backwards. Attached are a couple screenshots from the PDF of the users manual.
Finally, do a search for the VQ35, because Chebosto and perhaps others posted excerpts from some SAE articles about the VQ35 written by Nissan engineers.
Don't all the cylinders fire independently, one at a time? I don't know what the actual firing order is, but I was always under the impression that in most V-shaped engines, the cylinders fired one at a time. This helps reduce the vibration you get from the combustion pulses.
Not sure on this, so hopefully someone else can confirm.
Originally posted by 350ZTwinTurbo
jreiter - yes all cylinders fire independently.
Sorry, didn't see the bottom part of your post where you discussed the firing order of each cylinder. I must've totally overlooked it. I was mainly watching your animation, which showed them firing 3 at a time.
Yeah, all cylinders fire independently, one ignition per 120degrees of rotation.
If we take a V6 engine that has a firing order of 1-2-3-4-5-6, then it can be looked as follows:
For illustration purposes, suppose the following (which are not all true): 1) Spark occurs at TDC. 2) Only the intake valves are open from the top of the intake stroke, all the way to the bottom of the intake stroke. 3) Only the exhaust valves are open from the bottom of the exhaust stroke to the top. 4) All valves closed the entire compression and power strokes.
Now, if we start with cylinder one at TDC right at the time of the spark. Let's call this point 0*. This means number two is in a position where the crankshaft is 120* away from the point where this piston would fire, so lets call this point -120*, which means the cylinder is on the way up... At first look, it may appear that 120* before top dead center (BTDC) indicates the piston is in a position which is 1/3rd up from the bottom, since the piston moves from bottom to top, or where the crankshaft is moving from -180 to 0. But this is not true. The piston is actually only 25% of the way up when it is 120* BTDC. When the crankshaft continues to move (I am picturing it moving clockwise) to 90* BTDC, the piston is now 50% of the way up. Once it moves another 30 (60* BTDC) degrees, it is now 75% of the way up (some trig and cosine theory can figure this out).
So if you take a snapshot of the engine, at a point where, suppose cylinder 1 is at TDC just at combustion
Then cylinder 2 is going up, 25% of the way up the chamber on its compression stroke.
Number 3 is going down, 75% of the way down on its Intake Stroke.
Number 4 is at TDC in between the Exhaust and Intake strokes.
Number 5 is going up, 25% of the way up on its Exhaust stroke.
Number 6 is going down, 75% complete on its power stroke.
So basically, 1 and 4 go up and down together, as do 2&5 and 3&6.
I have included a drawing I just made… For clarity, I broke it up into two drawings so i didnt have to shrink it down to 640x480. The first post has 1,3, and 5, and the second has 2, 4, and 6... just picture them together on the same drawing...
All angles are measured with respect to cylinder number one being 0 degrees, and also with the untrue statement that this is the point of combustion. So as shown in the picture, cylinder two shows 120 degrees, meaning it is 120 degrees “behind” number 1.
I am an Electrical Engineer, not a Mechanical Engineer, but I think I have explained all this correctly…
1) spark occurs BEFORE the piston even reaches the top... the reason for this is so that the burn process starts early enough so that the power can be "applied" to the piston at the optimum time. You would think the speed that the air/fuel would burn would be so fast, but infact the ignition is not even close to an instant ignition with repect to piston position. We can do some math, 3.2 inches is the stroke on this engine (according to above post by D'oh). So the piston goes through the whole distance of the chamber twice for each revolution. This means, at 5000rpms, which is about 83rotations per second. So 3.2" x2 equals 6.4" each rotation, so that means every second, the piston has traveled a total distance of 531 inches, or 44feet per second. but its much worse than that b/c the velocity of the piston is not linear...anyhow...
2)The crankshaft drives the cams by a timing chain in the Z, some cars have a rubber (or even plastic) belt, but in the case of the belt, it does have teeth. The chain is linking the shafts which have gears on the ends, the shafts spin half the speed of the crank since the crank goes around twice, but each respective valve only opens once during that two revolutions.
3) the VQ is DOHC, so two cams above each "Bank" of cylinders. One cam opens the Ex valves, one opens theintake valves.
4) Don't know this, anyone out there have a shop manual for their Z?