4 cycle engine and number of cylinders

[KevGuy]

The 4 cycle engine has 4 stages: intake, compression, combustion and exhaust.

In a 4 cylinder engine, it's easy....1 cylinder on intake, 1 on compression, 1 on combustion and 1 on exhaust.

In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???

How does it look for a 6 cylinder engine?

 

[actionstan]

I think, and someone correct me if I am wrong, a 8 cylinder would have pistons firing closer together, but no two pistons would be in the exact same spot at the same time. I also believe(in my experience) this is why more cylinders = smoother idle.

 

[Kuato]

Originally Posted By: KevGuy
The 4 cycle engine has 4 stages: intake, compression, combustion and exhaust.

In a 4 cylinder engine, it's easy....1 cylinder on intake, 1 on compression, 1 on combustion and 1 on exhaust.

In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???

How does it look for a 6 cylinder engine?


Not exactly, depends on the manufacturer/firing order and is optimized. I'm sure they started way back with something like that plan though; but I don't know. Hopefully someone with the knowledge about crankshafts will swing by momentarily to enlighten us.

 

[ZeeOSix]

Originally Posted By: KevGuy
The 4 cycle engine has 4 stages: intake, compression, combustion and exhaust.

In a 4 cylinder engine, it's easy....1 cylinder on intake, 1 on compression, 1 on combustion and 1 on exhaust.

In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???

How does it look for a 6 cylinder engine?


The basic way to think about it is 2 crank revolutions (720 deg) divided by the number of cylinders. All the pistons in an engine, regardless of how many pistons, must go through their 4-stroke process in 2 crank revolutions. So that will tell you how many degrees each piston is "out of phase" from each other in the 4-stroke process.

So, for an 8 cylinder, 4-stroke engine there is a power stroke from one of the cylinders every 90 degrees (720/8). This also means that each piston is 90 degrees out of phase in terms of where it's at in it's 4-stroke process. So 4 of the 8 cylinders will be half way between strokes when the other 4 cylinders are at one end or the other (ie, TDC or BDC) of their stroke.

On a 6 cylinder, each piston is 720/6 = 120 degrees out of phase in their 4-stroke process.

On the 4 cylinder, it's 720/4 = 180 degrees ... and that's why each piston is at TDC or BDC at the same time.

 

[29662]

Typically yes. Assuming your talking about a typical v8(crossplane or flatplane crankshaft). Although in different phases of that particular cycle due to the cylinder configuration. The firing order is separated by 90deg of crank rotation so you would have 2 cylinders in any one cycle.

 

[29662]

Originally Posted By: ZeeOSix


On the 4 cylinder, it's 720/4 = 180 degrees ... and that's why each piston is at TDC or BDC at the same time.


Not exactly.
if your talking about an L4 with a Flatplane crankshaft(180 deg), then yes. If you're talking about a v4 or L4 that use a crossplane crankshaft, then no with respect to the TDC or BDC statement.

 

[ZeeOSix]

Originally Posted By: KevGuy
In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???


No ... as said before, each cylinder by default has to be 90 degrees apart, so no two cylinders can be on the same part of the 4-cycle process at the same time. Talking "typical" V8 design here, not some crazy design where 4 of the 8 pistons fire at the same time.

 

[Clevy]

Originally Posted By: ZeeOSix
Originally Posted By: KevGuy
The 4 cycle engine has 4 stages: intake, compression, combustion and exhaust.

In a 4 cylinder engine, it's easy....1 cylinder on intake, 1 on compression, 1 on combustion and 1 on exhaust.

In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???

How does it look for a 6 cylinder engine?


The basic way to think about it is 2 crank revolutions (720 deg) divided by the number of cylinders. All the pistons in an engine, regardless of how many pistons, must go through their 4-stroke process in 2 crank revolutions. So that will tell you how many degrees each piston is "out of phase" from each other in the 4-stroke process.

So, for an 8 cylinder, 4-stroke engine there is a power stroke from one of the cylinders every 90 degrees (720/8). This also means that each piston is 90 degrees out of phase in terms of where it's at in it's 4-stroke process. So 4 of the 8 cylinders will be half way between strokes when the other 4 cylinders are at one end or the other (ie, TDC or BDC) of their stroke.

On a 6 cylinder, each piston is 720/6 = 120 degrees out of phase in their 4-stroke process.

On the 4 cylinder, it's 720/4 = 180 degrees ... and that's why each piston is at TDC or BDC at the same time.



I cannot say for sure that every manufacturer is the same as far as firing and position however I did work on a triumph tr7 and I'm positive that engine had the 2 center cylinders in the exact same position and the 2 outside cylinders also in the same position.
So the outsides would fire simultaneously and so would the center pistons.

I vaguely remember in the early 80s ford changed the cranks on the Windsor engines to the high output version and I think that's when the Windsor engines fired as zo6 describes.
Weren't some inline 6 cylinders also designed with 2 pistons at the top simultaneously as well.
As far as that triumph goes it was my buddy's car. I want to say it was the stock engine but I can't say for sure.
ZO6 is right that some designs are as he describes but not all.
It was simpler to have 2 pistons at the top at once as far as geometry and maybe strength however today we can forge cranks stronger and with far greater accuracy so we now have engine designs that fire every 90 degrees.

I'm guessing that in an engine design that has 2 pistons at tdc simultaneously might be better for torque but not as good at spinning high rpm,and an engine that fires every 90 degrees might be better for attaining high rpm.
Just guessing though.
And please correct me if I'm wrong. I'm not positive and might have everything backwards.

 

[ZeeOSix]

Originally Posted By: 29662
Originally Posted By: ZeeOSix


On the 4 cylinder, it's 720/4 = 180 degrees ... and that's why each piston is at TDC or BDC at the same time.


Not exactly.
if your talking about an L4 with a Flatplane crankshaft(180 deg), then yes. If you're talking about a v4 or L4 that use a crossplane crankshaft, then no with respect to the TDC or BDC statement.


I'm talking about the common flat plane crank 4-cylinder configuration, and in terms of a V4 it would be again with each piston 180 deg apart in the 4-stroke process. Why would anyone build a V4 with 2 of 4 cylinders firing at the same time unless they like lots of vibration.

 

[29662]

Originally Posted By: ZeeOSix
Originally Posted By: KevGuy
In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???


No ... as said before, each cylinder by default has to be 90 degrees apart, so no two cylinders can be on the same part of the 4-cycle process at the same time.


Yes they would as each cycle encompasses 180 deg of rotation and the firing order is separated by 90 deg of crank rotation. They would just be in a different phase of any given cycle with the "paired cylinders" changing continuously, in regards to a v8.

 

[29662]

V4's use a 90 deg crank. They fire 1 cylinder every 180 deg of crank rotation, 720 deg in a 4 cycle rotation.

 

[ZeeOSix]

Originally Posted By: Clevy
I cannot say for sure that every manufacturer is the same as far as firing and position however I did work on a triumph tr7 and I'm positive that engine had the 2 center cylinders in the exact same position and the 2 outside cylinders also in the same position. So the outsides would fire simultaneously and so would the center pistons.


Just because two of the four pistons were at the top or bottom at the same time doesn't mean they both fired at that same time. A piston is at TDC twice and BDC twice in it's 4-stroke process. Multi-cylinder engines typically do not fire two or more pistons at the same time because it causes too much vibration. The pistons are usually fired by (720 degrees/# of cylinders) apart to give as many power pulses into the crankshaft as possible per crank revolution.

 

[ZeeOSix]

Originally Posted By: 29662
V4's use a 90 deg crank. They fire 1 cylinder every 180 deg of crank rotation, 720 deg in a 4 cycle rotation.


Yes, just like an I4 does. Like I said, 720/4 = 180.

Engine designs that fire multiple cylinders at the same time are odd-ball designs.

 

[29662]

Originally Posted By: Clevy
Originally Posted By: ZeeOSix
Originally Posted By: KevGuy
The 4 cycle engine has 4 stages: intake, compression, combustion and exhaust.

In a 4 cylinder engine, it's easy....1 cylinder on intake, 1 on compression, 1 on combustion and 1 on exhaust.

In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???

How does it look for a 6 cylinder engine?


Clevy, each one of those pistons on the TR7 was in a different cycle.

I agree with ZeeOSix on this one.

The basic way to think about it is 2 crank revolutions (720 deg) divided by the number of cylinders. All the pistons in an engine, regardless of how many pistons, must go through their 4-stroke process in 2 crank revolutions. So that will tell you how many degrees each piston is "out of phase" from each other in the 4-stroke process.

So, for an 8 cylinder, 4-stroke engine there is a power stroke from one of the cylinders every 90 degrees (720/8). This also means that each piston is 90 degrees out of phase in terms of where it's at in it's 4-stroke process. So 4 of the 8 cylinders will be half way between strokes when the other 4 cylinders are at one end or the other (ie, TDC or BDC) of their stroke.

On a 6 cylinder, each piston is 720/6 = 120 degrees out of phase in their 4-stroke process.

On the 4 cylinder, it's 720/4 = 180 degrees ... and that's why each piston is at TDC or BDC at the same time.



I cannot say for sure that every manufacturer is the same as far as firing and position however I did work on a triumph tr7 and I'm positive that engine had the 2 center cylinders in the exact same position and the 2 outside cylinders also in the same position.
So the outsides would fire simultaneously and so would the center pistons.

I vaguely remember in the early 80s ford changed the cranks on the Windsor engines to the high output version and I think that's when the Windsor engines fired as zo6 describes.
Weren't some inline 6 cylinders also designed with 2 pistons at the top simultaneously as well.
As far as that triumph goes it was my buddy's car. I want to say it was the stock engine but I can't say for sure.
ZO6 is right that some designs are as he describes but not all.
It was simpler to have 2 pistons at the top at once as far as geometry and maybe strength however today we can forge cranks stronger and with far greater accuracy so we now have engine designs that fire every 90 degrees.

I'm guessing that in an engine design that has 2 pistons at tdc simultaneously might be better for torque but not as good at spinning high rpm,and an engine that fires every 90 degrees might be better for attaining high rpm.
Just guessing though.
And please correct me if I'm wrong. I'm not positive and might have everything backwards.

 

[ZeeOSix]

Originally Posted By: 29662
Originally Posted By: ZeeOSix
Originally Posted By: KevGuy
In an 8 cylinder engine is it safe to say 2 cylinders on intake, 2 on compression, 2 on combustion and 2 on exhaust???


No ... as said before, each cylinder by default has to be 90 degrees apart, so no two cylinders can be on the same part of the 4-cycle process at the same time.


Yes they would as each cycle encompasses 180 deg of rotation and the firing order is separated by 90 deg of crank rotation. They would just be in a different phase of any given cycle with the "paired cylinders" changing continuously, in regards to a v8.


Read carefully what KevGuy asked. The answer is no. On a typical V8 there is no way that there are a pair of cylinders on the same exact part of their 4-stroke process.

Each 4-stoke cycle comprises 720 degrees (2 crank rotations), not 180 degrees. At any give time, all 8 pistons are in a different phase of the 4-stoke process ... they are all out of phase by 90 degrees of crank rotation (720/8 = 90).

 

[weasley]

Look up "big bang" engines to see how firing order does not have to be spread evenly around a crank's rotation.

 

[29662]

Originally Posted By: ZeeOSix
Originally Posted By: 29662
V4's use a 90 deg crank. They fire 1 cylinder every 180 deg of crank rotation, 720 deg in a 4 cycle rotation.


Yes, just like an I4 does. Like I said, 720/4 = 180.

Engine designs that fire multiple cylinders at the same time are odd-ball designs.


You're forgetting that an "L4" Flatplane has 2 pistons at the same position but in different cycles. In a v4 with a crossplane crank the pistons are not aligned like that because of the separation of the "v".

 

[29662]

Originally Posted By: weasley
Look up "big bang" engines to see how firing order does not have to be spread evenly around a crank's rotation.


I've got one. That's what I'm trying to explain to Z.

 

[ZeeOSix]

Originally Posted By: weasley
Look up "big bang" engines to see how firing order does not have to be spread evenly around a crank's rotation.


Yes, one of the odd-ball designs.

 

[ZeeOSix]

Originally Posted By: 29662
Originally Posted By: weasley
Look up "big bang" engines to see how firing order does not have to be spread evenly around a crank's rotation.


I've got one. That's what I'm trying to explain to Z.


I don't care about some odd-ball engine design ... I'm talking about the common typical engine designs that have been used over the last 30 years. KevGuy wasn't asking about a "big bang" engine design, he was talking about the typical 4, 6 or 8 cylinder engine design.

In the engines I'm talking about, the piston firings are spread evenly over the crank rotation to give it a smoother operation. So 720 degrees/# of pistons = degrees between firings, and degrees out of phase in the piston's 4-cycle process.