Understanding piston dwell around 0 degree crank angles

[96blak54]

Perhaps im over thinking it and perhaps my mind has to figure out the "whys". But in any case i feel knowledge is always power! Best to dive in with a brain full of ideas! So...anyhow....i got down to wondering about piston dwell when the piston is near( before, during, after) top dead center and bottom dead center. As in, "where is the pistons location relative to crankshaft crankpin angle". Associated with this dwell are valve event timing, ignition timing and exhaust blow down timing. The mechanical of the engine we can control like valve events, but we can not control the timing of fuel igniting, fuel burning, and exhaust leaving(we will call this one blow down)



This has pondered with me for a little while and i finally sat down, pen, and paper to figure it out. I wanted to know how much differences their are between piston dwell in a 4.6l and dwell in a 5.4l.



Starting off, my theory is that a 5.4l had more dwell at top/bottom than the 4.6l due to the longer stroke and longer connecting rod. "Where is the crank angle at a given piston location such as .050" from dead center? Are they the same between the 2 platforms?" Timing of ignition and the blow down are greatly affected with this dwell time and im wondering which has the advantage ..So pen and paper with a few good online calculator, i began crunching numbers.



First off,...



I start searching the crank angles for both platforms when the piston is .050" from dead center.



4.6l is 12° and the 5.4l is 11° while piston is .050"...



WAIT....what? This aint right. So i run the numbers again. To my surprise the stroke and rod ratio of the 4.6l has more crank angle piston dwell when at .050" before and after dead center. This doesnt seem right because when thinking about it, one pictures the crank pin circle of the 4.6l smaller than the 5.4l and the perimeter distance of this circle is shorter. Perhaps their is a change when at .100".



4.6l is 17° and the 5.4l is 15.5° at .100"



This dont make sense, unless we play in the part of stroke percentage and piston speeds. Still though....the 4.6l has more dwell at a given piston location. A whole 2° more when added together before and after dead centers. This means 2° more time for fuel igniting, burning, overlap, incoming rush and exhaust blowing down. Having a few more degree of dwell allows time for these functions. Also explains why the 4.6l is the better performer.

 

[OLD H2S]

What about wrist pin location? The 5.4 piston is way down the hole..

 

[96blak54]

Pistons are the same dimensionally. The pistons are over .100" down the hole atdc.

 

[96blak54]

Now that we have piston dwell time on the mind, lets cognitively put it together!



Lets start with overlap! Overlap is when exhaust and intake valves are open at the same time as the piston nears the top at the end of the exhaust stroke. Ideally... as the exhaust exits, fresh air gets pulled in from the intake. This is a gain of function known as scavenging and can be maximized by how the exhaust system is set up, but lets stay focused on dwell benefits.



Piston nears top dead center while exhaust stroke. Intake valve cracks open before piston reaches the top. Piston starts the intake stroke moving down the bore pulling in fresh air while exhaust valve is closing.



HYPOTHETICALLY HERE....Lets give a cam 24° of overlap. Lets split that number in half while piston at top dead center. Thats 12° before and 12° after. We use the SAME piston location for both 4.6l and 5.4l. After doing the math we find the the 4.6l has 2° more dwell than the 5.4l. This means more time for scavenging to pull in fresh air while at extreme rpms. More time than the 5.4l.



Fast forward to the compression stroke. Same piston approach....nearing the top, but this time squeezing the mix. So thats, piston squeezing, ignition igniting, flame propagation(this is where the flame is covering the bore diameter), combustion, piston being forced. We will focus 40° here. Split that in half, 20° before tdc and 20° after tdc. So thats 20° of advance ignition timing to light the mixture off. The 4.6l has almost 4° more dwell than the 5.4l.

 

[OLD H2S]

Dwell time is going to be the spot for detonation, as long as the mixture is moving no problem, when it stops bang. The first Npi head did not have much in the way of quench going for it.

 

[tvsn95]

the rod /stroke ratio is what makes the diff. rule of thumb the longer the rod the longer the dwell. all other being equal . thats why high rpm motors normally use long rods , to give burn time in the chamber. fuel burns at the same relative rate regardless of RPM.

 

[96blak54]

Good info ^

And yes the 4.6l rod/stroke ratio is greater than the 5.4l.

4.6l is 1.68 ratio and the 5.4l is 1.6 ratio

 

[96blak54]

Great! Moving on...



So we've covered the dwell time benefits for both top dead centers (TDC) of the 4 stroke cycle. Which only means now we cover the opposite dead centers at the bottom.



Picture the piston moving down the bore. Piston before bottom dead center (BBDC), piston bottom dead center (BDC), piston after bottom dead center (ABDC).



Remember the dwell timing of the 4.6l. I was using the piston at .050" before and after locations for reference when at the top dead center. Ill use the same for the bottom. Also note that using this piston location has no real significance when engine building. Im merely using the location as a tutorial in this thread to help understand piston dwell time and the associated attributes of the internal combustion engine.



Ok, piston on its way to a dead stop at the bottom when drawing in fresh air and fuel mix known as the intake stroke. Piston reaches the .050" BBDC and thats when we start to think about piston dwell.



Remember at TDC we had 12° of dwell from piston location .050" before and after. Since this scenario is a mirrored event. The bottom is the same as the top. So we know there is 12° of dwell within these locations.



As piston slows to a stop, approaching the bottom from a blistering unimaginable mid bore speed while drawing intake air in, a gain of function has been created through the intake manifold runner. The blister intake flash of incoming air/fuel being drawn in at thousands of feet per second, ....causing this mass to become a solid form that actually holds weight. This solid form rushing in as the piston slows is forcing itself through. Then as piston begins upward, this mass still holds weight....still forcing itself through. Just as the intake valve begins to slam shut.



Digging in deeper,...the valve slamming shut creates a reversion wave backward through the intake runner and when the timing is right, the wave comes back to the valve aiding the incoming rush, forcing it. Sort of a supercharged effect. This is known as Helmholtz resonance and its that intake growl you hear when the rpm is right.



Recap



The gain of function here during piston dwell is incoming rush becoming a supercharger for a brief sec as the piston changes direction.

 

[OLD H2S]

A way to think about it is the mix is not sucked in by the vacuum in the bore, the higher pressure in the manifold is PUSHING the mix into the bore fast and when the piston is at BDC the big push is still happening from the inertia of mixture mass with the valve cutting off the extrusion causing a stall in the manifold runner. This was easier when gasoline weighed more before the Smirnov spritzer we use as fuel now.

 

[ttocs]

so its working like an air pump?

 

[96blak54]

Stop trying to mess this thread up ttocs!

Its nowhere near an air pump! Im angered when people simplify an internal combustion engine as an air pump.

 

[Boostr1]

Stop trying to mess this thread up ttocs!

YEAH! I'm tryn to get learnt here...

 

[ttocs]

Its nowhere near an air pump! Im angered when people simplify an internal combustion engine as an air pump.

O really?

 

[96blak54]

If you have read this far im sorry. No visuals or picture aids makes for a boring thread, but i wanted to break down the connecting links of the 4stroke engine and also how why the 4.6l is so robust in power. Hopefully i have broken down explaining the gain of functions during piston dwell.



Other key aspects to remember about the modular platform are tiny pistons, light rods, internal balanced are a few. Flame propagation across the tiny bore piston doesnt need more advance timing compared to a bigger bore. The flame sweeps the bore, goes into compression, applying pressure on the piston sooner than it would if the bore was bigger diameter. Then their is less weight, less surface friction..etc. You get the idea.



So here we are at the final piston dwell. It is the 4th stroke in my description and what is considered the biggest gain of power when speaking camshaft wise, but its also never spoken of and ill try to explain why later.



Again, its timing of the non-mechanical to run its course during piston dwell is the main focus, so keep this thought in mind.

 

[96blak54]

Ignition btdc, ....flame propagation during dwell, ....and a few degrees after tdc of crank rotation, ....combustion happens, pushing the piston down the bore.



Now here is the funny about an engine. Just as there are points of which the piston comes to a slow and complete stop, their is an opposite where the piston speeds are fast. This area is found in the stoke middle.....refered to mid bore stroke.



At high rpm, piston speeds become so fast....the piston begins to outrun the combustion. Or i should say, the combustion to piston speed becomes moot. A longer stoked engine, this scenario happens at a lower rpm than a shorter stroke engine.



In example 4.6l and 5.4l. Comparing both mathematically to a set piston speed, the 5.4l will be lower in rpm to the 4.6l. If i recall correctly its roughly 1000rpm. So a 4.6l at 7000rpm has the same piston speed as a 5.4l at 6000rpm. Keep in mind the piston outruns the combustion.



We are at mid bore. The piston is begining to slow and then the exhaust valve opens. We will call this process "blow down". Blow down will refer to the pressures on the piston while exhaust escaping as the piston slows to bdc. In general, more pressure at blow down means more pressure on the piston during the end of the stroke ....equal more power! Yes, exhaust pressure is where the real power is!



When the exhaust lobe cracks open the exhaust valve, generally around 55°- 60° before bdc, THOUSANDS of exhaust psi burst out on that initial valve cracking off the valve seat. This means their is roughly 55° more crank rotation to bottom dead center! Thats 55° of time for the exhaust to escape....blow down. This is the BOOM we hear from the exhaust. And after the piston reaches the bottom, coming up pushing the remaining exhaust out, the valve is nearly at full open. Dont forget there was a load of no piston movement during dwell.



Now we got to ask "How much time is needed to allow exhaust pressures to escape?" Or should we ask "how much more power can be had delaying that exhaust valve opening?" Here in lies the power secret!



Hmmmm...55° worth?.... add in the piston dwell at .050"... maybe 61° worth. Cause by the time the piston is stroking the exhaust out, the valve is near full lift.

 

[96blak54]

What we dont want, is the engine having to do work pushing the exhaust out. Lucky for us, the exhaust volume is huge compared to what it was before combustion and it wants to escape into a bigger area. So it really doesnt want to hang around inside the cylinder basically moving on its own, but residual exhaust has to be pushed out and the engine has to do it. Keeping that volume moving out with piston speeds is the key.



Consider exhaust valve seat size in the blow down event. This is the orifice from cylinder to the manifold. A smaller seat/orifice would require more time for exhaust evacuation. A smaller size valve increases blow down, but earlier in the upper rpm. A larger valve reduces the scenario and requires more rpm, unless opened later. Opening later means all the exhaust pressures being applied at strokes end.



Let me try to make an understanding here.



Two stock identical 4.6l engines but one of them is boosted. The boosted engine is 100hp more than the naturally aspirated engine. Both having the same valve events, but why? In theory the boosted engine needs more exhaust valve timing. So it only makes sense the added volume going in means a bigger volume going out and why needing a little bit of more valve timing.



We think the power from boosted applications ly solely on the combustion aspects never thinking about the blow down. When in reality its both. Bigger combustion equals much bigger exhaust blow down. Meaning greater pressures at stoke end as exhaust event happens.



But im not focusing on boosted applications here. However, same rules apply!



Some say, "this doesnt make sense. If blow down pressures increase horsepower, why do we change the exhaust system to free flowing?"



The answer to this is the timing from valve barely opening all the way to the begining upstroke.....this is blow down. .

Pushing the exhaust out in that upstroke is where the free flowing exhaust benefits.



A stock 4.6l pi cam exhaust begins to open roughly around 60° bbdc. Or from tdc 120°. Fast or slow opening rate, once the valve loses seal off the seat. Nearly 70% of the exhaust volume escapes with tremendous force and it takes high rpm piston speeds to catch the blow down scenario. Thinking this way, we can move the valve opening point later, finishing up the power stroke with alot more pressure and then we utilizing piston dwell to finish out the mass majority pressure.

 

[96blak54]

Long story short....

If we place the piston an 1/8" before top and then 1/8" after the top.....this goes for the bottom of the bore too.

The 5.4l is 17* before and 17* after with a piston to cylinder head gap of 1/8".

The same 1/8" piston to head gap with the 4.6l has 20* before and 20* after. Making total dwell at 1/8" gap 6* more over the 5.4l.

More time for combustion, valve overlap scavenging when piston is at the top. More time for cylinder filling, exhaust evacuation while piston is at the bottom. More time allowing for more power potential.

What i hope you gain here is a deeper understanding of the cycles an internal combustion engine has. Also why the 4.6l is such a great design.

So apples to oranges, 4.6l to 5.4l....which platform would dominates all out power? That would be the 4.6l and its all because of the 6 degree more piston dwell time.

P.S
Ill also note that the coyote has the same stroke and rod length

 

[ttocs]

I think the lack of visual is what looses me. I get lost in the little details of the words. I learned that I needed a little visual in tech school to go with the words or it just does not stick to me... I wish I understood this more

 

[Boostr1]

Some whiteboard animation would be great...

 

[96blak54]

The 1st post has white board animation.