Competition Event happenings in cylinder bore.....

[RacinDuallie]

Let's hear the opinions on the events that happen in the bore as the crankshaft moves the piston up and down in the bore and how to optimize the events that are happening... ... ... . from a power building perspective- any takers?

 

[RacinDuallie]

I would like a clarification on boost psi containment with-in the cylinder as the piston goes thru its events and the ways we enhance that psi holding capabilities... ... ... .....

 

[Diesel Freak]

Suck Squeeze Bang Blow... . that's all ya need to know!!!

 

[banshee]

There's not much suck with 60 psi behind the intake valve.....

 

[Diesel Freak]

lol, its all relative.

 

[fest3er]

Suck Squeeze Bang Blow... . that's all ya need to know!!!

Perhaps it's a little more involved than that.



During compression, the air heats up to a temperature that's adequate to ignite the fuel.



The fuel has to be injected sufficently early so that it begins to burn just as TDC is reached. For #2 oil, injection has to be fairly early in normal engines, and *way* early in engines running high RPM or lowered compression. For gasoline or alcohol, injection can occur later because these fuels ignite easier.



The specific time of injection depends on many things, including charge air temp, fuel temp, the 'fineness' of atomization, the ignitability of the fuel, the 'velocity of the flame fron the fuel can sustain, the temperature of the block and head.



Injection timing can also depend on whether the system is mechanical or common rail. Mechanical systems 'shoot their wad' all at 'once'. CR systems can deploy 3-10 injection events, starting with a very short pilot injection to initially increase pressure and temperature, then succeesingly longer events as the piston moves down to control how fast cylinder pressure increases.



At BDC after the power stroke, the exhaust valve opens, and excess pressure is relieved. Then the piston moves up again, forcing the still-hot combustion products into the exhaust manifold, where pressure is typically somewhere in the vicinity of boost pressure, and thence through the turbo, where the exhaust gasses give up more of their energy to spin the turbo.



At TDC after exhaust, the exhaust valve closes and the intake opens. Well, OK. You purists out there are going to *insist* that I say the exhaust valve closes just before TDC, and the intake valve opens just after, because it's not good when pistons and valves meet. Which begs the question: why is there no NBA team named, "The Valves". Pistons vs. Valves would be a natural rivalry, offensive to only the most off-the-wall group using the most off-the wall dictionary that can be found, and one team or the other would nearly always lose. But I digress.



During the intake stroke, the charge air rushes into the cylinder.

At BDC after intake, the intake valve closes, and the compression stroke begins.



Now, you want to tune the intake? Install a number of fast pressure transducers along the charge air path and determine the pressure drops along the path. You think 50PSI boost means constant 50PSI pressure throughout the charge air system? Nope. As soon as an intake valve opens, the pressure will drop on the back side of that valve until the air behind the valve starts flowing to fill the gap, and the air behind that air. So you should be able to watch waves of low pressure move backward in the charge air system until the wave meets air that is already flowing forward.



Here's a weird thought. Put a blow-off valve near each intake port. Set each valve to pop off at max boost, and size each valve to move, say, 1/4 of the volume of air that each cylinder needs. The idea is to keep air moving in the manifold so as to reduce the 'standing waves' that can build up and rob power. I won't address handling the excess pressure that builds when the valve closes suddenly. (Ever hear your water pipes clank when you shut the water off quicky? Same concept. )



Another idea: change the intake valve timing so that it closes 40 degrees after BDC. Remember, at BDC, the piston has stopped moving, but air will still be flowing in. Might as well try to take advantage of that motion as long as possible, to cram extra air into the cylinder. Man, solenoid-operated valves are looking *so* useful right about now: leave the intake valve open until the pressure by the port is almost back up to the 'constant state' pressure farther back in the charge air stream. And close the exhaust valve, say, 30 degrees before TDC and open the intake valve for 10 degrees. Then open it again 10 degrees after TDC for the intake stroke.



Huh. I never picture the working of a piston before. Clearly it is sinusoidal. But it isn't a perfect sine wave in operation. Picture a piston with a 6" con-rod and a crank with a 2. 5" arm. So it has a 5" stroke. The odd thing is, because of the con-rod's length, the piston is 2" from BDC when the crank arm is at 90 degrees, rather than the 2. 5" one would expect from a pure sine wave. Of course, this also means that the piston's max speed is not when the crank arm is 90 degrees to the cylinder. Rather it happens when the crank arm and the con rod are orthogonal to each other, which is closer to mid-stroke.



Anoter thing I'd never pondered before is the effect of con-rod length on piston travel. A short con-rod (close to the length of the crank arm) will make the piston spend much of its time near BDC, and very little time near TDC. This is great for rapidly heating the charge air (via compression) and good for fast-burning fuels like alcohol and gasoline. On the other paw, a long con-rod (much greater than the length of the crank arm) will make the piston spend less time near BDC. For example. Consider the 2. 5" crank arm mated with a 3" and a 6" con-rod. I 90 degrees of rotation, the engine with the 3" con-rod will move the piston about 4" of its 5" stroke, whereas an engine with the 6" con rod will move the piston about 3" of its 5" stroke.



So the ratio of crank arm length to con-rod length is a fair indicator of how fast your fuel has to burn and how much compression you need to ignite the fuel. Near 1:1, fuel has to burn fast, but the compression ratio can be lower, because the charge air is compressed *really* fast, allowing less heat to be transferred to the head/block. Much greater than 1:1 (like 3:1 and higher), fuel can burn slower because it has more time to burn, but compression ratio has to be higher, because there's more time for the head/block to suck heat out of the charge air.



So it's more like Succcckkkkkkkkk, ssssssssqqqqquueeze, bang, bbbbbblllow. And the 'bang' can be either bbbbaaang or bbaanng, depending on the con-rod:crank arm ratio.



My tuppence.



N

 

[cojhl2]

Very good post fest3er,



I learned a lot in a few short paras. I dont know the amswers but I now know there are a lot of questions. .



From my standpoint, thank you for taking the time to write such a good response.



But I love this also!!!!

Suck Squeeze Bang Blow... . that's all ya need to know!!!

 

[RacinDuallie]

Neal,
Indeed! I concur. Very good post indeed. Now that there is attention relative to the inner actions happening within the power process, I would like a clarification on how we use different means to control boost psi containment within each cylinder... . such as o-rings, block reciever grooving and copper head gaskets and the proper way these containment issues are combated... ... .

 

[Turbo Thom]

And I thought Neal was just another pretty face. Was I ever wrong.

 

[RacinDuallie]

Preston,
Put yer wooly socks on, pull up the rocking chair, and fill up yer coffee cup!! Posts like Neal's are very enjoyable to read. I mean we do have a Banter forum for a slight release, but let's get some good educational threads here on the competition forums...
By all means ya all know I like to Banter every now and then. :-laf :-laf


Let's hear about the proper way to groove the cylinder head's deck surface for O-ringing and the proper protrusion the stainless steel o-ring wire and the proper layout of the proceedure and reasons for O-ringing the cylinder head as a means of compression containment with in the cylinder, and the reasons that the job should be done properly to avoid potential headaches down the road. Not to mention the added benifits in the power building process for a proceedure like this.

But on second thought, Preston, that suck, squeeze, bang, blow, sounds like a girl I used to know!:-laf

 

[Fletcher]

Perhaps it's a little more involved than that.



During compression, the air heats up to a temperature that's adequate to ignite the fuel.



The fuel has to be injected sufficently early so that it begins to burn just as TDC is reached. For #2 oil, injection has to be fairly early in normal engines, and *way* early in engines running high RPM or lowered compression. For gasoline or alcohol, injection can occur later because these fuels ignite easier.



The specific time of injection depends on many things, including charge air temp, fuel temp, the 'fineness' of atomization, the ignitability of the fuel, the 'velocity of the flame fron the fuel can sustain, the temperature of the block and head.



Injection timing can also depend on whether the system is mechanical or common rail. Mechanical systems 'shoot their wad' all at 'once'. CR systems can deploy 3-10 injection events, starting with a very short pilot injection to initially increase pressure and temperature, then succeesingly longer events as the piston moves down to control how fast cylinder pressure increases.



At BDC after the power stroke, the exhaust valve opens, and excess pressure is relieved. Then the piston moves up again, forcing the still-hot combustion products into the exhaust manifold, where pressure is typically somewhere in the vicinity of boost pressure, and thence through the turbo, where the exhaust gasses give up more of their energy to spin the turbo.



At TDC after exhaust, the exhaust valve closes and the intake opens. Well, OK. You purists out there are going to *insist* that I say the exhaust valve closes just before TDC, and the intake valve opens just after, because it's not good when pistons and valves meet. Which begs the question: why is there no NBA team named, "The Valves". Pistons vs. Valves would be a natural rivalry, offensive to only the most off-the-wall group using the most off-the wall dictionary that can be found, and one team or the other would nearly always lose. But I digress.



During the intake stroke, the charge air rushes into the cylinder.

At BDC after intake, the intake valve closes, and the compression stroke begins.



Now, you want to tune the intake? Install a number of fast pressure transducers along the charge air path and determine the pressure drops along the path. You think 50PSI boost means constant 50PSI pressure throughout the charge air system? Nope. As soon as an intake valve opens, the pressure will drop on the back side of that valve until the air behind the valve starts flowing to fill the gap, and the air behind that air. So you should be able to watch waves of low pressure move backward in the charge air system until the wave meets air that is already flowing forward.



Here's a weird thought. Put a blow-off valve near each intake port. Set each valve to pop off at max boost, and size each valve to move, say, 1/4 of the volume of air that each cylinder needs. The idea is to keep air moving in the manifold so as to reduce the 'standing waves' that can build up and rob power. I won't address handling the excess pressure that builds when the valve closes suddenly. (Ever hear your water pipes clank when you shut the water off quicky? Same concept. )



Another idea: change the intake valve timing so that it closes 40 degrees after BDC. Remember, at BDC, the piston has stopped moving, but air will still be flowing in. Might as well try to take advantage of that motion as long as possible, to cram extra air into the cylinder. Man, solenoid-operated valves are looking *so* useful right about now: leave the intake valve open until the pressure by the port is almost back up to the 'constant state' pressure farther back in the charge air stream. And close the exhaust valve, say, 30 degrees before TDC and open the intake valve for 10 degrees. Then open it again 10 degrees after TDC for the intake stroke.



Huh. I never picture the working of a piston before. Clearly it is sinusoidal. But it isn't a perfect sine wave in operation. Picture a piston with a 6" con-rod and a crank with a 2. 5" arm. So it has a 5" stroke. The odd thing is, because of the con-rod's length, the piston is 2" from BDC when the crank arm is at 90 degrees, rather than the 2. 5" one would expect from a pure sine wave. Of course, this also means that the piston's max speed is not when the crank arm is 90 degrees to the cylinder. Rather it happens when the crank arm and the con rod are orthogonal to each other, which is closer to mid-stroke.



Anoter thing I'd never pondered before is the effect of con-rod length on piston travel. A short con-rod (close to the length of the crank arm) will make the piston spend much of its time near BDC, and very little time near TDC. This is great for rapidly heating the charge air (via compression) and good for fast-burning fuels like alcohol and gasoline. On the other paw, a long con-rod (much greater than the length of the crank arm) will make the piston spend less time near BDC. For example. Consider the 2. 5" crank arm mated with a 3" and a 6" con-rod. I 90 degrees of rotation, the engine with the 3" con-rod will move the piston about 4" of its 5" stroke, whereas an engine with the 6" con rod will move the piston about 3" of its 5" stroke.



So the ratio of crank arm length to con-rod length is a fair indicator of how fast your fuel has to burn and how much compression you need to ignite the fuel. Near 1:1, fuel has to burn fast, but the compression ratio can be lower, because the charge air is compressed *really* fast, allowing less heat to be transferred to the head/block. Much greater than 1:1 (like 3:1 and higher), fuel can burn slower because it has more time to burn, but compression ratio has to be higher, because there's more time for the head/block to suck heat out of the charge air.



So it's more like Succcckkkkkkkkk, ssssssssqqqqquueeze, bang, bbbbbblllow. And the 'bang' can be either bbbbaaang or bbaanng, depending on the con-rod:crank arm ratio.



My tuppence.



N

So, what isomer branch structure would you suggest to facilitate optimum combustion?

 

[Hohn]

Perhaps it's a little more involved than that.


My tuppence.

N

Good post, Neal.

Rod/stroke ratio is something I think is very important.

You are correct in your comments on how geometry affects dwell time near TDC and BDC.

Max piston accleration always occurs when rod and crank arm are perpendicular (as you mentioned).

A shorter rod means that the piston is lower in the bore at every point other than TDC and BDC, and a longer rod means that the piston is HIGHER in the bore at every point other than TDC and BDC.

The way to represent them graphically is as a sine wave, but the longer rods will have a shallower slope near the midpoint. The shorter rods will be less "peaky" at the extremes, but spend more time near the end, and tend to rush through the middle range of travel. Yes, being the geek I am, I actually made an Excel sheet that graphed piston position vs stroke for a given r/s ratio, and compared the effects of long and short rods.

This has a lot of implications on performance. For one, it means that shorter rods will "suck" harder on the ports, and that a short rod engine will need larger ports. A longer rod makes more efficient use of restrictive heads, because the longer rod gives less peak suction, and instead spreads it out over the range of travel.

It also means that a shorter rod engine needs a different cam timing entirely. Shorter rods can use a LOT later valve closure on the intake, because the piston is so much lower relative to crank angle. This means that a big cam will have a lot more cranking compression in a short rod engine, all else being equal.

Really, the long vs short rod debate hinges on what you think is most important. If the compression and exhaust strokes are most important (piston moving up), then a shorter rod is best.

If the power and intake strokes are most important (piston moving down), then a longer rod is best.


Generally, a longer rod makes more power-- period. This relates to working better at high rpm, but also because of the effect on breathing and rod angularity (bore loading).

With optimized breathing, a rod ratio of abour 1. 75:1 is "perfect" and will give the broadest tq curve. Higher than this can make an engine "peaky" with a narrow RPM band. Lower than this intruduces too much angularity for best power. Realistically, anything from 1. 7 to 1. 8 is "good".

The lower the RPM range, the lower rod ratio you can get away with.


It's interesting to note that most Mopar V-8s have MUCH longer rods than Ford or Chevy engines. MOpar rod ratios:
318, 340: 1. 85
360: 1. 71
383,400: 1. 88
413,426,440: 1. 8

This is on average a lot higher than a Chevy (especially a SBC 400!) or a Ford.


Despite the low rpm range, I bet a 5. 9 would work well with a longer rod. A longer rod would allow earlier injection timing (the cylinder builds sufficient pressure/temp earlier in terms of crank degrees), which would give the sluggish diesel combustion more time. Also, the longer rod would produce a higher average temp/pressure 20° on either side of TDC, which is good for combustion speed and power production.


The only time a rod can be too long realistically is if you are making the piston super-short to achieve the long rod. The piston has to have enough compression height to maintain bore stability.

Of course, both a long rod and a stable piston mean you need a tall/ heavier block, which is currently out of favor from a packaging standpoint.

justin

 

[RustyJC]

At TDC after exhaust, the exhaust valve closes and the intake opens. Well, OK. You purists out there are going to *insist* that I say the exhaust valve closes just before TDC, and the intake valve opens just after, because it's not good when pistons and valves meet.

Most engines are designed with valve overlap where the intake begins to open prior to TDC and the exhaust closes after TDC. Valve overlap improves scavenging by allowing airflow from the intake through the (admittedly small) combustion volume and out the exhaust. The airflow out the exhaust also provides exhaust valve and seat cooling.



One downside of valve overlap - it fouls the IAT sensor with exhaust soot if I idle the engine too long with the exhaust brake engaged. This is because exhaust manifold pressure is higher than intake manifold pressure at idle, so the flow is reversed - exhaust flows back into the intake.



Rusty

 

[Turbo Thom]

Justin... ... ... You answered my question with your last sentence. How in the world do you keep the piston off the valves, when useing a longer rod? Like you said though, our engines are tall enough now.



So that being said, are the NARCAR engines equal now as to the stroke? They are all making equal HP, and the Chev has always been know as the short stroker.



Does you theroy work with diesels and gasers?



. . Preston.

 

[Hohn]

Most engines are designed with valve overlap where the intake begins to open prior to TDC and the exhaust closes after TDC. Valve overlap improves scavenging by allowing airflow from the intake through the (admittedly small) combustion volume and out the exhaust. The airflow out the exhaust also provides exhaust valve and seat cooling.

One downside of valve overlap - it fouls the IAT sensor with exhaust soot if I idle the engine too long with the exhaust brake engaged. This is because exhaust manifold pressure is higher than intake manifold pressure at idle, so the flow is reversed - exhaust flows back into the intake.

Rusty

Valve overlap has a HUGE effect on breathing.

In a highly modded NA race engine (read: big overlap, big cam), the "suction" of the exhaust on the intake is FIVE TIMES GREATER than the suction on the intake from just the piston going down the bore.

IOW, a race engine should see a LOT of effort in tuning the cam timing and exhaust (collector length, diameter, et al ad infinitum).

Now with a CTD, this is obviously not the case because 1) the turbo kills exhaust flow and makes pulse management a lot less significant, and 2) we aren't revving this engine to high enough RPM levels that would need such a beastly cam with 70°+ of overlap.

jh

 

[RustyJC]

Justin,



Agreed, but the post I was responding to stated that the Cummins had NO valve overlap - specifically:

Well, OK. You purists out there are going to *insist* that I say the exhaust valve closes just before TDC, and the intake valve opens just after (TDC)

Rusty

 

[Hohn]

Justin... ... ... You answered my question with your last sentence. How in the world do you keep the piston off the valves, when useing a longer rod? Like you said though, our engines are tall enough now.

So that being said, are the NARCAR engines equal now as to the stroke? They are all making equal HP, and the Chev has always been know as the short stroker.

Does you theroy work with diesels and gasers?

. . Preston.

NASCAR is a weird beast. Most NASCAR V-8s are built with slightly different bores and strokes on each side of the Vee. It's like two slightly different I-4s sharing a crank. One might be standard SBC 4. 0x3. 44, the other might be a little more undersquare.

Heck, in Pro Stock, you might have EACH CYLINDER have it's own bore and stroke, just slightly different. Per-cylinder CID will remain constant, though.

There's no NASCAR rule against "flat" cranks, or having divorced crank journals (each rod gets its own) instead of sharing one journal for two rods. Some teams run them (you can hear it in the exhaust, for sure), but most gave up on them when they found no real power advantage. Besides, race teams run on money, and flat cranks are just too much $$ in machining costs.

NASCAR teams will tailor the engine to the purpose if they can. Back before the "one engine" rule, they would build really special "heaters" for qualifying, that had another 50hp or so. Then they'd build a milder version for the actual race. After all, 500-600 miles is a LONG TIME when you're running 8800 rpm with a domestic V-8.

Different teams are always experiementing with different engine geometries. The current trend is to match it to the track. Superspeedways get the shortest stroke, longest rod engines and turn the most RPM. Shorter tracks with slower avg speeds or more RPM variance will get longer strokes and shorter rods.

Keep in mind, this is relative. Even a "short rod" NASCAR engine is somewhat long in the rods because it's still a race engine.

An example of a long-rod engine was the old 305 CID T/A race engines of circa 1970. The mopar race engines (mopars are my forte) were based on 340s (which are ALREADY a long-rod engine). But the class rules specified a 305CID (5 liter) displacement limit. To make the engine smaller, the DE-stroked a 340 to make it a 305. This took the stroke down around 2. 97" (IIRC). A 2. 97" stroke with a 6. 123" rod puts you at 2. 06 Rod ratio!! That's VERY long. And this was a ROAD COURSE engine!!



I can't see why the theory wouldn't apply to diesels as well. After all, it's a theory-- and if you can't prove it true, you can't prove it false!

Seriously, though. Some of the big-dog pullers and racers and such would be wise to consider de-stroking the Cummins a bit as they run the big RPMS.

Stock rod length is 7. 56" Stock stroke is 4. 72" That gives you 1. 6 for a rod ratio.
~~~~~
If we sacrificed a couple CID and dropped stroke down to 4. 65" and added that to rod length, what would happen?

First, displacement goes from 359CID down to 353. 6CID.

Rod ratio goes from 1. 6 to: (7. 63/4. 65)= 1. 65!

Cool! We've picked up . 05 rod ratio for only a loss of 6 cubic inches! Let's try a bigger sacrifice.
~~~~~~~
Let's de-stroke to 4. 55" This takes displacement down to 346CID. But our rod ratio is now up to 1. 69!


So, in this case, we've picked up almost a full tenth of rod ratio for a measly 13 cubic inches. Given the boost levels we run, this is a worthwhile trade, imo.



The other thing about increasing rod ratio is that as you go up, increases help less. For example, going from 1. 4 to 1. 5 is a huge improvement, but going from 1. 9 to 2. 0 is modest and probably not worthwhile unless your running extreme RPM and don't need much powerband width.





The final thing about a diesel is that they run a much narrower RPM range, which negates one of the only negatives of a longer rod.

NOTE: in the above examples, we kept the stock compression height on the pistons, which are way too tall and way too heavy. A piston that's "just long enough" would allow a longer with no loss of displacement.

Combine de-stroking with a shorter piston, and you could easily get a setup with the "perfect" rod ratio of 1. 75 with little to no negative consequences in performance or durability.

jh

 

[cojhl2]

Max piston accleration always occurs when rod and crank arm are perpendicular (as you mentioned). ,, **from an earlier post**.



Not to get in the way of a technical discussion when I am on the low end of the chain of knowledge here,, but I think when the rod is perpendicular with the crank throw we have max piston velocity. Max accel occurs at BDC when the angular velocities of the crank pin and rod provide an additive radial accel.

 

[COMP461]

NASCAR motors are 4. 155 bore 3. 300 stroke 6. 125 rods with 1. 88 Honda rod journals for the most part.

Pro Stock motors are 4. 70 bore by 3. . 60 with 6. 00 rod with the same 1. 88 Honda rod journal

The difference between a short track motor and a long track motors is mostly in the cam and manifold. The 180 degree crank was tried and was not worth it , 20 years ago , and has not been on the track since.



Most of these ideals were experiments or myths that were tried, and long gone years ago. Also in NASCAR money is no object, a typical motor program is upwards of 5 million. I only have a moment to write tonight, but will expound on this tomorrow, but the whole ideal is to try any thing, and everything, because you never know what will work.

 

[cojhl2]

Whoops,,,, I thought about this today,, Max accel of the piston occurs at TDC not BDC...