The rod bearings receive lubrication before any other moving part of the vehicle.

You are correct, I meant to say main bearings.

You are correct Pablo, engine rebuilds are not a frequent thing. In fact, very few modern engines ever make it to the ppoint where they wear out. Most of the time, the vehicle is scrapped or dies either because it was involved in a major collision, the transmission was never serviced and failed, the cooling system was not properly serived and a head gasket was blown, or various other things that people decided they did not want to fix.

So yes, replacing accessories, and other parts over the years often costs more than a rebuild.

 

I don't think you fully understand the operating mechanics of an engine, so I will test you with a little quiz.

1: How many cycles are there in a modern 4 stroke engine?

while there are only 4 strokes, there are actually 8 cycles.

2: Why are the cylinders honed instead of polished?

Even chrome plated bores are honed. Piston rings are never perfect and will never form a full seal from the factory. When you break the engine properly, it will wear down the piston rings either until the rings form a full seal, or until the hone is polished off. If you tear down and engine and find that part of the hone was polished, and other parts still had the hone pattern, that engine was never properly broken in. A proper break-in consists of high RPM, WOT, high load runs within the vehicles first 20 miles. If this is not done the engine will set in with a partial seal and both power and longevity are lost.

3: What is the #1 cause of mechanical engine failure?

Improper break in. Blow by from improper ring seating causes the oil to break down. This break down causes both chemical wear and can cause the oils viscosity to break down, causing poor lubrication the that parts that need it the most.

4: Do rod or main bearings break in?

You got this one right, if the crank journals touched the bearings for even a split second during engine operation, the bearing would be destroyed.

5: Does a larger exhaust system produce more average power power?

Right again. Larger exhaust systems can help the engine produce more power under high RPM, WOT conditions. For street use, the changes may not be noticeable, even detrimental. Was getting on the freeway the other day. There was this turbocharged Pontiac Grand AM with a 3 inch exhaust. I was in my 4.0L Aerostar. We were next to each other at the light. He was eager for the light to change. As soon as that light changed I took of and left him in the dust. As I look back in my mirror, everyone else is passing him too. Even a Geo Metro back there was passing him. About the time his engine really started to produce some serious power, and the turbo go some boost going, his transmission shifted. It would certainly appear he actually has a LOT less power than a stock Grand Am. That is related to the above statement that a 4 stoke engine has 8 operating cycles. The pistons don't pull air into the cylinder during this intake stroke, it is a 2 edged sword, the moment that valve opens, there is a brief purge cycle (the intake and exhaust ports are both open at the same time) fresh gases enter, and the remaining exhaust gases leave. You do not want any exhaust gases left in the chamber for the next power stroke, so this purge cycle occurs. The exhaust valve closes and the incoming gases literally force themselves into the cylinder, the intake valve in fact, does not close until the piston is on its way back up, and the intake gases are still forcing their way in when the intake valve closes. This mysterious force, that is driving these gases in is induction, or as it is called, natural induction. It occurs when the gases traveling through the intake tubes, manifold and ports are moving though at high speeds which gives these gases momentum. Same with the exhaust system. If the intake or exhaust pipes are too large, sufficient velocity to produce this effect is never achieved, or is only achieved at high RPMs. Smaller intake and exhaust designs result in more torque (power per stroke) available at low RPMs, which is desirable for vehicles intended for street use. Ever wonder why Ford's 5.4L V8 has so much more power than the old 5.8L? Check for yourselves, the 5.4 was designed for fuel injection and takes full advantage of this induction phenomenon. It has higher velocity ports than the 5.8 did.

 

This may be symantics, but a "cycle" in a piston engine is formally defined as a complete 4 (or 2) stroke sequence; One complete cycle contains the sequences we all know: suck, squeeze, bang, blow, and they can be done in two or four strokes. It seems like you're splitting each stroke, or phase, of the cycle into sub phases, and calling them "cycles". Your specific description is of the implementation of a modern Otto or Diesel cycle, but the terms are confusing.

A 2 stroke engine depends a lot more on the scavanging effect of the exhaust than a 4 stroke, but it still has the same basic phases of intake, compression, combustion, exhaust.

On the other hand, Mazda's Miller engine pushes some of the intake charge out of the cylinder during the first part of the compression stroke. It is one of the few engines whose compression ratio is less than the expansion ratio, netting a slight gain in efficiency over the conventional Otto cycle, which arguably is consumed by the supercharger that's needed to make the cycle work.

 

Mazda's Miller engine does not push some of the intake charge out at high RPMs however. But at low RPMs it looses that and without the supercharger, it does not produce much usable power. It is really not different than any other gasoline engine.

In a 4 stroke engine there is no suck cycle if the design is properly tuned. Yes air is draw in, but as an overall effect, once the gases are in motion, they pretty much drive themselves. Let me explain how this works as simply as I can.

The 4 stroke engine as shown is textbooks does not exist in automobiles. Only the simplest single cylinder designs operate on a 4 stroke 4 cycle principle. Whenever you have more than one cylinder on a bank, you get an effect called natural induction. This principle is based upon the velocity of gases in a tube.

Let me break down each cycle as it occurs in a modern engine

1: Suction. Lets start at the beginning of the intake stroke. The piston is beginning its downward motion. The intake valve is already open and the gases in the intake are already in motion and the chamber is already filling. As the gases come through the intake port and around the valve, they build up velocity. This velocity is important in the next cycle.

2: This is where our textbooks tell us the compression stroke begins as the piston moves back up. What they don't tell you is that the intake valve is still open. Now hold on, wouldn't that cause loss of pressure? Yes and no, the velocity built up in the early part of the first cycle prevents too much pressure from being lost. At low RPMs some of the pressure is lost, but at higher RPMs the intake velocity is high enough that there is no loss off efficiency.

3: Now we get to the compression cycle. The gases get compressed to improve the temperature and efficiency of the burn.

4: Pre burn. The last part of the compression is really not about compressing the mixture, but rather, the spark is fired and the flame front spreads. This part produces no power, but rather accelerates the burn rate. As the burning gases get compressed, the flame front travels faster, this is important because if the flame front does not spread quickly enough, the mixture near the outside edges furthest from the spark would not burn. Too much advance, and the gases begin to expand before the mixture finishes compressing, and the remaining mixture detonates via 'shock-wave'. This spontaneous burning across the entire surface creates the familiar knock or pinging sound and results in excessive heat as well as the mixture slowing the piston down.

5: The power cycle begins as soon as the flame front has spread across most of the pistons surface This is when the actual power is produced. The expanding gases drive the piston downward and turn the crank.

6: Exhaust phase. This phase occurs before the piston reaches the bottom of the cylinder. The gases are still burning and expanding, but losing energy. The hot gases rush out the exhaust port, which opens at this time. The hot gases create velocity in the exhaust port. This is important because it plays and important role in the next phase.

7: The scavenge phase begins just as the piston reaches bottom. The high velocity gases created sufficient momentum to create relative vacuum in the cylinder. The piston begins its upward stroke, but it is not pushing out exhaust gases, but rather it is being pulled up by the vacuum that has formed.

8. This next phase is know as the purge phase. The piston is still moving upward, and at this time the intake valve opens before the exhaust valve closes. This is important for several reasons. 1. It eliminates any remaining exhaust gases. 2. the vacuum is still present in the chamber and pulls gases from the intake, priming them and starts the intake gases in motion through the intake port This is important, because this velocity that forms is what causes the next phase to work.


If you pay attention, I have not broken the strokes down and called them separate cycles, because these events do not coincide with piston activity, but rather with valve timing and ignition timing. Things are happening seemingly out of sync with the pistons. as mentioned above, the intake valves are still open as the piston moves upward. Yet this is still considered to be part of the intake or suction cycle. The intake gases are still moving into the chamber and filling the cylinder. It is all a matter of the pros and cons of the different configurations, if you close the intake valve sooner, you get better compression and efficiency at low RPM, but penalize the higher parts of the power-band. Some automakers have figured how to get the best of both worlds. Honda's Vtec engine uses variable valve timing. Ford and Mazda use 'jet' valves that open fully at higher RPMs. Everyone is aiming for smooth progressive torque curves, that increase towards redline. By implementing valve timing tricks, BMW has come very close, with an engine that produces high torque at all RPMs that increase all the way to redline. Reports are that you can start out in any gear and smoothly accelerate (lower gear still give you a lot more go) without the customary lugging that is caused by the gases escaping through the intake valves at low RPM.

 

............

 

Again, the formal definition of a "cycle" is one complete set of events, whether you separate them into 4 or 8 seqeunces, or a continuum of fluid flow.

Engine builders have been making use of flow inertia to help fill and scavange cylinders for decades, sometimes so optimizing the effect at one speed that the engine is dead at others. Unless they use variable valve timing.

By the way, BMW's 400hp 5liter engine seems impressive, but its reliability is pretty poor.

 

A lot of BMW's stuff is unreliable. And that is correct that engineers have been using velocity tuning for decades, the amazing thing is, the engineers know all about it, and yet the vast majority or engine tuners and performance tuners know little or nothing about it. I didn't really break these down into more sequences, because each one is a distinctly different thing and cannot be blended into one of the neighboring cycles without leaving out important details about the actual function of the engine. I suppose this is off topic, I only finished it up because I left my earlier quiz on the subject. Ford's 'jet' valves like those found on most all Ford engines are a very reliable and consistent design. My Mazda B2600i uses the same concept. At first the stats seem unimpressive, 125 hp and 150 ft.lbs torque. But if you turned it into a V8, at a minimum of 250 hp and 300 ft.lbs. torque, it was ahead of its time, a 5.2L with more power than a Chevy 350 of the day. It had more power than the Ford 5.8L and the Chrysler 5.9 as well. Might have had cooling problems however. They built some prototypes, but the design never went into production. It used one intake valve, a rather large exhaust valve, and a 'jet valve' that opened as oil pressure increased which coincides with higher RPM. The design was reliable so long as quality oil filter with quality check vales were used, otherwise the hydraulic lifters would get plugged with varnish and fibers from the oil filter. It would have stood up well against todays engines, but should have had a 'jet' exhaust valve instead of a single larger port.

 

A little knowledge is a bad thing.

Ken

 

Actually, if you are tuning a vehicle to change its performance, these things do need to be considered. On my truck, I needed more torque to haul the loads I subjected it to. It has a 2.6L Mazda G6 engine. I replaced the factory intake hose and air box with a new one that had fewer bends, but a smaller diameter. Result, the truck may not have more horsepower, but it picked up considerable torque. It can haul loads exceeding 2,000 lbs in the bed up 5% grades at 75 mph or more in overdrive. Neither the Aerostar nor our Dakota can do that, and both of them have a lot more power than the Mazda 2.6l 4 cylinder engine. So I have tested these theories. Velocity tuning is often a bad thing for racing however, depending on the circumstances. A little knowledge may be a bad thing, but ignorance can be far worse. What you don't know can and routinely does hurt you.

Back to the original subject, if you have the oilpan dropped anyway, check the rod bearings. If you suspect there may be a problem it is better to fix it before the bearings fail, then to let them fail and take something else out with them. Bearings are meant to be disposable and are designed to absorb the damage of failure or small particles in the oil, leaving the crank unharmed. However, so long as oil changes are regularly performed, and a good high quality oil of the correct viscosity is used, and high quality oil filters are used (no Fram), the rod bearings should never wear out or fail. The reason you parts stores carry them, is because there are a lot of people out there who don't take care of their engines, and they seize up. Also, if antifreeze gets into the oil it can cause the bearings to fail.

 

ken,
"Sometimes too much knowledge canNOT be a good thing" Martha Stewart in prison


can I take the ankle braclet off yet or is there more in the lecture session?

 

Who is Martha Stewart?
Yep.... quote from old Russian moivie.... "As my old friend, he is dead, used to say, "witneses do not live too long life".....
Litvinenko new too much....

 

http://www.marthastewart.com/

 

Aeros are a GOOD thing, Martha is NOT

copper,
please close this thread

my oilpan has fell off...i think i am stuck in the rinse CYCLE and i know i have been STROKED
my crank bearings spun a long time ago and I can't git 'er done on POWER STROKE anymore

 

Cliff,

That's what modern medicine is for, and you know exactly what I mean.

Well, since this thread has served the purpose, and we have strayed far enough off topic, I'll close it. However, any of you are welcome to open up a new thread.

Regards