My 1955, 15th edition, Machinery's Handbook has factors of safety listed for single and double-acting engine piston & connecting rods ranging from 9 to 18. Somethings probably never change, or do they? What are current factors of safety for modern internal combustion engines? The largest factor of safety is for the reversing (tension/compression) load. Seems like tension would be the highest destructive force in race engines?

And, does anyone have an idea how the relationship between combustion force and crank position relate to torque at the flywheel. Seems to me the highest force is when the crank throw is far from perpendictular to the con rod TDC (bad mechanical efficiency for a lever).

Additionally, what sort of combustion chamber forces are present when an engine operates through the various stages of detonation (unaudible detonation, audible, carbon specs on plugs, aluminum specs on plugs)?

Any thoughts or direction to a website would be appreciated.

 

tension and compression are definately there. it is why wrist pins break, rods stretch and crankshafts snap (sometimes)

Inside the Internal Combustion Engine (ICE) is a very dynamic place. It is a bit more than what is in a statics class with vectors and such.

There are also lots of things that go on like detonation, balance issues, pulses and waves etc. Someone I know on a university SAE F1 team (or something like that) is designing the intake manifold for their car. the problem is that air velocity is reaching Mach 1 and is causing the intake manifold to break, or some nonsense like that. That doesn't really hold true for the common ICE though.

There is also thermal expansion. Set rings right and go spray. run lean, rings expand, close gap, no where to go but boomville.


There are also shear forces, obvously along with frictional forces (think spun bearing, windage etc) springs, etc.

As far as combustion force vs torque, keep in mind that as the piston lowers from TDC, the flame will eventually start to get weaker and the pressure is decreasing. You really want your timing to occur at the propper time. Id love to elaborate on that, but while doing a search today at work I came across a great thread about timing by an RKrause or something of that nature. Explains it well enough for me to reffer you there instead of explaining myself

here it is

http://web.camaross.com/forums/showt...ghlight=timing

 

"Safety factor" (SF)for engine design depends almost entirely on the application. A diesel truck engine which is expected to run millions of miles and maybe a billion revs with just replacement of the wear parts and no failures, has a far different SF from a Top Fuel engine which runs about 600-700 total revolutions in anger before being rebuilt, often with parts that have started to fail or have failed under load.

A higher SF implies stronger parts which often means larger parts. In a performance engine, bigger, heavier parts slow down acceleration, increase the inertial loads and make the engine larger and heavier. This decreases the acceleration of the vehicle which means more power is needed, so the engine is more highly stressed, so parts need to be stronger, etc. It's a viscious circle.

Going the other way by using stronger, lighter (and often MUCH more expensive) parts helps overall performance. A good example is the current Formula 1 engines, which get 900+ hp from 183 cubic inches NA, and weigh less than 220 lbs. Some probably are considerably less. Of course, they spin 19000 rpm and have 10,000 gs on the piston as it reverses. With pistons about the same diameter as a 5.7L LS1, that requires some pretty strong parts. A 9500 rpm Cup engine has about 5000 gs on the piston.

Race engines are designed with (just) enough SF so the engine can make it thru the race, with maybe a small margin. He who cuts the margin to the minimum often has the fastest car. A typical Nextel Cup engine might turn about 1.0 to maybe 1.2 million revolutions in anger in a race weekend. F1 races are shorter, but with twice the revs, they also turn about that many total revs. F1 is talking about "two-race" engines.

I agree that tension loads on the rod/piston at the top of the exhaust stroke are the most severe unless you are talking about a Top Fuel or severely overboosted and nitrous'd engine.

Normal combustion produces fairly steady loads on the piston which increase somewhat gradually then decrease smoothly. Detonation causes more of an impact load, sorta like getting hit in the teeth with a sledge hammer. Prolonged detonation is the kiss of death. Very short term mild detonation might be survivable in something like a drag race engine (or an Engine Masters dyno engine!), at least for a number of runs. For other applications you probably want to avoid it.

My $.02

 

No modern production engine SF then? Gas, diesel, whatever. I can fully understand the race engine math and reasoning.

All good information thanks fellas. Wow 19000 rpm! I like CART, but haven't watched it in years.

Or a pump gas drags engine. I see one of the fastest 540s used Brodix big duke cylinder heads based on a marine engine design specifically designed to reduce the tendancy to detonate. Personally, I think this contest is bad for the general hot rod population. It's implication is that anyone can run 8s & 9s on pump gas. These guy's can, although, I understand It's some blended, special formula "pump" gas that may or may not correlate to my 93 down the street. Most people will never achieve 8s or 9s on pump gas without some sort of short or long term engine damage. If had a nickle for every engine I saw damaged, and even I damaged, by trying to run pump gas, needless to say I'd be a rich man. The single best lesson I've ever learned is to avoid detonation altoghter (thank you randy lambert, big bad yellow Impalla, 1993 NMCA fastest street car shootout runner up fame).

Anyway, I'm still interested in the tremendous (slegdehammer to the teeth) cylinder pressure created during any stage of detonation someone knows something about. Perhaps there are some cylinder pressures people could point out that a production engine is capable of sustaining. Thanks in advance.

 

yea, i'm kinda interested in that to. i never thought about it in terms of exhaust vs compression/combustion stroke. i would've thought about decreasing the resistence in the exhaust stroke as much as possibly to pic up some power, but never thought about maybe having smaller exhaustports just for that reasson. cause now that i think about it its like a boxer throwing his hardest punch at a punching bag (compression/combustion). and then a boxer throwing his hardest punch at hit shaddow (top of that exhaust stoke). is it really that severe?

 

One of the interesting things with detonation is that you can see it on piston pins. They will be slightly flat spotted and go from a round shape towards a multi sided object. You can see the marks when you external hone a set of pins because it will take the high points off the pin first before it gets into the valleys. You can't see high RPM loads on the parts as well as you can see detonation loads.

As for the pump gas drags.... or any contest where pump gas is used it's really watched closely what gas is used, so any blending is not legal. I've seen some gas that was good under WOT, and probably better than the stuff you get at the store, but somewhere someplace that gas is whatever octane they say it is since every state has a different formula for what = what octane.

I think that the pump gas drags will go farther and farther. The biggest influence it will have is on combustion chamber design, those Big Dukes on that 540 or one of Jon Kaases Engine Masters motors show that. When guys are running a REAL 9.0+ DCR on pump gas they are doing something right. Things like the Pump Gas Drags and the Engine Masters only survive if there is someone to run them though, but they do bring things out that the average guy can use to go faster.

Bret

 

The high loading at the top of the exhaust stroke is the result of the inertia or "g" loading of the rod having to stop the piston from flying out the top of the block. The g loading is the same at the top of the compression stroke and the exhaust stroke, but on the compression stroke the combustion forces are trying to force the piston back down into the bore, so the rod has less loading on it trying to tear it apart.

Remember that Force = mass times acceleration or F= Ma. I quoted 5000 gs on a Cup engine at 9500. The Force or load on the rod is therefore proportional to the mass of the piston, pin, rings, etc, as well as the mass of the rod itself times the 5000 gs, which are proportional to the rpm. Here's where lighter weight (less mass) parts create lower loads trying to tear the rod apart. Of course to keep up the strength with lower weight, the material not only has to be used efficiently (design), it has to be very strong for it's weight. Read that as EXPENSIVE.

"......how fast do you want to go?"

 

well i could of told you that, lol. teach me something. is there anything odd that the teams do to prevent this? in the combustion stroke i'd assume that the teams would like to even out the power (as much as possible) through the entire stroke to prevent the piston from hitting too hard(and displace the load over time) at the bottom of the combustion stroke and at the top (but moreso the bottom). now i know in some apps they open the exaust valve on the downstroke of the combustion, so i guess that would take some force off the bottom of that stroke. but to me it seems like there would be more stress at the top of the exhaust stroke that anywhere else. how is it the same?

 

Ughhh can't do that clyde. Not unless you open the exhaust two different times in a cycle.

Remember that as the exhaust valve is closing the intake valve opens, which is how you get overlap and scavenging.


I would say that you would really want the load to hit hard, but you really can't. The burn in a chamber is really that a burn, not and explosion (unless it's Nitromethane) that's why you have ignition lead starting 40-25degs BTDC.

You could use a rod length change to effect the placement of the piston in the bore per deg of revolution.

As in make the parts lighter?????

Actually they can't. They have mass rules on the piston piston/pin combo and rod mass. (470g for piston/pin combo and 525g for the Rod) Other than moving mass around on the rod to reduce the bobweight there is not much they can do. Other than elminate things like rod bearings with DLC coatings but there is probably a rule that states they need to have XXg rod bearings now to cut that cost.

If I knew the big end weight of the connecting rods I could actually tell you how much F is on a Nextel Cup motor. Actually I did that before in another thread a while back. It might be a good idea to look up F=MA in the search function because it's actually quite applicable to this thread now.

Bret

 

Detonation is very strong force to be reconed with. We've bought pistons near $500 a peice (nothing special in terms of a crazy *** shape or anything, still circular, no fancy dish) and still have them crack with detonation. And this was a race motor. The piston was designed to absorb some (as much as you can really factor into a design/metal).

We aren't particularly happy with manufacturer but thats another story.

 

Well,ya ain't suppose to run even $500.00 pistons in detonation.Ya supposed to keep it tuned!!!! Pistons are not going to take that kind of abuse,regardless of who makes them,or how much they cost.

 

it wasn't consantly detonating! it happens on race motors durring a 24 hour race though!

 

SFs vary depending on the component and the duty cycle they see, as has been stated here before. It's not uncommon for certain areas of an engine to have a FEA SF of .7 or thereabouts if a good educated guess and several successful durability tests say it's OK. Other more critical areas may be at 1.5 SF.
No production engine I'm aware of (gasoline powered) can stand detonation encountered under high load for any length of time without terminal engine damage occurring. Under normal conditions peak average cylinder pressure for most engines is in the 900-1400 psi range. Peak average is a term normally used for taking a few hundred engine cycles (200 minimum or so) and then averaging the highest cylinder pressure seen during each cycle. This is obviously not the highest cylinder pressure seen, but is more representative for what you should expect the engine to see on average, which is more useful when predicting expected life of the components.

 

A Fast or Accel system or something similar can keep the engine at a pre selected tune on the fly.If you have that much veriation during a race(don't know how) you need to get this type of set up to prevent engine damage.

 

do you build for a lemans team? if so what team.