96speed

so me and my buddy are having issues.

lets suppose im driving home in my above average Z28 when behold, the heat from my header fries only one of my spark plug wires (assume no spark). the other 7 cylinders are pumping like champs. meanwhile, the cylinder with no spark is still being pumped full of air and fuel. what happens here? does the mixture combust completely due to the pressure of the piston and heat from the head and adjacent cylinders? does it combust only partially, leaving your hometown full of crap? or does it not combust at all, and exit through the exhaust valve? assume normal atmospheric conditions standard ls1 or lt1 CR, and premium fuel ( i dunno...91?)

i say the mixture does not combust.
My friend says in his words "some off it will burn."

so...i know compression ratio, air density, octane, temperature, and a couple other things determine the rate of burn. make a few assumptions.

who's right?

Cobalt

BTW: those werent my words (im the friend...or i was )

my thinking is that the CR of the motor (whatever an lt1 or ls1 is), and the heat from adjacent cylinders and the head above it, is great enough to ignite the mixture without a spark. like a diesel engine.

however, 93 octane gasoline doesnt burn like diesel fuel. does anyone know how much heat and CR is needed to combust an air fuel mixture without a spark. assume this is in a stock cubed lt1.


any engineers willing to make a few assumptions and bust out their calculators?

81ZMouse

There are so many variables that can affect this other than static compression ratio; quench height, chamber efficiency, engine temp, air temp, even bore/stroke/rod length. But, under normal operation, an SI (spark ignition) engine should NEVER combust on it's own. That is the sole difference in an SI and CI (compression ignition, diesel) engine.

We did build a 406 engine with about 11.5:1 compression that would almost start with no ignition, and this was with 92 octane fuel. Needless to say, it only lasted about 2 months before it knocked three pistons out. Like I said, if an SI engine operates without a spark, ie detonates, it is not long for the world.

So, in your example, basically what would happen is the fuel/air mixture would NOT ignite in the chamber, but it is very likely that it could ignite once it enters the exhaust, depending upon engine load and exhaust temp.

Shane

96speed

Looks like you are getting .

Serious: Wouldn't detonation be a much bigger problem if our engines would diesel so easily??

Ryan

Cobalt

i may be getting "owned"

but at least i can string together a coherent sentence!

OldSStroker

Interesting question.

If you are cruising at PART THROTTLE, the dynamic compression ratio can be very low, like 4 : 1 or maybe less, because it's not taking in much air. In this case, there's not much chance of the mixture burning.

If you are wide open throttle, racing your buddies 'Stang, (cause with you on 7 cyls. he might almost keep up), I still don't think you'll get any burn. The heat from the head, etc. is way below what you need to light the fire, and this ain't no diesel, as others pointed out.

IMO, you just pump out unburned fuel, and still smoke the 'Stang.

My $.02

Cobalt

i was under the impression that the CR was constant. what is this dynamic CR you speak of? it makes sense. the amount of air entering the cylinder is dependent upon engine speed, while the volume stays constant. right?

its been a long day. now im confusing myself.

either way, looks like i am getting "owned" so far.

chalk it up ryan, this is a first!!


judd

SStrokerAce

well dynamic comp ratio is what the engine actually sees.

I though I would add this.

A good way to calculate Dynamic compression ratio (at full throttle) is:

DCR = Clearance Volume + Cylinder Volume at Intake Valve Closing / Clearance Volume

Clearance Volume = Piston Dome/Dish, Chamber, deck and gasket volume

Static is the calculated comp ratio from volumes without accounting for cam timing or even PSI

You can have a 8:1(static) Blown engine with a 9:1 dynamic compression ratio, and a 14:1 static naturally aspirated engine with a 9:1 dynamic compression ratio, both will be very strong.

The high end for dynamic compression ratio is around 9:1


When you hear that a big cam, more specifically a cam with alot of overlap needs more compression, it's because the static level gets too low because not all of the intake charge stays in the cylinders.

add that into what Old Stroker said and you'll understand this a little better.

Bret

Injuneer

Another way to explain the reduced liklihood of compression ignition at less than full throttle would be to consider the "mass" of air/fuel mixture in the cylinder. Although the cylinder always draws in roughly the same volume of air, the density of the air is greatly reduced at part throttle by pressure loss through the throttle body.... that's why the TB is there.

At 15"Hg MAP, you are only putting 1/2 the mass of air in that you could at WOT/30"Hg MAP. The cylinder is effectively starting out the compression stroke with a significant vacuum, the cylinder needs to travel approx 1/2 of its compression stroke just to get the cylinder pressure back to atmospheric equivalent, then it actually starts to compress the mixture (relative to atmospheric pressure), but with only the last 1/2 of the compression stroke. Effective CR (referenced to atmospheric pressure) is only roughly 1/2 of physical compression ratio.... insignificant in terms of the pressures/temperatures required to auto-ignite the gasoline-based A/F mixture.

Another thing to keep in mind is the difference between "pre-ignition".... which would in essence be compression ignition.... and "detonation". High performance engines with rational compression ratios and adequate octane fuels are not so much subjected to pre-ignition as they are detontation. Detontation is a slightly different process than pre-ignition, because it involves the near simultaneous development of two flame fronts, one from the spark plug, and one from the auto-ignition of "end gasses"..... and there is where the difference lies. End gasses are the chemical compounds formed as the various components of the fuel break down and react to the intense heat and pressure of the compression process, including the rise in pressure already instigated by the original spark.

I would "guess" that when you lose one plug during normal driving, the fuel remains unburned, then the air/fuel mixture is pushed out of the cylinder into the exhaust. (We know that happens when a Top Fuel dragster engine "drops a cylinder".....). When it mixes with the exhaust, some of the fuel/air mixture might ignite from the hot exhaust gasses. In a "closed loop" fuel injected engine, the oxygen is seen by the O2 sensors as excess air = lean. The computer reacts by adding extra fuel, and now the healthy cylinders are running too rich and adding unburned HC's to the exhaust.......

Cobalt

fred,

i cried a tear after reading your post. *very informative!

well...ryan. i lost.

bummer.

96speed

Thanks for the info Fred!

Makes a lotta sense now!

Out of curiosity...how much compression would it take before pre-ignition happens? Let's assume WOT, 93 octane.

Ryan

ToddR

I'd say too many variables to give an exact pressure or temp.
Combustion chamber efficency,
Fuel mixture/ lean or rich
dynamic compression
Intake charge temp
Fuel octane rating
ect.ect.ect.

You can have a N/A engine run 8.5comp and 91 octane detonate
But, another engine with 10 to 1 and 7psi intercooled boost on the same 91 have none.

Corky bell uses relation of temperature of intake charge temp after compression. 1075F absolute But this can very substantially with variations in above examples.

This is equivalent to a temperature in a 13 to 1 compression on a "standard day"
We're talking about 525 degrees above an ambient temp of 90F guage.

Including compression induced temps.