Combustion Talk

The correct place for firing to occur is 10-20 degrees ATDC, variances in the flame front speed will cause the ignition event to be at a different BTDC time so the firing occurs in the range listed.

Pre ignition is caused by hot spots in the cylinder, no disputes.

There is a lot of talk about causes of detonation, and it is a vague term.

I stand by my statement that detonation is due to excess fuel, either due to over fueling or poor homogenization, as it is caused when the flame front hits excess unburned fuel. The speed of the flame front varies due to cylinder pressure, AFR and fuel quality. Flame front speed will increase with compression. Fuel and the oxygenation of said fuel varies the flame front speed. I am searching for the AFR effects on the flame speed and welcome discussion.

The sound of detonation and preignition are basically the same, but preignition causes more damage due to the push back on the piston as the crank is moving it up.

Here is a good article on the theory of combustion in a cylinder.
Untitled URL Link

just a thought but the top fuel guys really understand flame front, travel, compression , and timing trying to keep nitro methane lit is quite the science it would interesting to talk to the fuel injector/ blower /head -guy just a thought🛴

(quoted from post at 13:14:58 01/27/18) The correct place for firing to occur is 10-20 degrees ATDC, variances in the flame front speed will cause the ignition event to be at a different BTDC time so the firing occurs in the range listed.

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I'm a little slow. Can you explain this so I can understand it.

No problem, I have been studying this for a while, still learning every day.

The piston is pushed down by expanding gas due to a fire caused by the spark plug. It takes time for this expanding gas, or flame front to move, it must hit the piston at about 15 degrees after top dead center for maximum push and finish before the piston is at the bottom of the stroke. In theory all fuel must be burnt up by this time, but if there is excess fuel left, or pockets of rich mixture, they will explode or detonate. The better the mixing or homogenization then better the chance that the fuel will burn completely. You want to match the speed of the flame front to the pushing of the piston downward.

You sure do interesting things. I really enjoy your posts. I worked on control software in a paper mill (as well as the ERP and everything else since I was the only certified Oracle DBA with a CS degree in the plant) and thought that was interesting, but your work with engines and control software is really fascinating.

I just found this, easy explanation, even I can understand.

Lean mixtures burn faster than rich mixtures and they tend to produce more heat. Richer mixture needs more timing to completely burn and they tend to produce less heat. Too rich will cause knock, too lean with too much timing will cause knock. 0.85-0.86 Lambda seems to make the most power on the LNF so find the appropriate amount of timing for the fuel you are using and mods. Ie. tune the A/F for maximum power and base you timing off of that and your fuel grade. If you are knocking pull out timing in those areas you are getting the KR.

[b:2e085c673b]"The correct place for firing to occur is 10-20 degrees ATDC,"[/b:2e085c673b]
Can you give an example of such an engine?

Anybody ever watch those glass head small engines on youtube? There's a couple of videos in slow motion showing the combustion.

I will present my understanding from three sources: Automotive tech instruction (some from an Indy racer mechanic named George Whitehouse who crewed during the late 50s and early 70s on Offenhauser engines). Personal experience on the full range of automotive transportation from Mercedes to Pintos. And research into the process using test engines with glass cylinders.
My scraped together explanation:

Peak effective pressure in a tractor engine, maybe between 300 and 450 psi, happens (when things are correct) at between 10 and 13 degrees after TDC. This pressure peak is when most, 80-90%, of the fuel and air have been consumed and the temperature is as high as it can get. Forces on the piston, and rod/crank angles, can remove mechanical energy efficiently at that crank angle and piston speed.

Getting the peak pressure to cooperate toward this objective is complex, and the variables are nasty. Some factors are Octane rating of the fuel, Compression ratio, Fuel atomization, Chamber and piston crown temperature, Fuel air turbulence, Chamber squish areas, Chamber sharp edges (including spark plug electrodes and insulators), carbon or lead/ash deposits that can glow, Ignition timing, Spark placement in the volume shape of the chamber, and not least RPM.
Detonation (Knock) happens for two reasons but has one characteristic. It is the actual explosion (not flame front burning) of a specific volume of fuel air. An explosion here is defined as the simultaneous ignition of a volume of material. This is caused, in this discussion, by an initial ignition event that proceeds to burn through the available fuel air raising the pressure and increasing the radiative and compression related heat in the volume of the chamber. This pressure and heat conspire to set off the entire volume of a smaller volume of fuel air at the same moment in time. Each molecule of oxygen carbon and hydrogen in the volume reaches kindling temperature at nearly the same instant. (probably at the speed of sound in the compressed gasses, well above the speed of sound at one atmosphere pressure (one Bar) Probably as much as 4 times the mach-1 we think of as the speed of sound.

This increase in the heat and pressure is characterized as a shock wave that impacts the entire combustion chamber. This radically moves the peak pressure angle toward or before TDC. This pressure creates radically increased stress on components because they cannot expand (piston is nearly not moving or not moving). When repeated over and over in the same chamber, the detonation location can be considered to be stable. This location stability is easily detected by finding holes in pistons, blown burnt gaskets, melted ringlands and broken piston rings.
Stoichiometric fuel air mixtures (a mix with enough oxygen to oxidize all fuel) are the only ones that can "Burn" completely. Excess air (Lean combustion) results in remaining unconsumed oxygen, Excess fuel results in residual un-burned fuel. Excess fuel is cooler, and thus less prone to detonation, than stoichiometric mixtures. Lean is hotter because near all combustables burn. Rich is cool because some fuel is not burned producing less heat.
Pre-ignition is one source of knock. Hot sparkplugs, glowing carbon or deposits, overheated chamber edges or protruding gasket surfaces can be ignition triggers for pre-ignition. The spark then creates an additional flame front propagation, causing detonation to happen in more remote volumes in the chamber.
Spark Knock is the reverse of that. Early spark (assumes all those variables mentioned are constrained except timing) initiates flame travel through the chamber and (because the piston is still on the way up) the pressure increases to the point that, because of hot spots or not) parts of the remaining volume explodes.

Factors that make sense include: the fact that an engine running with timing that just barely initiates a tiny knock are actually timed perfectly. The tiny knock harms nothing. Thus spark Knock sensors are always adjusting and teaching the EPU (engine Control Unit) to keep a tiny knock happening within a range of intensity and frequency. Water injection cools the combustion process and (though increasing HC emissions) lowers detonation and provides a bit of converted energy in the form of steam. Higher engine speeds, compared to tractors, result in much less time to have detonation occur before the prime pressure peak.
Higher octane (resistance to burning) makes higher compression and advanced timing more efficient at the expense of the cot difference, and have no value in anything but a modified tractor. The R+M/2 method indicates our regular 87 octane is way higher than tractor gasoline of the 50s=60s.
Propane has high "octane" and can handle 10:1 compression efficiently at the tractor engine speeds.
Run-on after shutoff (dieseling) is a sign of preignition.
Other than one cylinder engines, every engine has a particular cylinder that will detonate before any other.
Stratified charge technology creates combustion that can reduce detonation and reduce fuel consumption but is expensive.
Variable valve timing is a valid attempt to widen the sweetspot for avoiding detonation and assuring cylinder filling.

I hope this is understandable. Jim

So how do you fire at 10-15* ATDC to get peak pressure at 10-13* ATDC?

I think it was a typo in the original post. I do't think David Believes spark occurs after TDC. Jim

In clarification, I am referring to firing as peak pressure, ignition is spark, it does take time for the flame front to travel from the spark plug to the top of the piston..

Jim,

That is a good point, we saw detonation happening on certain cylinders, it did seem to be more common on certain cylinders.

(quoted from post at 23:53:47 01/27/18) In clarification, I am referring to firing as peak pressure, ignition is spark, it does take time for the flame front to travel from the spark plug to the top of the piston..

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Yes it takes time.
We are talking about an internal combustion engine.
By definition, combustion is an explosion under controlled conditions.
Ideally you want the combustion to be complete when piston is at TDC. To achieve this, ignition must occur before TDC to allow
this.
In over fifty years of mechanic work, I have never heard of or worked on an engine that fires ATDC.
That's the reason I asked for an example of such an engine.

I know a few engines that start 28 ATDC and after the load is applied the ignition goes 10 BTDC. These are 2 cycle engines and this aids in getting the turbocharged to spin up. Lots of detonation going on during this time.

Oh, the old days of balancing engines.

Yes, we retard the timing on the big Clarks to 20-24 ATDC to heat up the turbo's, no torque but ton of waste heat.

You can't. Emission engines, particularly V-8s, in the late 70s used retarded ignition timing, to
increase burning time. This was before compression was lowered, egr perfected etc etc. Pre electronic
anything. Many engines had a vacuum retard diaphragm on the dist, so in high vacuum conditions, ie
traffic jams, engine idling, timing would be pulled back. Yes, power was way down, fuel mileage in the
tank, and very difficult for a tech to make run well. Every engine was a bit different, with the large
bore V8s being the dirtiest. Flame front would go out and border gas (the .008" or so mixture against
the cylinder wall being coolest and the cause of most hydrocarbon emissions) would not burn. Hemis went
away quickly because of their lack of turbulence in the incoming mixture that would not mix well.
Interesting discussion, almost need a white board in a discussion group..........

I figured that you meant *peak pressure* at 10-20 ATDC.

Have you looked any at Smokey Yunick's Hot Vapor Cycle Engine?

Basically he used a low pressure turbo as a homogenizer and _raised_ the temp of the incoming fuel air mixture. Supposedly was generated very good fuel numbers.

Haven't looked it up myself in a while.

This is a good article to understand the causes of detonation. I hope it helps.
Untitled URL Link

In the fire service they drill the need for Stoichiometric fuel air mixture into you when training on flammable liquid and gas fires. You can't have fire if there is too much fuel in the mix or too little. I have long wondered about competition engine builders and the work that they do on cylinder heads and piston lands. For example many heads come with eyebrows around the valves, and it is common practice to cut them down. Is this the right thing to do or is it defeating the mixing and interfering with flame front? Then too, diesels usually have specially designed pistons to enhance the swirl of the air as it is compressed so that the fuel gets injected into moving air and mixes faster in order to enhance the flame front. Then an engine builder puts in different pistons to increase or decrease compression depending on what type of puller they are building. What does this do to the flame front?

Something to consider is that peak piston velocity occurs at 90 degrees ATDC. I suppose if you had complete control over the pressure curve, you would want peak pressure to occur then, but realistically that's not possible because peak pressure is necessarily going to occur when the volume in the cylinder is small.

Another thing to think about is that the combustion process doesn't know anything about degrees. It is a function of time and pressure. True, there's a relationship between time, pressure and crank angle, but it's not linear and varies according to engine speed.

Something else to consider is incoming air enter the chamber thru ports at the bottom of the cylinder. Just like on a Detroit 471. Fuel gas enters thru the injection valve in center of head.

A lot of these engine are early 1950s technology thru the 1960s. Today 60 years later it's all about lowering nox and better fuel economy.

Detonation is the desired flame front from the spark plug colliding with another flame front that is not supposed to be there.
The secondary flame from could be from glowing carbon or pre-ignition.
Pre-ignition is the air/fuel mixture igniting prior to the ignition system providing a spark. Caused by lean mixtures, high heat in the cooling system or ambient air,glowing carbon, lube oil in the crankcase from worn rings or a faulty PVC system, low octane fuel or lugging the engine.

Nice comprehensive reply!

The heating of the intake charge was minimal in Smokey's engine . The primary purpose of the turbo's compressor wheel was to ensure all the fuel droplets were vaporized and thoroughly mixed with air.
Power and efficiency is from having the greatest possible average temperature/pressure difference from the end of the compression stroke. Along with time ignition starts vs the peak average temperature/pressure acting on the piston during the power stroke.
Eliminated lean and rich cylinders. Reduced HC, unburned hydocarbon emissions by not having wet fuel droplets on the intake or combustion chamber surfaces.
More or less made the gasoline engine behave like it was operating on propane.

Grinding off the eyebrows would do three things. One reduce the compression ratio (bad), decrease the area of the combustion chamber walls ( good), three, change the airflow around the valves from what the engineer designed, who knows? (good or bad) Ha ha Me, I would grind them off. If they had done much good, all engines would have them now days.
Combustion chamber design is very interesting to me. I had a Mazda diesel pickup that had precombustion chambers. The best milage I could get was around 29 miles per gallon. Heck, Chevy diesel pickups that were much larger were getting 35 MPG. That's why I say that precombustion chambers don't work.
Now on that huge motor I can see what they are trying to accomplish with the precombustion chambers. They inject a rich enough mixture, that will burn easily,into the precombustion chamber, then, the burning fuel enters into the main combustion chamber, which is very rich in oxygen, which should cause a more complete burn with no wasted fuel expelled in the exhaust. So, I can hardly wait to see if it works or not.

What Mazda is up to now...

Mazda SupCCI engine

http://edition.pagesuite-profession...619d02-359f-4fb5-aa92-b45d2d8b3352&pnum=1

I am not much of a diesel man still learning its interesting to see how gas engines have picked up on applying diesel fuel system strategy.

" Hemis went
away quickly because of their lack of turbulence in the incoming mixture that would not mix well."...........and too much surface area for that thin film of un-burned mixture. Today's Dodge "Hemi" chamber has only a minor resemblance to a hemisphere. I call them a 'HINO' (hemi in name only).
.....

(quoted from post at 11:08:11 01/29/18) " Hemis went
away quickly because of their lack of turbulence in the incoming mixture that would not mix well."...........and too much surface area for that thin film of un-burned mixture. Today's Dodge "Hemi" chamber has only a minor resemblance to a hemisphere. I call them a 'HINO' (hemi in name only).
.....

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I call 'em "semi hemi" Really gets the modern day "hemi" lovers up in arms!

(quoted from post at 11:08:11 01/29/18) " Hemis went
away quickly because of their lack of turbulence in the incoming mixture that would not mix well."...........and too much surface area for that thin film of un-burned mixture. Today's Dodge "Hemi" chamber has only a minor resemblance to a hemisphere. I call them a 'HINO' (hemi in name only).
.....

Click to expand...

I call 'em "semi hemi" Really gets the modern day "hemi" lovers up in arms!

Awesome discussion guys! Very educational.