Re: Combustion Talk

Posted by Janicholson on January 27, 2018 at 12:48:10 from (63.152.194.8):

In Reply to: Combustion Talk posted by David G on January 27, 2018 at 10:14:58:

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

Replies:

• Re: Combustion Talk David G 14:57:30 01/27/18 (0)

Add a Reply! You must be Logged In to Post Name: E-Mail: Subject: Comments: Optional Link URL: Link Title: Optional Image URL: Optional Video Title: Optional Video URL: Advanced Posting Options Email Notification: If you check this box, email will be sent to you whenever someone replies to this message. Your email address must be entered above to receive notification. This notification will be cancelled automatically after 2 weeks. No political comments, hate speech or bigotry of any kind will be tolerated. Violations will be removed and posting privileges may be permanently revoked without notice.   Advanced Posting Tools   Upload Photo  Select Gallery Photo  Attach Serial No List  Return to Post  Upload Photos/Videos Choose Files from your local computer Upload one or more videos to your post. Photo filesizes should be less than 300K and Videos, less than 2MB. Formats allowed are gif, jpg, png, ogg, mp4, mov, and avi. Be sure to use filenames without spaces or special characters, and filetypes of 3 digits lower case.

TRACTOR PARTS	TRACTOR MANUALS

We sell tractor parts!  We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today. [ About Us ]