Gasoline Combustion: Basics of Heat, Air, and Fuel Reactions

[thender]

Hello. I've gone round and round in my studies of engines and related technologies and I've come to the point where I would benefit from a better understanding of how combustion reactions occur and are sustained in gasoline engines. I have been looking for some basic introductory level information but I am mostly turning up engineering treatises.

I believe the combustion reactions in engines are of the "premixed" laminar and turbulent flame type.

The things that I need to understand about combustion reactions that are still unclear are how the fuel and air blend, how heat from a spark plug initiates a combustion reaction. What determines the speed of the reaction. What effect the compression of the engine has on supporting combustion, and how the heat energy of combustion is absorbed and transferred to the contents and surfaces of the combustion chamber.

There are lean and rich flammability limits for gasoline mixtures, I have them in a book but I don't understand why an overly rich or overly lean mixture will not burn, or why the rate of combustion seems to slow down exponentially as the mixture nears flammability limits.

There's a general belief that lean mixtures burn hotter, and some engine strategies use rich mixtures for cooling, I believe the extra fuel mass absorbs heat energy.

Exhaust gas recirculation is extremely important for both emissions and fuel economy. The emissions related aspect is again, I believe to absorb heat energy in the cylinder.

Moisture in the air affects engine tuning, why? I think it has to do with flame propagation. Some people have used water injection systems to reduce cylinder temperatures. Might be the same theory there.

Before you hit the lean limit when operating an engine it will become unstable. An important phenomenon where combustion becomes variable and partial burning and misfiring cycles begin occurs because slow burning mixtures cause instability.

But there has been a huge drive for running engines at ultra lean air fuel ratios for fuel economy, and now we have stratified charge lean cruise GDI engines.

Clearly there's a need to understand how the flame is initiated, how it propagates, and why.

What I think happens (in my uneducated ponderings), is that the air and fuel are inducted into the cylinder and mixed with as much turbulence, and sprayed as finely as possible, the heat vaporizes the fuel and the oxygen oxidizes it. Then the spark causes a combustion reaction to start and a flamefront travels through the mixture. The products of the combustion reaction heat the oxidized air and fuel nearby and so as one reaction finishes the next begins in a chain. As a result of combustion large amounts of energy is released which dramatically increases the pressure of the gas. The flamefront propagates outward, pushing against and passing through the unburned mixture.

I want to learn more about this but I don't even know the right terminology to search with and I am having a HARD time finding helpful subject matter.

Thanks for any help both directly clarifying the matter or indirectly simply pointing out helpful resources

-Andrew

 

[Baluncore]

Don't bite off more than you can chew, you must minimise the complexity of your approach. It would be really good if you had a very specific question to answer before taking up this study. Without that focus it will be far too complex. Keep reading those treatises. In three years you may really begin to understand the field, and so write a thesis on your own small corner of the subject.

 

[bigfooted]

Study a book that explains the fundamentals of combustion. A lot of your questions have nothing to do with gasoline engines and are actually mostly application-independent.

Based on your questions I suggest "Combustion Physics" by C.K. Law.

Additionally (not alternatively) you can study some of the material from the Princeton summer courses on combustion:
http://www.princeton.edu/cefrc/combustion-summer-school/archived-programs/

 

[Fahlin Racing]

Good subject! Many aspects to equal one process.

The things that I need to understand about combustion reactions that are still unclear are how the fuel and air blend, how heat from a spark plug initiates a combustion reaction. What determines the speed of the reaction. What effect the compression of the engine has on supporting combustion, and how the heat energy of combustion is absorbed and transferred to the contents and surfaces of the combustion chamber.[?Quote]

This is how I view it. Port Fuel injection or Carbureted engines we must pass the valve and worry about cam timing to limit reversion. Direct injected fuel you have injector characteristics, piston dome or dish layout, compression level, fuel and induction charge density in either situation. Watch wet flow videos if you can, that will be a visual of flow patterns when the valve or valves are open.

Once we are in the cylinder with our working fluid we can have
Single valve inlet
Double valve inlet
Triple valve inlet (VW?)

Anyways, the reason I mention here is this will dictate the in-cylinder mixture motion each valve arrangement will show different mixing motions. 2-valve heads (conventional american v8) produce natural swirl. 4-alve heads tend to favor tumble motions. 5-valve heads, I couldn't tell you what goes on. Remember you must take into account what the floor, our piston, will produce as well once its around TDC in regards to how the chamber is designed as well. Quench & squish areas,dishes or domes chamber design and spark plug placement.

As far as fuels in a nutshell, you can't just view the overall tune, rich or lean, but you must take into account once on the chamber. Where are the rich and lean areas within the cylinder too because the 'homogenous' mixture by theory doesn't stay so 'homogenous'. The volatility of the fuel, once you get into pre-ignition, detonation and regular burn cycle has been seen you will start seeing with the chamber and piston we are not trying to control the burn at a optimum angle to stay away from abnormal combustion but produce adequate heat production for expansion to drive the crankshaft that doesn't kill the engine's component materials causing you to rebuild the engine time and time again.

Your post, like mentioned by previous posters, has quite a bit of meat to it and combustion is definitely something that needs to be done in steps. Once the process is understood in its basic respect, then digging into minute details can be done.

When you say flame front travels through the mixture, you should replace the word through with consume. As far as what you see as propagate outward, I think of a diffusion flame front with diesels, they burn inwards. IMO the gasoline burn cycle is more of a horizontal direction either way, this you must view your plug position as well.

Lean is said to burn hotter, however I think otherwise unless I see results of a mixture with less fuel to make more usable heat than a rich mixture. I hope you know the Stoichiometric term and ratio 14.7:1. Anything above this is lean and anything below is rich. Again you MUST know what grade of fuel you will use too to help in tuning.

The world of combustion is a interesting topic. I haven't studied this subject for a solid time for atleast a year plus. I suggest have a dictionary or other resource to define words and ask. I still have things I haven't read, I read things from the scientific articles to engine building icons.

Some the motorsports side of things
Smokey Yunick - Hot Vapor engine (back when vaporization wasn't so great)
Larry Widmer's Swirl Power from the 80s Circle track in a very controversial at the time (Endyne)
Not sure how Grumpy Jenkins thought of Combustion but his stuff in drag racing was a force with his smallblock chevys beating big block chevys.
David Vizard has some stuff that is very basic and easily understood

Charles Fayette Taylor
Gordon P Blair

There are other names out there but for now that is it. I hope a partial view of what I see helps you.

 

[rcgldr]

Triple valve inlet (VW?)

and some Yamaha sport motorcycles. In the case of the Yamaha motorcyles, 5 valves per cylinder didn't result in better power output than 4 valves per cylinder used by their competitors (Honda, Kawasaki, Suzuki).

 

[MisterDynamics]

Basic Rules for Gasoline Burning In Piston Engines:

Too lean a fuel mixture = Flame front speed increases causing thermal runaway and overheating.

Too rich a fuel mixture = Flame front speed decreases, excessive carbon buildup in combustion chamber leading to hot spots.

Gasoline Flame Front Speed = 100 Ft/Sec

Air-To-Gasoline Stoichiometry Balance Limits =

8:1 = Most Rich before flame-out
18:1 = Most Lean before flame-out
12:1 = Start up Mixture for reliability
14.7:1 Optimum Mixture for peak fuel economy

EGR (Exhaust Gas Recirculation) Valves = Used to regulate fuel burn for either increased combustion or decreased combustion to prevent detonation and intricately linked to anti-knock sensors or devices. EGR Valves are also used to optimize and regulate fuel combustion at all operating conditions.

Octane has nothing to do with the amount of energy that is contained in Gasoline.

Octane slows down the speed of the Gasoline combustion flame front and higher Octane Gasoline (89 Octane Number and higher) should only be used in high compression Gasoline piston engines (9.5:1 CR and higher) to prevent detonation or “knock”.

If high Octane Gasoline is used in low compression piston engines (6:1 CR to 9:4 CR) there will not be enough compression to burn all of the Gasoline and the high Octane Gasoline will exit the engine unburned and wasted out of the exhaust. Low Octane Gasoline is specifically designed for low compression gasoline piston engines.

Regardless if Gasoline’s Octane Number is 100 or 86, there will always be 20,000 Btu/Lbs of heat energy in Gasoline and a fuel density of 6 Lbs per US gallon.

Gasoline is supposed to burn smoothly and not explode (detonate) or piston head damage can occur.

Flame fronts are supposed to be as turbulent as possible so that maximum BMEP can exert upon piston head area maximizing Brake Horsepower & Torque to the crankshaft.

Of course the engine and cylinder head should also be constructed from materials and techniques to handle high performance Gasoline combustion flame fronts, higher compression ratio and BMEP.

‘Flame Front Squish Zone Notch Grooves’ are a new patent invented by Somendar Singh.

These are cut into the area in the cylinder head interior surface between the valves to induce an increase in flame front turbulence resulting in an increase in fuel economy and power output.

A good project is to get a piston engine apart and go online and look up how to cut these “Squish Zone Grooves” into a piston engine to increase its power output and thermal efficiency.

Look up online: ‘Somendar Singh Squish Zone Notched Grooves’.

Everything else on modern automobile piston engines regarding Gasoline combustion are regulated by the engine emissions controls, such as MAF sensor, MAP sensor, O2 sensor, EGR, Anti-Knock, IAC, Valve Design, Camshaft Design, Induction Type, Fuel Injection Type (TPI, TBI, MPSTPI, CFI), Cooling System, Power Recovery, Ignition and ECM Computer.

As for non-modern cars and piston engines, the Gasoline combustion performance is limited to engine structural design, ignition system, fuel metering and how much airflow can enter the engine.

If you want to get more deeply into Gasoline Combustion Science, I suggest that you research deeply into:

Physical Chemistry
Thermodynamics
Organic Chemistry
Mechanical Engineering
Chemical Engineering
Aerospace Engineering

There also are pertinent Computational Fluid Dynamics (CFD) and Finite Elemental Analysis (FEA) software regarding real-time Gasoline Combustion reactions while burning inside of various heat engines.

There is an entire sub-science dedicated to the molecular and sub-atomic analysis of Gasoline combustion while burning inside of a heat engine, including flame front propagation and its quantum plasma effect.


Regards,

- MisterDynamics -

January 08, 2014

 

[Fahlin Racing]

Too lean a fuel mixture = Flame front speed increases causing thermal runaway and overheating

Please explain more, I have read that adding compression to a lean fuel air mixture producing a slower burn rate compared to a rich mixture.

 

[MisterDynamics]

Increasing compression will increase internal cylinder pressure and therefore increase internal cylinder temperature and hastening thermal runaway and overheating even more.

Usually like during a traffic jam where there is stop-and-go driving one tends to see an increase in engine temperature.

The fuel mixture tends to lean during stop-and-go driving causing an increase in flame-front speed and ultimately engine block over-heating.

During a traffic jam or excessive stop-and-go driving the electronic fuel injection system will tend to richen the fuel mixture to slow down the flame-front speed and prevent overheating.

During stop-and-go driving such as during a traffic jam one tends to see an increase in engine temperature followed by the electric cooling fan turning on.

Usually at this time the car's computer has changed the fuel mixture to a richer mixture ratio along with turning on the electric cooling fan. These two functions together along with a well-maintained cooling system brings the sharp rise in engine temperature back down to normal.

It would seem that by adding more compression the internal cylinder temperature would increase even more and make the overheating situation even worse.

Exhaust Gas Recirculation (EGR) Valves also play a role to limit flame front speeds from getting out of control by recirculating some exhaust gas back into the induction system to limit the Gasoline combustion speed regulating flame-front speed and thus preventing overheating.

EGR Valves are also used on present-day cars to prevent detonation and overheating especially when too low of an Octane Gasoline (86 Octane) is used in a very high compression (over 10:1) Gasoline Four-Stroke Piston Engine.Regards,

MisterDynamics

January 08, 2014

 

[Fahlin Racing]

How do you create more heat from less fuel over a rich mixture? I can see heat transfer rates increasing due to thinner boundary layer however I still don't see making more heat from less fuel though.

 

[MisterDynamics]

It's not so much "more heat" from less fuel than it is 'asymmetrical heat distribution'.

Gasoline Flame Front speed increases with leaner fuel mixture at idle RPM leading to hot spot temperatures absorbed into the cylinder wall & head that aren't as easily purged out through the exhaust valve port.

With leaner fuel mixture at idle RPM the heat distribution is more acute and concentrated within the cylinder wall(s) and head(s) along with lower airflow into the cylinders resulting in overheating.

The piston engine is an air-breathing machine. Lower RPM means less airflow through the cylinders.

That is why fuel mixture is made richer at idle to prevent this type of asymmetrical heat distribution.

It does seems that a richer fuel mixture at idle RPM would lead to increase in heat within the cylinder(s).

But in fact at idle RPM with less airflow moving into the cylinder(s) a richer fuel mixture will stabilize the Gasoline Flame Front speed and distribute it evenly.

This makes it easier to purge overall heat concentration out of the cylinder(s) through the exhaust port when at idle RPM when less airflow enters the cylinder(s) preventing overheating.Regards,

- MisterDynamics -

January 11, 2014

 

[Fahlin Racing]

asymmetrical heat distribution'

I will have to agree this is a key phrase when studying.

 

[JESherman]

So, what effect on combustion, knock, Octane used, in higher compression engine considering different rate-of-change piston displacement (rpm) with time approaching ignition?