A. The Combustion Events:
Combustion of the air fuel mix relates to the thermodynamic efficiency of the Otto Cycle process (4-stroke, naturally-aspirated, internal combustion engine).
The air fuel mix acts as a compressible fluid and the efficiency of converting it into usable energy in an engine is commonly related to only static compression ratio . This is why some people mistakenly fixate on CR.
However, several other factors affect combustion efficiency other than compression ratio:
I. Factors Affecting Burn Rate , Complete Burn, and Detonation Resistance
- fuel octane rating (self explanatory: straight chain carbon hydrogen bonds are easier to break than branched C-H bonds)
- ignition timing relative to cylinder pressures (Indicated Mean Effective Pressure [IMEP] ) and spark duration
- air fuel ratio and flow quality (fuel atomization and ionization)
- cylinder filling and mixing: turbulence (swirl vs tumble) , stratified charge, charge homogeny, charge density, valve timing effects on filling
- combustion chamber shape, squish/quench area, surface area to volume ratio, and bore (flame travel distance)
- spark plug location relative to the exhaust valves (charge ionization) or in chamber vs in port (direct fuel injection) and the number of spark plugs used
- heat loss through combustion chamber walls (heat flux cycles, heat transfer rates)
- cylinder emptying efficiency and residual burnt inert gases left in the combustion chamber (deliberate use of EGR for fuel efficiency at the sacrifice of performance, responsiveness, vibrations , and driveability).
II. Phases of Combustion
The most common misconception of the combustion event by some beginners is to assume that the intake charge (air fuel mix) is always thoroughly mixed and uniform as it fills the combustion chamber and that the combustion event occurs instantaneously.
Some people have the mistaken view that when a spark is initiated, the air fuel mix instantaneously explodes and pushes down upon the piston top. If this were true, however, the resultant instantaneous force over several cycles would destroy the engine internals. Although spark flame travel and combustion occur rapidly (in the order of milliseconds or thousandths of a second), they do not happen instantaneously.The combustion event occurs over several crankshaft degrees.
There are several distinct stages or phases of the combustion event:
Originally Posted on sdsefi.com
Let's examine the process and dynamics involved from the moment that the intake valve is fully open. With the piston moving down the bore, cylinder volume increases, cylinder pressure decreases, allowing the higher pressure in the intake tract to push the fuel/air mixture into the cylinder. As the piston starts back up and the intake valve closes, cylinder volume decreases and cylinder pressure increases.
When the crankshaft reaches about 30 degrees before top dead center, the spark jumps the gap between the plug electrodes. The purpose of the spark is to raise the temperature of a very small portion of the fuel/air mixture above its ignition temperature. This is the point where true combustion begins. As the exothermic reaction starts, the mixture directly adjacent to the spark plug is also ignited and the process rapidly progresses out from the plug in a roughly spherical shape. [Editor's Note: more on this effect of shape later below]
At about 20 degrees BTDC, the rate of heat release causes the cylinder pressure to rise above the compression line which is what the cylinder pressure would be at a given piston position without ignition. Notice that it has taken 10 degrees of crank rotation to generate this pressure level. This is known as the ignition-delay period.
The rate of pressure rise is a function of the rate of energy release vs. the rate of change of combustion space or cylinder volume. The rate of energy release is directly related to the flame propagation rate and the area of reacting surface. Flame speed is dependant on fuel/air ratio, charge density, charge homogeny, fuel characteristics, charge turbulence and reaction with inert gasses and the metal combustion chamber, cylinder walls and piston.
In technical terms, the pressure rise is referred to as flagregation. No two combustion cycles progress at the same rate or at a uniform rate. Some start slow and end slow. Some start slow and end fast. Some start fast and slow down. Generally, only the ones that end too fast will lead to knocking as the rapid pressure rise may happen too soon with the cylinder volume still decreasing or not increasing fast enough. Usually, not all cylinders will knock at the same time or on the same cycle because of this.
By the time the crank is at about 5 degrees ATDC, the cylinder pressure is about double that of the compression line. From this point to roughly 15 degrees ATDC the combustion process is very rapid due to the increasing area of inflamed mixture and the high rate of energy release. The peak cylinder pressure (PCP) occurs between 10 and 20 degrees ATDC on most engines and the combustion process is complete by 20 to 25 degrees ATDC.. The peak temperature within the combustion gasses will reach somewhere around 5000 degrees Fahrenheit and pressures may be anywhere from 300 to 2500psi depending on the engine.
Obviously it is very important to have the crankpin at an advantageous angle before maximum cylinder pressure is achieved in order that maximum force is applied through the piston and rod to the crankshaft. If the mixture was ignited too early, much of the force would simply try to compress the piston, rod and crank without performing any useful work. In a worst case scenario, the cylinder pressure would be rapidly rising before the piston reached TDC which would have the cylinder volume decreasing at the same time. This will often result in knock or detonation which is counterproductive to maximum power and engine life.
Detonation or knock is defined as a form of combustion which involves too rapid a rate of energy release producing excessive temperatures and pressures, adversely affecting the conversion of chemical energy into useful work.
It appears that we want 3 events to co-incide simultaneously:
the burning of the entire air-fuel mix to be finished by 20-25 crank degrees ATDC, the rapid expansion of the mix volume from a thermal reaction (explosion) to achieve peak cylinder pressures from 15-25 degrees ATDC, and the rod to crankshaft angle to be optimal for a maximum amount of torque to be produced from the downward push of the explosion (i.e. best rod to crankshaft angle occurs at 25 degrees ATDC).
B. Combustion Chamber Shape
From the above quote, you should have surmised that since the flame front travels from the spark plug at a fixed rate or speed through the mix to the outer borders of the combustion chamber (i.e. the bore and piston crown), if you increase the rpms, there will be less time for the flame front to reach the borders and completely ignite the mix. The flame travel at one speed needs more time to burn everything inside ...but it has less time to travel the distance from the plug to the borders with higher rpms....so you have to start the combustion process earlier by igniting the mix earlier to give it more time.
Therefore, in slow burning combustion chambers or at higher and higher rpms, you will need more advance on the spark timing (i.e. start the igniting the mix at a larger number of crankshaft degrees BTDC).
The problem with more advance are:
1. The piston is compressing the air fuel mix BTDC. If you ignite the mix at an earlier time BTDC, the exothermic reaction causes the mix to expand. So now, the piston must also overcome this expanding force [measured in engine cycle analysers as FMEP or Frictional Mean Effective Pressure] while it is still squeezing the mix.
2. The peak cylinder pressures and temperatures begin to climb more rapidly. Too rapid an increase in these 2 parameters can lead to auto-ignition of the mix (detonation).
Therefore, the goal of modern day combustion chamber designs is to achieve a burn (or flame front travel) that is fast enough to completely ignite the entire mix within the chamber without more spark timing advance and auto-ignition. Indeed, needing less spark timing advance (retard timing) is a sign of a more efficient burn.
How do we achieve this controlled, fast, complete burn without going too fast as to result in detonation?
Since the focus of the topic is on
combustion chamber shape and quench, I will not comment on the importance
of flow quality (i.e. atomization of fuel, swirl cylinder filling to achieve
a stratified charge, charge homogeny, charge ionization, and charge density)
on achieving fast burn. There was some allusion to the fact that I adhere
to Endyn's framework of combustion theory when in fact many of Endyn's methods
follow common advanced combustion theory that is also derived independently
from Endyn and accepted by leading race teams. So this is not a regurgitation
of the Rollerwave or Soft Head theory. There is such a thing as universal
truths or concepts about combustion theory which are arrived at independently
by various builders and mechanical engineers. Just because Endyn also states
these concepts in no way means that they have sole ownership to them...the
designs to achieve these? absolutely, that is what copyright laws are all
about. But the concepts? not.
now on to the discussion about combustion chamber shape....
III. Types of Combustion Chamber
A. Head Bowl Shapes:
1. Pent-Roof or Crescent (eg. B series and C series Hondas and Chevy Corvette ZR1 LS5 )
- Symmetrical, more compact design looking like the roof of a doll house with a centrally located spark plug and 4-5 valves/cylinder angled onto the flat part of the "roof" of the chamber. A relatively smaller chamber volume and the unique chamber shape produces a low surface area to volume ratio.
2. Hemispherical or Open (eg. Chrysler Hemi, Dodge-Chrysler 426 cid Cuda or Challenger )
- Symmetrical arc roof design resembling an upside down salad bowl with a centrally located spark plug and 2-4 valves/cylinder.
- Large bore and large combustion chamber volume to get more air fuel into the chamber necessitated very large piston domes (to achieve higher CR's ) which added even more surface area
- Very slow burn as a result of the large chamber volume and high surface area to volume ratio and therefore, necessitated tremendous ignition timing advance in the order of 45 degrees BTDC and required high rpms . They liked more and more advance. These twitchy engines readily detonated under these conditions to the bewilderment of their tuners. Timing was advanced further and further as more power was extracted until there was unexpected sudden detonation.
3. Wedge or Cantered (eg. Ford Escort , most older domestic V8's)
- Asymmetric design resembling a triangular wedge shape on profile or side view with 2 valves/cylinder over to one side at the highest part of the wedge roof and spark plug located near exhaust valves.
- Minimal quench area as compared to the pentroof and hemi designs
4. Bowl in Piston or Flathead (eg. old Ford V8's of the 50's )
- Symmetric flat top roof design with centrally located valves perpendicular to the piston crown , 2 valves/cylinder, and spark plug located off to the side.
- Often used in turbo applications due to high quench area
- Rarely used in modern day naturally-aspirated internal combustion chamber designs
B. Piston Crown Shapes
1. Domed (all motor : increases CR at the sacrifice of chamber volume)
2. Dished (forced induction: lowers CR at the sacrifice of increased flame distance and less quench area)
3. Flat top (either application: reduced surface area to volume)
The controversy lies in how much high CR contributes to detonation. Does increasing combustion volume, decreasing CR, and decreasing quench with a dished piston crown help or hinder fast, complete burn in forced-induction?
IV. What Do We Want In Terms of
The flame front travels away from the spark plug like concentric ripples or circular waves travelling away from a stone thrown into a calm pond.
We want the "ripples" or waves to reach the cylinder wall and piston crown (or floor of the combustion chamber) by 20-25 ATDC (at the most optimal rod to crankshaft angle that will help turn the crank with the greatest force or mechanical advantage).
In terms of combustion chamber shape, we want:
1. the most compact chamber size possible .
A shorter distance for the flame front to travel to the outer borders of the chamber will definitely make life easier especially with short recovery times for the flame to travel with higher rpms. We would like to see small chamber volumes and smaller bores if possible.
2. a low surface area to volume ratio.
Low surface area is correlated with shorter flame distances. Secondly, fast burn is fascilitated by having a low surface area to maintain cylinder temperatures high enough to assist in the burning process. You have to remember that the piston crown, cylinder walls, and head bowl surfaces are relatively cooler in temp. compared to the burning air fuel mix. The outer edges of the air fuel mix are in contact with these surfaces and are cooled by them during the combustion process. More contact with a bigger area of these relatively cooler surfaces will lower air fuel mix temperature.
So more area will cool the mix faster from the outer edges to the center of the mix.
We want a low enough heat transfer rate through the cylinder walls to help the burning of the mix but we don't want the temperatures to go too high so that the high heat ignites the mix prematurely before the flame front reaches it...this auto-ignition (igniting without the plug) is detonation.
The pinging you hear is when the auto-ignitions occur as the piston is coming up and is suddenly stopped or slowed down. For the most part though detonation cannot be heard especially at higher rpms when engine internal noise drowns out the sound. Most microphone-based knock sensors like Honda's only have enough sensitivity to detect knock up to 4000 rpm, after which time the microphone cannot tell the difference between knock and engine noise.
The issue of having "just enough " cooling surfaces inside the chamber to prevent the temperatures from jumping too high for conditions to fascilitate detonation is the basis of quench area.
Squish or Quench Area (also called artificial or mechanical octane boost) is the area along the outside edge of the head that is more or less flat or matches the angle of the crown of the piston closely. Its purpose is two-fold: 1) it acts to create a homogeneous mixing of the charge as it is compressed by the piston. A more homogeneous mixture burns faster with less ignition advance. 2) when properly set up, the squish area acts to cool the charge and the end (burnt) gases to help eliminate detonation.
Increased quench area can be obtained by milling the head deck and/or reducing head gasket thickness. Both of these modifications will further reduce the distance between the flat portion of the head bowl and the corresponding flat part of the piston crown.
So the top is even with the top of cylinder barrel, then the two cooler surfaces (flat portions of the head bowl and piston crown) close together reduces the possiblity of detonation and promotes turbulence which keeps the mixture homogeneous and produces a more complete and faster burn.
3. centrally-located spark plug with long spark duration
It has been shown that when a spark plug is placed at the bore center axis instead of being off to one side like in the Wedge or Piston in Bowl chambers, the burn is more complete for a given bore area. There has to be sufficient time for the spark to jump across the electrode in order to start the whole burn process and having a central location reduces the need for longer and longer spark duration. If you combine swirl mixing to achieve a homogeneous stratified intake charge (air fuel mix) with a central spark location, the burn efficiency increases.
4. a symmetric layout allowing for 4 valves or more per cylinder
Symmetric layouts allow for more valves to increase volumetric efficiency and allows the chamber to have an organized charge turbulence (by inducing swirl) upon filling (especially at WOT) but reduces quench area.
V. So which of these aspects of chamber shape are most important?
The Hemi combustion chamber design had such a large chamber volume and large surface area to volume ratio that it's design led to a very slow burn rate despite having a symmetric layout and a centrally-located spark plug.
It would need so much spark timing advance (up to 45 degrees BTDC!!!) because of the slow burn and a huge piston dome to achieve decent static CR's which increased even more surface area causing a vicious cycle.
Engine tuners for the hemi would keep on advancing the timing because they saw that they achieved more and more hp gains, as they advanced more and more. Meanwhile cylinder pressures were going through the roof as they advanced more and more. Then suddenly the engine would grenade with the next increase in advance. These engines were twitchy at high rpms (shorter time for flame front travel) and would grenade out of nowhere as the spark timing was advanced just that little bit more.
Most modern builders have come to realize that the smaller pent-roof shape with a piston crown that has the lowest possible area to achieve the desired CR is the best design despite a lower quench area compared to the compact Bowl in Piston (FlatHead) or Wedge chambers.
I guess the dogma that everyone clings
to is that static and dynamic compression ratios are THE determining factor
to thermodynamic efficiency (complete burn). Remember, efficiency means how
much you put out relative to how much you put in. So in terms of thermodynamic
efficiency, this means how much potential energy stored in the carbon-hydrogen
bonds of fuel and bonds of oxygen in the air can be converted to kinetic energy
in the form of expanding heat to push the piston down.
Peyomp hinted at the fact that I may present the views of Endyn in explaining combustion chamber shape. So what is Endyn's "unconventional" view on combustion?
If you read the Endyn archives using the search terms "combustion" and "compression ratio" and the Soft Head '99 article at their website, are the views that unconventional?
Endyn basically says that when it comes to making power, optimising the combustion burn is king while traditional hot topics (pun intended) suggested to be important in making power , such as improving cylinder filling and port flow numbers, take a backseat or lesser role (or even have an unimportant role ...for example, flow capacity).
The role of high compression ratio in causing detonation is downplayed by the detonation counter-measures Endyn emphasizes which is flow quality.
What is flow quality? As long as you have the intake charge speed up at the end of the filling event to result in a stratified charge, the compression ratio becomes less of a factor, since cylinder pressures and temperatures (the causes of detonation) are lower. Moreover, Endyn also tries to a) keep the intake charge atomized with small fuel droplet size ( which increases fuel droplet surface area) and b) use the shape of piston top to rapidly push the intake charge, as the piston is squeezing the charge, over to the hotter exhaust valve side of the chamber ( to induce surface ignition of the mix). Both of these methods also leads to a more complete and rapid burn. So the dampening effect of their chamber shape on peak cylinder pressures and the emphasis on flow quality purportedly reduces the negative effects of higher CR (in the order of 14-15:1 static CR).
Endyn emphasises startified charge, low surface area to volume and a compact chamber. This is not unconventional...in the 80's? yes it was unconventional and lean burn was heresy back then but in fact, it is very conventional these days.
Ironically, some surface area is increased at the piston crown top in the Rollerwave configuration as the dome and exhaust trench adds area, in exchange for speeding up the mix again as it enters the chamber at the intake port throat. So charge homogeny and mixing even takes precedence here over area to some extent.
Your comments to any of the above points are welcome. To be clear, I do not work for Endyn and I don't get any sponsorship from them. I don't believe everything they say without skepticism. I merely am pointing out their ideas and factors to modern combustion theory unrelated to Endyn that are out there in the public record (on the web even!). They are not that oddball as they make themselves out to be.
it's a lot to digest but if you get the stages of combustion at the various crank degrees, the main factors that affect combustion (chamber shape is only one factor ), and the aspects of chamber shape other than CR that contribute to a more complete burn, you will have gotten the jist of this post.
It's no co-incidence that the NSX, Ferrari 350, and latest Vette use a 4 -5 valve /cylinder pent-roof combustion chamber with semi-domed pistons.
bottomlines: you want low surface area and compactness (i.e. smaller chamber volume) .....too slow of a burn is a setup for lower thermodynamic efficiency and detonation at high rpms (like in low displacement Hondas) as CR increases. The flame can't travel the longer distance of a large volume chamber with higher rpms and more area slows the burn speed down from cooler chamber temps.
the thing that bends people's noodles is the de-emphasis of high CR and relatively reduced quench area.
everyone hears more quench area is good...yes to a certain degree but not so much as to increase your chamber volume and surface area to cause slow burn from too much of a cooling effect.
everyone says lower CR is needed in forced induction at high boost...yes to a certain degree but choosing too deep of a dish on the piston to lower CR increases surface area, increases chamber volume...both of which causes slower burn....and a deeper dish also reduces quench area at the same time...so you get kicked in the balls twice with a deep dished piston ( tempted to put in pizza instead of piston there weren't you?). A lot of aftermarket piston manufacturers lower CR by reducing the compression height (distance from the piston pin center to the top of the piston) instead of deepening the dish on the piston top surface.
the Chrysler Hemi chamber is the poster
child of how not to make a chamber shape these days.
PLEASE don't get me wrong: it is an ingenius design developed in an era when high octane gasoline was plentiful and the mentality of " bigger and more is better" ruled the day and people's consciousness.
It's 1950's technology at it's pinnacle (and already had one re-incarnation or comeback in 1970 when the Cuda and Challenger kicked some serious ass). Remember the 50's was an era of high post-World War 2 optimism with lots of gas around and the Saudis were just figuring out what to do with all this oil they had.
I am not spitting in the face of the incredibly intelligent men who designed this Hemi chamber. On the contrary, I have incredible respect for what they brought to the table. It was an important link to get where we are today. We have learned more. It's the evolution of combustion chamber design.
The OPEC oil embargo, fuel efficiency, computer era (CAD CAM design technology, ECU's in passenger cars, and computerized engine cylce analysers), happened in between 1950 and now. We had conservation added to our consciousness. The 80's IMSA/FIA Endurance and FIA Formula One road racers had to deal with fuel restriction rules to develop more efficient combustion engines. NASCAR was merrily continuing with the old way of thinking.
We learned that big is not always better...especially not bigger chamber volumes ....and we learned why. The Japanese were at the forefront of lean burn technology and fuel efficiency. If you are not going to use exhaust gas recycling (EGR) back into the chamber to increase fuel efficiency , how are you going to do it?...especially when EGR kills performance (hp) and driveability....
Stratified layers of progressively leaner air fuel ratios as the piston fills...this is how. You can add less fuel, use a smaller chamber size, use smaller displacement, use high rpms...to get the performance without big fuel, big ports, big displacement, big chamber volumes, big CR's...no more big.
Fast burn, lean burn, small chambers, high rpms....this was the new design driven by the climate of conservation.
So we can now run as fast as some of the pony cars but drive a further distance because of better gas mileage... The 1970 426 cid Cuda Hemi is fast but would be parked at the side of the highway or track because it never got to the finish line running out of gas...we would be at the finish line ahead....isn't that called progress? This is no tortoise and hare story. We can run as fast as the hare and be as patient as the tortoise.
Just look at the automotive history of engine design and combustion theory over the past 30 years and you will see how we got to a pentroof compact design and lean fast burn.
To go back to a big chamber Hemi design these days would be a mistake IMHO...I hear with all this retro craze and making cars look similar to what they did in the 60's-70's, Daimler-Chrysler may bring back the Hemi (again)...I hope they update the design (better turbulence filling and less area)...you know that there are some Harley Davidson engine builders still trashing Japanese bike engine designs and promoting the glories of a big Hemi design even today...they still cling to the past with a strong grip oblivious to the change in awareness for the climate that we have today.....
here is an example:
by Tuan from Hondavision.com