Fire Under PressureFebruary 21, 2019
A candle flame is an example of combustion related to that which takes place in an engine. But because the candle flame burns in the atmosphere, it differs from engine combustion, where the gas exchange process occurs internally under higher than atmospheric pressure.
A flame can have two distinct regions: pre-mixed and diffusion. The burning candle experiences a diffusion flame because it occurs at the interface between the fuel and the oxidant. With a candle, the fuel is melted and evaporated by radiant heat from the flame and then oxidized by the air.
The Bunsen burner offers a more complex example of combustion; it has both pre-mixed and diffusion flames. It consists of an air regulator, a fuel source and a cylindrical tube. The flame generated nearest the base is pre-mixed. Air entering at the base of the Bunsen burner is not sufficient for complete combustion. Consequently, a second flame above that point is established at the interface where the air is diffusing into the unburned fuel. This is responsible for the Bunsen burner’s flame-within-a-flame appearance.
Even though the combustion event inside an engine is quite a bit more complicated, the basics still hold true. Gasoline, as a hydrocarbon-based fuel, needs to be atomized and emulsified to burn–it has to be broken down into small particles and mixed with air. Alone, in liquid form, it will not burn. When atomized, gasoline has a laminar burning velocity of approximately 0.5 meter/second (m/s) or 1.64 feet/second (f/s). As a comparison, when acetylene is mixed with air, it burns at a rate of 1.58 m/s or 5.18 f/s. The slow laminar burning speed of gasoline poses an interesting problem when gas is used as a fuel for an internal combustion engine.
This is best represented using metric measurements. The time for a gasoline-fueled flame to travel within a cylinder with a 100mm diameter and an ideal central location for ignition is 100 milliseconds. There’s a problem, though: When an engine of this dimension is running at 3,000 rpm, there is only a window of 10 milliseconds for the combustion event to take place. Obviously, another force must be at work, because we all know that a gasoline engine can run substantially faster than 3,000 rpm. The secret is to increase the burn velocity.
Early on, Detroit learned that the flame in an engine travels at a rate of 10 to 25 m/s. This is substantially faster than the velocity stated earlier, but it is the reason gasoline can be used as an automotive fuel. To increase burn velocity, turbulence can be introduced to the combustion event. In an engine, this is accomplished by the induction and compression process, along with the design of the combustion chamber.
During pre-mixed combustion, the effect of the turbulence is to break up or wrinkle the flame, creating burnt gases in the unburned region and vice versa. This effectively increases the flame area and speeds up combustion. Though diffusion is usually associated with a compression-ignition engine, such as the diesel, it can also occur in a spark ignition engine under certain circumstances: The fuel is injected in a fine spray and the turbulent air motion sweeps the vaporized fuel and combustion products away from the fuel droplets, speeding up the burn velocity.
The actual combustion event that causes a flame to be established and expand against the piston is very complex. A full grasp of the chemistry involved is not required to understand it, but the laws of thermodynamics–the study of energy and its transformations–need to be touched on briefly.
Thermodynamics consists of two statements deemed laws: The first says that energy cannot be consumed or destroyed; only its state can change. This can be applied to an engine and how energy is turned into heat, then motion and finally back to heat.
The second law of thermodynamics is more complex, but can be summarized by saying that energy follows certain guidelines and never deviates from them. For example, heat, on its own, will travel only from hot to cold; for anything else to happen, an external energy force has to act on the system. The laws of thermodynamics apply directly to a combustion chamber due to thermal transfer into the casting and engine coolant, along with the effect of the compression ratio on thermal efficiency.
A common analogy that compares an engine to an air pump establishes that engine output is more substantial when more air is pumped. This cannot be denied, but it is a one-dimensional statement that ignores the fact that, without an efficient combustion event, the air by itself can do nothing. For this reason, early automakers quickly recognized the need to examine the impact the combustion chamber has on an engine.
The Combustion Chamber
In 1673, Christian Huygen, King Louis XIV’s water keeper, invented the first engine. It was developed as a better means to transport water from the Seine River to the grounds and gardens of the Palace of Versailles. This gunpowder-consuming single-cylinder external combustion behemoth was welcomed by the peasants and oxen that were used as water carriers prior to its existence.
As the internal combustion engine slowly progressed from these humble roots, it was discovered that efficiency and power could be increased with a controlled process in a closed environment. Early combustion chambers were little more than covers for the cylinder until Sir Harry Ricardo made a major breakthrough in combustion chamber design in the early 1900s.
Ricardo’s monumental invention was the turbulent cylinder head for a side-valve engine, which set new standards for compression ratio, running at 6.00:1. At that time, the fuel had an octane rating of only 60 to 70. Over the next several decades, engineers and inventors explored the impact of the combustion chamber on the character of the engine. Today, combustion chamber design and technology are constantly evolving and producing smaller, higher specific-output, more fuel-efficient engines.
A number of criteria drove combustion chamber development. The distance the flame needs to travel had to be minimized. This can be accomplished by reducing the distance from the spark plug electrode to the incoming charge. This allows for higher potential engine speeds, which will produce more power. It also provides less time for something to go wrong. Abnormal combustion, better known as detonation, is more likely with a slow combustion process, because it allows time for an additional flame front to start.
The spark plug should be centrally located in the bore and nearest to the exhaust valve, as these are the most turbulent and hottest parts of the combustion chamber, respectively. Additionally, the exhaust valve should be as far from the intake valve as possible, limiting thermal transfer to the fresh incoming charge.
There needs to be sufficient turbulence to promote rapid combustion–but too much turbulence can create problems, transferring heat away from the chamber and promoting noisy combustion. This turbulence is generated by design and can be induced externally, in the intake port, or internally, using squish pads. The clearance between the deck of the cylinder head and the piston is identified as the squish region. It acts to cool the intake valve and is best located near it.
The design of the valvetrain and the number of valves used impact the combustion chamber; valve placement, size and actuation all affect combustion. Many engines have very poorly designed combustion chambers due to economic pressures. No older domestic pushrod V-8 engine with inline valve placement (other than the Chrysler Hemi) allows for a central spark plug location. The chamber is often designed to use a long-reach spark plug that places the electrode tip near the center, even though the entry point is at the perimeter. Many cylinder heads place the spark plug electrode at the perimeter of the bore, and some early Pontiac V-8s actually had a bias toward the intake valve, allowing function to take a back seat to ease of manufacture.
The ideal engine would have a high compression ratio for thermal efficiency and throttle response, but would need to work in unison with a combustion chamber that has a fast burn rate. This is essential to increase the engine’s octane tolerance and limit the production of the emission oxides of nitrogen (NOx), the poisonous gas responsible for photochemical smog; NOx production was the impetus for the introduction of emission controls in the early 1960s.
Three elements combine to produce NOx: heat, pressure and exposure time. They are the bane of every engine designer. A high compression ratio increases the production of NOx by its elevated cylinder pressure and the heating of the charge as it is forced into a smaller region. This phenomenon can be cheated by the implementation of a fast burn rate, eliminating the third element, exposure time, in the recipe for NOx. The best examples provide a balance between octane tolerance and high compression ratio with quick burn rates. Other factors come into play, too, such as the material used and the spark plug location. Starting the flame in the center of the bore allows for a quicker, more even burn, which translates into higher cylinder pressure in fewer degrees of the crankshaft’s rotation past top dead center (TDC).
To produce the most power, it is necessary to have the cylinder pressure rise as quickly as possible, allowing it to expand against the piston for as much of the stroke as possible.
Most performance cylinder heads are aluminum castings, because of aluminum’s light weight and ability to dissipate more heat and allow a higher compression ratio. But it is often overlooked that it is easier to produce power with a cast-iron cylinder head if all design factors are the same. This is due to the superior thermal efficiency of cast iron. When switching from iron to aluminum, the engine will require about one additional point in compression ratio to maintain the same thermal efficiency. This is because of the cast iron’s ability to hold the heat and use it to expand against the piston.
Of great concern to the combustion engineer is the surface-to-volume ratio of the combustion chamber. Surface-to-volume ratio determines the amount of heat loss into the casting and water jacket of the cylinder head, and is a factor in reducing hydrocarbon production. It is desirable to have as small a surface area as possible, relative to the volume occupied by the chamber. This can be derived with the following calculation:
surface-to-volume ratio = surface area/volume of chamber
Hydrocarbon emissions in designs with high surface-to-volume ratios are created because the outer layers of the mixture are being cooled in the region of the chamber walls. The flame cools as it approaches the chamber wall, extinguishing itself and leaving a layer of unburned fuel behind. Hemispherical combustion chambers offer the best surface-to-volume ratio, and tests conducted by Chrysler in 1950 showed that, in order to match the thermal efficiency of a 7.0:1 compression ratio hemi-head engine, its previous combustion chamber would need a compression ratio of 10.0:1 at 1,200 rpm, 9.4:1 at 2,000 rpm, 8.9:1 at 2,800 rpm, or 8.5:1 at 3,600 rpm. The required compression ratio drops as engine speed increases, due to gains in volumetric efficiency at higher piston speeds.
Types of Combustion Chambers
Most of us are familiar with the terms “open” and “closed” when referring to a combustion chamber; they were popularized by Chevrolet with its big-block engine series. We don’t necessarily know what the designations mean, though. They are not proper engineering terms to identify a combustion chamber and are arbitrarily used to describe the squish-to-bore-area relationship. A combustion chamber is nothing more than a cavity in the cylinder head casting (with the exception being the bowl-in-piston designs that are used in many diesel engines).
The relationship between the size of the combustion chamber and the size of the bore quantifies whether a chamber is opened or closed. An easy way to determine this relationship is to place the proper head gasket on the deck of the cylinder head to orient the bore position. If a large amount of the deck surface is exposed to the bore, the chamber can be considered closed. The portion of the head’s deck that is outside the combustion chamber but exposed to the bore is used as a squish region, creating internal charge acceleration that stimulates the charge and increases the burn velocity as it rushes to escape this area as the piston sweeps toward TDC. This is considered internal charge acceleration because it is created in the bore.
To properly identify a combustion chamber, all aspects of it, including its shape, are considered.
Hemispherical or Pentroof
A hemispherical chamber design offers the least amount of compromise for the efficiency gained. The valves are placed at the bore perimeter and, in the case of the Chrysler Hemi V-8, at an included angle of 58-1/2 degrees from the crankshaft centerline. This position also allows for huge airflow gains–it moves the valves away from the wall and unshrouds quickly, creating a more efficient cross-flow movement of the charge during overlap while limiting thermal transfer from the exhaust valve to the fresh charge.
The Hemi design offers the best surface-to-volume ratio and also creates a very short direct exhaust port, which is essential to limiting heat rejection into the coolant. Having a central spark plug, the hemispherical head offers excellent octane tolerance. At the perimeter of the bore, across from the valves, are small squish pads to help move the end gas over to the spark plug and increase burn speeds.
Valve placement in pushrod designs requires dual rocker shafts, but lends itself very well to dual overhead-cam configurations. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer. The incoming charge also generates a high rate of tumble.
Used over the years by almost every manufacturer, this chamber resembles an inclined bathtub recessed into the deck of the cylinder head. Inline valves are normally tilted to accommodate the sloping roof of this design. Spark plugs are located on the thick side of the wedge and are usually positioned midway between the valves. The inherently steep walls work to mask the air/fuel flow path; they deflect it and force it to move in a downward spiral around the cylinder axis. During the compression stroke, the squish area reduces to such an extent that the trapped mixture is violently thrust from the thin to the thick end of the chamber.
The bathtub, or heart-shaped, designation is generally reserved for any chamber that is not a wedge or hemispherical. Most domestic pushrod engines have used it in varying forms. In some instances, the shape of the combustion chamber was almost oval, with the latest trend being the efficient heart shape. The deck of the cylinder head that overlaps the piston forms two squish regions: a large area across from the spark plug and a smaller region on the opposite side. Its crescent shape earned it the nickname of the “heart chamber.” The valves are inline and are partially masked by the chamber wall, being more exposed on the spark plug side, while the area across from the major squish region is generally tapered and does not have the steep wall of a wedge style. Spark plug location is maximized by biasing toward the exhaust valve and is as central as possible, working under these limitations. Heat transfer from the close proximity of the valves limits volumetric efficiency and octane tolerance.
To the best of our knowledge, this style has not been utilized by Detroit on a gasoline engine in the last 60 years. It consists of a flat cylinder head deck with a single row of valves facing a circular cavity cast into the piston. An annular squish region is created around the piston perimeter. Known for very turbulent combustion, it works well for diesel engines but was deemed excessively noisy for American standards.
With the end of World War II and the increasing availability of low-cost, high-octane gasoline, Detroit was often able to utilize a higher compression ratio while not having to maximize the combustion chamber efficiency. As time went on and the horsepower race began, the combustion chamber became a hidden, but key, component in winning on both the track and in the showroom.