As you might expect, pistons are subjected to more heat, pressure and movement than nearly any other engine part. They have to be built for durability. With engines constantly evolving, carmakers keep finding new ways to built better pistons, including altering their size and composition. The piston is at the heart of how an engine creates motion. Adding pistons to an engine or making them bigger increases displacement, which means the engine generates more power because it burns more gasoline.
The number of pistons in an engine isn't the only important thing about them, however. The shape of a piston has a big effect on the internal combustion process. Let's first talk about different piston shapes. Pistons that are elliptical, or shaped like ovals, become more circular when heat is applied. This allows for improved sealing when the cylinder meets the combustion chamber for greater efficiency.
Some pistons are tapered or conical, which lets the piston move more freely regardless of how much heat is present. Others are "barrel" shaped, which is smoother and generates less noise and harshness as it moves [source: University of Windsor ]. Next, let's learn more about how the top of a piston affects performance, fuel economy and engine longevity.
As engines have evolved over the years, pistons have evolved with them. They're getting shorter and lighter, and use smaller skirts — the cylindrical "body" of the piston. Newer pistons are often made of aluminum alloys comprised of more silicon than in the past. Its function is to create internal charge acceleration that stimulates the end gas and increases the burn velocity as it rushes to escape this area as the piston sweeps toward TDC. It is considered internal charge acceleration because it's created in the bore.
To properly identify a combustion chamber, all its aspects including shape need to be considered. For our purposes we limit the discussion to those found on most production engines in America.
Hemispherical or Pentroof A chamber of this design is considered to offer the least amount of compromise for the efficiency gained. The valves are placed at the bore perimeter and, in the instances of the original Chrysler Hemi, at an included angle of This position also allows for huge airflow gains since it moves the valve away from the wall and unshrouds quickly. This creates a more efficient cross-flow movement of the charge during overlap and limits thermal transfer from the exhaust valve to the fresh charge.
As mentioned previously, this design offers the best surface-to-volume ratio and also creates a very short direct exhaust port, essential in limiting heat rejection into the coolant. Having a central spark plug, the Hemi 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. With pushrod designs, the valve placement requires dual rocker shafts but lends itself very well to dual OHC 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.
Mickey Thompson experimented with Hemi heads on Pontiacs in the s and you'll recall that the division designed an experimental aluminum Pontiac Hemi engine, reported on in the March issue of HPP. Wedge Used over the years by almost every manufacturer including Pontiac, this chamber resembles an inclined bathtub recessed into the deck of the head.
Inline valves are normally tilted to accommodate the sloping roof of this design. The spark plug is located on the thick side of the wedge and is usually positioned midway between the valves. 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.
Bathtub or Heart-shaped The bathtub designation is generally reserved for any chamber that's not a wedge or hemispherical. Most domestic engines of pushrod design have used it in varying forms. In some instances the shape of the combustion chamber was almost oval, with the latest trends 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 has nicknamed it the heart chamber. The valves are inline and are partially masked by the chamber wall being more exposed on the plug side. 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 as central as possible, working under these limitations.
Heat transfer from the close proximity of the valves limits volumetric efficiency and octane tolerance. Bowl in Piston To the best of HPP's knowledge, this style has not been utilized by Detroit on a gasoline engine in the last fifty years but is common in Europe.
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. Making Sense of It All Since we don't have the means to create our own cylinder head, we're forced to work with what is available.
The theory of combustion chamber design and function was touched on only briefly here; many have spent their entire lives studying this with new discoveries each day. Our reasoning was to establish that more than flow numbers need to be considered when choosing a Pontiac cylinder head.
How the combustion chamber uses the airflow is just as important as the flow value itself. Even the worst combustion chamber design can be improved upon by smoothing the walls and surface of the chamber to increase flame speeds, reducing the volume of the squish region with a zero deck or thinner head gasket, and indexing the spark plug. The worth of these simple tricks is diminished, as the design of the chamber becomes better, but should not be forgotten.
Airflow numbers are easily obtained on a test bench but a trained eye is needed to identify a more efficient combustion chamber. A good rule is to query the manufacturer on the amount of spark advance his cylinder head would require with your combination. The more lead it needs, the greater the propensity for detonation and the slower the burn speed.
Head Games Here is the evolution of the combustion chamber for high performance Pontiac engines. Special thanks to Jim Taylor and Mark Erney for their assistance in obtaining this photography. As you can see, the "basic" design of Pontiac's fully machined combustion chamber was little changed over its history.
It is actually a combination of the wedge and bathtub style. However, its size and the valve placement were modified as needed. The early Tri-Power heads shown feature 1. The chamber was also relieved on the intake side.
For , the chambers were opened up, reducing the shrouding of the valves. In mid , the round-port Ram Air II heads debuted and as you can see, the chamber shape was subtly changed as well as compared to D-port heads.
It was done by opening up the area around the valves on the spark plug side and adding a small scallop above the plug hole. A special treat is to see a Ram Air V head. Valve sizes would be a whopping 2. Chamber size varied by year and application, with early heads using a cc chamber in most cases; some chambers were as small as 67 ccs.
The and later heads exhibit a much larger chamber to reduce compression with the 96 head featuring a 96 cc volume and the HO heads featuring large cc chambers. The 6X head reveals the smaller 1. On one hand, I am not out to increase sales of said guide since I'm not one of its supporters in any way. On the other I am certainly not attempting to belittle the efforts of the featured company as they have long been at the forefront of performance A-series engines and components, nor do I wish to be-little the grandiose efforts of an eminently brilliant engineer who produced a valuable plethora of information to the A-series masses.
I do, however, feel a little more information than has been presented is needed to provide those general readers with some much-needed relevant and tangible information.
I am not about to start conjuring up a vast array of test data either, since this is all relative more later - simply some basic information to consider. There is absolutely no denying the fact that a certain number of folk will always be drawn by something that is 'different'.
It's generally what helps vendors keep selling wares over a protracted period. Reincarnations of innumerable goods will attest to this. Cylinder head chamber shape is no exception - more than any other part of the cylinder head because it is something that is easily seen with the naked eye. But all that glitters isn't necessarily gold…. The chamber is responsible for four basic events. On the face of it, the simplest answer is to move anything that could be viewed as an obstruction to this parameter as far away from the valves as possible.
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