Combustion Chambers (and a little engine theory)
The combustion chamber is the area inside the engine where the fuel/air mixture is compressed and then ignited. It is generally formed on one side by the shape cast into the cylinder head, and on the other side by the top of the piston. When the piston is at top-dead-center the chamber is at its smallest dimension, and this is the time when the fuel/air mixture is at its most unstable condition and ready to be ignited. The better the combustion chamber is designed the better the engine "breathes;" that is, the more efficient the overall flow of air through the engine.
The engine's overall efficiency is determined by the shape of the chamber, shape of the top of the piston, location of valves and spark plug and overall airflow through the intake and exhaust. This article will concentrate only on the various types of combustion chambers and not on the myriad of dynamics that affect flame propagation.
Types of Combustion Chambers
To properly identify a combustion chamber, all its aspects including shape need to be considered. For purposes of this article we will limit the discussion to those found on most production engines in America.
Hemispherical or Pentroof - The "Hemi" Combustion Chamber
A chamber of this design is considered to offer the least amount of compromise for the efficiency gained. Effectively, a hemispherical combustion chamber is one half of a sphere cast into the bottom of the cylinder head. The valves are placed at the outside of the bore area and at a specific angle from the crankshaft centerline. Optimizing this position allows for huge airflow gains, since it moves the valve away from the wall to keep it from creating turbulence.
This creates a more efficient cross-flow movement of the charge during valve 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.
Hemispherical chambers generally have a central spark plug, which offers excellent octane tolerance. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer.
Used over the years by almost every manufacturer, this type of chamber resembles an inclined basin 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.
The relatively steep walls in such chamber design force the air/fuel flow path and deflect and force it to move in a downward spiral around the cylinder axis. During the compression stroke, the compressed air/fuel area reduces to such an extent that the trapped mixture is violently thrust from the thin to the thick end of the chamber. This builds up significant kinetic energy, which when ignited contributes to overall power.
Bathtub or Heart-shaped Combustion Chamber
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 later trends being the more efficient heart shape.
An example of this would be the current LS1 by Chevrolet. 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 makes it look like a heart chamber.
The valves are inline and 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, making the overall design very efficient for producing power. However, heat transfer from the close proximity of the valves limits volumetric efficiency and octane tolerance.
Bowl in Piston
This approach is essentially the reverse of a chamber-in-head design. It isn't popular with U.S. engine designers, although it was in the early part of the 20th Century in flat-head engine configurations. European engine designers do still occasionally use this design, however.
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 From All This
How the combustion chamber uses the airflow is just as important as the flow itself. Of equal importance is the shape of the top of the piston, since any protrusion into the combustion chamber will slow the flame speed. The ideal design is a dished or flat piston, since theoretically the flame front can spread evenly across its surface.
All that said, the name of the game in engine design is thermal efficiency. The ideal engine should have a high compression ratio for thermal efficiency and throttle response but needs 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). 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. The way to limit the production of Nox is to have a fast burn rate, but this is difficult to accomplish in the real world.
Looking beyond emissions and octane tolerance to produce power, it's necessary to have the cylinder pressure rise as quickly as possible, allowing it to be used to expand against the piston for as much of the stroke as possible.
Notice How Small Today's Radiators Are?
Look under the hood of any new vehicle and you'll see a pretty small radiator, even though your car/SUV might have 250-300 horsepower. Contrast that with your old classic V8-Whatever. The engine might have produced similar horsepower, but the radiator was as large as the engine itself and the car still overheated on warm days, right?
That's because of thermal efficiency. That old engine produced far, far more heat than it did horsepower, so it needed a big cooling system. Today, by contrast, that (considerably smaller) engine in your vehicle is much more thermally efficient. Its combustion chambers, intake, exhaust, cooling jackets, etc. are many times more efficient at producing power as a function of displacement and, therefore, produce less heat to be dissipated.