Curl said:
What is the reason for making bowl shaped piston crowns?
Some factors in deciding the shape of the piston crown are the valve angle and placement, which determine the shape of the combustion chamber in the head, and the desired compression ratio.
The majority of gas engines use a flat-top piston and shallow included angles on the valves (wedge heads and modern 4 valve heads), and the compression ratio is adjusted by varying the depth (and thereby the volume) of the combustion chamber. This basic configuration, along with a mostly central spark plug location, gives a relatively efficient burn by allowing the flame front to smoothly progress through the fuel/air mixture. A fast consistent burn that reaches all the outer edges of the chamber at the same time is the goal.
By having flat surfaces on the piston that match flat surfaces on the head and having the piston almost touch the head at TDC, turbulence can be generated to stir up the burning mixture and give faster more complete combustion. This is called squish or quench, and helps ensure that no pockets of the mixture lag behind the main burn. These pockets can either end up unburned or can cause the engine to be sensitive to detonation, both conditions lowering efficiency and raising emissions and the later possibly causing engine damage.
If a domed piston is used in a large chamber to get the compression ratio desired, the dome can temporarily block off some of the flame front and cause erratic and incomplete combustion, as described in the first article I linked. For example, a head with steep angles between the valves (old Chrysler Hemi) has a large combustion chamber and requires a large dome to get the compression ratio up. The burn on these engines is not very consistent or efficient so is not well suited for low emissions. The popularity of the Hemi at the time was mainly because the ports flowed well and the engines were able to make a fair bit of power for their size.
A current rule of thumb for present engines is that a dome of more than about 0.150" is going to start causing problems and costing horsepower. This is more of a concern with engines that are used for the higher levels of racing where every bit counts but should still be considered when deciding on a particular engine package.
On the other side of things, the Ford 1600 cc Cortina GT engine is an example of a successful racing engine that used a flat "combustion chamber" and a piston with a large dish. It was the mainstay of Formula Ford for several decades, mainly because of its rugged design and reasonable cost. The central dish allowed for a decent consistent burn and had a squish ring around the perimeter of the piston to also help the burn.
The power was good for the day ('60s) but the straight-up valve design required for the flat head limited the port flow and cam timing and thus power. These limitations are why gas engines are not designed to use pistons with large dishes; most modern 1600cc engines produce considerably more than the 110-115 hp that the GT engine made in race trim, and are much cleaner as well.
Large domes or dishes expose more surface area of the piston to the high combustion temperatures generated, requiring more cooling to keep temperatures reasonable and wasting heat that otherwise could power the vehicle. As a result, both extremes are avoided unless a particular shape has a benefit that offsets the heat loss.
Did that help?
