Combustion Chamber Shapes
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Fresh post by MacDizzy
There are good and bad combustion chamber shapes for different reasons. If we’re talking about high performance, a good shape will be one that will burn most of its mixture in a small amount of time. That shape will usually be a low and flat dome. With a squish area between 35%-45% of bore area, depending on its displacement and operating rpm.
A bad shape, for high performance applications, is one that is tall. It doesn’t really matter if it’s tall and rounded or tall and squared off. That fact that it is tall means the burn time is going to be longer. That translates into a slower, weaker, rise in cylinder pressure. Some of these designs are good for other reasons, however.
One of the reasons to make a chamber with a slow burn is so that the power delivery will be smoother. With a smoother power delivery,
Engine life is enhanced. The piston and bore and main bearings will all last longer.
Chambers with a thick squish area can usually be improved by tightening up the squish thickness. One of the first things that will happen is power will increase due to the increase in compression ratio. If you modify a combustion chamber in this way, be sure to calculate the UCCR before the modification, so you’ll know just how much material to remove without raising the compression ratio too high for the fuel you intend to run.
Fuel mileage will also increase due to more of the fresh mixture being burned – less going out the exhaust port.
There have been engines designed with all kinds of different combustion chamber shapes. Most of them are used to combat some undesirable running condition, or poor Engine design, usually related to an inability to get rid of its heat.
An example of this is if the Engine is shrouded, and must survive on its own cooling fan, blowing air across its cooling fins from one direction only. Or maybe the Engine is placed in such a position as to not receive a cool airstream as it runs. These types of conditions will have engineers doing all kinds of things to get cooling under control.
One of the most popular things to do is to simply lower the compression ratio. Lower compression ratio’s make less heat.
Another popular thing to do is to design a thick squish area. Thick squish areas will usually lower the compression ratio, and provide a decent amount of cool gasses that do not burn. Though the tailpipe will be dirty, it will be cool and dirty.
Still another is to design a deep bowl, or pyramid shape dome. These shapes leave a lot of time for the mixture to burn, so they are more efficient. Their heat rise does not spike quickly, but their burn is more complete. This design is often used on smaller displacement cylinders, in order to take more advantage of as much of the displacement they have.
The offset chamber is another way to calm the effect of too much heat. A design like this puts the combustion chamber on the side of the Engine that runs the coolest, typically the inlet side of the Engine. Medium and large bore engines are sometimes designed this way, particularly if the Engine suffers from some form of shrouding, or if it might, through the course of running, have its cooling fins packed with dirt or mud.
All of these designs may or may not have positive squish. That is, a squish area angle that is greater than that of the piston dome.
Let’s say a typical piston can have its dome averaged out to be 11°, using a protractor. If the squish area is parallel to this, 11° also, less fuel is wasted – more of it becomes part of the power making process. This is a high output design, but tough on pistons and other Engine internals. It’s good for fuel mileage however, in lower compression ratio’s.
In a positive squish condition, the squish area of the combustion chamber should be greater than the piston crown angle. Typically, 1° to 1.5°. When the chamber angle is cut like this, peak power will go down a little, but piston life goes up a lot.
The squish area really needs to be at least 2° to 3° greater than the piston crown angle for better advantage. In this case, 13° to 14°. In extreme cases this angle can be as high as 10° greater than the piston crown angle.
The effect of this is multi-fold. It (may) lowers the compression ratio, burn more slowly, and provide cooler end gasses. In effect, it acts like a huge triangle shaped combustion chamber.
The squish area ratio, that is, the percentage of the combustion chamber that is squish area has a large effect on the performance and heat too.
A 50% squish area ratio (SAR) means that 50% of the combustion chamber is combustion chamber, and 50% of it is squish area – in percentage of area. Ideally, it would be best to have a small SAR, like 40%. But, small SAR’s mean the central, let’s call it, burn area, becomes larger. And a larger burn area only work best on small engines.
The reason this is true is because fuels burn at a relatively constant rate. Whether you pour the fuel in a 500cc single cylinder Engine or a 50cc single cylinder Engine, its fuel is going to burn at the same speed.
With that in mind, consider a running Engine of both of those sizes. To keep it simple, let’s say the spark plug fires at 20° BTDC on both engines.
Twenty one crankshaft degrees after the spark plug fires, when the piston has just passed TDC (Top Dead Center), cylinder pressure should be at its peak in both engines. But, due to the 50cc Engine having a 50 mm bore, and a short fuel burn distance (25 mm in every direction from the central spark), and the 500cc Engine having a 100 mm bore, and a long fuel burn distance (50 mm in every direction from the central spark), the small Engine is much, much, closer to its ideal burn. Its flame front has burned all of the available fuel, its cylinder pressure has risen the most it can. The 500cc Engine will take another (as much as) 15 crankshaft degrees before the cylinder pressure will peak.
That is why smaller engines are more efficient. In the same amount of time, more of their mixture is burned.
Squish areas are less important on small engines, because the flame front is fast enough to burn their mixture well. They will burn more of their mixture, as a percentage of what’s available than a larger Engine will.
The other important dimension to a combustion chamber is the squish area thickness. That is, the distance between the piston, at TDC (Top Dead Center), and the combustion chamber (head), in the area of the squish – the edge of the bore.
The thicker it is, the less efficient it becomes. The thinner it is, the more efficient it becomes. If it is too thin, pre-ignition can become a problem due to creating local hot spots, even if the compression ratio is OK for the fuel it is running.
A local hot spot can be a small piece of carbon that is stuck to the piston crown or dome, becoming superheated. This glowing hot piece, acts like a spark plug, igniting the mixture much sooner than anticipated.
It is important to consider as many things as possible when designing a combustion chamber squish thickness. Another very important thing to consider is the relative condition if its parts.
If the Engine is fresh, with new main bearings and crankshaft and rod bearings, including the piston pin bearing and associated pin, then one could logically assume that minimum squish thickness can be attained.
We consider this because, though seemingly solid, metals are elastic. When the Engine is spinning, these super stiff parts move and stretch and get pulled all out of shape. If the squish area is too thin, the piston will make contact with the combustion chamber at high rpm. This will cause detonation, because the compression ratio will rise to a level higher than anticipated. It can also cause pre-ignition, as well as a severely damaged Engine – usually the crankshaft bearings, piston, bore and dome.
It seems, the major developers of engines have discovered that a single cylinder Engine of about 125cc’s is the closest to ideal. Well, ideal for practical purposes.
In the real world we have 125 cc engines, and 250 cc engines and 500 cc engines, and others. In the interest of keeping things light and simple, a compromise is made. And that compromise is efficiency.
To keep the engines relatively light, their displacement is grown. Their power goes up, but not in a linear fashion to their displacement. What I’m saying is that realistically, a 500 cc single cylinder Engine is not 10 times more powerful or efficient than a 50 cc single cylinder Engine.
Some direct fuel injection two strokes have made their way into the market. Their combustion chamber shape likely has more to do with packaging the injector parts into its combustion chamber than any real attempt to extract maximum efficiency. Given enough time however, I’m sure they will continue to improve.
The fact is that if we want direct injection two stroke engines we must continue to purchase them as they are being developed. As big as the manufacturers are, they can not afford to design the ultimate Engine before bringing a revenue building product to fruition. We must buy their “development”, if you will.
I also want to mention that combustion chamber shapes designed for high fuel mileage and high output are similar, because they waste little fuel.
Engines that turn more constant rpm’s typically use tub shaped chambers, because they do not need to be as responsive but need good torque. Slower burning shaped typically make better torque and fewer rpm’s.
Engines, like ours, on our motorcycles and ATV’s, the kind we want to be really snappy when we blip the throttle, are usually designed around approximately 45% SAR, with the compression ratio relative for the fuel its running.
Smaller engines, like scooters and mopeds will still be relatively snappy with less or no squish area due to their smaller bores.