The exhaust system, how it works - TKART - News, tips, tech about karting

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

The exhaust of the 2-stroke engines of racing karts (called “expansion” engines due to their geometry) is a component made of sheet steel with a thickness from 0.6 to 0.9 mm. It is connected to the discharge bend, also made of steel, which in turn is fixed to the cylinder exhaust pipe, that is, the short passage that allows the exit of flue gases from the combustion chamber.
If we wish to look at its geometry, the following can be identified: a first segment with a constant section; then a divergent pipe (cone); a new straight segment and a converging part (counter-cone). Finally, the needle valve terminal for the discharge of the exhaust gases.
Its main function is to optimise the washing and draining phases, namely those that allow the replacement of the burnt mixture with a fresh one. In fact, as is known, 2-stroke engines do not have intake and exhaust valves that regulate timing like 4-stroke engines, but it is the piston with its

reciprocating motion that opens and closes the transfer ports (intake fresh mixture in the combustion chamber) and discharge (expulsion of the ignited mixture). Therefore, if on the one hand, 2-stroke engines are very simple in terms of mechanics, on the other hand they cause major complexities precisely in the washing and wastewater phases. Suffice to say that the 4 cycles of the operation of an internal combustion engine (intake, compression, combustion and exhaust cycles) are performed with a single turn of the crankshaft, and there are moments in which the transfer ports and exhaust are open simultaneously, to the point that there is a direct passage from the carburetor to the exhaust. This makes it possible for part of the fresh mixture is expelled without being burned, or, on the contrary, part of the ignited mixture remains in the combustion chamber. The exhaust serves to reduce these phenomena, which would cause performance degradation.

THE WASHING PHASE IS CRITICAL IN A 2-STROKE ENGINE, WITH POSSIBLE LOSS OF FRESH MIXTURE AND THE REMAINING GAS BURNED IN THE COMBUSTION CHAMBER AFTER THE EXHAUST PHASE”

Exhaust layout for a 2-stroke engine

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

EXHAUST SYSTEM

FLUE GASES WAVE

NEGATIVE PRESSURE WAVE

Depending on the angle of the cones, the wave of negative back pressure generated by the cone is broader but less intense (smaller angle), or narrower and more intense (greater angle).

POSITIVE WAVE

Even the wave of positive back pressure, generated by the counter-cone, varies depending on the angle of the walls of the pipes, resulting broader but less intense if the angle is smaller, or narrower and more intense if the angle is greater

Exploiting the laws of physics, it is possible to generate pressure waves at the exhaust that allow an adjustment of the outgoing gas flow. In fact, according to these laws, during the combustion phase, when the piston opens the exhaust port, a wave of positive pressure is created that moves along the exhaust bend, from the cylinder to the exhaust. Each time the wave encounters a pipe segment with a wider diameter, it generates another wave that has negative pressure (that is, less than the existing pressure in the exhaust pipe) and a motion in the opposite direction compared to the positive pressure wave.

THE EXHAUST SYSTEM - HOW IT WORKS