TWO-STROKE QUERIES
Many thanks for the interesting article on two-stroke engines (CAR August 2020). I have the following tech‐ nical questions regarding two-strokes:
1. Expansion chambers are used for tuning purposes. How does one calculate the size and shape of this chamber to increase power?
2. What causes a hole in the piston top when running a two-stroke without an exhaust and, therefore, without backpressure? I ask because it has happened to me.
LOUWRENS OOSTHUIZEN
Kleinmond
We are glad you enjoyed our article. Let’s jump straight in to answer your questions.
Question 1: Not a trivial question at all. The optimum ex‐ haust design on a two-stroke engine involves gas dynamics, organ-pipe mathematics and some black art, shall we say. The main parts of the pipe consist of the downpipe, diffuser (diver‐ gent cone), belly section, baffle converging cone and the stinger (final, straight section of pipe). The dimensions and lengths are linked to the engine capacity, port locations (including size and crank-angle degrees of duration) as the basic inputs. Then the desired power delivery of the engine and operational speeds influence the cone angles and lengths. It is possible to tune for a high-peak power figure during a narrow engine-speed range (racing) or a low-peak figure over a wider operating range which results in a more user-friendly application.
The principle behind the tuning is to create a low-pressure wave reflection at the diverging cone to return and hit the combustion chamber as the new air-fuel charge is delivered via the transfer port to aid filling of the chamber. The next aim is for the high-pressure wave reflection created at the convergent cone to return and stop the fresh air-fuel charge from escaping down the exhaust and increase the pressure inside the combustion chamber before the ports close. If you are not familiar with organ-pipe calculations from first prin‐ ciples, some online calculators will do the calculations when you input the required variables.
Question 2: A hole in the piston can be caused only by knock damage, excessive heat, lean running or another sort of mechanical failure like the spark plug tip falling off (or piece of valve in a four-stroke). We would guess that because the exhaust was removed, the standard gas dynamics (as de‐ scribed in answer 1) were compromised which meant heat was not carried away as efficiently as before. More fuel would have escaped causing a lean burn, raising the heat even further, resulting in the damaged piston.