We were sent this article from Dale Cooper, Lecturer in Motor Vehicles at East Surrey College as an entry to our June Competition.
Using the Pico scope to explain the 4-stroke cycle
Whilst covering power and torque with a BTEC level 3 group, I decided to try and take a snapshot of what was happening inside the cylinder in order to show what takes place over the full 720° of crank rotation on a single cylinder engine. Figure 1 shows the basic evidence gathered by the colleges Pico Scope unit, the blue trace is from the ignition system and shows a typical output with the long and short tails of the actual and wasted spark (unfortunately I neglected to invert the signal so the spikes go downwards rather than up!). The red trace is from an AC inductive crankshaft sensor reading from a standard missing tooth trigger wheel. The whole assembly was mounted on a Honda CBR 125R and timed so the leading edge of the first tooth and the spark where together in order to more easily reference top dead centre (TDC).
Red trace - standard AC inductive crankshaft position sensor.
Blue trace - inductive pick up on HT lead showing actual spark (long tail) and wasted spark (short tail)
The reason a single cylinder was chosen was because it shows clearly the 540° of crank rotation where the parasitic losses of the induction, compression and exhaust stroke absorb the inertial energy from the crank, this can be seen in the different levels of amplitude on the red trace, a low crank speed is shown by a low peak height and a high crank speed is shown by a high peak height.
So what was learned from this?
Following the graph from left to right we come to the area where the missing tooth, ignition spark and combustion occur. Ignoring the high spike of the first tooth, what can be seen first came as a little bit of a surprise. As combustion occurs you would expect a very sharp rise in crank speed as the rapidly expanding combustion gases force the piston downwards. However, by placing the crank speed trace between two parallel lines, it can be seen that the crank rotates at what appears to be a near constant speed until the exhaust valve opens just before bottom dead centre (BDC), as indicated by the vertical dotted line. Where the speed drops off dramatically.
After BDC the crank, now full of energy, accelerates until approximately two thirds of the way through the exhaust stroke where it peaks and then slows slightly as it moves through TDC and the overlap period. This spike in rotational speed is possibly due to the piston moving towards where the ineffective crank angle begins to take effect and the piston height to crank rotation ratio is reducing, requiring less energy to move the piston in the cylinder.
As the crank rotates through TDC and into the engines induction stroke it can be seen to slow at a fairly constant rate until it reaches BDC, then barely accelerating at all through compression as the greatest amount of energy is absorbed from the rotating mass of the crankshaft. Once again as the piston moves towards TDC on compression the crankshaft speed picks up as all the compression of the fresh mixture has taken place and the crank and piston enter their ineffective regions once again.
On conclusion this little experiment shows that, not excluding the energy required to overcome the laws of motion in accelerating the mass of the crankshaft, when combustion occurs the piston receives a long push down the cylinder bore rather than a short sharp shock. For the students, it also backed up the argument in favour of smoother running multi cylinder engines, as whenever another combustion process was started within 180° or less the torque output would become closer to becoming constant.
Technical Author @ Pico Technology UK