It’s not always about diagnostics....

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It’s not always about diagnostics....

Postby AuthorMike » Thu Jun 06, 2013 2:30 pm

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).

Waveform.png

Fig; 1
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)

CBR crankspeed test.psdata
(318.67 KiB) Downloaded 469 times



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.

Pic.jpg


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.
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Re: It’s not always about diagnostics....

Postby Fat Freddy » Fri Jun 07, 2013 3:34 am

Hi Dale

Nice write up.
I just thought I would have a play with your data file and this is what I came up with.

One of my thoughts is. Is it possible the air gap is changing to give you the change in amplitude.
Also is it possible a couple of teeth have been bent togeather (circled area) ?

Also I can see what looks like piston acceleration on the first part of the compression stroke. Possibly caused by the cylinder being below atmospheric pressure?
Just some thoughts.

Edit: Where in relation to TDC was the missing tooth? Generally it is quite a few degrees before TDC?

[Image


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Re: It’s not always about diagnostics....

Postby AuthorMike » Fri Jun 07, 2013 7:50 am

Hi FF,

Not sure if Dale is a member on the forum, but I will let him know you have left a reply,


Cheers,

Mike
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Re: It’s not always about diagnostics....

Postby Dale Cooper » Fri Jun 07, 2013 8:09 am

Hi FF

Thanks for your comments

The timing was referenced to the firing point of the engine which is around 10 deg BTDC, this made it easier to reference when combustion started.

Although the timing equipment looks a little 'Heath Robinson!' it was all trued up in a lathe before use so the air gap shouldn't vary that much. The only other explination I can think of to cause this effect is the balancer shaft on the engine?

As, like everyone, I'm still learning the pico system how did you get the trace to show actual engine speed rather than the amplitude of the AC output.

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Re: It’s not always about diagnostics....

Postby Robski » Fri Jun 07, 2013 9:44 am

how did you get the trace to show actual engine speed rather than the amplitude of the AC output.

RPM math channel.

The timing was referenced to the firing point of the engine which is around 10 deg BTDC

How did you come to this (accurately) ?

And surely the references to engine position in degrees are typo's ??
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Re: It’s not always about diagnostics....

Postby AuthorMike » Fri Jun 07, 2013 11:09 am

Robski wrote:And surely the references to engine position in degrees are typo's ??


This may be my fault, I think when I copied from Dale's MS Word file to the forum, the degrees symbol became a "0".

I have changed it back! :oops:
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Re: It’s not always about diagnostics....

Postby Dale Cooper » Fri Jun 07, 2013 11:30 am

Robski

Your right they are formatting errors and thanks mike.

On the engine there is a window that is obscured in the picture that allow you to view the two refernce marks on the rotor, one is for TDC and the other is the 'F' mark for the ignition timing.

The crankshaft was positioned so that the 'F' mark was aligned in the window and the timing disc that I fabricated and attached to the crank was locked into place with the edge first tooth aligned with the center of the AC inductive sensor. When the trace showed the spark being delivered by the ignition system the waveform from the crank sensor started show crank speed.

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Re: It’s not always about diagnostics....

Postby Bilmekanikeren » Sat Jun 29, 2013 1:07 am

I am not so sure that using the amplitude of the CKP signal as a measure of crankshaft speed is the best (or a valid) measure.
I believe it would be better to graph CKP frequency vs. crankshaft angle and use that as a measure of crankshaft speed vs. piston position.
In order to derive piston speed based on the crank speed, the piston rod angle is needed. This would have to be calculated based on rod length, crank angle, and crank stroke.
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Re: It’s not always about diagnostics....

Postby Valhalla » Sun Jun 30, 2013 12:11 am

It's interesting that there is an approximate relationship between frequency and amplitude of the trace on one cycle, but not on the opposite cycle (whatsoever). The pattern from cycle-to-cycle appears similar, so is it a coincidence only that it fits the inferred piston speed? As mentioned, I would only use the frequency of tooth-passing to derive crankshaft speed.
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