Understanding Crank Signal Misfires

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Mpylypchuk
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Posts: 7
Joined: Thu Jan 05, 2023 9:20 pm

Understanding Crank Signal Misfires

Post by Mpylypchuk »

Hi there once again.
I been reading an old post from Ben Martins. https://www.picoauto.com/library/case-s ... ic-process

In this example he shows a waveform from the crank sensor and a frequency to plot accel and decel.

I've come across similar waveforms on vehicles but I don't fully understand why the misfire in his example works the way it does.

By looking at the acceleration and deceleration it doesn't make sense to me that the fault is a sticking or malfunctioning injector. The crank is accelerating all the way up to 6. But he does prove his point with the fuel pressure sensor so I'm not arguing with that. To me it just seems like with the way that waveform behaves, it's more of an issue of compression, rather than an injector fault since the crank is progressively getting faster and faster until after cylinder 6. The lack of deceleration compared to other cylinders is what kind of seems to stand out to me. I feel like I'm more used to seeing a sticky injector or poor performing injector with a rapid drop in crank speed after that injector.

If anyone can chime in and explain why this behaves this way it would be much appreciated. I just want to understand the behavior of the engine in this situation better.
Thanks guys.

Mpylypchuk
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Posts: 7
Joined: Thu Jan 05, 2023 9:20 pm

Re: Understanding Crank Signal Misfires

Post by Mpylypchuk »

Here is the picture in question
Crank misifre capture.png

ben.martins
Pico Staff Member
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Joined: Tue May 16, 2017 1:02 pm

Re: Understanding Crank Signal Misfires

Post by ben.martins »

Hello and apologies for not getting back to you sooner.

Thank you for questioning this capture, it's the only way we learn.

Lets start with the crankshaft speed and how misfire detection works. Ignoring the waveform for a second and just go with the theory. We know that the engine ECU has a target RPM that it is constantly trying to achieve. In a heavy diesel engine like the one in the case study this is done purely by injection timing and quantity as the intake butterfly is always open. The crank sensor feeds back the speed to the engine ECU in order to correct injection if the engine speed is faster or slower than the target.

During the compression stroke and as the piston comes up to TDC, the crankshaft will start to slow down as the pressure inside the cylinder reduces the piston speed and therefore the crankshaft. As injection takes place, and we have good combustion, the pressure rises rapidly and forces the piston back down the cylinder speeding up the crankshaft.
Crank Maths.jpg
This means there is a constant speed up and slow down of the crankshaft during each compression and combustion event. Due to a scopes ability to capture fast moving signals we can display the crank sensor as a frequency, which is what we have done in the maths channel below.
crank maths PS7.png
This 'frequency' is also monitored by the ECU which, along with a cylinder id such as the camshaft, it can determine a misfire and identify the cylinder at fault.

If we go back to the case study a number of steps had already been carried out to point away from a compression or mechanical issue. A relative compression test showed an even pull of current during cranking. A crankcase pressure check displayed no obvious pulse issues which could be a result of excessive blow-by. It was only when we looked at the exhaust could we see that there was a disturbance in the pattern. As the exhaust pulse is the result of everything (combustion is a result of air in the cylinder, fuel and compression to raise air temperature to ignite the air fuel mixture) then providing all cylinders are contributing the same, there will be an even output at the exhaust. Where it is not even, looking at the crank sensor along with a cylinder ID and a math channel can help isolate a cylinder in the event that no fault codes are present.
Crank maths with misfire & rail pressure  PS7 cyl 6 slow down.png
In the capture you have above, we can see that the crank speed is gradually, on average, increasing through the firing order before getting to cylinder 6. Following the slow down for compression and then injection, the crank speed starts to slow down which means the following compression event will take the crank speed even lower.

Even in the event of a no fuel being in the cylinder, there will still be a natural spring effect on the piston as it has just compressed the air in the cylinder. As the piston goes over TDC, there will be some pressure to push the piston back down but at a slower speed than if combustion had taken place.

This slow down is seen by the engine ECU and so fuelling is altered in the attempt to get the crank speed back to the target RPM. However, each time on cylinder 6 the speed does not increase in the same way as the other cylinders.

Please remember that this is big diesel engine with a large flywheel which means a lot of the slow down is counteracted by the inertia stored in the flywheel. This may be the reason why the slow down is quite subtle and the possible explanation as to why a misfire detection fault code was not triggered.

I hope this helps and please let me know if you need any further information.

Kind regards

Ben

jueja
OneWave
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Joined: Sun Sep 19, 2021 4:20 am

Re: Understanding Crank Signal Misfires

Post by jueja »

The waveform also reveals the max speed a not working cylinder achieves is not only lower when compared to a working cylinder but this peak in speed takes place a considerable amount of crank degrees/time earlier than in the working cylinder. The reason being less energy liberated by the "air spring only" action resulting in a weaker and shorter push.

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