In many entries, I mention “jittering”. What is it, what damage can it has done – today’s topic.
Jittering is a short-term fluctuation or some parameter – short-term difference from ideal (required/inquired values). In a slang – “twitching”. Actually, jittering is typical for each “analog” value. This inadequacy to perfect value is made both by a real inadequacy of parameter to ideal and it’s measuring inaccuracy. In turn, the measuring inaccuracy is created by inaccuracies of sensors (which are measuring the exact parameter), also by inaccuracies by ADC (analog to digital converter) hubs, and also by external circumstances – changes in supply voltage, disturbances from other hubs in wires, etc.
If the system is correctly constructed, the jittering of sensors and ADC is significantly lower that jittering of the real parameter itself, it means, their impact to measurements in insignificant. But we have to remember, that the voltage supply (onboard) has to be high-quality, connections from sensors to the management module – in good order. For example, the damaged alternator can cause problems to many systems, and also – increased jittering for measured parameters.
We move to a more exact topic – BMW petrol engines.
Which parameters have jittering?
As already mentioned before – all, who are “analog”, for example:
- fuel, low-pressure;
- fuel, high-pressure;
- VANOS position;
- Valvetronic position;
- oil pressure;
- measured consumed air mass;
- temperature (coolant, inflowing air, etc.);
- the position of the crankshaft, etc.
Additionally, for example, VANOS jittering is formed from several inaccuracies:
- angular play in camshaft actuator;
- angular play in VANOS hub itself;
- jittering of VANOS position, which is formed due to jitter of oil pressure;
- jitter, which forms due to inaccuracies of VANOS sensors itself.
Due to this cause – in case of problems, all possible causes of increased jittering of each hub have to be evaluated.
In turn, for example, processing of temperature data has its specifics – jitter of processing of sensor data is eliminated by software tools. That’s why, when using diagnostics tools, we don’t see continuous changes of value, for example, 20.3 .. 20.2 .. 20.3 .. 20.4 oC, but the average value: 20.3 oC.
What is the allowed range of jittering?
Typically jitter is few % of the absolute value of the parameter. But the experience plays a very important role in evaluating jitter. Only comparing the visible (using diagnostics tolls, or in some other way measuring the exact parameter) jitter with one, the “healthy” engine has, we can make correct conclusions.
For example, correctly performing a low-pressure system of N43/N53 series engines maintains the pressure of 5000 +/-30 hPa, it means, 5000 +/- 0.6%.
In turn, the oil pressure in idle for these engines is maintained in the range of 2600 +/-200 hPa, which means: 2600 +/-7.6%.
Rough run data for the evenly performing engine (idle, Stratified charge) indicate +/-0.200 units (0 efficiency is +7.000 units; N53 series engine), which means: mechanical efficiency of cylinders is +/-2.9%.
What does it mean – “short-term” difference from ideal?
If we talk about the performance of the engine and it’s unevenness, the “length” of jitter typically is measured in parts of second or several seconds. If the difference from ideal is longer than several seconds, DME has to be able to eliminate this inadequacy, instead, very short differences (hundred part of second, thousand part of second) from ideal will not get identified (we don’t even talk about portrayal and possibility to notice it visually) and very often it’s not even possible due to mechanical inertia.
What are the consequences of jitter?
If the jitter is in a lower range – it’s existence is a normal phenomenon and is foreseen during the designing of the engine (constructors both calculate and measure jitter to evaluate, what exact conditions cause it and affect it most, take actions to reduce the jitter, evaluate the impact of jittering of each hub to the even performance of the engine).
But, if the jitter exceeds allowed range, the uneven performance of the engine increases rapidly. Additionally, jitter is in no way compensated by adaptations (for example, LTFT) or “rapid response” tools (for example, STFT/integrators). Of course. that jitter in no way can be eliminated by the management of the problematic hub (otherwise there will no be a jitter). Exactly, for this reason, the jitter has such disruptive impact.
A situation can be compared with rolling part of the car. Very slow changes in the hight of the suspension are compensated by self-leveling hydraulic systems. Swifter changes – electronic stabilization systems. Even more swift changes – springs and shock absorbers. But, when driving the road, covered in small, but deep holes – all systems are helpless, and driving is very uncomfortable.
finally – the modern DME are performing some procedures (creation of adaptations, tests of chemical and mechanical efficiency – to even the performance of cylinders) only when data of all systems correspond to inquired ones. Increased jitter can be (and is) a cause, why DME postpones service procedures, mentioned before, and DME works on emergency mode (mode of limited functionality).
Summation of jitter.
Different systems of the engine management are very often interrelated. For example, VANOS is managed by oil – it’s pressure stability (and jitter), logically, will have an impact on the performance of the VANOS hub.
High-pressure pump receives fuel from the low-pressure pump, logically, that the performance of the high-pressure system will be (and is) dependent on the stability of the low-pressure system (jittering of pressure).
Of course, when developing the engine, engineers are trying to make in such a way, that the impact of jittering problems would be minimal. Unfortunately, it’s not possible to exclude this dependance completely. It’s clear, that VANOS cannot work without oil pressure, also the rail pressure will be 0 (it means, there will be no pressure at all).
Also when evaluating the impact of jitter, the experience will be very helpful. For example, low and high (rail) pressure systems for the N53 series engine are built quite correctly. High-pressure jitter increases for only 1.5 .. 2 times (against the low-pressure jittering). But the tendency is still unpleasant – rail pressure system, which is depending on a low-pressure system, don’t smooth down the jitter of the low-pressure system, but actually is increasing it!
Jittering of oil pressure in the range of +/-20 .. 25% causes serious problems in all systems, depending on it (VANOS, Valvetronic/HVA). For example, by such jittering of oil pressure for the N53 series engine, serious problems with the even mechanical efficiency of cylinders (idle, Homogeneous mode) to +/-20 .. 25%, it means, the engine is not able to perform normally.
In which case we have to pay attention to the jittering?
If the engine has problems maintain even performance also when everything is as if in order. Oil and fuel pressure jitter can be the cause (and is) of this problem!
Additionally, we have to remember – DME doesn’t record the error messages regarding increased jitter! DME “waits”, till the required parameter, will reach the ideal value. And – regularly and not even waiting too long – also “expects” this coincidence. Unfortunately, after a moment the measured value doesn’t corresponds to ideal anymore, DME starts to “repair” the situation, and, while the situation has not improved, postpones several procedures (tests, mentioned before).
Typical features, that the jittering of oil or fuel pressure could be ones to blame:
- Rough run data in idle (especially – Homogeneous mode) indicate increased (above +/-5 .. 8 %) uneven performance of cylinders, although the spark plugs, ignition coils, and injectors are in perfect technical condition;
- the engine doesn’t perform individual adaptations of cylinders (../F5/Shift+F6/F1 live data are indicating the inadequately small difference between opening times of injectors and multiplicative type adaptations);
- the engine don’t performs chemical tests of cylinders (../F5/Shift+F6/F3 all indications are 0.0%).
