In several entries tests of chemical efficiency are mentioned – in this entry, I will explain, what are they, and why they are necessary.
While driving (small/medium/large load), for N series engines, the average fuel mixture of each bank is controlled by wide-band probe – average fuel mixture is maintained precisely: according to the value required. But this probe doesn’t see the fuel mixture of each cylinder, which “belongs” to it.
Why should it be seen? Only then, when the fuel mixture is optimal (fuel mixture of each cylinder, which belong to one bank) – only then the engine will have max torque, will not vibrate, CO catalytic converters will have the smallest load, the content of CO/HC in exhaust gases will be the lowest.
What defines the differences of fuel mixture of cylinders on the run? Main cause – injectors. Especially – piezo injectors (the parameter scattering of them is larger than for injectors of “old” type).
Yes, small differences will be caused also by differences of Valvetronic hub between cylinders, but on the run, the intake valve opening is relatively high (several mm), accordingly – if the engine can run idle satisfactorily, on the run – differences of cylinders air fulfillment will not be huge. Or, shortly – Valvetronic will not be the cause of largest differences.
Logically the question appears – how can we measure this distribution of the fuel mixture (or more precisely – flowrate of the injectors)? Measuring the speed of flywheel (more precisely – fluctuation of the speed) turning don’t works – measuring results will be too much affected by, for example, uneven road, additionally – then larger the speed of flywheel, the smaller the impact of performance of one cylinders one performance cycle in total energy (and measurement need to be performed for shorter time – “next” cylinder starts working sooner).
The solution is quite smart and simple at the same time.
What is it?
Let’s assume, that the bank has 2 cylinders (for example, N43, N46 and other series engines) – the simplest case. To make this less complicated – don’t take in account the reaction time of the injectors (it’s described by offset adaptations) – let’s suppose, that the tests are performed in conditions of long injector opening (the engine has a large load, the impact of offset adaptations is small).
Let’s increase the opening time of the injector for one cylinder by 10%, but the opening time for the injector of other cylinder let’s reduce for 10%. Should the average proportion of the fuel mixture change or should remain the same? If injectors of both cylinders work perfectly – yes, the average proportion of the fuel mixture will not change.
But as a sample we will check the case, when the flowrate of one injector (in this sample – of the first cylinder) is for 8% higher than for second injector (difference/situation is realistic – nothing special).
So, in the situation, when Lambda of the bank is 1.00 (perfect – Stoichiometric mixture), actually the first cylinder has fuel excess of 4%, but in the second cylinder – an excess of air of 4%. Excess of fuel and air are burned directly after getting in the exhaust.
Or, if we view the injected fuel amount in absolute values: in the 1-st cylinder 104%, in the second: 96%. Average injected fuel amount: (X1+X2)/2 = (1.04+0.96)/2 = 1.00. Everything in order, the average fuel mixture is perfect.
Increasing the opening time of the injector of the first cylinder for 10%, the fuel amount, injected by it, will be 1.04*1.10 = 1.144, but if we decrease the opening time of the injector of the second cylinder for 10%: 0.96*0.90 = 0.864. And now – let’s check, what’s the average injected fuel amount (X1+X2)/2 = (1.144+0.864)/2 = 1.004.
Here, the average fuel mixture is not ideal anymore – it’s not 1.00 anymore! Yes, by flowrate of injectors with 8% measured difference is only 0.4%, but – it’s visible!
How could we increase the precision of measurements?
At first – part the measurement in two steps:
- on first the fuel mixture correction is: +10%/-10%,
- on second the fuel mixture is corrected “vice versa”: -10%/+10%.
The precision of measurements is doubled, it means, by the difference of flowrate of 8% the difference of Lambda will be 0.8%!
At second: repeating such measurement cycles for several times, resulting in the average. For example, 10 double cycles one after other: the possible error decreases to 3 .. 5 times!
Slowly changing – adjusting the parameters (adaptations) of injector flowrate, we have to reach a situation, when Lambda value don’t changes, changing the test steps (correction of “polarity”). If Lambda doesn’t change – obviously, the fuel amount, injected by both injectors, is equal. Additionally, this method minimizes the impact of imprecision of Lambda probe (it’s not important, is the real and/or measured Lambda 0.99 or 1.00: we are interested only in changes in Lambda value).
Using methodology, described above, MSV/MSD engine management systems are evening the performance of cylinders in all performance modes (in small, middle and large load – in all range of RPM).
These efficiency testes for exact work mode (required torque and RPM) are repeated regularly (after a certain count of motor-hours) and, if the uneven running of the engine is detected (for example, a strong difference of Lambda from an ideal value, misfires, etc.) – are performed immediately.
Test results of MSD80 management system chemical efficiency tests can be seen here: ..F5/Shift + F6/F3 (relative values: last/previous test session)
And: ../F5/Shift + F6/F1 (4th .. 6th lines, absolute values)
Unfortunately, these tests cannot be used in Stratified charge mode – the fuel mixture is significantly different from Stoichiometric, the air flow in inlet manifold fluctuates significantly – Lambda probe readings are not so precise anymore. What does MSD80 do?
MSD80 switches to a Homogeneous mode for a moment (several minutes – several cycles of several tenths of seconds), performs measurements of injector efficiency/flowrate (chemical efficiency of cylinders), after finishing the test switches back to Stratified charge mode and uses parameters, measured before, for Stratified charge mode. As the engine efficiency, when it switches between Homogeneous and Stratified charge modes, changes for only 15 .. 25%, the injector measurements are close matching and can be well used for both modes (in current driving conditions).
So – don’t be surprised, that, when driving evenly, the engine periodically switches to Homogenous injection mode. One of the reasons – performance of tests of the chemical efficiency of cylinders! MSD80 takes care, that we, users of N43/N53 series engines, have even and pleasant driving.
Note: MSV/MSD control units uses CJ120 chipsets (manufactured by Bosch) for Lambda measurements. Around Stoihiometric mixture (Lambda = 1.00) these chipsets provide high accuracy of measurements thanks to self-calibration feature of internal OPamps.
