In this entry – why Stratified charge is so important for N43/N53 series engines? Or, more exactly – why for other BMW common rail petrol engines everything is fine without using of Stratified charge?
As I have already mentioned in other entries, only in Stratified charge mode DME of N43/N53 series engines can measure a range of very important parameters of the injectors: leaking in the closed condition, reaction time, atomization quality. Without these measurements (without Stratified charge) data of injectors are incorrect – the vibration of the engine, misfires, problems with fuel mixture, trim errors of the Lambda probes, etc. start.
The amount of fuel, injected by injectors, in idle (by an identical opening signal from DME) could differ for 3 .. 4 times!
But the question – how N63 can do without Stratified charge – is logical. Or, for example, N54/N55 series engines? They have the same HPFP’s and the same piezo injectors!
The fundamental difference – Rail pressure:
- N53 series engines: 200 Bar in all driving conditions;
- N43 series engines: 150 Bar in idle, till 200 Bar by RPM above 1500;
- N63/N54/N55 series engines: 50 Bar in idle, 50 .. 200 Bar by higher RPM/required torque (depending on engine/SW release).
Note: Rail pressure of the N43 series engine in idle is reduced due to insufficient flow-rate of HPFP.
Here, the Rail pressure in “other” engines (the ones, who do not use Stratified charge) is reduced for 4 times in idle and low torque mode! Exactly in these modes, the injectors work in modes of short openings and with a small amount of injected fuel (and, accordingly – the leak has the highest impact).
4 times reduced Rail pressure changes the situation radically!
The injector management signal for N53 series engines in idle looks as following:
As we see, the injector is opened and closed for a long time – even for a long time while it is in (electrically) “open” condition. In this “transitional” period the injector is half-open, the amount of fuel injected by it – unstable, beam – could be of low quality (with bad atomization). This is only a management signal – it does not show mechanical inertia. Mechanical inertia causes additional problems.
Reducing the Rail pressure for 4 times, the situation improves radically – the impact of the opening/closing stages reduces very significantly. Also, the impact of the leaking of injectors (in closed condition) is significantly lower.
Such – much smaller differences of the injector parameters – DME “handles” much simpler – such advanced measurements as for N43/N53 series engines are not necessary.
Why for N43/N53 series engines the Rail pressure is not reduced till 50 Bar?
It has several reasons:
- to save fuel, these engines use a Stratified charge also in idle. Stratified charge requires a pressure of 200 Bar, otherwise, this mode is not possible (the injectors will inject fuel insufficiently fast);
- these engines have no turbocharger, reaction to the accelerator pedal is immediate. If the pressure in idle would be reduced to 50 Bar (and the Stratified charge would be turned off), the sense of “zero mass flywheel” would disappear, because the increasing of Rail pressure requires a certain time. By larger required torque the Rail pressure has to be increased for the injector to be able to inject all required fuel during the intended time;
- also in idle when the Stratified charge is turned on, N43/N53 measures atomization of the injectors, leaking, and reaction. These parameters are necessary also for injector adaptation maps for other (higher) torque conditions. If the engine will not use Stratified charge in idle, data for adaptation maps of injectors will be missed.