Direct injection and Stratified charge

Before we move to more interesting issues, I will tell in short about the basic differences of N43/N53 engines.


What is Direct injection?

BMW N43 and N53 series petrol engine

In the picture above you can see cross-section of BMW Direct injection petrol engine. Main differences:

  • injectors are located not in the intake manifold before induction valve, but directly in combustion chamber;
  • petrol pressure in injectors is increased up to 200 bar (in normal operation mode) instead of several bar for “regular” petrol engines;
  • injections happens directly before fuel ignition and even in the process of ignition, in portions of 1 .. 3 pcs; in case of small and medium load – in portions of 2 .. 3 sections and huge air “leftover” (it is called Stratified charge);
  • to ensure necessary instant reaction, ”usual type” injector (electrical coil + spring) is replaced with Piezo injector;
  • spark plugs with wider air gap (~1,6 mm) for higher energy spark are used;
  • 1st spark is used for fuel ignition and additionally up to 4 discharges in fuel combustion process;
  • ignition coils with increased (to 40KV) voltage and significantly higher amount of accumulated energy.

Note: in description above only one of few possible engine operating modes is mentioned. When and which operating mode is used by N43/53 series – information will follow.

Stratified charge (engine works with lean mixture, it means: Lambda is highly over 1,0).

Only N43 and N53 (from all engines of BMW) use Stratified charge operation mode.

As we know, to perform successful fuel ignition, air/fuel is necessary in proportion 14,6/1.

Necessary amount of fuel is defined by opening duration of nozzle, while necessary amount of air is adjusted with throttle. In case of small and medium load in the intake manifold, tenuity is created (the engine “sucks” air during expansion cycle, but throttle is “resisting”). To create this tenuity, the engine has to perform extra work. This work is inexpedient, it means, it waists energy (consuming extra fuel etc.) If it’s possible to avoid such waist of energy – why not do that?

The Direct injection allows to execute “slightly” different principle of operating:

  • throttle can be opened “wider”, by this reducing the tenuity in the intake manifold and also energy necessary to create it. N53, for example, allows to intake 3 .. 3,5 time more air than would be needed for fuel combustion, it means, Lambda = 3 .. 3,5;
  • if the nozzles inject fuel a moment before fuel ignition and in the immediate vicinity of spark plug, normal fuel ignition is possible: even if the average mixture (in all combustion chamber) is lean (with Lambda to 3 .. 3,5, it means, air “leftover” is 3 .. 3,5 times), in direct vicinity of spark plug it is “rich” enough (Lambda below 1) and ignites very well.

Stratified charge creates significant gain for N53 engine: 25% fuel economy in load of 0 .. 10%, 20% fuel economy in load of 10 .. 30%. Percentage of economy decreases to 0%, when 60% of load is reached. For example, if we drive with 90 .. 120 km/h, N53 works with load till 30%, it means 20% economy comparing to “regular” BMW engine. Fuel economy for N43 is a bit smaller, but in average it creates at least 15%.

Additionally to fuel economy, such engine can maximize performance in area of middle load – in each cycle fuel is injected in 2 .. 3 portions, each portion being injected in it’s own – more optimal injection moment.

The “price” of Stratified charge.

Each exhaust system of such type for producer (and, of course, for buyer) costed several thousands Euro. The cost of special NOx catalytic converter – over EUR 3000, it’s resource: 50’000 .. 80’000 km. The cost of NOx sensor – EUR 450 .. 500, resource: 80’000 .. 100’000 km. Injectors – significantly more expensive than “regular ones”; high-pressure fuel pump (from the beginnings it, and injectors also, was persecuted by failures (several withdrawals etc.).


Related entries:

Stratified charge performance check

Rough run. Trim errors. Hidden cause

NOx catalytic converter regeneration and desulfation