This time – about one very interesting nuance of starting the engine.
As already mentioned in previous entries, Cranking can be longer, if, for example:
- there are problems with fuel pressure;
- there are (or have been) problems with oil pressure;
- IBS system tests the state of the battery (then the engine will not be allowed to start for some 4 .. 5 seconds).
I will start this “cranking entry” with little historical deviation, which is directly connected with today’s article. More than 20 years ago, I had BMW E30 with modified M20B25 stroker engine and LE-Jetronic management. I remember, that the car started from half-turn if there was a pressure in the fuel system. The ignition was managed from the separate module, as in old engines. All injectors had common management, individual management of separate cylinders was not intended.
Then one of my friends purchased E30 with an M20B25 engine, but a newer version and already with more modern management – Motronic 1.1. This already was a digital management system – no mechanical idle regulators, no additional enrichening of the fuel mixture in case of a cold start. Also, the ignition system was managed from DME. But this engine took more time to start! Sometimes this peculiarity was less pronounced, sometimes – more. I remember, that on that time, when I understood, why it’s like that, I made a conclusion – yes, this car won’t start, if, for example, there will be a need to start it by towing. This engine already had RPM and crankshaft position detection sensor (initially these engines had two separate sensors to detect RPM and the position of the crankshaft, later – one/common sensor was used both for RPM and crankshaft). Continue to read, and you will understand, why I mentioned exactly this innovation.
Where hides the point of this nuance? Why the engine has to be started for a longer time? Why this additional “hold” changes and is not exactly understandable, on what it depends.
In the image: the flywheel wheel and sensor, which detects the position of the crankshaft and it’s turning speed. These data are one of most important, for DME to be able to perform – it gives information, in which moment to inject the fuel in the cylinder, in which moment to ignite it; to identify the speed, with which the crankshaft rotates.
One of the teeth is “skipped” – using this, the position of a crankshaft is defined. Every time, when starting the engine, DME detects the position of the crankshaft afresh. Possibly, you will oppose – isn’t it possible to “remember” this position from the previous driving session? Unfortunately, no! There can be conditions, which “destroy” this calculation. For example:
- car with turned-off ignition (even disconnected accumulator battery) has undergone some maintenance, in a result of which the position of the crankshaft has been changed;
- car (with manual transmission) has been towed towards/ backward (the gear knob was not left in a neutral position). Even if DME would “see”, that the crankshaft’s position has been changed, DME will not be able to distinguish turning the direction of the crankshaft;
- if the car has an automatic transmission, and it has been transported (towed) etc.
So – there are obstacles (situations) when an incorrect initial position of the crankshaft is possible – different from one, which is “remembered” by the engine when finishing the previous driving session. Due to this cause, the “old” position cannot be used, and every time, when starting the engine, the position of the crankshaft has to be defined from the beginning.
In the image above you see the worst possible scenario – to detect the position of the crankshaft, it has to make the full turn! Only, when the crankshaft will perform a full turn, the “skipped” teeth will turn to the sensor, and DME will know – the “zero” position is reached.
N series engines have two camshafts, which are equipped with VANOS. Also, positions of VANOS mechanisms (camshafts) have to be detected, before starting the engine (in this case as starting we understand the moment when the fuel is injected and ignited). As the camshaft turning speed is 2 times less than crankshaft, DME could require till 2 full “empty” turns, till it’s ready to start. Only after detecting the positions of crankshaft and camshaft, the fuel is injected and ignited for the first time in the cycle.
The turning speed of starter in the moment of starting can vary, but averagely it’s 100 .. 200RPM and even less in the first moment of start (in the first second, when it slowly starts to turn the engine – overcoming the mechanical inertia of the flywheel) – it’s clear, that these required 2 “empty” turns to require an additional second or even more. During this period the engine will not even try to start!
Even more – if, for example, the battery has aged, the start – with reduced torque (especially topical in cold winter time) and the engine is turned slower than necessary, the positions of crankshaft and camshaft could not be confirmed or can be confirmed only during next turns of crankshaft (when required RPM are reached). In this situation, the starting can last even longer!
In any case – this additional hold, which is typical for all modern engines, depends on the starting position of the crankshaft, and it has a random nature. Sometimes the engine starts very quickly, sometimes it requires more time. If the starting doesn’t last longer than 4 seconds – most probably, everything is OK with the engine (it’s starting)!
