Glow 1
kfz-tech.de/PDM59
Although we have recently been told that it is possible to initiate self-ignition in a diesel engine by means of external ignition and thus influence its timing, we will assume pure self-ignition in the following discussion, i.e.,
without any electrical or electronic components.
This is only necessary for cold starts, i.e. whenever the engine temperature is below the normal operating temperature or 60°C. This requires thermal energy, which, incidentally, does not always have to come from
electricity. Anyone who has ever observed the (cumbersome) start-up process of a Lanz Bulldog with an external burner knows how wonderful it is to have heat at the touch of a button.
The doesn't complain about the time that passes, which is hardly noticeable today but used to be very noticeable earlier before you could finally set off.
Der schimpft dann auch nicht über die Zeit, die heute kaum aber früher sehr merklich vergeht, ehe man endlich losfahren kann. Here too, the diesel engine has almost caught up with the gasoline engine. So a
battery is needed to start a diesel engine when cold. In addition, you will need at least one slightly stronger cable and a fuse suitable for the high currents. The diesel engine can still be recognized by the slightly more
powerful battery.
| Here you can see how the glow plug protrudes into the combustion chamber. |
However, the plant appears to be capable of considerable performances. If the diesel fuel is sufficiently fluid and the electrical system is in top condition, cold starts are possible even at -30°C and even lower temperatures.
Well, the engine often sounds like a dog whimpering in pain. Its engine speed isn't exactly breathtaking either, but it will start at some point, hopefully before the battery runs out of power.
A minor difference compared to gasoline engines: unless it starts on the first or second attempt, it seems to prefer not to start at all. This is due to its spark plugs, which, especially in the days of carburetors, tended to get
wetter and wetter and thus less willing to ignite. The diesel engine, on the other hand, seems to improve its situation during several start attempts, e.g., with, albeit slight, heat input through the compression of air.
The introduction of direct injection has significantly reduced cold start problems. There is no longer a real secondary combustion chamber whose surfaces directly dissipate much of the laboriously generated heat back to
the coolant. If the depression in the piston is considered a secondary combustion chamber, it no longer has direct access to the coolant. All heat remains inside and culminates during repeated starts.
No, diesel engines still need a glow plug system. In the engine, this consists of glow coils which, in a kind of short circuit with sufficiently low resistance (e.g., 1 ohm), become so hot so quickly
that a start at around 0°C requires only a relatively short pause. In addition, a diesel engine no longer reacts directly to the turning of the ignition key, but waits for the pre-heating time on its own.
There are many other forms, such as 'keyless go', to which the diesel engine must also adapt. Added to this is a possible start-stop system that decides very arbitrarily when the engine should run and when it can be
switched off. Important indicators include the battery status, engine temperature, and outside temperature. Sometimes, the corresponding control unit apparently decides that the red phase of the traffic light has lasted long
enough and starts the engine prematurely.
So starting a diesel engine has become a complex matter. And it doesn't stop with the start. Beru writes that today, a glow phase of up to 3 minutes can still be assumed after the engine has been started. And now the
environment comes into play, and with it the Malfunction Indicator Light shown below, with its form replicating that of an engine.
Since it is man-made, or rather brain-made, it is almost as dominant as the automatic start-stop system. For example: It is entirely plausible that it lights up when the afterglow does not work. However, if you leave your
vehicle unused for a long period of time, e.g., due to a long trip, and it no longer starts, the light will also be on. This is understandable, because it reports too little energy, e.g., for afterglow.
Here again is the entire possible glowing process shown in the image. Beru states that preheating is only necessary for direct injection engines above 0°C, but this probably refers to older models without turbochargers.
After all, the geometric compression ratio, which used to be well over 20:1, is now heading towards 15:1. Since charging has little effect at starting speed, a modern
direct injection engine needs a little more preheating energy. You can tell by the breaks at the start, even when temperatures are just above freezing.
| 1 - 4 | Glow plugs 1.-4. cilinder |
| 5 | Control unit - Engine management |
| 6 | Control unit - Automatic glow time |
| 7 | Supply line 6 mm2, Fuse 50 A |
| 8 - 9 | Glow control + Feedback |
| 10 | Control of low and high heating output |
The circuit diagram is pretty straightforward, isn't it? First, somewhat unexpectedly, the individual wires from the glow time control unit to the glow plugs. To go one step further, these start at 1.5 mm2 up to the
connection point and cover the somewhat longer path with 2.5 mm2. The power supply between the two control units with 6 mm2 and a 50 A fuse.
This fits perfectly with the resistance of the individual glow plugs, which is between 1 and 1.1 ohms. The exact function of the two control lines between the engine and glow time control unit cannot be determined with
complete accuracy. But somehow the latter has to be informed that it is supposed to start at all. The second line would then be a kind of feedback. The fact that something like this is necessary can be seen in the example
of the MIL warning light above.
First of all, one would not expect any connection between a MIL light coming on and the glow that is still considered to be 'preheating'. What does a deficiency in this area have to do with
unfavorable exhaust gases? But if you think one step further, you'll figure it out. Just imagine that such a vehicle that would not start was forced to do so in tow of another vehicle. Now there is no afterglow, and what comes
out of the exhaust pipe does not meet the standards at all. That explains the MIL check, which is likely triggered in some way via the second line.
But there's more to it than that. And to explain that, we need to take a brief detour. We need some kind of mistake or a long period of non-use of the vehicle in question, e.g., due to a long vacation. Sure, then the battery is so
weak that the car won't start anymore. Whether the MIL mechanism already recognizes this state remains unclear at this point. The owner ensures that the battery is recharged. So far, so good.
And then, slightly delayed, the MIL lamp, now perceived as deceitful, lights up after all. One might think of the particulate filter again, for example, but that is far from the case. It is allegedly due to a lack of voltage or
energy in the ignition system and in the electrical fuel delivery system. If the repair shop doesn't keep a cool head here, the customer will pay an unnecessary bill, because the solution is: reset the lamp
and nothing else.
And it remains until another error occurs. It was the battery that caused this lack of voltage. But it's a mistake that was fixed long ago. And yet the lamp continues to burn merrily. If you don't have an OBD scanner with a
deletion function, you'll have to take your car to a repair shop to have the error light deleted on a car that is now completely intact. Is that the purpose of a MIL indicator light?
