The Diesel engine has quite a hard time and the manufacturers also have a hard time with the Diesel engine, given the fact that it is almost twice as expensive to produce as the petrol engine. Indeed, the difference is somewhat more balanced, if the petrol engine is equipped with direct-injection, although the pressures occuring in the two injection systems are still very different.
At least, one has managed to regulate the cold starting much better than was previously the case. In the, at that time normal, side-combustion-chamber-process, the combustion chamber was practically divided. The main part of it was found in the cylinder head, pretty much directly surrounded by coolant. This brought with it a great deal of thermal dissapation, which was why this type of engine relied completely on electric heating.
Indeed, due to the enormous turbo-charging the ever shrinking static compression ratios are being given a hard time. In the meantime, there are already Diesel engines with a compression ratio of 14 : 1. Since when cold-starting, the turbo-charger cannot contribute much towards the increasing of the compression, cold-starting is slowly becoming a problem again.
Todays direct fuel injection systems are much less demanding. Utility vehicle engines generally need no pre-heating, and motor car engines, almost solely when the temperature is below freezing point. Because the glow-pins of today heat up considerably faster and, e.g., are triggered by opening the door, the engine can be started earlier, the well known 'one minute of silence' which was valid for the older Diesels until one actually started the engine, has disappeared completely. The Diesel engine has, at least in this respect, become, an worthy competitor for the petrol engine.
In one respect however, it is still lagging behind, that is the exhaust-gas emission, which, particularly (directly) after cold starting, is very unfavourable. For this reason, the task of the glow-pin has changed, from heating-up before starting, more towards the after-burning. Apart from that, the sometimes very unpleasant noises made by a cold Diesel engine have also been greatly reduced. To sum it all up, one can now say that the Diesel engine can basically be started just as quickly as the petrol engine and also, apart from the cold-start phase, no longer produces loud operating noises.
Examining this more closely, one discovers that the low temperature leads not only to less heat generation at the end of the compression stroke, but also to much lower starting RPMs, which even further lowers the temperature. The rod glow-plugs, e.g., in four-valve engines, which reach from above, deep into the piston area, try to combat this. The small amount of space available makes a very small diameter necessary. Nowadays, glow pipes with a diameter of just 3 millimeters are possible.
No system can can get by without taking the neighbouring factors into consideration. Thus, the position of the glow pipe in the combustion chamber is very important, oddly enough, more in the area where the air-speed is low through swirling, and where too much heat is dissipated from the glow plug too quickly. Of course, the type and the position of the injector blind-holes must also harmonise with the whole construction. The key-word nowadays, is application, the adaptation of the suppliers component to the respective engine.
The same as the spark plug, the glow plug is basically made up of two parts, one of which is pressed into the other during the production. Outside is the sleeve with the threading for the cylinder head, here also known as the plug body, and inside, is the glow pipe with the heating coil which is embedded in an absolutely vibration resistant material. The extremely thin wire filaments are separated by less than one millimeter from each other. The distance from the glow pipe itself, is somewhat larger. Between this and the sleeve, rings are also positioned for the sealing and insulation.
Electrically seen, the two glow coils, the heating coil at the tip and the regulator coil further inside, form a (relatively low) resistance for the vehicle battery. This means, a high amount of current can flow here, which is also the reason why 30 to 80 Ampere fuses are used. This current reaches its peak value at the beginning of the glowing phase. The glowing begins in the heating filament. In 2 to 5 seconds it spreads to the regulator coil. It's PTC-quality (Positive Temperature Coefficient) protects the coils, because together with its warming, through the increase in resistance, the current flow is slowed down.
Should of course, such a glow-process be triggered by, e.g., an opened driver's door and not be followed by starting the engine, then this must be recognised by the control device responsible for the glow equipment and the whole process must be switched off. This control device can of course, with assistance from the engine parameters, determine how long the glow-process should carry on after the engine has been successfully started. Low noise- and exhaust gas developement opposed to battery drain. The possiblity even exists of re-activating the glow pins after a longer period of thrust operation.
At least the afterglow is measured after only minutes, whereby the pre-heating only takes seconds. It can take some time, before the engine reaches a useful working temperature. This amount of tension does not fit in with the glow pins, which prevents testing with a direct connection to the battery. In the car, the tension and current supply is regulated through a control device. This can supply the glow pins at the beginning, for a very short time, with on-board tension. The electronics are able to cut this back the moment it could cause damage.
Nowadays, by the way, the regulation is also done individually, because of the construction or through the processes taking place, the conditions in the individual cylinders, after running for longer period, can vary. Thereby, in approx. 2 seconds, temperatures of 1000°C are reached. A wide field is opened if the glow-rod, like the spark-plug, is also used as a sensor. Thus, an ion-current and the temperature are now measured. It's quite possible, that now the combustion quality can be transmitted to the control device where various reactions can take place.
Unfortunately, it is sometimes so well sealed and so tightly stuck in the cylinder head, that workshops have difficulties releasing it, even bit-by-bit, without causing damage to the head. Mostly, one is so pleased to have changed the defective one without further negative results, that the advice from the manufacturers, to change them all, is ignored. The workshops have to get used to working more and more with a torque-wrench for the dismantling, e.g., to avoid snapping the glow pins off. The best thing to do is, use rust remover, correctly warm up the engine and/or the adding of extra current ... 04/12