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By Jake Venter on ToyotaZone
Many motorists are comparative newcomers to diesel engines and tend to regard these engines as being very similar to petrol units, except that they use a different fuel. Nothing could be further from the truth, because diesel engines differ in a number of ways from petrol engines.
Most of these differences centre on the combustion process. A petrol engine burns its fuel in one fireball expanding outwards from the spark plug, but diesel combustion takes place as a sequence of small fires that centre on vaporised fuel droplets.
Petrol has to be configured to ignite only when it’s exposed to a spark, but diesel fuel must be endowed with the ability to ignite when it gets hot enough, even if there is no spark. The result is that petrol’s octane rating, which is a number that indicates it’s resistance to self-ignition, is replaced in diesel fuel by a cetane number that indicates its ability to self-ignite.
Cetane is a fuel with very high self-ignition qualities, and is given a cetane rating of 100, while methyl naphthalene, which has very poor self-ignition qualities, is given a cetane number of 0. A fuel’s self-ignition qualities are evaluated by comparing it to various mixtures of cetane and methyl naphthalene in a test engine. The mixture that performs the closest to the fuel under test determines the rating.
This means, for example, that a mixture of 45 per cent cetane and 55 percent methyl naphthalene will be given a cetane number of 45, and if the test fuel performs like this mixture it will be given the same rating. South African diesel fuel has a minimum cetane rating of 47 at the coast and 48 inland. This value ensures that the period between injecting the fuel and ignition starting – the ignition delay – is minimised, leading to easier starting, improved combustion and smoother idling.
This expresses the amount of energy that can theoretically be released when a fuel burns. Diesel fuel has a slightly higher calorific value than petrol, (about 38 vs. 34,5 megajoule per litre) at 20 degrees C. This is often used to explain why diesel engines are more economical, but this is only a small part of the story.
The density of diesel fuel plays a larger role in determining its performance than in the case of petrol engines, for a very interesting reason. Modern petrol and diesel engines utilise an injection system that supplies fuel at a constant pressure to the injectors. The amount to be squirted in is determined by the electronic control unit (ECU), and this device takes various parameters into account before sending a signal to the injector.
This signal determines the time the solenoid or piezo-electric injector has to stay open. In the case of a petrol engine the ECU relies on the lambda sensor in the exhaust system to measure the amount of oxygen present during the previous combustion, so that the injector time can be tailored to deliver the correct mixture.
This means that the system can cope with varying fuel densities. Most diesel engines do not have such a feedback system, so that the amount of fuel delivered cannot be varied to cater for a change in fuel density. To counter this, some engines have a fuel heating system that ensures the fuel temperature does not change much. The density stays more or less constant, ensuring that the amount of fuel delivered under the same engine conditions does not vary much.
Diesel fuel contains a proportion of paraffin compounds to ensure clean combustion, but these compounds tend to crystallize at low temperatures leading to clogged fuel filters. This process may start at temperatures as high as 0 degrees C, so that special additives are needed for diesel fuel sold in the winter.
These usually do not limit the formation of crystals, but prevent their growth, so that clogging is prevented. The temperature above which filter plugging will not occur is called the cold filter plugging point. In South Arica, winter grade diesel’s plugging point is -7 degrees C (Highveld) and -5 degrees C (coast). The equivalent values for summer grade diesel is -2 degrees (Highveld) and l degree at the coast.
Another important quality of any fuel is the flash point. This is the temperature at which a combustible liquid gives off just enough vapour for the gaseous mixture above the liquid to be ignited by an ignition source. For diesel this temperature is 69 (Highveld) and 67 degrees C (coast) but for petrol this temperature is so low that it’s simply rated as being “less than ambient.”
This explains why a stored can of petrol is far more dangerous than some stored diesel fuel. However, as little as 3 per cent petrol added to diesel fuel will reduce the flash point to room temperature, making the mixture no longer as safe as diesel would normally be.
A lot has been written lately about the amount of sulphur in our fuel. The super grade is now at a low 50 parts per million (ppm), and this will help to reduce sulphur related problems, such as the formation of excess acid and carbon. If you’re still worried about the sulphur in our fuel, just keep in mind that any good diesel oil will contain an anti-acid additive. Even so, there is a very good counter measure. Change the oil more often, because most of the acid ends up in the oil.
[yellow_message] Some of the latest diesel engines may be damaged if 500 ppm diesel is used. (Consult the owner’s handbook.) If you own such an engine you should be careful to always fill up with 50 ppm fuel. [/yellow_message]