Uber Technical

Whats The difference?

Basics First  Aside from additives such as ethanol, which is made from corn, the petrol and diesel fuel  used today derive from the same source–crude oil. Although they share the same parentage (just like engine oil and some plastics, ), they have very different properties.

Petrol is often seen of as being a single chemical, but it’s actually a complex mixture of hydrocarbons that generally have 4 to 12 carbon atoms in their molecules. In the case of petrol made from Brent Crude, 5 compounds make up 29% of the petrol fraction, whereas 20 compounds make up 59% of the petrol fraction . Diesel fuel is a light oil that is heavier than petrol, composed of hydrocarbons generally having between 10 and 20 carbon atoms in their molecules . The point is petrol and diesel fuel aren’t the same.

petrol is  burned in spark-ignition internal combustion engines, in which a source of energy (a spark) is required to start the ignition process. Simply put, these engines mix petrol vapor and air, compress the result, apply a spark to initiate combustion, and then force the byproducts of combustion out as exhaust. petrol is well suited to this process, although sometimes either it or engine operating conditions can cause “knock,” an unwanted and destructive condition in which a portion of the fuel ignites spontaneously due to compression alone, without the aid of the spark.

Knock is a serious problem for petrol engines. It puts particular strain on the timing belt  The resistance of a particular petrol blend to knock is given by the octane number, with a higher value indicating greater resistance. Over the years people  come to think of higher octane numbers as indicating higher energy/more power. That’s not  true,  needless to say that self-ignition is the main  difference between petrol and diesel engines. In a petrol engine you don’t want the fuel to self-ignite–that is, to burn without benefit of a spark plug. In a diesel one you do.

Diesel fuel is typically burned in compression-ignition internal combustion engines, which draw in air and subject it to high compression (much higher than in a petrol engine), causing it become super heated. At peak temperature and pressure, diesel fuel is injected into the cylinder and self-ignites (autoignition). After combustion is complete, the combustion byproducts are exhausted out of the engine. Sometimes, such as on cold days,  glow plugs are used to provide extra heat for starting, but these play no role once the engine is started.

Diesel fuel is not rated by octane number but something called a cetane number. The cetane number gauges the ease with which the diesel fuel will autoignite when compressed. Higher cetane numbers indicate easier self-ignition and better engine operation, whereas a higher octane fuel would be more resistant to self-ignition under compression.

That’s the bones of the problem with mixing petrol and diesel. In a diesel engine you want controlled self-ignition, whereas in petrol engines  you want to avoid self-ignition, and have the fuel ignite only when the spark fires.

What happens when you use Petrol in a diesel engine? – it don’t work! . Since petrol is designed to be resistant to self-ignition, petrol in a diesel engine either won’t ignite or will ignite at the wrong time. Some diesel engines run leaner than petrol engines (meaning that the air-fuel mix has a higher proportion of air than a petrol engine). That increases the chances that the petrol won’t ignite and that unburnt fuel will be sent into the hot exhaust system–where, unluckily , it could ignite, leading to possible exhaust damage.

Some types of diesel engine use the diesel fuel as a lubricant for the fuel pump (remember, it’s a fuel oil).


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