what happens if i make a mistake when refueling

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It is possible that, with the rise in fuel prices, some drivers have decided to change their traditional tank filling with 98 octane gasoline to one with 95 octane gasoline. A decision with which to save a few euros at the end of the month but that can leave some of them unhappy.


First of all, what we need to know is that there is no substantial difference between 95-octane and 98-octane gasoline, although they do not have to offer the same performance. The secret is in its ability to be compressed.

Some basic notions

If we look at a simple explanation, in a gasoline engine we find a cylinder by which a piston moves up and down. When the cylinder is at the bottom, which is known as its bottom dead center (BDC), the free space of the cylinder fills with air and fuel. With the rise of the piston, the air and fuel are compressed until they are reduced to what is called the combustion chamber, when the piston is at its top dead center (TDC). It is in this small space that the fuel is ignited with a small spark and the explosion generates the energy that moves the vehicle.

The higher the compression, the greater the energy obtained from the fuel. For this reason, diesel engines, which assimilate higher pressures, manage to obtain more energy than gasoline for the same number of liters of fuel. And, in the same sense, in a gasoline engine (whose compressions are usually between 8:1 and 12:1, generally) we will obtain more energy the more the air and gasoline mixture can compress.

This is where the decision to refuel with 95 or 98 octane gasoline. 95-octane gasoline supports more modest pressures during combustion, so detonation occurs sooner. However, 98 octane gasoline does withstand higher pressures and, therefore, is especially suitable for high-performance engines, generally with ratios greater than 10.5:1.

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So which one do I choose?

With this description, it is possible to think that it is better the 98 octane gasoline, because we will get a better engine performance. However, this is only possible in those vehicles with engines designed to work at very high pressures. If our car is not a sports car designed to extract maximum power, we will be throwing money away, because we will not be getting all the possible performance from the fuel.

We do have to be careful, however, if our car is specially designed to run on 98 octane gasoline. In this case, if we fill up with 95 octane gasoline, a breakdown will not occur, but the engine will be more lazy and will not reach its full potential. This is due to detonation of gasoline will occur before that the piston reaches its PMS, so it will receive energy before time and, therefore, will not work at its maximum performance.

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To detect this problem, cars usually have a sensor that adapts the engine times to adapt to this situation and prevent major breakdowns in the car. However, it must be taken into account that, in an engine prepared to run on 98-octane gasoline, using 95-octane gasoline will cause less power, higher consumption and a higher volume of emissions. In addition, in the long run, if you always refuel with 95 octane gasoline, it is more likely that small imbalances or premature wear will be generated.

In addition, to make 98 octane gasoline more attractive, which is also significantly more expensive, manufacturers often advertise better additives for this fuel, although it is difficult to find the details of them or get the results announced. The best thing, therefore, is to check in the car manual what fuel our vehicle uses and act accordingly.

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And be careful with the diesel.

Before closing, a little reminder of a much more serious situation. Pour diesel into gasoline it can cost us a lot of money, so if we usually use cars with different fuels, it is important to take this into account.

First of all, it’s complicated put diesel in a gasoline car, because the mouth of the diesel hose is larger and will not enter. If, despite everything, we have refueled diesel (in older cars it is more likely to fall into this error), it is best that we do not start the car in any case. We must call an assistance service and mount the car on the crane to take it to a workshop. There they will empty the reservoir tank and check that nothing else has been damaged.

If on the contrary, we have started the car and we have circulated with it, it will have stopped shortly after resuming the march and the smoke that will have come out of the exhaust pipe must be very white. During this time the car has had to drive limply until finally coming to a complete stop. This time the filling of the tank will be much more expensive, because the emptying in the workshop will have to add a cleaning of spark plugs, injectors and all the components through which the diesel has passed.

Photos | Susana Vera/Reuters and Zakaria Zayan

It is possible that, with the rise in fuel prices, some drivers have decided to change their traditional tank filling…

It is possible that, with the rise in fuel prices, some drivers have decided to change their traditional tank filling…

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