In the vehicle's technical documentation, you can often find the instruction: “recommended fuel — A-95, acceptable — A-92.” Or like this: “recommended — A-98, acceptable — A-95.” The higher the octane number, the higher the fuel price. Let's figure out how critical it is to use gasoline with a high octane number and whether there is a real benefit from using “low-octane” fuel.
What is the octane number
Gasoline is a combustible mixture of light hydrocarbons obtained during oil distillation with the selection of certain fractions at specified temperatures.
Fuel is characterized by several key parameters, among which are distinguished:
- the proportion of aromatic hydrocarbons, benzene, and oxygen-containing organic compounds. Aromatics help increase the octane number, but lead to increased carbon deposit formation, aggressively affect rubber and plastic elements, and benzene increases exhaust toxicity and also increases carbon deposit formation.
- sulfur content. When sulfur burns, sulfuric and sulfurous acids are formed, which corrode metal surfaces, exhaust system elements, and catalysts.
- octane number. This indicator reflects the fuel's ability to resist self-ignition when compressed in the combustion chamber, that is, its detonation resistance.
The octane number is determined in laboratory conditions by two methods — motor and research. For this, a single-cylinder engine with an adjustable compression ratio is used.
The tested gasoline is poured into the engine, the detonation level is recorded, after which reference mixtures (isooctane with n-heptane in various proportions) are sorted through in the same installation until a similar detonation intensity is achieved. The isooctane content in such a reference mixture is the octane number of the fuel being studied.
The difference between the motor and research methods lies in the engine operating conditions during testing (the motor octane number is always lower than the research octane number). Different countries use different measurement and labeling methods, but at domestic gas stations it is customary to indicate the octane number obtained by the research method.
Why is detonation dangerous
In a gasoline engine, fuel is mixed with air (combustion is impossible without oxygen), and this mixture is supplied to the combustion chamber, where it is ignited by a spark. With increasing temperature and pressure, oxidation of hydrocarbons begins, peroxide compounds and their decomposition products are formed, which are prone to self-ignition.
If the mixture does not have the required detonation resistance, a thermal explosion occurs, causing self-ignition of the fuel.
Simply put, after igniting the mixture with a spark plug, another explosion occurs in the cylinder. The flame spreads hundreds of times faster, and the shock wave is reflected many times from the cylinder walls and piston — this is detonation.
Detonation combustion leads to overheating, accelerated wear, and even local damage to the engine. At the same time, power drops, exhaust becomes smokier, fuel consumption increases, etc.
The occurrence of detonation depends not only on the composition of gasoline, but also on the design features of the engine and ignition system.
What happens when using low-octane fuel
If you fill with gasoline with an octane number lower than required by the vehicle manufacturer, the fuel will burn untimely, power will decrease, and the engine will begin to wear out — detonation will occur.
But then why do manufacturers indicate two values — recommended and acceptable octane numbers?
The point is that in injection engines, the ignition timing is adjusted automatically within a certain range. This range is enough for the engine to adapt, for example, to both A-95 and A-92.
It is important to understand that “recommended” and “acceptable” fuel are not the same thing, and the engine's ability to operate for a long time on “acceptable” fuel is largely determined by its design.
Modern engines are becoming more environmentally friendly and economical, but at the same time they do not lose power. This is achieved by increasing the compression ratio, which, in turn, places higher demands on the detonation resistance of the fuel.
Therefore, for modern engines with a high compression ratio (approximately — more than 10), characterized by a small volume with high power, the use of low-octane gasoline will definitely lead to detonation. To suppress detonation in extreme modes, a knock sensor is installed in engines, the signals of which the controller uses to reduce the ignition timing. Detonation disappears, but the engine begins to operate under increased load: the fuel ignites later, burns out already in the exhaust tract, destroying the valves. In addition, power drops and fuel consumption increases.
“Old” engines — with a large volume and a low compression ratio (approximately — up to 10) — are more tolerant of a decrease in the octane number and are less prone to detonation.
Information about which gasoline (or which gasolines) is suitable for the engine is indicated on the inside of the fuel filler flap and in the vehicle's technical documentation.
Is there any savings
As already mentioned, when the octane number decreases even within acceptable limits, fuel consumption increases and power decreases.
However, these inconveniences can be compensated by the lower cost of the fuel itself. Whether there will be a real gain in costs in this case (that is, increased consumption multiplied by reduced price) — it is impossible to say unequivocally, for each case it is individual.
The fact is that fuel consumption when using one or another grade of gasoline, in addition to the design features of the engine, also depends on the ignition system settings, driving style, operating conditions, and many other factors.
It can be said for sure: for modern engines, it is preferable to use high-octane fuel (even if low-octane is allowed in the documentation). This will protect the engine and exhaust system from unnecessary loads and reduce the risk of breakdowns and expensive repairs.
Engines from the 80s–90s are less demanding on the octane number, and using A-92 instead of A-95 (if allowed by the documentation) will not harm the vehicle. Here it is worth recalling that Chinese manufacturers often use outdated Mitsubishi or Toyota engines when creating new cars — such engines also calmly treat low-octane fuel.
Summary
- You should fill the vehicle with the fuel for which the engine is designed.
- In modern cars, the ignition system can adapt to fuel, but low-octane fuel reduces power and increases consumption.
- For new highly boosted engines, the use of low-octane fuel is highly undesirable, even if the manufacturer allows it — the engine's service life is significantly reduced.
- For engines of old design, the use of low-octane fuel is quite acceptable.
- Savings when switching to 92nd (or 95th) gasoline depend on many factors and are calculated individually for each vehicle.
