Technical analysis on evaporation, calorific value and performance of each fuel at high temperatures
The arrival of summer and high temperatures raises a recurring question for flex-fuel car drivers: gasoline or ethanol, which fuel yields more in intense heat? The choice, which is normally based on the famous “70% rule”, can be influenced by technical factors that intensify with increasing temperature. This article presents a detailed analysis of the properties of each fuel and its behavior under heat, to help you make the most efficient decision for your vehicle and your budget.
Fundamental properties: gasoline vs. ethanol
To understand which option is best, you must first know the physical-chemical characteristics of each fuel, which determine its performance and behavior in the engine.
- Gasoline (Regular and Additive)
- Calorific Power: Gasoline has a higher calorific value of approximately 32 MJ/L (megajoules per liter). This means that each liter of gasoline stores more energy, which translates, in practice, into greater autonomy (kilometers per liter).
- Volatility: It is less volatile than ethanol. Its evaporation rate is lower, an important feature when the fuel tank is heated by high ambient temperatures.
- Density: Slightly less dense than ethanol.
- Hydrated Ethanol
- Calorific Power: Ethanol has a lower calorific value, around 22.7 MJ/L. For this reason, the vehicle’s consumption is naturally higher with ethanol, requiring the injection system to supply a greater volume of fuel to generate the same energy as gasoline.
- Volatility: It is a highly volatile fuel. It evaporates much more easily, especially under heat, which can lead to significant losses even before burning in the engine.
- Latent Heat of Vaporization: Ethanol has a high latent heat of vaporization. When it passes from the liquid to the gaseous state inside the intake manifold, it “steals” heat from the air, cooling the air-fuel mixture.
The impact of heat on performance and the engine
High temperatures directly affect the behavior of fuels, from storage in the tank to burning in the cylinders. The intense heat of summer exposes the advantages and disadvantages of each.
The main villain of performance in the heat is the evaporation. The power system of a modern car has a canister, an activated carbon filter that absorbs fuel vapors to later burn them in the engine, preventing them from being released into the atmosphere. However, on days of extreme heat, the rate of ethanol evaporation can overwhelm canister capacity, resulting in actual fuel losses through evaporation. Gasoline, as it is less volatile, suffers less from this problem.
On the other hand, ethanol’s ability to cool the air-fuel mixture becomes an advantage on hot days. A cooler mixture is denser, allowing more oxygen to enter the combustion chamber. This can result in a more efficient burn and a slight gain in performance, as well as helping to prevent pre-ignition, known as “pin knocking”. Engines with a high compression ratio, especially turbocharged ones, can benefit from this effect.
Cost-benefit analysis: the 70% rule in summer
The standard rule for deciding between gasoline and ethanol is simple: if the price of a liter of ethanol costs up to 70% of the price of a liter of gasoline, alcohol is more financially advantageous. This calculation is based on the difference in calorific value between the two. However, the evaporation factor in intense heat can change this math.
As ethanol evaporates more easily, part of the fuel you put in the tank is lost before it is used. This loss is not recorded at the fuel pump or on-board computer. Therefore, for a more accurate analysis in the summer, it may be prudent to adjust the calculation.
- Gasoline
- Pros: Higher yield per liter and lower evaporation loss at high temperatures.
- Contras: Higher price per liter.
- Ethanol
- Pros: Lower price per liter and potential for better performance due to intake cooling.
- Contras: Higher consumption and significant losses due to evaporation in the heat, which reduce the real yield.
Considering evaporation losses, the 70% parity may not be enough to guarantee ethanol’s advantage in extreme heat. A margin of safety, using a factor of 65% to 68%, can provide a more accurate reflection of actual cost-benefit during the hottest months of the year.
For most drivers focused on maximum range and cost predictability, gasoline tends to be the most rational choice during intense heat. Ethanol evaporation losses can offset the savings obtained at the pump. For those looking to extract maximum performance from the engine and are not so concerned about small variations in consumption, ethanol can continue to be an interesting option, especially if its price is well below the 70% limit in relation to gasoline. The final decision must consider the price at the pump, your driving style and vehicle usage priorities.
