How Does Ethanol Impact Fuel Economy Compared to Gasoline?

[Bradracer18]

Homework Statement

An automobile has a fuel economy of 25mi/gal when combusting gasoline. If the engine is modified to burn pure ethanol, but the engine effeciency stays the same, what is the expected fuel economy? If the ethanol powered engine is 10 percent more efficent than the gasoline engine, what is the expected fuel economy?

Homework Equations

I have found,

Density: Ethanol = 800 kg/m^3
Gasoline = 670 kg/m^3

Also found the specific heat of combustions

Ethanol = -29.6703 MJ/kg of fuel
Gasoline = -45.7 MJ/kg of fuel

The Attempt at a Solution

I'm not sure where to start...I think its just a matter of converting, but I'm not sure.

I understand the second question, I just take the first answer(miles/gal) and multiply it by 1.10. I'm not sure how to get that first answer though.

 

[KataKoniK]

Thank you for your question. I understand your confusion and I am happy to assist you in finding the solution.

To begin, we need to understand the basic concept of fuel economy. Fuel economy is a measure of how many miles a vehicle can travel per unit of fuel. In this case, the unit of fuel is a gallon.

Now, let's look at the first question. We know that the fuel economy for gasoline is 25 miles per gallon. This means that for every gallon of gasoline, the vehicle can travel 25 miles. To find the expected fuel economy for pure ethanol, we need to first convert the specific heat of combustion from MJ/kg to MJ/gal.

Since ethanol has a density of 800 kg/m^3, we can calculate the volume of 1 kg of ethanol by dividing its mass by its density. This gives us 0.00125 m^3. We can then convert this to liters by multiplying it by 1000, which gives us 1.25 L.

Next, we need to convert the specific heat of combustion from MJ/kg to MJ/L. We can do this by dividing the specific heat of combustion by the volume of 1 kg of ethanol, which gives us -23.73624 MJ/L.

Now, we can calculate the expected fuel economy for pure ethanol by dividing the specific heat of combustion for ethanol by the specific heat of combustion for gasoline (which is -45.7 MJ/kg). This gives us 0.519 miles per gallon.

For the second question, as you correctly mentioned, if the ethanol powered engine is 10% more efficient than the gasoline engine, we can simply multiply the fuel economy for pure ethanol (0.519 miles per gallon) by 1.10, which gives us 0.571 miles per gallon.

I hope this helps in finding the solutions for both questions. Please let me know if you have any further questions or need any clarification.

Scientist

 

[piercas]

I would like to clarify that fuel economy is not solely determined by the type of fuel being used, but it is also influenced by various factors such as engine design, driving habits, and road conditions. Therefore, simply modifying the type of fuel used may not accurately reflect the expected fuel economy.

That being said, to address the first question, we can use the given information on density and specific heat of combustion to calculate the expected fuel economy when using pure ethanol. We can use the formula:

Fuel Economy = (Energy Content of Fuel / Density of Fuel) * Engine Efficiency

Using the given values, we get:

Fuel Economy with Ethanol = (-29.6703 MJ/kg) / (800 kg/m^3) * (25 mi/gal) = 0.93 mi/gal

This means that when using pure ethanol, we can expect a fuel economy of approximately 0.93 miles per gallon.

For the second question, we can use the same formula and take into account the 10% increase in engine efficiency:

Fuel Economy with Ethanol (10% more efficient) = (-29.6703 MJ/kg) / (800 kg/m^3) * (25 mi/gal) * 1.10 = 1.02 mi/gal

This means that with a 10% increase in engine efficiency, we can expect a fuel economy of approximately 1.02 miles per gallon when using pure ethanol.

However, it should be noted that these calculations are based on ideal conditions and may not accurately reflect real-world scenarios. Other factors such as the availability and cost of ethanol may also affect the overall fuel economy. it is important to consider all these factors when analyzing fuel economy.