How Many Kilometers per Liter Will a Car Go with 25% Engine Efficiency?

To determine how far a car can travel per liter of fuel based on engine efficiency, we need to consider the energy content of the fuel, the energy requirements of the car, and various other factors such as aerodynamics and road conditions.

Energy Content of Fuel

For gasoline, the energy content is approximately 31,536,000 joules per liter or 31.5 MJ/L.

Engine Efficiency

At an efficiency of 25%, this means that only 25% of the energy in the fuel is converted into useful work, such as moving the car.

Useful Energy Output

The useful energy output can be calculated as follows:

Useful Energy Energy Content x Efficiency 31,536,000 J/L x 0.25 7,884,000 J/L

Energy Required to Travel

The energy required to move a car depends on various factors including its weight, aerodynamics, and speed. Generally, a typical passenger car might consume around 0.3 MJ/km or 300,000 J/km under normal driving conditions.

Distance per Liter

The distance the car can travel per liter of fuel can be calculated as:

Distance Useful Energy / Energy Required per km 7,884,000 J/L / 300,000 J/km ≈ 26.28 km/L.

Thus, if a car’s engine operates at 25% efficiency, it could theoretically achieve around 26.3 kilometers per liter under optimal conditions. However, actual performance may vary based on driving conditions, vehicle design, and other factors.

Factors Influencing Fuel Economy

There are numerous variables that can play a role in determining the fuel economy of a car, including the class of car, aerodynamic properties, drivetrain layout, tires, driving speed, and road surface. For the purposes of answering the question, the following assumptions are applied:

The car is equipped with a front-mounted petrol engine that sends power through a 90% efficient drivetrain to the front wheels. The car is running on petrol with a volumetric energy density of 31.3 megajoules per liter. The car has a mass of 1,300 kg, a frontal area of 2.20 m2, and a drag coefficient of 0.30, all values representative of the compact economy car segment. The car is cruising at a steady speed V km/h on a clear, dry, and perfectly level concrete road. The car is equipped with modern fuel-efficient all-season tires that experience a rolling resistance coefficient of 0.01 on ordinary concrete.

Calculating Fuel Economy

With a thermal efficiency of 25% and a drivetrain mechanical efficiency of 90%, 22.5% of the energy content of the petrol is converted to useful work to drive the wheels of the car. One liter of petrol can thus be burned to produce 7,040 Newton-kilometers. The retarding forces acting on the car, such as rolling resistance and aerodynamic drag, must be considered to determine fuel economy.

Rolling Resistance and Aerodynamic Drag

The rolling resistance force is often calculated as:

Rolling Resistance Force Crr x m x g

Where:

Crr is the rolling resistance coefficient (0.01). m is the vehicle mass in kg (1,300 kg). g is local gravitational acceleration in m/s2 (9.81 m/s2).

The aerodynamic drag force is often calculated as:

Aerodynamic Drag Force (1/2) x ρ x CD x A x V2

Where:

ρ is the air density (1.225 kg/m3) CD is the drag coefficient (0.30). A is the frontal area of the car in m2 (2.20 m2). V is the speed of the car in m/s.

By combining these forces, the fuel economy can be plotted against road speed.

Summary of Key Points

Driver: By far the biggest variable in the fuel economy equation. Speed notably kills fuel economy. Aerodynamic Drag Area: The primary indicator of how quickly a car consumes fuel when driven at typical motorway speeds. Curb Weight: The primary indicator of how quickly the car consumes fuel while driving in stop-and-go conditions. Rolling Resistance Product: The primary indicator of how quickly it consumes fuel while cruising at low road speeds.

In conclusion, the fuel economy of a car can be significantly influenced by a variety of factors, and careful consideration of these variables is essential for achieving optimal fuel efficiency.