Understanding the Engine
Internal combustion engines are often called aspirated engines because they are "air-breathing" machines. These engines burn a combination of air and fuel mixed at the stoichiometric ratio 14.7 to 1.17 In the past, fuel and air were mixed in the carburetor, relying on the venturi effect of the pipework to render the fuel into an aerosol.18 Modern systems rely on fuel injection, a process governed by the car's engine computer unit (ECU).19 A variety of sensors feed into the ECU providing real-time information so the computer can manipulate engine performance for optimal emissions. The ECU can regulate fuel flow precisely to match air intake or even adjust the spark timing for various RPM and engine loads.
A rough approximation of fuel use can be made through calculations on engine sensor data. Estimates on fuel consumption will be approximately equal to dividing the amount of airflow through the engine by 14.7, the stoichiometric ratio.20 Engine mass airflow may be determined either with an MAF (Mass Air Flow) sensor or through calculations on the MAP (Manifold Absolute Pressure) sensor.
(in case the server goes down, the equations in the image are:
Engine Mass Airflow = RPM * (Manifold Air Pressure / Absolute Temperature)
Fuel Mass = Engine Mass Airflow / Stoichiometric Ratio
Using these equations, I can compare the fuel requirements for different driving speeds. To collect the necessary data, I configured the DigiMoto software to log data on the engine's RPM, intake temperature and manifold pressure. For the tests, I drove the Jeep at 45, 55, 65 and 75mph along the same stretch of Riverwatch Parkway. To ensure consistency, logging only took place after the appropriate speed was reached and locked into with cruise control.21 During the drive, the windows were left up for consistent aerodynamics and the air conditioning was left off to eliminate extra engine variables. The engine intake added approximately 13° of heat to the ambient air temperature resulting in 103° Fahrenheit (312.5° Kelvin).