Most people know the basic premise of how an internal combustion engine operates. An air/fuel mixture enters the engine where it is ignited, exerting force on the crown of the piston, forcing the crankshaft to rotate, which in turn rotates the wheels via a transmission – all fairly basic. What many fail to consider however, is the means by which the car measures the amount of air it is taking in, in order to properly meter fuel.
In the world of fuel-injected engines, there are two popular methods of measuring airflow – speed density and mass airflow (MAF) systems.
Speed density has been around longer than MAF systems, and use a range of data to calculate airflow. The inlet air temperature and manifold pressure is measured via a manifold air pressure (MAP) sensor, which allows the ECU to calculate the density of the air present. With that data, the correct amount of fuel can also be calculated.
The car’s ECU is equipped with data called a volumetric efficiency table that allows the car to estimate how much air the engine is taking in at a given speed. An oxygen sensor is placed in the exhaust stream that measures the air/fuel ratio and allows the ECU to make corrections as needed.
Because speed density systems do not directly measure airflow rates, they have become less popular for new car manufacturers, who instead use MAF sensor systems.
A MAF sensor sits directly in the intake air stream, in a section of the intake with no bends or turbulent areas – allowing for accurate air measurements. Most MAF sensors operate using a heated wire that is in turn cooled down by the incoming air. The car then sends a certain voltage to the wire to keep it at a constant temperature, and that voltage is directly proportional to the airflow into the engine. MAF-equipped cars typically still use O2 sensors in the exhaust stream as a way to check and ensure the MAF sensor is working properly.
Speed density offers an advantage in that it is better at handling high-revving engines than a MAF sensor is. Most engines – especially those of smaller displacement – that rev above 8 or 9000 RPM are better suited to speed density.
Conversely, MAF systems deal with rapidly changing environmental or engine conditions more effectively than a speed density setup.
Speed density is still preferred by many modified car enthusiasts, because tuning is simplified and because the wire within a MAF sensor can become an obstruction at very high power-levels.