Speed Density

What is a Speed Density engine managment scheme?

Speed Density systems calculate the amount of air entering a motor using sensors that monitor engine operating conditions. The Ideal Gas Law (PV = nRT) is the basis for this type of engine management. Instead of directly measuring the amount of air entering the motor ("n" in the above equation), speed-density systems estimate it using n = PV / RT, and then applying additional corrections. Fuel tables in a honda ECU (from a theoretical standpoint) are Volumetric Efficiency (VE) tables.

Basic operation:

• A Map Sensor is used to measure the pressure of air in the intake manifold (DENSITY term in speed-density). This provides the P term in the Ideal Gas Law calulation.
• The engine's rotational speed (SPEED term in speed-density) is used to estimate the volume of air entering the motor by multiplying how much air is sucked into the cylinder each stroke (usually displacement divide by two) by the number of engine revolutions. This provides the V term in the Ideal Gas Law calculation.
• The Intake Air Temperature Sensor measures the temperature of air entering the engine, providing the T term in the Ideal Gas Law calculation.
• R is a constant
• Knowing P, V, R and T, the ECU can then calculate an estimate of n (moles of air entering motor) from sensor data and n = PV / RT

Because speed-density calculates airflow instead of directly measuring it, additional factors are often used to provide a better estimate. The Tps Sensor is key in many of these - TPS tip-in enrichment is used to provide more fuel that the Ideal Gas Law calculation suggests in order to provide fueling for a rush of new air prior to the Map Sensor reaching a steady state. Off-throttle leaning is used to prevent the motor running rich after sudden decreases in throttle.

Here are some links to more information about speed-density systems:

• http://www.asashop.org/autoinc/jan97/techtotech.htm - decent link, not TOO techy.
• http://www.wincom.net/trog/efi.html - quite technical but practically oriented
• http://www.simcar.com/literature/sae940759/sae940759.htm - highly technical paper on practical use of speed density systems for air/fuel management. Has excellent figures illustrating the role of secondary correction factors, like TPS accel/decel.
• One Two Three Four Five Microsoft Powerpoint presentations from a College Engineering class on gas laws, thermodynamics and engines.