I replaced a Mass Air Flow sensor on a Volvo 850 yesterday. The diagnostic protocol I completed before condemning a $350 part reminded me once again how computers have taken over every aspect of engine management, up to and including “drive-by-wire” systems that actually control the throttle in response to inputs from the gas pedal, engine speed, gear range, etc. In other words, when you press on the gas, there is only an electrical connection to the engine from the pedal. Instead of a cable, there are two range sensors and a single redundant sensor to tell the computer how far down you have pushed the gas. The computer refers to its ‘look up’ tables and decides how much to open the throttle plate. Luckily for us there are redundant sensors and a ‘limp home’ mode that allows the car to be driven in event of a failure caused by an errant 44oz Coke.
The Mass Air Flow sensor I replaced in the Volvo uses a precisely heated wire to tell the computer how much air is flowing into the engine. As outside air flows over the wire, it cools it down slightly, providing an extremely precise varying voltage signal to the computer. Using this information the computer adjusts the amount of time it holds the injectors open, making sure you have good acceleration combined with low emissions and good fuel economy. What normally fails in one of these technological miracles is the “hot wire”. If the voltage signal is off, the computer thinks you are going down the road with the throttle whacked wide open, when in fact you are maneuvering in the parking lot. All of a sudden you are stalling, your fuel economy is down, and the car is hard to start.
Ironically, to meet the needs of the catalytic converter, the fuel mixture needs to be slightly richer than a fuel mixture that would provide the best fuel economy. You can see the tightrope the engineers and designers are walking. If they build a car with great fuel economy, it will be harder to control emissions (as well as being small). If they build a car with great fuel economy and low emissions, it is likely to be expensive, and small like the Toyota Pruis. The great thing about the intensive computer control of every aspect of the engine and transmissions operation is just how well they can make the cars run and still get decent fuel economy and low emissions. Those of you old enough to remember the early emission controlled cars of the ‘70’s will also remember how poorly they ran and what lousy fuel economy they got, especially the big American cars. My son is driving a full sized 2015 Dodge pickup that gets and an average of 18-19 miles per gallon, approximately twice the fuel economy of the ’75 pickup he drove for several years. Not only does it get good fuel economy, but it is extremely reliable as well, and had the same truck been built several years ago, it would have qualified as a ULEV or Ultra Low Emissions Vehicle!
What the future will bring is up in the air, or still on designer and engineers desks. All we can say for sure is that intensive electronic control of automotive systems is here to stay.
