Why do some engines use both TPS and MAP sensors?

Question

I ask because I've always thought it peculiar that the OEM LS1 throttle body has provision for both MAP sensor and TPS.

Since intake manifold pressure depends on how open the throttle plate is, I would imagine that throttle position can be inferred from the MAP reading (assuming an airtight intake - let's keep failure modes like vacuum leaks out of this discussion).

Isn't it redundant to have both MAP and TPS on an engine management system? If so, why do some manufacturers like Chevrolet have both on their engines?

Answer 1

http://thedodgegarage.com/turbo_pfi.html

The 84-95 Dodge FWD cars (2.2/2.5/3.0) use a speed density system.

The MAP MAP sensor is the primary sensor in determining how much fuel the engine needs. This is the second most important sensor to the ECU after the distributor's HEP(Hall Effect Pickup) sensor. Basically, the engine can flow a given amount of air, so this value is hard coded in the ECU. The ECU can calculate how much fuel is needed based on the hard coded flow rate, manifold pressure, and RPM.

The TPS serves 2 purposes - identify quick changes and WOT(wide open throttle). When the throttle is opened or closed quickly, it can a couple engine revolutions for the manifold pressure to reflect the throttle position. The ECU will adjust accordingly by giving more or less fuel until the MAP sensor readings catch up with the TPS readings. This is similar to the accelerator pump on a carburetor. When the ECU senses WOT, it will turn off the A/C, turn on the cooling fans, and use a different fueling and timing table. In most vehicles, the WOT flag could be set by the MAP reading sensor reading near barometric pressure, but these ECUs were also used on turbocharged vehicles.

Answer 2

While these two sensors are related to how the engine runs, their function and what they provide for engine management are completely different.

MAP (or Manifold Absolute Pressure sensor)

The MAP provides the computer with information as far as the density of the air. This tells the engine how much air is actually getting into the engine. This, along with the Mass Airflow (MAF) sensor (if so equipped) and O2 sensors, tells engine management how much fuel to disperse into each cylinder to keep the air/fuel ratio somewhere near stoich so the engine will run at its best with fewer emissions.

TPS (or Throttle Position sensor)

The TPS is basically there to provide the computer with the driver's input. What does the driver want to do? Larger TPS reading provides for more load to be put upon the engine and for the vehicle to go faster. It can also indicate to the management system if the need to downshift the transmission is needed to provide the vehicle response the driver is wanting. Of note, with most vehicle manufacturers moving towards "drive by wire" (no direct connection between the throttle pedal and the throttle body), there is no need for a TPS. Since the computer controls the throttle, it knows already where the throttle position is at, because it is directing the show.

Mind you, these are the general reasons for having each of these sensors. While you could possibly infer throttle position due to manifold pressure, having both sensors allows the engine to be more responsive. If running just off of the MAP sensor to determine these things, the computer would always be reactive and trying to keep up with the demand. There would have to be large assumptions built into the programming and I'd assume a larger more powerful engine management system would have to be applied to compensate. Having both sensors there gives the computer the exact want of the driver as well as the amount of air flow going into the engine to provide the driver with a much better driving experience.

It should be of note, there is also the same argument made for having both a MAF and a MAP sensor installed on the vehicle. These two share a lot of the duties to help with engine management. Many GM vehicles were produced with both sensors (and still are). Without a MAF, the engine management can run in what is known as speed density mode. While this mode works, having the MAF in place provides a more precise measurement of incoming air for the computer, and thus better fuel management, economy, and lower emissions. It does provide a restriction in the intake tract, though, which is the tradeoff.

Answer 3

I suspect the reason is for vehicles which operate "fly-by-wire" throttle pedals. Without a TPS, you'd have no idea how much throttle angle the drivers foot was asking for so the engine wouldn't know how far to open it's throttle flap.