Piston Slap: MAP-ping Engine Load

>>>“And, in a modern car (’08 Civic, stick), will the computer control prevent me from wasting gas by pushing the gas pedal beyond the point where I’ve reached 99% load?” This was a slightly ambiguous sentence. What I meant was, will it prevent me from wasting gas if I push the pedal beyond the point where the engine has reached 99% load. It's quite obvious that I can push the pedal beyond that point. What is open loop vs closed loop operation? What loop? What what are MPFI and DI? Thanks.

@David C. Holzman Open loop= running solely off the preprogrammed fuel maps. Closed loop= using the MAP/MAF and O2 sensor(s) to optimize the base fuel maps. MPFI- multi-port fuel injection (in the intake header). DI- direct injection into the cylinder.

@bumpy ii Thanks bumpy ii, that's correct. Open vs closed loop refers to any control system. A control system is open-loop if it lacks feedback. A closed-loop control system references a feedback mechanism to adjust its input to achieve a desired output. In this case, the O2 sensors close the loop. The engine control unit (ECU) always references MAP/MAF and/or throttle position (TPS) to determine where on the pre-programmed fuel map to start, and references the O2 sensor (or not) to adjust the "fuel trim" to run stoichiometric. When at very high load the assumption is made that you wish for maximum power which is achieved at a richer than stoichiometric mixture, typically 12.5:1. Standard O2 sensors switch high/low at 14.7:1 to help achieve stoichiometric but do not have a readable variance in output when you get to 12.5:1 mixture so the ECU ignores the feedback and runs off the program. Higher-dollar lambda sensors will provide a readable variance in output over a wider range of mixtures and closed-loop control can be maintained with intentionally rich or lean mixtures but this is usually confined to exotics, race cars, or extra-cost efficiency seekers.

It should be said that cars/trucks which use a UHEGO (wideband) as the upstream O2 sensor do not need to go open loop at WOT/full load. Since the UHEGO is able to accurately measure the air/fuel ratio across a wide range, the ECM targets a richer than stoichiometric a/f ratio at high load values. It is still in a closed loop mode using the UHEGO input to drive Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) for feedback. The late-model Jaguar and Land Rover stuff with which I am familiar works like this. I do not know what Honda does.

@cdotson Modern engines only run in open loop during initial startup. As an example, the Ford Coyote 5.0 runs in closed loop even during full throttle conditions. The use of a scan gauge allows the reading of both wideband lambda sensors (one for each bank), and shows a solid ~14.0 under nearly all conditions. (Deceleration pegs the air/fuel ratio, due to fuel cutoff.) Furthermore, drive-by-wire systems no longer directly map pedal input to throttlebody angle. Rather, the computer produces a "torque target" based upon pedal input and looks that value up in a multi-dimensional array. Throttle position, camshaft advance, spark advance, fuel injector duty cycle, fuel pressure, and myriad other parameters are modified to produce the desired "torque target." Wideband O2 sensor feedback allows correction of the fuel trims.

@David C. Holzman Yes, it will prevent wasting fuel. If no more air can go in, then it won't put more fuel in either when you push the pedal farther. The amount of fuel, for the most part, just comes from the MAF/MAP reading along with the RPM. It's really an air pedal, not a gas pedal.