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Originally Posted by Setec Astronomy
It's not as simple as running rich, but that's part of it. The basic explanation is that you're converting the fuel to heat. The heat expands the combustion products, driving the pistons up and down. A vehicle is not very efficient at capturing this heat; in comparison to say a modern condensing boiler/furnace in your house, which captures so much of the heat for heating your house that it can use a plastic (PVC) exhaust pipe. In your car, a lot of the heat goes out the tailpipe, and a lot of it is absorbed by the various metal parts of your engine, and the coolant and oil. If you do a lot of short trips, you keep using the fuel energy to "re-heat" all those parts of your engine, especially during the winter when they cool down quickly.
The open loop/closed loop different is similar but different. The O2 sensors and the catalytic converter don't operate properly until they reach a certain temperature. Since the engine control computer can't read the O2 sensor it can't be sure the combustion process is tuned properly, and therefore uses a pre-programmed "open loop" fuel map based on throttle position, MAP, etc. As noted, this mixture is rich.
I think the easiest way to visualize this is to think about the heater in your car. How does this work? Hot coolant is run through a heat exchanger, and air is drawn over that heat exchanger (your heater core) to be warmed and then enters the passenger compartment. If you drive for 20 minutes, stop for a cup of coffee for 3 minutes, and get back in the car, your heat will still blow hot. If you get out for 2 hours to have dinner, it will blow cold. You need to put the heat back into the coolant in order to take it out to warm the air. The heat that you put into the coolant is energy from the gas in your tank, so the more times you do it the more energy you use. That's a little oversimplified, but whatever.
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You have the theory down of energy that can't be destroyed but converted.
Open loop the o2 sensors are not used (scan tool will display 450mv). After the ect reaches 160 deg the ecu determines if it should go into closed loop based on engine and ambient temperature parameters. The reason that the engine runs rich is because the air is denser when cold than when warm. The ecu picks this up based on maf, intake air and ambient temperature sensors. As the engine reaches operating temperatures and the o2 sensors are warm enough they begin to monitor o2 readings and the ecu begins to adjust fuel trim values, thereby adjusting injector pw to provide near perfect 14.7:1 ratio. This would lean the mixture from when you started the vehicle and the ecu read baro and used a calculated value opposed to real-time readings.
The quicker an engine reaches operating temperature the better. For every gallon of gasoline used a gallon of water is produced due to condensation that the oil absorbs throughout the crankcase and is blown into the combustion chambers through the pcv system (early cold morning white smoke from tail pipe, then goes away after warm). So the ideal operating temperature of any engine is 210-220 deg at the temperature water begins to boil.
The question in the topic is fuel economy based on a 2-3 minute drive. To begin with how long do they leave the vehicle running at start up? It has been proven that a vehicle should be left to warm up for couple of minutes and then driven lightly. Reason being if the car is left to sit out for 20 min before driving off well thats gas burned from a vehicle sitting there.
It being such a short distance is it stop and go traffic?? If so the transmission works in gear ranges. in first gear most gear ranges are anywhere from 4:1 (6 speeds) to 2.5:1 (4speeds) which means the engine will have to rotate 4 times before the wheels rotate 1 producing 4 times the torque but requiring 4 times the load. Contrary an overdrive gear is anything under 1:1 like a .7:1 which means the engine will only rotate almost 3/4 turn when the driven wheels have already rotated 1 this creates a .7 torque requirement and only producing .7 times the load.
Finally the most important transmission related factor would be the torque converter clutch. A torque converter uses centrifugal force to double torque. It slings fluid starting from the rotor in the center and hits the vanes at an angle to were they shoot out and hit the cover ends creating a spinning motion much like a water reel that uses water pressure to spin the head and water the lawn within a circular radius. The converter is only used when torque is most needed, at lower speeds. After the vehicle has been driven for a while and the transmission has reached overdrive several times the ecu will determine if weather or not to engage the clutch and stop torque multiplication. If it receives an ok it eliminates torque multiplication and increases fuel economy because of the less load it places on the engine on top of the economy increase because of a overdrive gear. This would require the vehicle to be DRIVEN for at least 10-15 min.
The Last and final thing would be are vehicles in tune?? With 1 spark plug or injector fouled or plugged will dwindle fuel economy because of what the ecu attempts to do to adjust it. The vehicle must be in good shape if they expect it to get the recommended mileage.