PeachParts Mercedes-Benz Forum - View Single Post - How is fuel used during downhill with no throttle applied?

mplafleur · #11 02-22-2002, 01:41 PM

There are several strategies used to determine fuel delivery in a vehicle. Typically it is load and air flow, or engine speed and air density along with throttle position. These inputs are used in table lookups and equations with other modifiers such as air charge temperature, coolant temperature and the ego sensor. Remember, we may in closed-loop control and trying to maintain a stochiometric (14.64:1) fuel-air mixture. And of course, there may be a high RPM cut-off and a high speed cut-off. In a no-load or closed throttle situation, the idle-speed control logic will try to maintain rpm within a certain range.

Steve is correct, on hard decels, fuel may be cut off, momentarily. This time can be measured in the milliseconds. When coasting, the engine, if under no load, will try to maintain idle speed. The engine controller does not care if the car is sitting or moving. It will inject fuel at the prescribed amount to maintain idle. To remove fuel would mean having to restart the engine at a later time. For reasons of trying to obtain low emissions, auto manufacturers try to refrain from restarting vehicles. This is a high emission event and also adversly effects the catalytic converter.

Besides, if you were going downhill and wanted to accelerate, you don't want to have to restart the engine, then start the acceleration process which is what the engine strategy would have to do. That isn't efficient nor does it give the performance that cusomers demand and expect.

I remember discussions on this topic during the energy crisis in the '70's. I though also that it was not good for an automatic transmission to be left in neutral while coasting downhill. This was both for lubrication and cooling reasons. In a manual, it is OK.

Now I haven't worked in engine strategy since the 80's, but I haven't been aware of any significant changes in basic fuel control. Electronic throttle control has appeared in the last few years, but that is a whole different topic. (and one I can't talk about). I'll check with others and verify this.