Quote:
Thanks for the help in advance guys
I hope I can communicate my question clearly enough
It’s common knowledge that a dirty air filter causes poor fuel economy.
What I can’t rectify in my head the that if your car meters the fuel for the amount of incoming air and that any restriction in the intake system means and equivalent amount of air you will not have to compress, why the loss in fuel economy? Is it that the car has a hard time compensating? Or not enough flow to mix up the incoming fuel?
And along that line of thinking. Assuming you accelerated at the same rate would it be more efficient to keep your rpm’s as low as reasonable (say you have a manual) and open the throttle more?
If I could I would prefer knowledgeable responses as opposed to anecdotal responses
Again thanks for your time
OK, I see this as two main questions, so let's see: In the old days with carburetors a restricted air filter would cause a progressively lean condition which also caused progressively worse MPG this was because of the mechanical nature of a carb. Basically, there is an orifice that allows fuel through it and into the air stream. The engine creates a low-pressure area as it runs and the air-fuel mixture get pushed into that low pressure area by atmospheric pressure. When the air filter is restricted there is less air flow yet the engine is still creating the same low-pressure area. The fuel gets pushed into this low pressure area the same as before but there is less air to mix with it, hence, a rich condition.
You would think that fuel injection would not have this problem due to the computer compensating and that is basically true. However, other factors came into play at this point, one is that a restricted air filter is causing an increased pumping loss in the engine. Remember, the engine is basically an air pump that creates power by the compression and burning of the air-fuel mixture. The more work it takes the engine to get that mixture into the cylinders, the less power output it will make. To compensate for the lower output the engine will need to run at a higher RPM. We are talking slight amounts under a slight restriction of the air filter. Nothing noticeable by seat-of-the-pants, but there none-the-less. The other factor is that if the air filter is restricted the engine is no longer running at a "perfect" state of tune. The computer will compensate to a great degree as the air filter goes from new to "full". Once the air filter reaches a saturation level beyond its servicable lifetime, the system no longer is running at optimum. The rpm will need to increase to provide the same power as before, the fuel trim will decrease as the air flow decreases and you now have a less than efficient system, hence a reduction in MPG. The engine is fighting for air and will become sluggish. The transmission may not shift properly because the TPS (throttle position sensor) shows you want to go faster and/or need to downshift yet the RPM and MAF/MAP sensors say that the car is not ready and/or capable.
You second question deals more with the optimum RPM to run the vehicle and asks about low RPM being ideal. Basically, to answer your question: it depends on the engine. Every gasoline and diesel automotive engine is designed around a particular RPM band. An example would be a HD diesel engine in an OTR tractor-trailer that is made for mega-torque at low, low RPM vs a race bike designed for high output at (relatively) high RPMs. The camshaft, valve layout, head(s) and the bore and stroke are designed for the intended "duty" of the motor. The high rpm race bike will have a very short stroke, big bore, high lift cams, multiple (and smaller compared to a 2-valve head) valves and straight ports. The diesel will nearly be the opposite on most counts.
Each engine is going to have a sweet spot of BSFC (brake specific fuel consumption) where the RPM of the engine is at its most efficient. For the diesel, it will be LOW, for the race bike it will be HIGH. For the average passenger car it will be low, but a little higher than the typical diesel engine.
So to answer your question, the most efficient rpm of the engine is the BSFC. Let's just say that a particular engine is most efficient at 3,000 RPM. The way to get the most efficiency is to quickly get to 3,000 RPM and stay there. This is a little too simple however because it does not take into account gearing or the coefficient of drag (cd). If the vehicle had a true contstantly-variable transmission (CVT) and was always run at the optimum RPM, the most efficient speed would be the speed at which going slower or faster would increase comsumption. Likely when cd started to show a large increase.
I personally find the best MPG when I accelerate at an "average" pace. Not granny slow or racer fast. With the new electronic transmissions a "middle-of-the-road" amount of acceleration will allow the transmission to work with the engine to give near optimum shift points. On my manual transmission vehicles I usually aim for 2500-3000 shift points if I'm not racing.
I hope this gave you some insight.
