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Linus Gump -> RE: H3 actual gas mileage (7/1/2006 7:43:43 PM)
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quote:
ORIGINAL: Michael
Mileage isn't necessarily better with cruise control...
RPM doesn't have as much of an impact on fuel economy as you may think. It all depends on the load placed on the engine. This will affect how much manifold pressure, or vacuum, is going into each cylinder on the intake stroke, (modulated by the throttle butterfly). A high RPM setting with low manifold pressure (high vacuum) can result in better economy then a low RPM and high manifold (low vacuum) pressure. As an exaggerated example, think of driving around at 15 MPH in high gear and trying to accelerate. You get low RPM and high manifold pressure (low vacuum). On the other hand, think of cruising around at a steady 60 MPH in high gear. You now have high RPM and low manifold pressure (high vacuum). Aside from possible damage to the engine in the first part of the example, it is obviously more economical to operated in the relm of the second half of the example. The other end of the spectrum has us cruising around in low gear at a relatively high speed with the engine screaming with RPM. Now you have high RPM and a relatively low manifiold pressure (high vacuum), but you now have more RPM then the engine needs and it is wasting fuel.
The PCM monitors manifold pressure via the MAP sensor, and it monitors RPM usually off of the flywheel. Once the RPM starts to change (the degree of change is dependant on the programming of the PCM) the PCM opens the throttle valve to allow more air (higher manifold pressure/lower vacuum) and fuel into the engine to overcome the greater load. Once the throttle is opened all the way and the load continues to get bigger, or the load is added faster than the throttle can compensate for, the PCM tells the trans to downshift. It if is a manual trans, then the operator must make that decision. The PCM can detect these changes quicker then you can, and modulate the fuel/air mixture more precisely then you can.
Rememer that POWER= PLAN (Pressure in the cylinder, Length of stroke, surface Area of the piston, and the Number of times per unit of time measure) For any given RPM and load setting, the engine has a specific fuel consumption. Sometimes the most economical setting can be a high RPM setting, and sometimes it can be a low RPM setting.
Your idea of trying to maintail the same speed in hilly terrain is exactly what the PCM is trying to do. You just have the ability to see the increase in grade ahead of time and can add more throttle (manifold pressure) before the load increases. But, you dont know precicely if you added too much or not enough from the optium setting until you are on the hill and modulating the throttle. Both systems will allow you to coast down one hill and up the other, thus reducing the power requirement to climb.
As for the octane rating, you are partially correct. The different grades of fuel do have a different octate rating. That is the primary distinction between them. The higher the octane, the more resistant the fuel is to detonation. Detonation occurs from preignition or hot spots in the engine that cause the fuel to explode rather than burn smoothly. These explosions can be detrimental to the engine, and to fuel economy. High compression engines, and high performance driving require higher octane. Daily driving typically will not require a higher octane, unless the engine is designed for it (higher compression). You are correct about the energy content of the fuel, but that has nothing to do with octane. Bear in mind, those differences are within the allowable specifications set forth by the EPA and are considered to be acceptable.
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