Quote:
Originally posted by Maxvla
well now... this thread has developed very nicely and i havent even read it in a while.
i basically just skim read the last 40 posts or so i think the debate is whether going downhill burns more gas or not.
think of it this way. when you are driving normally the rpm's go up as you speed up going downhill, even if you arent pushing the pedal. if you take the car out of gear what happens?? the car returns to idle even as the speed increases. therefore i deduce that the engine's rpms are only increasing due to the wheels speeding up which are hooked to the trans which is in gear and is connected to the engine. this is also why you can use engine braking.
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I'm not arguing against that. You're talking about the CAUSE, I'm talking about the EFFECT.
Everyone agrees that the cause of the rise in rpm is the fact that the momentum of the car (combined with gravity when going downhill) causes both the engine of the car to speed up and the car itself to slow down. However, there are two viewpoints on the EFFECT, one is that the fact that, since the gas pedal is not being depressed, there can be no more fuel being delivered to the chamber than what the idle would deliver, the other is that the rpm's are linked to fuel consumption.
The former argument is logically flawed. The fact of the matter is that there is a certain minimum amount of air/fuel mixture that MUST be present in the chamber to fire. Anything less would not ginite and you would lose that cylinder, etc etc. Your engine is basically a glorified, FREAKING BIG air pump. When the piston hits the bottom, a vacuum is created at the top, which is how air/fuel is drawn from the valves. The more often the piston creates this vacuum (read: the higher the rpm) the more powerful the force is that is sucking the mixture through the throttle body, which opens it wider than idle. The only difference between this situation and when you depress the gas pedal is WHY the throttle body opens: when using engine braking, it opens because more air is being sucked through it. When you depress the pedal YOU are opening the throttle. It makes no difference HOW, just the fact that it is being opened means that more fuel is beind delivered, which means greater consumption.
You must also condier the fact that the engine is applying force to the vehicle during engine braking. When going downhill in a high gear, the force being applied is minimal if it's there at all. When going in a low gear, the force is great, which is shown by the fact that the vehicle slows down. You simply cannot apply force without energy, and that energy is converted from chemical to mechanical by way of combustion. The chemical energy comes form the burning of fuel, which has to be present to exert the force. It cannot simply be drawn from thin air.
But the ultimate test is this: drive one tank where you do all engine braking, then drive a tank when you do NO engine braking. While both extremes are unlikely in the real world (as everyone will do SOME of both) the test will show which one will yeild better gas mileage (and by virtue of that, which method consumes less gas).
If you want to engine brake, that's fine; it's not going to kill you and I do it too; just don't tell me that you're getting the same milage as someone who makes proper use of the equipment put there for the specific purpose of slowing down and stopping: the brakes
