Quote:
Originally Posted by raycers_honda The assumptions are right.Also some times much before approaching a stop light i shift the car in neutral and let it roll upto the stop light.Dont know whether that helps in fuel efficiency in the long run.Not much use of brakes here but only a slight tap to reduce the speed gradually. |
Quote:
Originally Posted by AutoIndian Please do not do this, "shift the car in neutral and let it roll upto the stop light". This makes the car to roll on free wheels, you don't get any assistance of engine braking & stopping the car in case of an emergency would be far more difficult. There is hardly any fuel saving if you coast your car on neutral till the traffic light rather than in higher gear. It is not at all worth taking the risk. When you are nearing the traffic light in higher gear (4th or 5th) just take your foot off the accelerator pedal & let the car coast on the higher gear, due to the inertia it can easily travel upto the signal, w/o you giving any accelerator input |
Not only the safety factor. It is my belief that this actually burns more fuel instead. MPFI vehicles have a fuel cut off based on throttle operation. It means when the foot is off the throttle, the fuel supply is cut off from the engine, and the engine is run by the inertia of the wheels. It is something like a reverse process, where the movement of the wheels generates the force to keep the pistons moving and the engine running.
However, I do not vouch for the accuracy of the above mentioned facts. I had read them on some website, but do not remember where.
Considering that the scenario stated above is correct, by shifting into neutral and rolling to a stop will actually consume more fuel. This is because in neutral the engine is disconnected from the wheels, and thus the momentum of the wheels cannot work to keep the engine running. Thus, fuel supply to the engine must be given, only to keep the engine running, and not to move the car. The car moves on its own inertia, which is wasted since it is not being used to run the engine.
Same concept also applies to the scenario of driving downhill in neutral. Safety considerations apart, it means burning more fuel, as opposed to driving in gear, where as long as the car is rolling and there is no throttle input, fuel supply would be cut off.
I think this was also mentioned somewhere on the forum by another member. Will provide a link to that post if I can find it..
EDIT 1: In fact, this can very well be verified by simulating the situation. Try the following:
1. Stop on a slope, facing downhill. (The slope should be of decent gradient. Very gentle slope might not work).
2. Shift into first, and start accelerating. Slowly, but gradually.
3. When the speed reaches around 10-12 kmph (or whatever speed you generally shift into 2nd), leave the throttle, depress the clutch, shift into 2nd. Release the clutch, but do not accelerate. The car will continue to move forward. Also, you can see the RPMs increasing slowly, as the car rolls downhill without any throttle input.
4. When the engine reaches optimum RPM in 2nd, (by optimum RPM I mean whatever RPM that you generally upshift at), shift into 3rd, and continue without using the throttle. Again, the RPMs can be seen increasing.
Continuing this process, we can see that as the speed of the car increases, owing to downhill momentum, so does the engine RPM wrt to the current gear, without any throttle input whatsoever. This means that the increase in engine RPM is due to the rotation of the wheels, which drives the engine pistons. This is a reverse process. This means, that the engine is running without any fuel supply (or maybe minimum fuel supply).
Caution: While accelerating downhill, do not keep shifting up only to test this phenomenon. Increase your speed only till a comfortable limit, where you can drive at the required speed without needing to use the brakes to keep the speed in check. Safety first!!