Quote:
Originally Posted by ::CMS:: While idling, the fuel is pumped to keep a particular rpm say 800 and the same is not required when the engine uses the flywheel to keep the same rpm ie, less fuel is consumed till it reaches the idle rpm where the anti-stall kicks in. |
So far correct.
Let me just put "Load" into simple terms. It is the amount of fuel required to turn the engine at a given RPM.
A car with just me at 1500 rpm and 50kmph will be using much less fuel (hence much less load on the engine) than when the car is doing the same 1500 RPM and 50kmph with 5 People (more fuel required = More load on engine).
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Originally Posted by ::CMS:: I have observed, while moving at say 60km/hr, on neutral the load stays around 50% and at the same time costing in the proper gear makes the engine load ~25-30%, the moment you put the gear to neutral the load goes upto ~50%. The same happens when you lug the engine, it goes to ~80-90%, hence we say more fuel is consumed if you lug the engine or if not in proper gear based on the rpm. |
One small thing. Here by coasting you obviously mean Throttle closed and not maintaining 60kmph.
That means you are deceleration from 60kmph in gear. Therefore engine is getting a part of or entire energy from the momentum of the car (Hence slowing down - apart from road, air resistance etc). Hence the lower load percentage shown compared to neutral.
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Originally Posted by babu.sundaram On contrary, just think in plain physics. It needs energy to turn the engine. More energy is needed when you turn it faster. Idle rpm is lower than the in-gear rpm. So, more energy is needed when engine is in-gear. |
Yes, but when coasting (0 throttle) in gear, the wheels are driving the engine. Therefore no/less fuel is injected to turn the engine over.
But when in neutral, only fuel is responsible for turning the engine over albeit at the idle rpm. Only fuel is responsible for overcoming the inertia, friction or all the running parts of the engine.
Quote:
Originally Posted by babu.sundaram Given that fuel is the only source of energy (no regenerative mechanism kicking in like braking situations) in-gear must consume more fuel. Isn't it? |
Again depends on what situation you are talking about. If you are coasting (0 throttle) you are invariably decelerating. Hence engine breaking or regeneration occurs.
If you are talking about maintaining a given speed, this conversation is moot.
Simple physics,
Neutral - no energy to wheels - so no maintaining of speed - fuel burnt at idle speed.
Part throttle to maintain speed - energy to wheels from fuel burnt - speed maintained.
So for a given distance to be covered in given time, Part throttle speed maintenance is more fuel efficient than "in-neutral-speed-loss and more-throttle-speed-gain" cycles.
Quote:
Originally Posted by babu.sundaram Neutral with no throttle input would be the lowest fuel consumption case, except when engine braking is used (where vehicle's momentum is used up to turn the engine). |
Yes, but to what end. You will be going nowhere. Its the lowest fuel consumption when your motive is to simply turn over the engine.
To get from A to B in a given time, there are two ways (idealized).
Accelerate to the required average speed and maintain in gear.
Accelerate to a much higher speed and at a calculated time, shift to neutral and let the momentum get you to B.
I think it is obvious that the first case will be more efficient in real world terms when resistances are taken into account which increase non linearly with speed. And in the second case, you will be running the engine on idle with absolutely no contribution to your motive of reaching the destination. Plus the fact that we need to slow down regularly makes the case for engine braking - more fuel efficient technique.
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