[DieselFan]

Engine can't run on Zero Fuel. There is a power stroke for each 4 strokes. So at idle if you are at 800 RPM say you would execute 400 power strokes (One revolution of the crank shaft has 2 strokes I presume). In each of these power strokes we have fuel injected by Fuel pump into the engine. So Fuel gets consumed by a running engine on a continual basis whether you press the throttle or not.

Some folks have updated that engine consumes zero fuel in gear when you do not press the accelerator. My question then is how does the car move forward in gear 1 or 2 without pressing any accelerator?

Originally Posted by DieselFan

Engine can't run on Zero Fuel. There is a power stroke for each 4 strokes. So at idle if you are at 800 RPM say you would execute 400 power strokes (One revolution of the crank shaft has 2 strokes I presume). In each of these power strokes we have fuel injected by Fuel pump into the engine. So Fuel gets consumed by a running engine on a continual basis whether you press the throttle or not.

Some folks have updated that engine consumes zero fuel in gear when you do not press the accelerator. My question then is how does the car move forward in gear 1 or 2 without pressing any accelerator?

Engine can run on zero fuel. When driving downhill in gear, the momentum of the vehicle or the wheels is transferred to the engine which keeps the engine rotating. As long as engine's RPM is maintained above threshold value due to rotating wheels, ECM will detect it and no fuel will be injected. Why should any fuel be injected in this case when there is no need to do so. This is what modern cars are meant to deliever.

[anupmathur]

Originally Posted by DieselFan

Engine can't run on Zero Fuel.

LOL, you're right!

The problem here is: It is not running like an engine, but as a 'compressor' being driven by external forces!


And, it is important to realize that this is just the basics we are talking about.
There are many advanced compression/engine braking techniques and devices in use in different vehicles meant for varied applications.
Many of these techniques do use fuel during engine braking!

[Guna]

Originally Posted by gaurav.28ch

Engine can run on zero fuel. When driving downhill in gear, the momentum of the vehicle or the wheels is transferred to the engine which keeps the engine rotating. As long as engine's RPM is maintained above threshold value due to rotating wheels, ECM will detect it and no fuel will be injected. Why should any fuel be injected in this case when there is no need to do so. This is what modern cars are meant to deliever.

I have a question:
If no fuel is injected, where does the energy to compress the air come from? Probably from the momentum (Kinetic energy) or from power generated by other cylinders.

If it is coming from the kinetic energy, the car is going to loose momentum as it is getting consumed for compressing the air which is not generating as there is no ignition or 'power stroke'. So you need to ignite fuel to keep the momentum going, unless the intention is to slow down.

If it is coming from another cylinder, wont it result in imbalance? All four cylindrs ned to be running to get the right balance? Also this is not helping in saving fuel.

Originally Posted by Guna

I have a question:
If no fuel is injected, where does the energy to compress the air come from? Probably from the momentum (Kinetic energy) or from power generated by other cylinders.

If it is coming from the kinetic energy, the car is going to loose momentum as it is getting consumed for compressing the air which is not generating as there is no ignition or 'power stroke'. So you need to ignite fuel to keep the momentum going, unless the intention is to slow down.

If it is coming from another cylinder, wont it result in imbalance? All four cylindrs ned to be running to get the right balance? Also this is not helping in saving fuel.

You are right here. If driving in gear on a flat surface, kinetic energy of rorating wheels would ultimately be exhausted in in compressing air, which we call engine braking. Surely after sometime you need to press on throttle afterwards.
But when driving downhill in gear, car will have sufficient kinetic energy available continuosly to keep the engine RPM's above threshold value though a portion of energy will defitely be lost in compressing air. But this energy will be replenished as you are going downhill.

[anupmathur]

Originally Posted by Guna

I have a question:
If no fuel is injected, where does the energy to compress the air come from?

This energy comes from the momentum of the car. The purpose is to decelerate quickly by offering resistance to the car's motion.

Originally Posted by Guna

If it is coming from another cylinder, wont it result in imbalance? All four cylindrs ned to be running to get the right balance? Also this is not helping in saving fuel.

(explained above) Further, our cars have no system to cut off specific cylinders. Either they all fire or they all stay off.

[ImmortalZ]

I'm surprised to see how hard this concept is to grasp. Come on guys - it is easy to comprehend. Take a small fan, like a computer case fan. Plug it in, it spins. Put your vacuum cleaner into blow mode and point it at the fan. It will spin. And if you plug a multimeter to the pins, it'll show that the fan is generating voltage even.

Similarly, give an engine fuel and air (and a way to detonate that fuel), it will push the pistons down and end up spinning the fly wheel. Spin the fly wheel with an external force and the pistons will move up and down without needing any explosions. No fuel, no explosions. No explosions, nothing to overcome the frictional losses inside the engine. Result? The energy given into the engine is expended. Wasted. Engine braking, yo.

The kinetic energy to move the cylinders, spin the fly wheel, whatever, comes from the wheels. The wheels, forced to spin by momentum. Momentum created by the engine itself when it was blowing up dinosaurs.

It is a simple movement of energy in different directions.

[Guna]

Originally Posted by ImmortalZ

I'm surprised to see how hard this concept is to grasp. Come on guys - it is easy to comprehend.

I think there is a bit of conversation going on , no comprehension issues

[supremeBaleno]

Since weekends are usually spent off the net, could not reply.

Originally Posted by ImmortalZ

I'm surprised to see how hard this concept is to grasp.

In theory, lots of things sound pretty simple. But in practice, it could be very different.

Assuming for a moment that your logic is bullet-proof, I have 1 query. Let's say I get a 1-2 km downhill stretch all to myself to test this theory and I start off at the top. I start the car which would ofcourse require fuel to be burnt and as I start the descent, I resort to engine braking and do the full descent without tapping the A-pedal. By your logic no fuel should be used since I am using e-braking and also not hitting the A-pedal.

If so, there should be no exhaust gases since ZERO fuel is burnt. Do you say that for the full 2km stretch, there would be no traces of fuel-combustion ? This can be easily checked using a simple set-up rigged to the exhaust pipe.

If yes, I would agree with the theory, but I would need proof to believe it. Ofcourse, I dont expect anyone to prove this for me - dont want this thread to end up on the lines of the alternator vs battery thread. Just saying that if solid proof is available, I will embrace the theory.

I have another query which is something I want to ask the experts :

What about the sound difference between an engine running via combustion of fuel and another one that is being run via the momentum from the wheels ? Won't it be different ? Every car engine running by fuel-combustion will have a unique sound. If we cut-off fuel to the engine and run it via wheel momentum, will the car engine still sound the same ?

A car engine has the same sound during normal running and during engine braking ie. it sounds 'live' - don't know how to say this. I doubt that it would sound the same way after fuel is cut-off and when no combustion is happening.

Hope this is not too naive a query for the 'experts'. I believe that there is no such thing as a stupid question and its better to have your doubts cleared, especially when dealing with something that completely differs from what you have known for long.

[blacmagic]

SB, the zero fuel thing kind of makes sense and looks simple to implement too. please do note that Im going on a porely common sense basis here. it's not difficult to program an ECU to do this:
if RPM > idling, supply 0 fuel
else if rpm <= idling, supply fuel as usual

[sps]

Originally Posted by ImmortalZ

Similarly, give an engine fuel and air (and a way to detonate that fuel), it will push the pistons down and end up spinning the fly wheel. Spin the fly wheel with an external force and the pistons will move up and down without needing any explosions. No fuel, no explosions. No explosions, nothing to overcome the frictional losses inside the engine. Result? The energy given into the engine is expended. Wasted. Engine braking, yo.

Good explanation immortalz.
In some vehicles, when we go in third gear or second gear the vehicle automatically moves in a certain speed say 20 Km/h without pressing the accelator pedal. So this means that fuel is being automatically pumped. During city drive this feature is quite useful.
Do you think that we will get max fuel efficiency if we use this mode while driving in the city.

[vasoo]

Originally Posted by sps

Good explanation immortalz.
In some vehicles, when we go in third gear or second gear the vehicle automatically moves in a certain speed say 20 Km/h without pressing the accelator pedal. So this means that fuel is being automatically pumped. During city drive this feature is quite useful.
Do you think that we will get max fuel efficiency if we use this mode while driving in the city.

I think in most modern cars, the fuel supply is regulated by the ECU rather than purely by the a-pedal. Otherwise it is difficult to explain cases where the car easily moves off the block when you lift the leg off the clutch without even touch a-pedal. I am sure fuel is consumed here, same is the case with the "coasting method" practiced by many FE aficionados. Basically no a-pedal does not translate to 0 fuel. I remember in older carb-based cars, mechanics used to adjust the carb/a-pedal cable settings to alter the idling speed of the engine.

FE issues seems to happen mostly when the a-pedal is used out of sync with the engine/car dynamics. Say if you are moving the car in 4th gear at 80 kmph, and then suddenly you floor the a-pedal. This may result in a surge in supply of fuel to the engine, depending on the ECU. However the chance of this getting translated to real power depends a lot on the engine. If you have a powerful engine, the car lunges forward, you gain miles and you offset the FE impact to some extend, else, you just wasted some fuel in unburnt form through exhaust. Same is the case with braking. If you are going at 80kmph and brakes suddenly, the power generated by engine is wasted on the brakepads as heat. But if you plan the event juduciously and use the e-braking to decelerate slowly, you stand to gain, both from better use of fuel and from saving brake system from wear and tear.

[anupmathur]

For understanding how and why the fuel-cut is used, please read: Stealth 316 - 3S Fuel Injection Control and Fuel Cut

See the section on fuel-cut towards the end.

While this explanation is for two specific cars, it describes all the functions quite clearly.
All FI cars work in much the same way.

[longford]

Well I am not sure if the parties veering either extremes are correct..

On my Accent 2001, the default engine rpm is around 750.. On my commute and fro I have absolutely clear roads of 10 kms each way and a nice bridge as well.. When I reach the bridge top, I shift it to neutral to use the kinetic energies.. At 60kmph downhill it does go quite a distance comfortably...
The engine rpm goes to 750 the moment i shift to neutral...

So i guess the fuel may not be pumping but the rpm is also not the right measure becos at 60kmph, the car is definitely over the default 750 rpm..

Yeah.. the rolling is quite silent too as compared to the fuel combustion based rolling..

great thread!!!

cheers

[janitha]

Originally Posted by longford

Well I am not sure if the parties veering either extremes are correct..

On my Accent 2001, the default engine rpm is around 750.. On my commute and fro I have absolutely clear roads of 10 kms each way and a nice bridge as well.. When I reach the bridge top, I shift it to neutral to use the kinetic energies.. At 60kmph downhill it does go quite a distance comfortably...
The engine rpm goes to 750 the moment i shift to neutral...

So i guess the fuel may not be pumping but the rpm is also not the right measure becos at 60kmph, the car is definitely over the default 750 rpm..

Yeah.. the rolling is quite silent too as compared to the fuel combustion based rolling..

great thread!!!

cheers

Once you are rolling down in neutral, how does the engine and rpm come into picture? Wont it i be just idling? Secondly, isn't it dangerous?