[adya33]

Originally Posted by typeOnegative

That my friend is extremely dangerous. For two reasons:

1. The car can easily go out of control if there are too many bends and curves.
2. The frequent braking that you have to do will result in over-heating of the braking system thus considerably reducing the braking efficiency. This means you will be in a tight spot on a long slippery slope. Pardon the eloquence.

You are right its dangerous.
Thats why I said in my post maximum practically possible gear
Practically meaning which will not put your life in danger.
And you can always switch to lower gear if you feel uncomfortable in higher gear.

I guess we should be in lower gear as possible. Lower the gear the more will be engine breaking power.

I am not talking about getting maximum braking but about getting maximum FE

Originally Posted by prabuddhadg

Sorry, I meant on 2nd gear or any other gear. Someone mentioned he goes downhill on 2nd gear without using gas.

No. Car will not go downhill when switched off and in gear. But what that member meant was he goes downhill in 2nd gear without using gas peddle (aka accelerator) with engine running.

Originally Posted by sreenivass

This seems like an intelligent control mechanism, do other car makers follow this ?

Almost all cars with ECU have similar mechanisms.

[rohitbagai]

Originally Posted by santosh.s

Interesting... can you please explain why does it happen so?

To understand why this is, you need to understand how the engine decides to deliver fuel. On any vehicle with an oxygen sensor the driver decides how much air to admit to the engine and the computer decides how much fuel to deliver. So in other words by pressing accelerator most cars we regulate the air inflow directly and the fuel inflow is regulated by ECM accordingly. On carbureted vehicles, this is accomplished by the mixing control, which could be a solenoid but is usually a motor that continually opens and closes a port. On fuel-injected vehicles, this is controlled by changing the fuel pressure, pulse width, or just the duty cycle of the fuel injectors.
Either way, the ECU is constantly monitoring the oxygen sensor. The O2 sensor, when heated, generates electricity because oxygen ions are attracted to it, and pass across it, inducing a current. More voltage means a leaner mixture, which is to say that it has more oxygen in it because less of the oxygen has been burned. Less voltage means a richer mixture, which means more of the oxygen has been burned. When we take our foot off from the accelerator pedal it restricts the airflow to the engine. So the proportionate oxygen ions are reduced. Lesser oxygen ions will generate lesser current and ECM will inject more fuel to keep the engine alive and maintaining stoichiometric burn ratio. This is going on continually while you drive.
Generally speaking, each revolution with the throttle closed uses a fairly finite quantity of fuel. As a result, engine braking will cause most vehicles to consume more fuel. Also the point is that the ECM is trying to run as close to a stoichiometric burn ratio as possible and so with the throttle plate closed, that's going to be more fuel per second when the engine turns at a higher number of RPMs…and we all know while we use engine breaking downhill we are at higher rpms at lower gear.
I guess I am clear by now.
I know there are many modern engines that sense it and cut off the fuel and spark in such a situation. But majority of our engines don’t have that feature.

[rohitbagai]

Originally Posted by amu1983

>> if no fuel is injected into cylinders how does an engine keep alive (running)

To the best of my knowledge, no extra fuel goes into the engine, and that's why you get the 'braking'. If there was any fuel going in, the engine would then be powering the wheels. Try this. Press the gas during engine braking, and the car comes to life in a second.

It is still powering. But the difference is ...when the throttle is closed the power is way below the power generated by the thrust of the vehicle translated by the wheels to the fly wheel. As soon as you open throttle it overpower the backward thrust from the wheels and hence accelerate (comes to life...in your words)

[mithun]

The process of starting a diesel engine is less affected by temperature. When a petrol engine is started from cold it needs a lot of fuel to make it run properly. If you do short trips all the time then you'll get far below the fuel economy that you'd normally get. Since emissions are proportional to the amount of fuel consumed, you'll be producing higher pollution levels.

Diesel engines last a lot longer than petrol engines. This is because petrol destroys lubrication and diesel fuel doesn't. Cold start-ups are a real killer for petrol engines because of all that extra petrol needed to start a cold engine.

Diesel engines are ideal for short journeys because their efficiency is almost the same irrespective of whether they are cold/hot.

[typeOnegative]

Originally Posted by adya33

You are right its dangerous.
Thats why I said in my post maximum practically possible gear

Fair enough. I misunderstood. I am sorry.

[santosh.s]

Thanks Rohit for an in depth explanation. However I find it little hard to believe that majority of today's ECMs are relying only on oxygen sensor. I am no expert, but I believe they are much more capable than that, because almost all manufacturers advertise that ECM in their MPFI engines monitor multiple variables. Oxygen sensor is obviously one of them, but typically there are other parameters too, like engine RPM & temperature for example. I can give you at least a couple of familiar examples where I believe ECM controls the amount of fuel when we don't even touch the accelerator pedal (which is equivalent to air intake, as per your explanation)-

1. When car is at rest and engine idling, put it into 1st gear and slowly release the clutch. As RPM drops, fuel supply is automatically increased a bit to compensate and the car would actually start moving (unless the clutch is released too fast, in which case engine stalls). If you press the clutch again, the RPM momentarily shoots above idle and returns to normal again.

2. When A/C cuts in and off, RPM keeps switching between two preprogrammed values (during idling, of course).

3. After a cold start, RPM hovers well above its normal idling value for a while, before returning to normal.

All I am trying to say is that in above cases, many factors other than oxygen sensor also must have been taken into account by ECU.

Originally Posted by rohitbagai

When we take our foot off from the accelerator pedal it restricts the airflow to the engine. So the proportionate oxygen ions are reduced. Lesser oxygen ions will generate lesser current and ECM will inject more fuel to keep the engine alive and maintaining stoichiometric burn ratio.

If I got you right, you meant "less" fuel here, right?

I know there are many modern engines that sense it and cut off the fuel and spark in such a situation. But majority of our engines don’t have that feature.

To make the picture clearer and realistic, can you give a few examples in each case? I am wondering because the "feature" to cut off fuel when accelerator is released and RPM exceeds idling seems like almost a trivial job for the kind of processing any ECM is supposed to do.

[santosh.s]

Originally Posted by mithun

The process of starting a diesel engine is less affected by temperature.

Diesel engines last a lot longer than petrol engines.

really confused.
As per my knowledge, it is difficult to start diesel engines in cold atmosphere. That's why they are equipped with electric heater, but petrols don't need that heater... No? (I am not talking about FE here)

Regarding the 2nd point, I think it should directly translate to lesser maintenance for diesels, but I am sure that it is quite the opposite. Can you please clarify?