Drive Accord Honda Forums banner
1 - 20 of 36 Posts

·
Registered
Joined
·
59 Posts
Discussion Starter · #1 ·
Either during coasting/engine braking, and if it does shut off, how long till it does?

Owners manual seems to not have anything on this
 

·
Registered
Joined
·
1,320 Posts
There is some website that talks about the VCM system and it seemed to indicate that there was no fuel being used when coasting and breaking. Scroll down to 3. on this link and see if you get the same impression:

http://www.world.honda.com/automobile-technology/VCM/
 

·
Four Doors/Two Pedals
2020 Honda Accord Sport 2.0T 10AT
Joined
·
1,172 Posts
I just looked at that website and it does say that when braking, all cylinders are at idle and the fuel supply is stopped.
 

·
Registered
Joined
·
480 Posts
Yes, if you have your foot off the gas and the transmission gearing is keeping your engine above about 1500 rpm, then the fuel is completely cut off.
 

·
Registered
Joined
·
1,038 Posts
A gasoline internal combustion engine requires at least three things to operate:

1. Air
2. Fuel
3. Spark

If any one of the three are shut off, the engine stops running.

If you are driving down the road, with your engine running, the fuel is being delivered to some, if not all of the cylinders to keep the engine running. How can you tell if the engine is running? Look at the tachometer. If it is above 0, your engine is running. Pretty simple, isn’t it?

Now where you are getting confused is looking at the VCM graphic, which of course only applies to the V6 automatic transmission cars.
This graphic only explains how the VCM system operates, not how the manual transmission V6 coupe, which does not have VCM, operates.

Take a look at the VCM graphic, #3, When does VCM Operate?

Open up the #4 Accelerating to Merge tab. Look at the color of all six pistons, which is Orange, showing all 6 cylinders operating equally.

Now look at #3, Braking to Decelerate. Look athe color of the pistons. 3 are Orange, 3 are Blue, indicating the 3 cylinders are deactivated, and no fuel is flowing to those cylinders, BUT FUEL IS BEING DELIVERED TO THE OTHER CYLINDERS TO KEEP THE ENGINE RUNNING.

At no time while you are driving is the fuel completely shut off to all 6 cylinders at the same time.
 

·
Registered
Joined
·
1,569 Posts
If anything, you get minimal fuel delivery to the cylinders. Just enough to keep some combustion. If you did not have any combustion you would not feel the engine running, and the exhaust would be quiet.

Just take a drive on an open road or parking lot, and while coasting turn kill the engine. I am sure you will notice a change.
 

·
Registered
Joined
·
1,038 Posts
If anything, you get minimal fuel delivery to the cylinders. Just enough to keep some combustion. If you did not have any combustion you would not feel the engine running, and the exhaust would be quiet.

Just take a drive on an open road or parking lot, and while coasting turn kill the engine. I am sure you will notice a change.
No, you will not get "if anything, minimal fuel delivery to the cylinders", you will get normal, i.e., what is programed into the computer for speed and load of the engine at that RPM. Nothing more, nothing less.
 

·
Registered
Joined
·
480 Posts
If you are driving down the road, with your engine running, the fuel is being delivered to some, if not all of the cylinders to keep the engine running. How can you tell if the engine is running? Look at the tachometer. If it is above 0, your engine is running. Pretty simple, isn’t it?
This isn't true. If you're coasting in gear most modern engines will completely shut off the fuel flow. The transmission/gearing keeps your engine spinning and zero fuel is required so the fuel is completely cut until you either hit the gas pedal or the rpms drop below about 1500. As soon has you hit the gas or the rpms drop low enough, the ecu will resume fuel flow. Meanwhile, your tach shows how fast the engine is spinning regardless of whether fuel is flowing or not.
 

·
Registered
Joined
·
916 Posts
This isn't true. If you're coasting in gear most modern engines will completely shut off the fuel flow. The transmission/gearing keeps your engine spinning and zero fuel is required so the fuel is completely cut until you either hit the gas pedal or the rpms drop below about 1500. As soon has you hit the gas or the rpms drop low enough, the ecu will resume fuel flow. Meanwhile, your tach shows how fast the engine is spinning regardless of whether fuel is flowing or not.
This is the correct answer. One way you can tell is if you have a stick shift leave the car in gear as you are slowing down to a stop. Just before you come to a stop you will feel a slight surge. This is the fuel going back to the engine as you fall below the min rpm for fuel cutoff.

The real question is does coasting in gear with no fuel use more fuel than coasting in neutral with the engine idling? Does the longer coast make up for the greater fuel use. I did some calcs a while back and it does seem that coasting in neutral uses less fuel as the approx 300 mpg you are getting is pretty minimal use.

Little off topic but rule of thumb for mpg in neutral is 4 times your speed. I have seen 300 mpg plenty of times coasting down hill with my scanguage.
 

·
Registered
Joined
·
916 Posts
BTW you brake a car with the middle pedal (or left). You break one with a sledgehammer. ;)
 

·
Registered
Joined
·
480 Posts
This is the correct answer. One way you can tell is if you have a stick shift leave the car in gear as you are slowing down to a stop. Just before you come to a stop you will feel a slight surge. This is the fuel going back to the engine as you fall below the min rpm for fuel cutoff.

The real question is does coasting in gear with no fuel use more fuel than coasting in neutral with the engine idling? Does the longer coast make up for the greater fuel use. I did some calcs a while back and it does seem that coasting in neutral uses less fuel as the approx 300 mpg you are getting is pretty minimal use.

Little off topic but rule of thumb for mpg in neutral is 4 times your speed. I have seen 300 mpg plenty of times coasting down hill with my scanguage.
Yeah I agree that coasting in neutral is usually more beneficial since you can coast so much further without slowing down as much. My rule is that if I need to slow down anyway (coasting to a red light, etc.) then I will leave the car in a high gear to benefit from the fuel cut. But otherwise neutral is the way to go. Or a combo of both depending on the situation.
 

·
Tampa FL
Joined
·
373 Posts
I am not sure about automatics; but whit manual transmissions; you can turn off the engine while coasting (great when coming down a hill) and as long as you are still in gear everything still works; so if the fuel injection shuts off during coasting, it is the same thing, just my 0.02
 

·
Corvalis TTX
Joined
·
4,609 Posts
I can't speak for the 9th gen Accords with DI, but:

- I have been told by Honda engineers that my 8th gen V6 will shut off fuel completely when coasting and certain other conditions are met

- I have seen the engine management code for some Ford and Jaguar cars and I can confirm that they absolutely do shut off fuel when coasting when certain other conditions (road speed, the amount of time the throttle has been closed, engine temperature, gear, RPM, etc) are met. Fuel will not be shut off when coasting in neutral or with the clutch pressed in
 

·
Even My Mower Is a Honda!
Joined
·
4,339 Posts
This isn't true. If you're coasting in gear most modern engines will completely shut off the fuel flow. The transmission/gearing keeps your engine spinning and zero fuel is required so the fuel is completely cut until you either hit the gas pedal or the rpms drop below about 1500. As soon has you hit the gas or the rpms drop low enough, the ecu will resume fuel flow. Meanwhile, your tach shows how fast the engine is spinning regardless of whether fuel is flowing or not.
This is the correct answer. One way you can tell is if you have a stick shift leave the car in gear as you are slowing down to a stop. Just before you come to a stop you will feel a slight surge. This is the fuel going back to the engine as you fall below the min rpm for fuel cutoff.

The real question is does coasting in gear with no fuel use more fuel than coasting in neutral with the engine idling? Does the longer coast make up for the greater fuel use. I did some calcs a while back and it does seem that coasting in neutral uses less fuel as the approx 300 mpg you are getting is pretty minimal use.

Little off topic but rule of thumb for mpg in neutral is 4 times your speed. I have seen 300 mpg plenty of times coasting down hill with my scanguage.
This ^^^

Most cars have been doing it since OBD-II was introduced in 1996.

Jay
 

·
Corvalis TTX
Joined
·
4,609 Posts

·
Super Moderator
Joined
·
7,853 Posts
A gasoline internal combustion engine requires at least three things to operate:

1. Air
2. Fuel
3. Spark

If any one of the three are shut off, the engine stops running.

If you are driving down the road, with your engine running, the fuel is being delivered to some, if not all of the cylinders to keep the engine running. How can you tell if the engine is running? Look at the tachometer. If it is above 0, your engine is running. Pretty simple, isn’t it?

Now where you are getting confused is looking at the VCM graphic, which of course only applies to the V6 automatic transmission cars.
This graphic only explains how the VCM system operates, not how the manual transmission V6 coupe, which does not have VCM, operates.

Take a look at the VCM graphic, #3, When does VCM Operate?

Open up the #4 Accelerating to Merge tab. Look at the color of all six pistons, which is Orange, showing all 6 cylinders operating equally.

Now look at #3, Braking to Decelerate. Look athe color of the pistons. 3 are Orange, 3 are Blue, indicating the 3 cylinders are deactivated, and no fuel is flowing to those cylinders, BUT FUEL IS BEING DELIVERED TO THE OTHER CYLINDERS TO KEEP THE ENGINE RUNNING.

At no time while you are driving is the fuel completely shut off to all 6 cylinders at the same time.
I wish I can find an explicit document to show all the injectors do shut off given the correct set of conditions, this has even been confirmed for as early as 7th Gen Accords by FredSVT member who is a master Honda technician.

The VCM is used while the car is still maintaining a certain speed by shutting down 3 of the 6 injectors, not when engine is coasting, i.e. in gear, no throttle position input etc.

If anything, you get minimal fuel delivery to the cylinders. Just enough to keep some combustion. If you did not have any combustion you would not feel the engine running, and the exhaust would be quiet.

Just take a drive on an open road or parking lot, and while coasting turn kill the engine. I am sure you will notice a change.
The amount of fuel required to maintain optimal combustion cannot be changed, the fuel control ECM has to allow proper amount of fuel to be sprayed into the cylinder to allow proper combustion under load, if only "minimal fuel" is allowed at higher RPM, then you would encounter a lean burn condition, i.e. detonation to cause damage to engine components.

This isn't true. If you're coasting in gear most modern engines will completely shut off the fuel flow. The transmission/gearing keeps your engine spinning and zero fuel is required so the fuel is completely cut until you either hit the gas pedal or the rpms drop below about 1500. As soon has you hit the gas or the rpms drop low enough, the ecu will resume fuel flow. Meanwhile, your tach shows how fast the engine is spinning regardless of whether fuel is flowing or not.
Correct in every count!! :thmsup: Especially the RPM on the tachometer, it only shows how many revolutions, not if combustion is taking place.

This is the correct answer. One way you can tell is if you have a stick shift leave the car in gear as you are slowing down to a stop. Just before you come to a stop you will feel a slight surge. This is the fuel going back to the engine as you fall below the min rpm for fuel cutoff.

The real question is does coasting in gear with no fuel use more fuel than coasting in neutral with the engine idling? Does the longer coast make up for the greater fuel use. I did some calcs a while back and it does seem that coasting in neutral uses less fuel as the approx 300 mpg you are getting is pretty minimal use.

Little off topic but rule of thumb for mpg in neutral is 4 times your speed. I have seen 300 mpg plenty of times coasting down hill with my scanguage.
BTW you brake a car with the middle pedal (or left). You break one with a sledgehammer. ;)
:notworthy :lmao:

I am not sure about automatics; but whit manual transmissions; you can turn off the engine while coasting (great when coming down a hill) and as long as you are still in gear everything still works; so if the fuel injection shuts off during coasting, it is the same thing, just my 0.02
Pretty much, power steering, air conditioning, alternator will all still work as long as they are driven, even while coasting, even the brake booster should work since it just requires vacuum from the engine as long as the engine is turning regardless if it's coasting while in gear without any fuel being sent through the injectors.
 

·
Car so nice, bought twice
Joined
·
4,062 Posts
I can say with 100% certainty that on the 8G I4 no fuel is used (almost always) when coasting above a certain speed (probably RPM / gear related). If you know what you're looking for, you can often feel the car "surge" just a tiny bit when ignition starts again. My understanding is that the ecu knows where the camshaft is, so it knows the exact moment when to restart ignition to not damage anything.

I've never tested coasting in neutral, but I assume that the .3 gph used in N isn't recovered by the longer coast distance (no drag on the wheels from the engine/tranny/belt driven accessoris).
 

·
Corvalis TTX
Joined
·
4,609 Posts
I can say with 100% certainty that on the 8G I4 no fuel is used (almost always) when coasting above a certain speed (probably RPM / gear related). If you know what you're looking for, you can often feel the car "surge" just a tiny bit when ignition starts again. My understanding is that the ecu knows where the camshaft is, so it knows the exact moment when to restart ignition to not damage anything.

I've never tested coasting in neutral, but I assume that the .3 gph used in N isn't recovered by the longer coast distance (no drag on the wheels from the engine/tranny/belt driven accessoris).
My understanding is that they don't cut off the fuel when coasting in neutral because the engine will stop turning over and all engine driven accessories will no longer work.
 

·
Registered
2013 Touring 6 spd MT
Joined
·
264 Posts
It is hazardous to coast on neutral as you are losing vacuum support for your brakes, just in case you need them.
 

·
Super Moderator
Joined
·
7,853 Posts
It is hazardous to coast on neutral as you are losing vacuum support for your brakes, just in case you need them.
Are you talking about coasting with engine turned off? If so, yes, the vacuum will eventually bled out from the brake vacuum booster, however this will take a few brake applications, the vacuum in the booster won't just suddenly disappear as soon as engine is turned off.

Although you will lose power steering as soon as engine is turned off while in neutral.

But if you are talking about coasting with engine still running, even in idle, then vacuum is continued to be produced.
 
1 - 20 of 36 Posts
Top