Free-wheeling

@ all....I'm referring to your latest standard fuel injected vehicles.

I have seen several people mention the onboard monitor for the fuel, and I have driven several vehicles with onboard monitors (all 2010 models and above). When going downhill at 100 I observe the monitor drops drastically (on average per 100km over the tank cycle) than compared to just letting the car go in 5th gear without accelerating. Now if many posters here were of the opinion of using the onboard computer, and I have tested this and the results are against this whole argument, then which theory is right.....a mechanic or the onboard system (considering that each time you fill up the vehicle the onboard system resets the fuel consumption therefore you can see exactly per tank what you are getting....owe by the way I have also concluded from the onboard system that bp gives best mileage for some reason).
 
snail said:
@ all....I'm referring to your latest standard fuel injected vehicles.

I have seen several people mention the onboard monitor for the fuel, and I have driven several vehicles with onboard monitors (all 2010 models and above). When going downhill at 100 I observe the monitor drops drastically (on average per 100km over the tank cycle) than compared to just letting the car go in 5th gear without accelerating. Now if many posters here were of the opinion of using the onboard computer, and I have tested this and the results are against this whole argument, then which theory is right.....a mechanic or the onboard system (considering that each time you fill up the vehicle the onboard system resets the fuel consumption therefore you can see exactly per tank what you are getting....owe by the way I have also concluded from the onboard system that bp gives best mileage for some reason).
Click to expand...

You cannot make a judgement by how much you perceive the average consumption readout dropping; if your car doesn't have a current consumption readout, your particular OBC is useless in this instance. Regardless, the mechanic doesn't know what he's talking about* as cutting the fuel supply is a verifiable function of modern fuel injection & and an engine obviously needs a supply of fuel to remain at idle when disconnected from the drivetrain. Read Gnome's post at #39 again slowly - it's a good one.

*The mechanic referenced was talking about engine braking and consequent wear; he's still wrong, but it's not related to consumption as such.
 
Just to add a bit to the discussion. I've logged the injector timing, RPM and throttle position of a '98 Polo 1.6 (does definitely not have the most sophisticated engine management system around) and when coasting in gear, the time that the injectors spent open was between 75-80% less than when the car is idling. This means that the car uses less fuel when coasting in gear than in neutral.
 
With regards to fuel consumption:
You use less fuel going downhill in gear than in neutral in fuel injected vehicles since the duty cycle to the injectors is cut by a huge amount in the process. Extra fuel isn't required in order to drive the internal components of the engine around as would be the case in neutral (engine idling under own power).

With regards to drive train wear:
Yes, the rotational forces are reversed in the drive train but they should not cause any damage.

1. The rotational forces on the cambelt are exactly the same since the crank shaft is driving the belt in the same direction. The rotational forces are not magically reversed. The cambelt will not run slack or any nonsense like that.

2. The rotational forces are reversed in the gearbox but most modern gearboxes use helical cut gears (except reverse which is normally a noisy spur gear) which operate exactly the same in either direction. The difference is that the end thrust on the shafts in the gearbox are reversed but gearboxes are designed to handle end thrust on shafts in both directions.

3. The thrust on the pinion gear and shaft in the diff is also reversed but as above, differentials are designed to handle this.

With all of the above consider that reving the engine to 5000 RPM and dropping the clutch produces HUGE forces on the drive train (which it can handle fine in most cases) so some mild reverse forces experienced during engine breaking is going to do nothing to the drive train. Drive trains normally have large abuse tolerances built into them.
 
Last edited:
Some times it would be nice to be able to give posts ratings. ^ that post deserves a 5 star rating.
 
Last edited:
i see everyone here is saying in fuel injection cars coasting in neutral costs more money than staying in gear which from the explanations above i can understand.

Now my car is an ancient golf with a carb, what happens in terms of cars with carbs? Is coasting without the gear in better than keeping it in gear and using no throttle?
 
m4dh4tt3r said:
i see everyone here is saying in fuel injection cars coasting in neutral costs more money than staying in gear which from the explanations above i can understand.

Now my car is an ancient golf with a carb, what happens in terms of cars with carbs? Is coasting without the gear in better than keeping it in gear and using no throttle?
Click to expand...

The carb might not cut the fuel supply when down gearing but you are still using those revolutions to run the alternator which supplies power to the electronics. If you put the car in neutral while decelerating it will use the battery more which in turn needs to be charged when accelerating again.
 
I read this, and initially I thought the whole "stay in gear and take your foot off the gas instead of putting it in neutral" was a load of BS, but I tested it and true as cookies, when I leave my car in gear the on-board computer reports 0l/100km, and the moment I put it in neutral it changes to anything between 0.2 and 0.4l/100km.

Fascinating!
 
TheGuy said:
The carb might not cut the fuel supply when down gearing but you are still using those revolutions to run the alternator which supplies power to the electronics. If you put the car in neutral while decelerating it will use the battery more which in turn needs to be charged when accelerating again.
Click to expand...

ermmm... no, the alternator should still provide enough current to run the car while it's idling.

The carb probably does not cut off the fuel properly though. I'd still do the in-gear thing though, purely from a control and wear-and-tear-on-brakes point of view.
 
m4dh4tt3r said:
Now my car is an ancient golf with a carb, what happens in terms of cars with carbs? Is coasting without the gear in better than keeping it in gear and using no throttle?
Click to expand...

Carb's don't cut the fuel supply when under engine breaking like fuel injection systems do.

The butterfly position in the throttle body will be at the same position as at idle but the engine revs will be higher due to the drive train driving the engine.
This means that there is more vacuum in the throttle body which in turn increases the amount of fuel being drawn into the engine.
The result is often some nice popping or backfiring noises from the exhaust on a steep downhill.
That is unburnt fuel igniting in the exhaust. Very inefficient. :)

However it also depends on the type of carb being used. There are lots of variations in design and implementation.
 
You must log in or register to reply here.
Back
Top
Sign up to the MyBroadband newsletter
X