Why is using neutral or (freewheeling) bad

Dr.Millah

Hey guys.

I must say i do this quite often, and i regularly hear that its bad and dangerous. But i dont understand why?

Could someone explain what the problem is? and does it have negative effects on the car etc?
Thanks

Jimbo

Not so much negative effects, but going down a hill the engine acts as a brake and controls the speed of you car

Esel

Off the top of my head:

You do not have control, apart from braking and steering. There's a skill to bringing the engine revs up before you put the car in gear again, which is not a suitable procedure in an emergency.

What happens if the engine cuts out while you are coasting?

It's bad for an automatic gearbox for definite.

I'm sure there will be more knowledgeable pointers from some of the mavens here!

Dr.Millah

yea, that is one of the thing i heard, that the breaking capibility of the car is reduced? is this right?

Also, having the car in neutral and having the clutch down, are they different? if you were in gear and had the clutch down would that be the same as having the car in neutral is what im trying to say?

colm_mcm

when freewheeling, you don't get the effect of engine braking (the car slowing you down when you take your foot off the accelerator), and you aren't as in control of the car as if you had the car in gear.

fuel savings aren't worth it either, in fact you could be using more fuel.

Dr.Millah

esel wrote: »

Off the top of my head:
There's a skill to bringing the engine revs up before you put the car in gear again, which is not a suitable procedure in an emergency.

!

I should have mentioned that i rev match before going back into the gear after neutral.

colm_mcm

why bother putting it in neutral?

Dr.Millah

i have alot of use for it when driving as i am a petrol saving freek. mostly its around the city in slow moving traffic, say i needed to move 20 foot foward i would just use the cars power to get maybe 5mph then put it in neutral to freewheel the rest (being on flat or downhill roads of course)

peasant

Most modern engines have a fuel cutoff on engine breaking and are very miserly with the fuel when you only feather the gas.

Idling, on the other hand, is a different proposition.

tuxy

Dr.Millah wrote: »

i have alot of use for it when driving as i am a petrol saving freek. mostly its around the city in slow moving traffic, say i needed to move 20 foot foward i would just use the cars power to get maybe 5mph then put it in neutral to freewheel the rest (being on flat or downhill roads of course)

How much has your mpg incresed since you started doing this?

Dr.Millah

peasant wrote: »

Most modern engines have a fuel cutoff on engine breaking and are very miserly with the fuel when you only feather the gas.

Idling, on the other hand, is a different proposition.

So idling ( in neutral) could burn more than in gear?

peasant

indeed it could

Dr.Millah

tuxy wrote: »

How much has your mpg incresed since you started doing this?

Hey,
im not fully sure as i didnt take note of my mpg before a few months ago.
But at the moment i could get 400miles all in and around cork city, and 90% of trips under 5 miles for 45 litres of petrol. 45 litres of petrol is approx 10gallons so i get around 40mpg.
But my car is only 1000cc so that also contributes to such high mpg. some of my trips i wouldnt ever let my rpm go above 1500 aswell.

Dr.Millah

peasant wrote: »

indeed it could

interesting. I was unaware of that. thanks

Esel

Honda 50 ftw.

ianobrien

Dr.Millah wrote: »

i have alot of use for it when driving as i am a petrol saving freek. mostly its around the city in slow moving traffic, say i needed to move 20 foot foward i would just use the cars power to get maybe 5mph then put it in neutral to freewheel the rest (being on flat or downhill roads of course)

You're not saving fuel. You do know that at zero throttle, with the revs over a certain figure, no fuel is being injected (a figure of 1600 to 1900rpm, depending on car), so by coasting, you're using fuel (idle) while leaving it in gear and above a certain rev, you're using no fuel.

You can feel it kicking in if the revs drop. It's at about 1800rpm in my 2005 Almera.

JHMEG

OP, you need to read up on Hypermiling here:

http://www.cleanmpg.com

It's the art of freewheeling, anticipating traffic etc.

blackbox

Dr.Millah wrote: »

I should have mentioned that i rev match before going back into the gear after neutral.

If an emergency arises that requires you to accelerate to avoid it you will not be able to react as quickly.

congo_90

power steering and braking become less effective. As others have mentioned an idle engine uses more fuel. not to mention the fact you effectivly have no control in terms of speed regulation except gravity and the current speed of the car. you're also putting excess wear on brakes compared to lighter braking needed when the car is in gear

Biro

I have a trip computer on my car, and I use 0.0 litres per 100km's when slowing down in gear with no throttle, and it increases if I clutch in or go to neutral, as the car needs to provide fuel to the engine to keep it from stalling. The momentum does that when in gear. You're saving nothing, you're costing yourself more.
Holding the clutch in for long periods isn't great on the clutch either.
If you are going to the lengths of saving a few millilitres of petrol anyway then you have to ask yourself is this journey really necessary? Also may be time to trade down to save a few more bob if times are that hard.

colm_mcm

congo_90 wrote: »

power steering and braking become less effective.

neither the power steering or braking system become less effective when coasting. steering and brakes more likely to be used due to car gaining momentum on hills though.

Marcus.Aurelius

colm_mcm wrote: »

neither the power steering or braking system become less effective when coasting. steering and brakes more likely to be used due to car gaining momentum on hills though.

I don't think that's correct. Best practice at Institute of Advanced Motorists is to brake using your gears. (for instance, if you engage a gear and leave the acc pedal alone, the car's speed drops anyway, and faster than if you were coasting)

colm_mcm

maoleary wrote: »

I don't think that's correct. Best practice at Institute of Advanced Motorists is to brake using your gears. (for instance, if you engage a gear and leave the acc pedal alone, the car's speed drops anyway, and faster than if you were coasting)

I'm with you, but congo90 seems to suggest that the steering and brakes systems themselves become less effective, rather than the car being harder to control.

*Kol*

peasant wrote: »

Most modern engines have a fuel cutoff on engine breaking and are very miserly with the fuel when you only feather the gas.

Idling, on the other hand, is a different proposition.

Opel Vectra for example. If you coast in gear the fuel is cut off. You can see it on the trip computer the instantaneous fuel consumption goes to 999.9 mpg.

WHITE_P

In addition to not actually saving fuel, you are putting extra ware on your brakes when stopping, engine braking is by far better than having to apply the brakes every time you want to come to a stop. In traffic if you gauge the pace of the traffic around you, you should only have to brake lightly.

As for travelling on open roads etc. (OP: I know you only mention city driving), if you were free wheeling along on the open road you are NOT in proper control of the car.

One of my brothers was doing a job where he was required to have a driver with him, the company used forigne lads, and one of them kept free wheeling around corners on open road, after the third near accident in one day, my brother had enough and asked your man what he was at, (got the usual reply, I no understand, english not good), I told him to take it up with the H&S rep in the company as it was a real risk to his life being in a car with this guy.

tuxy

*Kol* wrote: »

Opel Vectra for example. If you coast in gear the fuel is cut off. You can see it on the trip computer the instantaneous fuel consumption goes to 999.9 mpg.

How do you coast in gear? If you are in gear you would not be coasting.

NiSmO

This post has been deleted.

Pete67

Power steering and brakes are indeed less effective with the engine idling. When hydraulic power steering is installed, it is quite easy to stall an idling engine by turning the steering wheel suddenly, as may be required to avoid a collision. This is due to the sudden load placed on the engine by the power steering pump. The vast majority of modern cars are nearly impossible to steer without the benefit of power steering - just try to steer a car around a corner when it is being pushed if you don't believe this.

On the other hand, with the engine in gear, the combined inertia of the moving car and rotating engine will prevent the engine stalling until the speed over the road is so low that the engine cannot continue to run.

The effectiveness of the cars brakes are greatly boosted by vacuum servo assist. On petrol cars, the vacuum comes from the inlet manifold, and is highest when the engine is being driven at reasonable rpm by the road wheels with the throttle closed. On diesels, the vacuum assist comes from an engine driven pump, again the higher the engine speed the more vacuum is available for braking assistance.

Coasting is never a good idea.

Dr.Millah

Thanks for the informative replys everyone.
Always helpful!

JHMEG

WHITE_P wrote: »

you are putting extra ware on your brakes when stopping, engine braking is by far better

You are putting extra wear on your engine by using it for braking, and brake pads are cheaper to replace.

peasant

JHMEG wrote: »

You are putting extra wear on your engine by using it for braking, and brake pads are cheaper to replace.

Not exactly true. After a long, hot idling period in heavy traffic there's nothing like a good downhill stretch (if you can get it) with a bit of engine breaking to cool things down again.

colm_mcm

peasant wrote: »

Not exactly true. After a long, hot idling period in heavy traffic there's nothing like a good downhill stretch (if you can get it) with a bit of engine breaking to cool things down again.

Engine braking - good
Engine breaking - bad :mad:

JHMEG

peasant wrote: »

Not exactly true. After a long, hot idling period in heavy traffic there's nothing like a good downhill stretch (if you can get it) with a bit of engine breaking to cool things down again.

Or the fan.

peasant

colm_mcm wrote: »

Engine braking - good
Engine breaking - bad :mad:

ooops ..ze spellink again

Pete67

Extra engine wear from engine braking is marginal if it exists at all. The engines moving parts are lubricated. The braking effect comes primarily from the energy absorbed in compressing air and discharging it through the exhaust. An engine can do this almost indefinately without ill effect.

On the other hand, sustained braking using wheel brakes can result in severe overheating of the friction materials in the pads or shoes, can distort or warp discs and drums, and in severe cases can lead to total loss of braking effect. This can happen when to brake fluid is heated to the extent that vapour forms in the wheel cylinders, and is much more likely when the fluid is old and has absorbed a quantity of water from the atmosphere over time. Hence the oft ignored requirement to replace brake fluid periodically.

Anyway, this is getting away from the OPs original question which related to coasting. Apologies for that.

peasant

JHMEG wrote: »

Or the fan.

All the fan does is shovel more air over the hot coolant. I prefer the faster turning water pump for more effective cooling. Faster coolant flow means better dissipation of heat from the entire engine.

Steve

JHMEG wrote: »

You are putting extra wear on your engine by using it for braking, and brake pads are cheaper to replace.

-snip- sorry.

/note to self: read the thread properly.

JHMEG

peasant wrote: »

All the fan does is shovel more air over the hot coolant. I prefer the faster turning water pump for more effective cooling. Faster coolant flow means better dissipation of heat from the entire engine.

The fan blasts cooler air over a radiator that's extremely efficient at dissipating heat. That's why the fan goes off again so quickly.

Faster coolant flow - but only through an efficient radiator - will cool the engine a tad quicker until the thermostat closes (a cold engine is less efficient), then you can circulate the coolant at a million miles an hour and it won't cool the engine.

The fan/radiator/thermostat do a fine job and are adequate.

JHMEG

Pete67 wrote: »

Extra engine wear from engine braking is marginal if it exists at all. The engines moving parts are lubricated. The braking effect comes primarily from the energy absorbed in compressing air and discharging it through the exhaust. An engine can do this almost indefinately without ill effect.

When you are dissipating the kinetic energy of a 1.2 tonne car through the wheels, clutch, transmission and engine then of course things will wear quicker.

Dr.Millah

When ye say 'engine breaking' do you mean the car slowing itself when its in the gear. Or coming down gears to slow the car down, because the latter has bad effects on the transmission.

Lord Nikon

*Kol* wrote: »

Opel Vectra for example. If you coast in gear the fuel is cut off. You can see it on the trip computer the instantaneous fuel consumption goes to 999.9 mpg.

Not a very accurate trip computer then.

I too from time to time try to economise on fuel consumption. Best I can get is 47mpg.

Best place to coast in neutral is going up a slip road off a motorway, I also anticipate traffic too.

Pete67

When you are dissipating the kinetic energy of a 1.2 tonne car through the wheels, clutch, transmission and engine then of course things will wear quicker.

That would be true if the kinetic energy is dissipated by converting it to heat via friction. Thats is not the case here. There is no energy dissipated by the clutch unless it is being slipped. The mechanical efficiency of the gearbox/differential is around 98%, so very little energy is dissipated here either. There is no additional energy dissipated through the wheels, as they are rotating at the same rate whether engine braking is being used or not. That leaves the engine, where energy is dissipated primarily by compression of air, generating heat which is lost to the engine cooling system and the exhaust.

You are far more likely to wear out the clutch thrust bearing by constantly changing in and out of neutral or coasting with the clutch depressed.

galwaytt

coasting means you're a passenger in the vehicle, and not in control of it - on corners you will always understeer more than if under power, and this is an unstable condition, and you have no option to change that condition either easily or readily.

Consider: if you coast on your test, you fail. Coast on your IAM test, you fail.

Engine braking is very effective at slowing you down. Relying on brakes alone means you're braking is solely down to pad performance. By using engine braking you're spreading the torque reaction (slowing) through the whole drivetrain - and no, it doesn't wear it more. Proof of this is the proliferation of brake shops in the US - automatics have poor engine braking, and so pads/discs/shoes are consumed as much as gas........takes as much energy to stop a car as to move it in the first place. Stand at the bottom of Lombard Street in San Francisco, and you can smell the pads.......

Trucks can have air brakes to use in lieu of the road brakes, utilising the huge compression avaialable on big diesels.........

peasant

ermm ..air brakes are only ever an addition to regular brakes, mostly used for maintaining constant speed downhill. You'd have a hard time stopping a truck on airbrakes only.

(Unless you mean the regular brakes on a truck which are operated via compressed air)

JHMEG

Pete67 wrote: »

That would be true if the kinetic energy is dissipated by converting it to heat via friction. Thats is not the case here. There is no energy dissipated by the clutch unless it is being slipped. The mechanical efficiency of the gearbox/differential is around 98%, so very little energy is dissipated here either. There is no additional energy dissipated through the wheels, as they are rotating at the same rate whether engine braking is being used or not. That leaves the engine, where energy is dissipated primarily by compression of air, generating heat which is lost to the engine cooling system and the exhaust.

You are far more likely to wear out the clutch thrust bearing by constantly changing in and out of neutral or coasting with the clutch depressed.

I'm not so sure all that is true. eg, as well as compression there is natural friction in the engine. If there wasn't we wouldn't need oil.

My point it an engine/drivetrain rotating at high speed and under the load of engine braking will wear quicker than one that is idling.

peasant

JHMEG wrote: »

I'm not so sure all that is true. eg, as well as compression there is natural friction in the engine. If there wasn't we wouldn't need oil.

My point it an engine/drivetrain rotating at high speed and under the load of engine braking will wear quicker than one that is idling.

Yeees ...but on engine braking you are missing the explosions

galwaytt

you don't need oil for friction reduction (entirely) - it's tecnnically possible to build an engine that doesn't need it, but using the likes of PTFE films, friction modifiers etc.

However, you need oil as your bearings are hydrodynamic - not mechanical. By dint of that design they are very low friction wise. Besides, they're designed to be at full effect on minimum oil pressure, i.e. idling. Otherwise it'd seize every time you came to a halt..........modern engines are low friction engines. Bikes even more so (slipper pistons)

colm_mcm

If an engine and transmission can handle acceleration, then it can surely handle engine braking.

JHMEG

colm_mcm wrote: »

If an engine and transmission can handle acceleration, then it can surely handle engine braking.

Yes they can, but not forever.

peasant wrote:

Yeees ...but on engine braking you are missing the explosions

True but you have the same force exerted on everything anyway, just from the other end.

galwaytt wrote:

you don't need oil for friction reduction (entirely)

Yes I do. You let me know how you get on with plumbers tape and I'll carry on using oil.;)

kaizersoze

peasant wrote: »

ermm ..air brakes are only ever an addition to regular brakes, mostly used for maintaining constant speed downhill. You'd have a hard time stopping a truck on airbrakes only.

(Unless you mean the regular brakes on a truck which are operated via compressed air)

I presume he/you meant to say exhaust brake.

Capital Slow

ianobrien wrote: »

You're not saving fuel. You do know that at zero throttle, with the revs over a certain figure, no fuel is being injected (a figure of 1600 to 1900rpm, depending on car), so by coasting, you're using fuel (idle) while leaving it in gear and above a certain rev, you're using no fuel.

You can feel it kicking in if the revs drop. It's at about 1800rpm in my 2005 Almera.

I don't understand!

Do you mean your engine is running at 1600 rpm at nothing? That's impossible, the engine needs fuel to keep running, isn't it?

Just say that's true, my other question is, my car is a 97 KIA, how do I know immediately whether my engine has the same feature as your relatively new car apart from trying it out?