Engine Breaking?

milgo · 21 Jul 2006 at 19:10

There must be fuel being used. When you press the accelerator you are just pumping in more fuel, if you were running along and there was no petrol surely the pistons would just melt?

Enfield · 21 Jul 2006 at 19:11

Engine braking uses more fuel seeing as you increase the revs when you down change.

And you mean braking not breaking.

paradigm · 21 Jul 2006 at 19:13

Enfield said:
Engine braking uses more fuel seeing as you increase the revs when you down change.

And you mean braking not breaking.

Not on a modern car it doesn't. The ECU is clever enough to work out that it doesn't need fuel to turn the engine, FFS read the thread.

milgo said:
There must be fuel being used. When you press the accelerator you are just pumping in more fuel, if you were running along and there was no petrol surely the pistons would just melt?

No, because its not taking in air either.

PiKe · 21 Jul 2006 at 19:14

Enfield said:
Engine braking uses more fuel seeing as you increase the revs when you down change.

And you mean braking not breaking.

LOL no it doesnt.

Rev's have no relation to fuel, the rpm increases because you've changed down and theres more force turning the engine.

L0rdMike · 21 Jul 2006 at 19:14

Enfield said:
Engine braking uses more fuel seeing as you increase the revs when you down change.

And you mean braking not breaking.

How though?

The accelerator pedal controls airflow and with out the pedal down you wont be getting a lot.

PiKe · 21 Jul 2006 at 19:16

L0rdMike said:
The accelerator pedal controls airflow and with out the pedal down you wont be getting a lot.

Seeing as this appears to be a newbie's 101 in the workings of an engine...

Not much air = not much fuel.

lmfy2k · 21 Jul 2006 at 19:16

Jeremy Clarkson said on one of the top gear episodes that when you are approaching some traffic lights and you let the engine brake i.e leave it in what gear and coast to the traffic light line, (remembering to press the clutch before engine stalls!)

that time it was coasting cost you £0 in petrol!

i heard him say it, cant remember what episode it was on though

milgo · 21 Jul 2006 at 19:16

But at idle the throttle body butterfly is shut and air is still drawn in isn't it?

paradigm · 21 Jul 2006 at 19:17

milgo said:
But at idle the throttle body butterfly is shut and air is still drawn in isn't it?

No, because without the throttle being depressed, there is no openings for ANYTHING to get into the combustion chamber, not just fuel.

saitrix · 21 Jul 2006 at 19:18

Enfield said:
Engine braking uses more fuel seeing as you increase the revs when you down change.

And you mean braking not breaking.

No it does not. You have momentum to keep the engine turning over so no fuel needs to be sucked in to keep the engine turning over. As soon as you put your clutch in it has to use the usual idle setting to keep it ticking over.

teaboy5 · 21 Jul 2006 at 19:21

thefullcollapse said:
yes but school is only 3 miles away

You live 3 miles away from school? get out of the dam car and walk you lazy sod. Should save you a few bob. And £100 a month doesn't seem to be a lot i spend around 250 myself.

Enfield · 21 Jul 2006 at 19:24

Oh right wicked.

I always engine brake, bloody car still uses lots of petrol though!

milgo · 21 Jul 2006 at 19:25

Hmmm, but you need oxygen for the combustion so how does it tick over if nothing is getting in?

Joe T · 21 Jul 2006 at 19:26

Also remember the difference between engine braking and clutch braking. Changing down puts a lot of pressure on the clutch - you are a effectively braking with the clutch. Once you are in gear, you are then using engine braking.

saitrix · 21 Jul 2006 at 19:27

milgo said:
Hmmm, but you need oxygen for the combustion so how does it tick over if nothing is getting in?

Pure momentum of the wheels turning the crankshaft over.

PiKe · 21 Jul 2006 at 19:27

How the frig do you spend £100 a month when you only drive 6 miles a day then?

£36 lasts me a whole month commuting to and from work and popping out to see friends.

saitrix · 21 Jul 2006 at 19:29

PiKe said:
How the frig do you spend £100 a month when you only drive 6 miles a day then?

£36 lasts me a whole month commuting to and from work and popping out to see friends.

Because he don't just do that journey.

On a side note I use to drive to college 1 mile.

Still got through 25 litres a week due to all the country road driving.

NathanE · 21 Jul 2006 at 21:41

Shocking to see so many people clearly not having the faintest idea of how an engine works :eek:

IMO this stuff should be examinable during a driving test!

I bet the same people think the "rev' limiter" can magically dissipate kinetic engine too

So go easy with that engine braking guys

Smiley Man · 21 Jul 2006 at 21:41

milgo said:
Hmmm, but you need oxygen for the combustion so how does it tick over if nothing is getting in?

once the revs drop to idle the engine will revert to a tickover setting and allow fuel and air in to control the engines idle speed, regardless of throttle position, the car will then idle along or stall

(if clutch is up)

similar to when tickover is higher when you first start the car, the auto-choke keeps the fuel/air mix rich until the engine has warmed up

MrMatteh · 21 Jul 2006 at 22:43

lmfy2k said:
Jeremy Clarkson said on one of the top gear episodes that when you are approaching some traffic lights and you let the engine brake i.e leave it in what gear and coast to the traffic light line, (remembering to press the clutch before engine stalls!)

That was when he drove the diesel A8 up to edinburgh and back

i recall him saying that if you clutch in and coast, you use fuel as the engine needs feul to turn over. if you use engine braking, modern ECUs use no fuel

not sure if this is diesel specific or not.

In mine, if im doing 20/30mph and i clutch in and coast up to the line, my engine will keep to about 2krpm, as its expecting me to change into a new gear. As soon as i stop, it goes down to idle.

So, clutch in coasting uses more petrol than engine braking i would say (for my car, clearly)