Engine Breaking?

The_Dark_Side · 28 Jul 2006 at 00:19

if fuel is cut completely then this must produce the same effect as turning the ignition off then back on with the throttle open right?

thefullcollapse · 28 Jul 2006 at 00:32

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.

i have a life

The_Dark_Side · 28 Jul 2006 at 00:52

Clarkey said:
Hmm I think the motors forum nee to do some serious reading up on engines because it has become apparent that 90% know absolutely nothing.

The_Dark_Side said:
if fuel is cut completely then this must produce the same effect as turning the ignition off then back on with the throttle open right?

no reply?
i was hoping you'd have answered my question by now.

silversurfer · 28 Jul 2006 at 01:45

Thanks for the vid but the car isnt in gear is it, so its slightly different I think.

I use engine braking because it provides the most control over the car. Crashing is more expensive then a bit of a petrol so hopefully I have my priorities right there. On our nissan, any engine braking at any revs or speed shoots the mpg computer upto a maximum 85mpg Ive noticed where as idling or taking the car out of gear would not show a mpg figure at all. I imagine they are roughly equalivant anyway.

Even with engine braking, there is no proper load or torque required so the mix could be run extremely lean. So long as it doesnt detonate its not important as it cant stall.
Turning off the ignition off alltogether with engine in gear would rapidly slow the car I think, so wheres the difference?

The_Dark_Side · 28 Jul 2006 at 01:49

silversurfer said:
Turning off the ignition off alltogether with engine in gear would rapidly slow the car I think, so wheres the difference?

with the ignition off you know there is definitely zero fuel running through the system and the ignition isn't operating at all.
therefore if this method slows you down quicker than merely closing the throttle this means there is SOME fuel getting through when you release the loud pedal, rather than zero fuel movement during both processes.

Clarkey · 28 Jul 2006 at 02:58

lol, thats a fun one for a carbed car

Redline it and turn the ignition off while keeping it at *** till about 2k rpm then turning it back on. KaBOOOOOOOOOOOOOOOOOOOOOOOOOM

chino · 28 Jul 2006 at 09:34

Just a quick note about throttles...

I'm pretty sure that an SI (petrol) engine has a throttle, i.e. it throttles the amount of air, but a CI (diesel) engine does not have a throttle in the strict sense of the word as it controls the amount of fuel being injected.

I'm also pretty sure (thinking back to my IC engine lectures) that on engine braking it is possible that all of the fuel is cut to the engine, hence 0% fuel usage, but it all depends on the ECU and how it is set up. It will definitely use less than coasting though.

Plus on a practical note, you have much more control over the car in gear than out.

NathanE · 28 Jul 2006 at 09:48

chino said:
Plus on a practical note, you have much more control over the car in gear than out.

Another practical note is that it also saves you fuel money and brake pads.

Lashout_UK · 28 Jul 2006 at 10:16

.......but since is when putting extra load on your powertrain - engine, box and clutch worthwhile, when a set of pads is dirt cheap! It's not worth it.

The fuel saving, I can bet you, is so minor that you might as well not bother, either.

PiKe · 28 Jul 2006 at 10:18

Lashout_UK said:
.......but since is when putting extra load on your powertrain - engine, box and clutch worthwhile, when a set of pads is dirt cheap! It's not worth it.

Based on that you shouldn't accelerate then either, just use the forces of mavity to make your car 'go'.

TheOtherPhil · 28 Jul 2006 at 10:22

silversurfer said:
Thanks for the vid but the car isnt in gear is it, so its slightly different I think.

It does exactly the same when driving along, no matter what gear! The ECU doesn't know what gear you're in, it just cuts the fuel on zero throttle and reapplies it when the revs get to 980rpm to maintain idle (in my case) or you press the throttle down again. There is no magic, it really is that simple

Lashout_UK · 28 Jul 2006 at 10:22

No, because the powertrain is there to provide forward motion and designed for it, whereas the brakes are there for you to slow down.

PiKe · 28 Jul 2006 at 10:26

Similar forces being applied, just by the wheels rather than the engine.

chino · 28 Jul 2006 at 10:27

Lashout_UK said:
No, because the powertrain is there to provide forward motion and designed for it, whereas the brakes are there for you to slow down.

What's to say the powertrain isn't also designed with engine breaking in mind?

I'm willing to bet that it is a serious consideration, especially with all the NVH (noise, vibration and harshness) issues which surround the powertrain when power is taken off.

TheOtherPhil · 28 Jul 2006 at 10:36

I think what Lashout_UK is trying to get accross is that you don't go downshifting through the box to slow yourself down and he is right. Yes, take your foot of the throttle and allow the gearbox to slow you (this is called accelerator sense) before finally using the brakes if you need to stop.

When approaching a hazard (which can be anything) like a roundabout, you continue in the same gear as you approached it. Back off the throttle until you are ready to go again and then brake down to the required speed. Only when it is clear to go again do you select the right gear to provide controlled acceleration to negotiate the hazard. This is called block changing and is taught to advanced drivers.

Gears are for going, brakes are for stopping. You won't get a more economical way to drive.

NathanE · 28 Jul 2006 at 10:40

^ Of course. Using just engine braking to stop is idiotic. But there's nothing wrong with it in changing engine speed in anticipation of a corner, junction etc.

Most people will apply a little braking in combination with some engine braking.

Lashout_UK · 28 Jul 2006 at 10:41

Oh I'm sure it's well within limits but my point is that the brakes are there to slow you down so why load up your powertrain for the sake of a 30 quid set of brake pads.

In my eyes I think that engine braking puts unnecessary wear on the powertrain - after all, the brakes are much better at stopping the car than any engine braking could be (unless you want an uncontrolled stop

) and the parts for the brakes are much cheaper than any powertrain componenents.

What's the point in reducing the life of the engine and transmission by doing something it's not intended to be used for - the effects of it could shorten it's life considerably.

Now this isn't saying that I don't do it, I do - where I live has quite a few hills so to save baking the brakes I use engine braking in the one road use that is recommended, for long descents where heavy braking could over power the braking system and overheat the discs and pads, so selecting the correct gear to maintain my descent and avoid this issue

Just my 0.2c.

>edit< TheOtherPhil just beat me to it there

It's even an advanced driving technique that - Not coming down through the gears approaching a stopping position, instead just remaining in gear and selecting neutral or clutch down and first when finally you have come to a halt and using the combination of brakes and that gear's coast-off

chino · 28 Jul 2006 at 10:56

Lashout_UK said:
...What's the point in reducing the life of the engine and transmission by doing something it's not intended to be used for - the effects of it could shorten it's life considerably...

That's the point, it is designed for it.

I'm not saying you should change down through the gears to get maximum engine braking, just that it should be used to help reduce fuel consumption.

In summary...

1. Engine breaking reduces fuel consumption as long as your car is able to reduce fuelling.

2. You are in more control when in the correct gear approaching a junction/obstacle/etc, than when you coast.

3. Powertrains are designed with engine braking in mind.

This advanced driving technique you mention is engine breaking. Slightly badly worded, as it could be taken that you are advocating coasting.

TheOtherPhil · 28 Jul 2006 at 10:59

NathanE said:
^ Of course. Using just engine braking to stop is idiotic. But there's nothing wrong with it in changing engine speed in anticipation of a corner, junction etc.

Correct, we use the term IPSGA

Information
Position
Speed
Gear
Accelleration

Take the information from the road, hazard and the surroundings, Position your vehicle in a suitable place on the road to negotiate the Hazard. Get to the right speed to negotiate the hazard (coast down in the current gear or brake). Only when you are at the right spead do you select the appropriate gear before accelerating away from the hazard.

If you are already at the right speed in anticipation of a bend etc, then select the correct gear to negotiate it

If you have to drop 20 mph, use the brakes....this makes for a faster drive and is cheaper to replace.

saitrix · 28 Jul 2006 at 11:03

I just use the engine braking that I have in the gear I'm already in. I also use my gears for hills so I don't have to ride my brakes.

TheOtherPhil, Damn I couldn't remember IPSGA, then again I'm useless for remembering things like that.