Brakes to slow, gears to go! The only reason racers downshift the way they do is so that they are in the right gear to accelerate hard out of the next corner.

Double declutching – clutch down, gears into neutral, clutch up, blip the throttle, clutch down, into next gear, accelerate.

Totally poinless on a synchro mesh gearbox. Its only usefull on a crash box, the clutch up in neutral blip the throttle bit gets the lay shaft ( i think) up to the same speed as the main shaft so that the gears mesh smoothly.

I look at braking as a waste of energy, you’re converting the fuel you’ve just used to accelerate into heat at the brake pads. Accelerate less, look ahead and match your speed to the conditions. If you see the traffic slowing up ahead, ease off the throttle, so many people seem to tear up to a red traffic light and slam the brakes on at the last minute.

You have to brake in my car but then its an auto plus its also a diesel and they have next to no engine braking when lifting off as they don’t generate a vaccum on the intake like a petrol engine. This is why you will catch up with a petrol car who has lifted off even though your are not pressing a pedal.

What utter tosh, who told you that? Like for like, a diesel engine produces far more engine braking, due to the much higher compression ratio. The reason you have to brake in your car is all to do with the autobox, not the engine.

What utter tosh, who told you that? Like for like, a diesel engine produces far more engine braking, due to the much higher compression ratio. The reason you have to brake in your car is all to do with the autobox, not the engine.

Errrr, no? Sory t omake you sound a bit thick but think about what the compression ratio is
a) there to achieve
b) how it is achieved
c) where all the energy used compressing the air goes when the piston goes down again.

Part of the reasons why diesel is so effiient is the engine doesnt have to suck air through a restriction, it always gets a full cylinder (or more in a turbo) of air. When you close the ‘throttle’, which doesnt realy exist, there isnt anything to throttle on a diesel, all it does is stop the fuel. The engine therefore takes a full charge or air, squashes it and recovers most of that energy when it expands again.

A petrol engine when the throttle is closes pulls on a prety high vacum (the throttle has closed off the air supply) so it does a lot of work drawing anyth air in, this warms up so the energy recovered sucking the piston up again is less than that used to draw it into the cylinder.

To get arroudn this some old diesels have whats called a jake brake which raises the decompression cam at th top of the compression stroke on the on a diesel to let the air out and stop the energy being recovered. These were banned as they sound like a machine gun.

Slight hijack, but, when you’ve actually stopped (and I’m addressing the seeming majority of drivers here) could you release the brakes and let the lights go out. Put the handbrake on if you are going to be stopped for oooh, I don’t know, more than a second. They are amazingly dazzling up close in the dark, especially on a crowded dual carriageway, and mask the brake lights of others who are still slowing.

Totally poinless on a synchro mesh gearbox. Its only usefull on a crash box, the clutch up in neutral blip the throttle bit gets the lay shaft ( i think) up to the same speed as the main shaft so that the gears mesh smoothly.

Almost, its the input shaft that needs spinning up to speed again (the bit that’s atached to the clutch.

Imagine it in 3 parts, engine, gearbox input, gearbox output. the output always spins at the speed of the rear wheel, the engine is the engine and the input shaft conects the engine and the gearbox, the gearlever connects the input to the output, you could infact achieve a pretty similar effect by bolting the gearbox straight to the engine and putting the clutch between the box and the axle, but it makes sense doing it the other way as the flyhweel the cluch pushes against is on the back of the engine.

Double clutching does nothing more than wear out the clutch reslease bearing on a modern car.

A friend once drove from Knaresborough to York and back (about 28 miles) with no brakes.

And I mean no shoes on the drums at all. All round.

He was a silly boy but managed it.

The engine therefore takes a full charge or air, squashes it and recovers most of that energy when it expands again

I do not think that is true. There is certainly a lot of engine braking on every diesel I have driven. More than in petrols at similar revs I would say.

I was taught to use engine braking (change down the gears to slow down)as you car is in more control.

Since then (5 years) I have always used engine braking. It always used to really annoy me when I was in the car with the mrs and she used to not use the gears, on the brakes all the way, then slam it into 1st whilst the car is still moving 🙄

Even wiki agrees with me

http://en.wikipedia.org/wiki/Jake_brake

Although it does imply that they might be fitted to engines now so maybe modern diesels do have them.

On the original topic I leave it in whatever gear I’m in and coast allong untill I have to use the brake pedal to stop. No need to shift down unless it’s a big hill and you need to be in 1st/2nd to keep the speed under controll.

well, given that i failed my first driving test attempt for accruing 5 minors for depressing the clutch pedal too early when braking i now use the transmission to brake all the time in conjuction with using the brake pedal.

Totally poinless on a synchro mesh gearbox. Its only usefull on a crash box, the clutch up in neutral blip the throttle bit gets the lay shaft ( i think) up to the same speed as the main shaft so that the gears mesh smoothly.

How about blipping the throttle on a normal downshift to match revs? I’ve done it for the past few years to make progress smoother. Is that bad for the clutch/gearbox/engine?

Tinas – if I am in first or second in my car and I lift off at higher revs, I get jerked forwards because of the amount of engine braking. There’s no machine gun sound. All the diesels I have driven from old 80s designs to modern CRs have done the same thing.

Your explanation sounds very reasonable but it doens’t seem to match experimentation.

It’ll depend on your age and what you are driving/riding.

I was taught (car) to slow down using brakes and then select the correct gear needed.

I was also taught (m/c) to be in the correct gear at all times.

I still do both, although auto in a car for years now.

Maybe turbo’s have a similar effect, once the fuel stops and the turbo stops spining it would act as a restriction like a petrol engines throttle?

well, given that i failed my first driving test attempt for accruing 5 minors for depressing the clutch pedal too early when braking i now use the transmission to brake all the time in conjuction with using the brake pedal.

Yeah they fault you if you cruise too far with the clutch down, as you are not in proper control of the vehicle.

Doesn’t mean you have to transmission brake though.

Re: age.

I learnt to drive in, oh, 1990-91 maybe? And I was taught to block change down – ie, brake to scrub speed, and dip the clutch at the last moment before it ‘chugs’ to stop it stalling, going straight from 4th/5th etc to 1st/2nd. I think it was a relatively new thing at that time.

In practice, I’ll slow on the brakes but step down through some gears (maybe 6-3-2), not at high revs to transmission-brake, but at moderate RPM in readiness for a change in conditions such as traffic lights changing.

given that i failed my first driving test attempt for accruing 5 minors for depressing the clutch pedal too early

That’s called coasting, that’s different. You are (arguably) not in full control of the vehicle. You don’t have to slam it in second at 50mph in order just to keep the engine engaged.

Those of us taught to drive years ago were conditioned to move our way up and down the gear box systematically – 4, 3, 2 and 1. But that’s because vehicles in those days had inferior brakes and we needed to bring the cars to a halt by using the drag of the transmission.

These days, we recommend “block changing”. Use the brakes to slow down, then, when the speed is right, choose the gear that is best for that speed. So you may move from fourth, say, straight to second, after you have finished braking. Brake pads are cheaper to replace than clutches. You can also block change to move up, skipping intermediate gears. So remember “gears to go, brakes to slow”.

So say the Institute of Advanced Motorists. Who are probably wrong if we ask the double declutching TJ 🙄

Tinas – perhaps, yes. I’d think that the restriction of the turbo’s exhaust turbine would be much more though.

There’s also possibily something thermodynamic going on – high compression would heat up the intake air a lot, and then some of that head ends up in the cooling system so you get less back on the expansion stroke. Double the compression would result in much more heat leaving the intake charge.

Tinas. Interesting theory. But unfortunately, nonsense. Maybe it would work on a conveyer belt? The problem with your theory is;
A) under engine braking conditions little/no combustion is taking place, so no ‘energy recovery’, in fact greater energy required to turn engine compared to a low compression petrol engine, which is obtained from the kinetic energy of the vehicle via the drive train. Just try turning a diesel engine Over by hand compared to a petrol one.
B) the main reason that diesels Seem more efficient in comparison to petrols is that diesel as a fuel is a heavier and therefor more calorifically (sp?) dense fuel. If we measured fuel by mass rather than by volume it would be a different story.

All engines are basically compressors, powered by the ignition stroke. When that ignition isn’t happening, either when turning over in a no fuel state or on the over run in a low fuel state, the resistance (engine braking) is proportional to the compression ratio. Try driving down a 1:1 slope utilising only engine breaking in a diesel vs a petrol landy and you will know I am right.

Source: many years of messing around with both petrol and diesel engines, mainly in landrovers.

Sorry to make you feel a bit thick like, but every day is a school day…

the main reason that diesels Seem more efficient in comparison to petrols is that diesel as a fuel is a heavier and therefor more calorifically (sp?) dense fuel. If we measured fuel by mass rather than by volume it would be a different story.

No it wouldn’t, not by much anyway, given a high enough compression ratio you could build a ‘diesel’ to run on petrol and it would still beet the normal petrole engine.

under engine braking conditions little/no combustion is taking place, so no ‘energy recovery’, in fact greater energy required to turn engine compared to a low compression petrol engine, which is obtained from the kinetic energy of the vehicle via the drive train. Just try turning a diesel engine Over by hand compared to a petrol one.

Yes diesels have higher compression, but that compression then pushes the piston down the far side, otherwise air sprung forks would never rebound in your theory.

All engines are basically compressors,

No they’re not, a compressor would open the inlet valve on the downstroke, then the exhaust valve on the upstroke, thus it’s being forced to do work on the air on both strokes. In an engine the compression and combustion strokes take place in a sealed cylinder, so in a diesel it recoveres (a large proportion of) the energy contained in the compressed gas, the inlet and exhaust stokes are comapratively work free. In a petrol engine the engine braking is done by the inlet stroke sucking against he manifold vacum which is sealed by the throttle butterflys being a tight fit with the manifiold.

Try driving down a 1:1 slope utilising only engine breaking in a diesel vs a petrol landy and you will know I am right.

Source: many years of messing around with both petrol and diesel engines, mainly in landrovers.

source: designing stuff with much bigger pressures than a diesel engine for a living.

My focus quite happily crawls down 1in3’s in 1st.

Engine braking is all braking effects from the engine. Basically any drag is a braking effect. I agree that your explanation of the intake restriction and its effects. However, there are many other effects. First, a petrol engine
is still burning fuel. The amount of fuel and air in the cylinder is still close to ideal. Point being that it still produces power. The engine does breath quite a bit still. Sure, a diesel uses some fuel too but it is extremely small b/c the inj. pump goes to idle settings off throttle. Another point is that the compression of the air on the up stroke and expansion (air spring) is not adiabatic and is a function of compression ratio. Hence, higher compression does add a significant braking effect. Remember we are almost talking 3x here. Diesels take quite a bit of energy to keep spinning.

What’s your focus got to do with it? Comparing like for like, 2.5 diesel (turbo or otherwise, doesn’t seem to matter) vs 2.5 petrol in a vehicle of the same weight, with the same gear ratio with the same flippin engine block, the diesel has WAY more engine braking. Real life bud, sorry.

From an aussie 4×4 site, this seems to sum it up;

Engine Braking
Another distinct advantage of the diesel engine is its engine braking. The design of the diesel engine means that it sucks in air unrestricted and the amount of diesel fuel injected determines how much power the engine makes. A petrol engine requires a constant fuel/air ratio so the amount of air drawn in by the engine is regulated by an inlet buterfly in ratio to the fuel being used. What this means is that at idle, a diesel engine still draws in a large amount of air (RPM x engine capacity) while a petrol has the butterfly almost closed and there is very little air being drawn in.

When using  engine compression for downhill braking and  factoring in the compression ratio of 20:1 for a diesel and only 9:1 for a petrol, the diesel engine offers significantly greater resistance to an increase in engine RPM.  The diesel engine offers significant resistance as large quantities of air are still being drawn in through the unrestricted air intake and being compressed to 20:1. The petrol engine has the air intake butterfly closed during downhill braking and so only minimal quantities of air is being drawn in and then, its only being compressed to 9:1. This is why a petrol engine tends to “run away” compared to a diesel.

Also, it’s a bit daft to state that the engine braking effect from a petrol engine is due to the vacuum formed in the inlet manifold behind a closed throttle. Imagine how strong the throttle butterfly would have to be built if it was going to accept the braking forces of a large vehicle slowing from a significant velocity. My money is on compression ratio, sorry.

All engines are basically compressors,

No they’re not, a compressor would open the inlet valve on the downstroke, then the exhaust valve on the upstroke, thus it’s being forced to do work on the air on both strokes. In an engine the compression and combustion strokes take place in a sealed cylinder, so in a diesel it recoveres (a large proportion of) the energy contained in the compressed gas, the inlet and exhaust stokes are comapratively work free. In a petrol engine the engine braking is done by the inlet stroke sucking against he manifold vacum which is sealed by the throttle butterflys being a tight fit with the manifiold.

I didn’t say they were perfect compressors, I said they were basically compressors. If you were to drive an engine via its output shaft and connect a pressure vessel to the exhaust, it would compress air. Not as efficiently as a two stroke compressor, granted, due to two strokes being redundant.

source: designing stuff with much bigger pressures than a diesel engine for a living.

Well lets hope you understand that stuff better then… 😯

Engine overrun.

How about blipping the throttle on a normal downshift to match revs? I’ve done it for the past few years to make progress smoother. Is that bad for the clutch/gearbox/engine?

I do that on my Ducati simply because it sounds awesome! Does’t harm the engine or box but will use more fuel.
I’ve done a few trackdays though and i found i was a lot faster when i stuck to the gears to go brakes to slow idea. Most racing bikes will have a slipper clutch these days which allows you to change down as fast as you want without worrying about blipping the throttle or locking the back wheel. All they are changing down for it to be in the right gear to accelerate out of the corner.

What’s your focus got to do with it? Comparing like for like, 2.5 diesel (turbo or otherwise, doesn’t seem to matter) vs 2.5 petrol in a vehicle of the same weight, with the same gear ratio with the same flippin engine block, the diesel has WAY more engine braking. Real life bud, sorry.

Ok how about a 1.9 diesel manual and a 2.0 petrol manual. I know which on produces engine braking also all morden cars controlled by the ecu cut fuel on a closed throttle above a set rev level. He’ll my 1986 205 gti used to do this and if you did an emergency stop the engine cut out.

Ok how about a 1.9 diesel manual and a 2.0 petrol manual.

not really a fair comparison, gear ratios will be totally different to allow for the roughly 40% lower redline in the diesel. Although my 1.9 Peugeot noticeably has a LOT more engine braking than a similar petrol.

Your 206 gti kinda proves my point actually, high compression (for a petrol) and low gear ratios (which modern diesels dont have, lessening the noticability of engine braking) means lots of engine braking when not being fuelled.

A friend pointed out to me about 5 years after i passed my test to blip the throttle and i could go down through the gears smoothly.Ive always done it since. Couldnt get the knack of it at first but its here to stay. Ive never fekked a clutch,gearbox or engine. One car i had done around 130k in it, the wee 106 diesel i had done 120k in it (around 160k before it was sold with original gearbox and clutch)the current diesel i have is now on 216k (got it with 23k on the clock) and the gearbox feels like new,never had a clutch. The other car,a 106 gti, ive put 80k on it and blipped the throttle to death down through the gears.. its due to blow up but never seems to. I glance at the rev counter when i blip and know where to blip it to for 4th 3rd and rarely 2nd.Blipping into 1st on that car no matter how well the revs were matched would put me through the screen face first down the bonnet! That car is still on the original gear box too. I`d never stop doing it.

Same as people say above. I’m old so I learnt to to engine brake. But had a couple of lessons with advanced driving instructors in the last few years, and it is clear that the methods taught have changed.

Even with a good well matched “blip” on the bike.. ive read (some top kawasaki engine expert) that the stresses it puts through the gearbox is not good at all.He reconed you would eventually thrash it to death on the bike.

I dont mind doing it on my cars that are worth around £300 & £900. Besides,they will die of some other problem long before one caused by a blipping gone bad!

I tend to only “engine brake” in a low gear when going down hills and trying to hold the car back without needing to brake. I try to anticipate speeds and minimise braking. Braking is a waste of fuel, but sometimes it can’t be helped 😀

A STWer yesterday… [video]http://www.youtube.com/watch?v=b7ll_s3ka88&feature=related[/video]

Wheres the flower?!

I’m surprised that gearboxes can handle the stresses of harsh acceleration with aplomb, but not the stresses of deceleration. Unless you’re dumping the clutch at redline revs, I’m struggling to see how engine breaking can be any worse than vigorous driving.

Ah yes… the air charge drawn into a petrol engine under almost closed throttle will be very low pressure, so the piston will be sucked up instead of working against atmospheric pressure. What’s the actual inlet manifold pressure differential?

On the subject of compressors, it has occurred to me that a diesel compressing the same amount of air all the time is very wasteful considering it only uses a little bit of that air. So it would be muc more efficient to have a small V8 and under normal driving leave one side of it unused and leave the valves open, so you’re not compressing any air. Large American petrol trucks do this so the tech exists.

Never seen the words V8 and efficient used in the same sentence before, unless ‘not’ is in there too… Lol! I’m pretty sure some big eighties American saloon cars did the deactivating of cylinders thing, (not one bank, that would be horrible, the front four I think) but I don’t think it caught on. Good idea in principle but complicated in execution, you need separate camshafts for the valves of the cylinders that will be deactivated and the engine needs to be configured in such a way as when running on less cylinders it isn’t horribly out of balance.

It has caught on in the trucks. And the reason for de-activating one bank is that it’s far easier to do – just have a clutch on one of the cam pulleys I suppose.

Anyway I am talking about a 2.0l V8, say, which would give normal performance and good economy.

These were banned as they sound like a machine gun.

close but not quite. the machine gun noise has to be silenced, but lorries still have Jake brakes.

Of course diesels have engine braking. anyone who lets off the accelerator in any diesel vehicle knows that. but its not the same as a petrol, where the braking is naturally through the throttle. it is “artificially induced” through either a throttle on the exhaust side of things (increasing pressure on the upstroke), or on turboed vehicles, closing the turbo off to restrict the flow of air into the cylinder in the first place (creating a vacuum on the intake stroke)

pretty sure you can hear the rattle of pressure release valves whenever I let the foot off the accelerator in my HDi, though cant be certain. i might look into it…

Anyway I am talking about a 2.0l V8, say, which would give normal performance and good economy.

Yeah I reckon that would be feasible, but would be a very expensive engine to produce. Two cylinder heads, clever gubbins and lots to go wrong, not to mention very small components with high stress loads. Easier to develop the inline 4. Which is what has happened, largely.

“artificially induced” through either a throttle on the exhaust side of things (increasing pressure on the upstroke), or on turboed vehicles, closing the turbo off to restrict the flow of air into the cylinder in the first place (creating a vacuum on the intake stroke)

Give over! What would be the possible incentive to engineer a complex system of shutting off turbos or extra valves just to provide a little diesel engine with engine braking, which IT ALREADY PRODUCES IN SPADES because it is a diesel and has a compression ratio of 20:1!!! A jake brake is only neaded when holding back 40odd tonnes down extended grades and you don’t want to overheat the road brakes. Your HDi Does. Not. Have. an additional engine braking system, although of you can hear rattling on the overrun I would agree that you should look into it, or chop it in quick…

Are you trolling me??? Lol

You really don’t get how a compressor works do you? have you ever seen one when it is off load? they turn really easily. An engine when open to the atmosphire and driven from the crank shaft squezzes the air which is then free to return to the volume it was before before its pussed out into the atmosphere. On a car you have friction and an amount of restriction from an exhust but this is the same regradless of fuel source. A compressor sucks air and the compresses this into either a system or reciver just like pumping air into a tyre. the air has no where to go and this makes it hard the higher the pressure.
my 205 gti wouldn’t have had any engine braking due to my foot being on the clutch liek the teach in the driver test.
Also how come its fine for you to compare landys with different egines but not for any one else?

I thought the same thing when I first heard it. I wrote the editor of the GM Diesel page and this is what he wrote back:

A gas engine has more engine braking than a comparable displacement diesel because at low throttle levels a gas engine is working against a closed throttle plate. A diesel has a wide open intake manifold without a throttle plate.

Think of a diesel piston and cylinder like an air cylinder. If you press the piston up to near TDC with the valves closed, it takes a lot of pressure, right? But guess what, after rotating beyond TDC all that compressed air now pushes down on the piston to accelerate it to BDC.

A gas engine will generate about 25″ of vacuum with the throttle closed, so when the piston comes up to TDC, very little pressure is there to push the piston on the down stroke. Then, when it wants to pull in a fresh charge, the engine has to pull against the vacuum. All this consumes energy and creates what is termed “compression braking”.

Of course, a diesel does produce some compression braking due to mechanical losses and heat generation, but a similar displacement gas engine will always generate more “compression” braking.

taken from here