engine braking

My petrol engine has a bang on overrun even above idle speed.

So you've not driven a diesel car then?

Lift off the "throttle" in a diesel with the car in gear and it will decelerate faster than in neutral, the lower the gear, the greater the deceleration.

Remarkably like a petrol car in fact. As a driver, I don't give a monkey's where this drag is going on but all earth drivers would call this "engine braking".

Tim

A "jake brake" is an exhaust brake.

In article , Duncan Wood writes

snip

snip This is a fascinating thread. Here's my 2 penn'th; I've recently bought my first diesel (Skoda Octavia 1.6). There's a 30mph limit on a local hill; -not very steep- this IS Norfolk! In 5th with my foot off the accelerator the speed increases; in 4th with foot off the accelerator the speed stays at 30mph. So it does seem that the engine is providing some retardation.

I learn to drive in the 60s so was taught to use engine braking, but generally don't use it for approaching junctions, roundabouts, etc. I do change down before sharp bends so as to get more acceleration as I drive round and away from the bend.

One advantage of using the brakes that occurs to me is that it activates the brake lights to let a following driver know that you are slowing down which may be a good thing in some circumstances.

I have driven desiel cars for 25 years vans too , what your experiencing is drive train drag, completly different to engine braking.

Petrol engine braking is caused by vacuum

So you're telling us that dipping the clutch would make no difference whatsoever.

So why does my car decelerate faster on the over-run with the engine engaged? If it was all drive train drag it should make no difference.

Tim Tim

Of course it would the engines designed to drive the wheels not the other way round

But you're all telling me there's no such thing as engine braking!

Tim

No it isn't, it's a compression release, an exhaust brake closes the exhaust off

Once upon a time people looked at the sky and believed that everything they could see circled the Earth. You may be good at the spanner stuff but time and again you never show no inclination to look beyond things as they seem. When underlying principles are concerned all you do is stick your fingers in your ears and go la la la.

And based on the available evidence, that was the correct thing to believe.

The problem is, *your* 'underlying principles' contradict what my real- world experiences show me.

Turning your criticism of me around somewhat, why don't you abandon your theories, and try something practical?

Take your car to the top of a short hill. Disable it from starting, perhaps by removing the fuel pump fuse. Turn the ignition on. (On the basis that you might need that to get speedo output.) Put it in, say, 4th gear, and have an assistant push you over the brow.

Note the speed half-way down, and again at the bottom.

Now repeat the experiment with the plugs removed, and come back and tell us how much faster it went the second time. How will your theories explain that?

Or if your car has a rev-counter that works without the engine running, again disable the engine from staring. Crank it for 10 seconds, and see what rpm is reached. Now remove the plugs and try again. Why does it crank so much faster the second time if not due to compression?

Chris

Wheras you seem to have failed to progress beyond GCSE & the world of a perfect gas.

So what? I repeat, anyone with a clue does it.

there is absolutely no point in making four or five downshifts merely to stop at some lights.

There's also the fact that the compressed air is very hot (while compressed). And a not-insignificant amount of that heat leaks into the enclosing metal, and then into the coolant.

John

There was no evidence hence it was a belief.

Not my theory. Simply tried and tested principles explained by Newton, Boyle and others. If a behaviour is observed and a conclusion made that is contrary to these principles, the experiment or conclusion must be wrong.

As a practical experiment, I have been observing my diesel car more closely in the last couple of days. I never noticed anything particularly different about it immediately after a switch from a petrol car - down a particular incline it has exactly the same top speed and subsequent roll out on the level in 4th as the petrol car had in 5th (to make the gearing equivalent - AC off too). The compression theory doesn't fit. With my closer observation I now notice behaviour that suggests the software inserts a delay before overrun cutoff.

I am glad to see, this time, a civil reply from you after your "nonsense" and "don't have a clue" without explanation of what you perceive to be wrong.

It is fashionable to 'drive on the brakes' nowadays with ABS & good tyre grip plus the turbo diesel power to reaccelerate..

I learnt to drive on a motorcycle which does force you to learn to anticipate conditions ahead of you rather than just react-react-react - Smooth driving suits a motorcycle e.g. using the engine braking approaching corners (non existent on 2 stokes)

I also worked on a farm years ago where engine braking was vital for example when negotiating a steep hill with a grass cutter and trailer full of grass behind you....braking was out of the question & risked a double jack knife, you just let the equipment behind try to push the engine revs up (to 2200 rpm!) remarkably safe. Modern tractors have ABS and 4 wheel (not 2) braking so I guess you could now just hit the brakes...

Technology does allow the jerky method of driving....

Peter Hill wrote:

Funny that Peter, I've driven diesels as part of my old job for many years and found that they had plenty of engine braking, and a couple of years ago, I swapped from petrol to diesel for my private transport and that has plenty of engine braking as and when the need arises.

It's how you drive a vehicle that counts, drive everywhere looking and thinking no further than the bonnet (or even steering wheel with many drivers) and those drivers then tailgate so close they get high on the exhaust fumes of the car in front then there is no option but to use the brakes constantly for all slowing down (one of the main reasons for many traffic jams BTW).

But get the 'nut behind the wheel' to use a little brain power and look (and think) ahead as far as possible (combined with keeping at least 3 seconds from the vehicl in front) and 'engine braking' comes into its own and driving becomes less fraught.

I was taught the above some 45 years ago by an ex-wartime captain turned driving instructor (amongst other very good advice) who abhored sloppy driving (and drivers) and it's paid off handsomely over the years in easier driving and good fuel economy.

My philosophy has always been that the engine/drivetrain combination is und er no obligation to provide any braking effect, and that the braking system alone should be designed to be capable of providing all required retardati on. My main practical concern is that other than in top gear, the gearbox works as a reduction unit, the differential always so. On overrun however, both units are being driven as multipliers, which I suspect places greater strai n on the gears causing accelerated wear. Given the choice I much prefer to impose wear on the brakes and replace the m more often, than to replace a transmission component. Never having had to do the latter over very high mileages, assures me that I'm right.