engine braking

You've invented two types of friction?

See my earlier reply.

In article , shazzbat writes

I think the idea is that brake pads are cheaper than clutches, but I agree with you. I've always used engine braking and chosen the highest gear most appropriate to the road speed without labouring the engine, and it makes for smoother, fluider driving. That said, I took lessons and passed my test 30 years ago.

Problem with "brakes to slow" is that the driver doing this is endlessly flashing their brake lights at the vehicle behind.

And cars are heavier. And headline torque for a given capacity has increased which allows a heavier, higher geared car to perform better. If nothing else it means the engine's contribution to friction in the system has reduced.

It is not possible to increase headline torque of a non-forced induction engine without reducing the energy it keeps for itself. This energy is going to be the same whether it is supplied by fuel or by a reduction of kinetic/potential energy, ie, "engine braking".

You don't half talk some bollocks, m'lud.

No it hasn't. Unless you're talking forced induction.

If you like pantomine, very well. Otherwise I suggest you look at some data.

Here's a couple for starters

1974 Lancia Beta 1592cc 95ft-lbf

That caveat was given later on.

I know that, but my point was that if you have a set starting point BTDC, the weights won't retard it past that point- they'll only advance it further, or not at all, dependent on speed.

Well, as ever, if you know better...

By the way, there is a difference between the pressure change across a turbine and that due to a volume change of a cylinder. The former is lossy. Perhaps this is what is confusing?

the thing that is so wrong is the idea that retarded ignition can push the pistons backwards, overadvance can do this, but not retardation. As an aside the MK3 escort had vacuum advance AND vacuum retard mechanism on certain early models with auto gearbox (there were two opposing diaphragms with a sort of vacuum switcher that turned on the retard side under certain conditions).

utter bullshit.

But we're talking about engines, where (as with anything that actually does it) it is lossy, even if you don't lose heat to the engine you'd lose it in the air passing through the engine.

True.

But 50% of cars are now Diesel. So 50% of the new drivers are going to jump in cars that have no engine braking. Why teach something with a caveat? "If you get a Diesel it won't slow down much when you take your foot off the gas, you will have learn to press the brake".

I think there are now quite a lot of instructors using Diesels.

Is everyone on here a die hard, Diesel over my dead body, petrolhead?

Diesels have no engine braking ??? The ones I have driven do, in fact they have quite a lot of it, something to do with enormously high compression ratios , which is why they can get hot enough to ignite their diesel merely by being spun over. I wonder where all that heat comes from, perhaps from squashing air, which takes a lot of effort and therefore gives a quite marked engine braking effect.

I've owned diseasels since 1997, and driven them since about 1990. Trust me, they have plenty of engine braking, despite their gearing often being higher than the equivalent petrol.

Given that you're evidently from Mars, how would you know?

Tim

Diesels have no engine braking ??? The ones I have driven do, in fact they have quite a lot of it, something to do with enormously high compression ratios , which is why they can get hot enough to ignite their diesel merely by being spun over. I wonder where all that heat comes from, perhaps from squashing air, which takes a lot of effort and therefore gives a quite marked engine braking effect.

I'm a little rusty on my 1970's A level Physics, but in a diesel on the overun (no fuel injected), what actually happens?

I think compression creates lots of heat, decompression without ignition surely does the opposite (refrigeration)? So the net heat generated is zero. (???)

So the energy the diesel engine puts in to compressing the air actually just goes into pumping the air out of the exhaust, much like a petrol engine.

Cheers,

Gareth.

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.

You get some of it back on the expansion stroke, but as the valves don't open at the optimum times for this a chunk of the stored energy just ends up wasted.