Driving technique for braking in the city

[Philippe Schutt]

Downsizing + turbo, super-long gearbox, EGR, car weights.

[Matt113]

good question, I have an astra 1.7 cdti (recent engine so) when I leave the highway or even downhill (not far from my home there is a descent which leads to a red light so people generally let themselves go in engine brake) I have to brake almost all the time because in engine brake I catch the front car (I specify that the car does not brake , it lets go in engine brake).
and there is a lot of road having a slight slope that I do in engine brake without problem so consumption 0.

[Did67]

Speaking of engine brake, how to explain, despite the total absence of injection, the big decrease in engine braking on recent diesel?

I am not a specialist, but I think there are two things:

1) The engine brake is used its engine as a "compressor". There has been a tremendous increase in power for a given displacement, thanks to high pressure direct injection, turbo, 4 valves per cylinder, etc ... But displacement and compression ratios are the same. So we have engines that are tremendously more powerful than the ones we knew 30 years ago, but they still basically have the same engine braking (whereas we would expect them to brake as much as they accelerate). In large limousines, the smoteurs are on average much smaller displacement than before! So much less engine braking - although the acceleration is much better !!!

In addition, the cars got heavier: more complex engines, catalytic converters, airbags, equipment, air conditioning ...

2) I think that there is a "delay" effect of the computers: if on the mechanical systems, the sgaz arrival was cut as soon as we lifted the foot, I think that the computers are late (I do not know if this is intended - the time taken by the data from the lambda probe t that the computer "averages" them to avoid oscillations (?????). This is in any case what I observe on the computer of my C5. It never cuts suddenly, but in stages .... On the other hand, once the injection has been cut, I don't see how the engine would slow down less than it can, no longer receiving any more fuel ... After that, c is that the car is very heavy and the engine very small (2.0 l anyway in this case - but the majority of smotors are 1.5 !!! It was the displacement of the R4 GT at the time).

[Woodcutter]

[...] But the cubic capacity and the compression rates are the same. [...]

Not quite, the compression ratio tends to decrease on the most modern engines.

[...] 2) I think that there is a "delay" effect of the computers: if on the mechanical systems, the arrival of sgaz was cut as soon as one eased the foot, [..]

There's no "gas inlet"on a Diesel ...

[...] It was the displacement of the R4 GT in time). [...]

What's this ?

[Did67]

Not quite, the compression ratio tends to decrease on the most modern engines.


There's no "gas inlet"on a Diesel ...

The lower compression ratio, I didn't know. But we still have to stay in the same order of magnitude, right? Otherwise, we would lose performance ... I haven't read technical sheets for a while.

For the gas supply, of course, it is a slip, I meant the injection of fuel ...

[Woodcutter]

[...] The decrease in the compression ratio, I did not know. But we still have to stay in the same order of magnitude, right? Otherwise, we would lose performance ... I have not read technical sheets for some time ....

I was very surprised to see that for example the D4D engines (in version 2.2l) of Toyota were at 16: 1 for example ...
An effect of increased turbo pressures?

[Did67]

An effect of increased turbo pressures?

This seems to me a good hypothesis. The compression ratio is a volumetric ratio.

It is clear that the same final pressure in the cylinder at top dead center (which is responsible for the auto-ignition of the fuel and determines the efficiency) can be obtained with a better filled cylinder at low dead center (turbo, 16V cylinder heads ) despite a lower compression ratio ...