Isn't it more efficient coasting in gear/engine on?

[euromodder]

Originally Posted by cRiPpLe_rOoStEr

You might know engine-braking is not so great in Diesels.

Engine braking is best in diesels due to their higher compression ratio.

No wonder trucks are usually fitted with those exhaust-brakes...

They carry a lot more weight as well .

[mechman600]

Originally Posted by euromodder

Engine braking is best in diesels due to their higher compression ratio.

You obviously have never driven a diesel. It is harder to push the piston on the compression stroke on a diesel because of the higher compression ratio, but remember that it is pushing against a spring that pushes back just as hard on the way back down, effectively cancelling itself out. No throttle plate means far less pumping loss to slow the engine down.

Try driving a 10 year old diesel pickup down a hill (before they had standard equipped exhaust and/or transmission brakes): painful. The gas equivalents have much stronger braking.

[Arragonis]

Originally Posted by mechman600

You obviously have never driven a diesel.

You obviously haven't looked at his garage

[mechman600]

Oops. My bad.

[ecomodded]

With my diesel in 5th(its top gear) it coasts along pretty nice really, so much so that i was surprised at the lack of engine/gear drag on my momentum.
Some months ago here on ecomodder there was a similar thread, that inspired me to try some in gear coasting on roads i normally only neutral coasted on.
My car coasted well in the in gear tests, normally I coasted up to 53 mph on a steep hill shaped like a U(down one side of it and up the other)in gear it managed to coast too 49 mph.
On steep longer hills The engine braking is not near enough to slow the car.
It may have something to do with the low RPM of the motor as well.

[cRiPpLe_rOoStEr]

Originally Posted by euromodder

Engine braking is best in diesels due to their higher compression ratio.

Into newer Diesels, not just the throttle-plate but also the management of the e-VGT increased the engine-braking feature enormously. But the ones without throttle-plate still have virtually no engine-braking.

[euromodder]

Originally Posted by mechman600

You obviously have never driven a diesel.

I've mostly driven diesels for the last 18 years.
Both directly and the older indirectly injected diesels.


All of them had far better engine braking than comparable petrol engines (comparable in power or displacement).

[Arragonis]

Originally Posted by cRiPpLe_rOoStEr

Into newer Diesels, not just the throttle-plate but also the management of the e-VGT increased the engine-braking feature enormously. But the ones without throttle-plate still have virtually no engine-braking.

Which diesels have a throttle plate ? Just interested.

Diesels have more engine braking, higher gearing may mask it somewhat. A diesel in 4th may brake as much as a petrol in 4th but the rpms would be very different with the petrol being higher. Higher rpms = more compression strokes, Diesels have more compression for each stroke.

They also have bigger bangs (but fewer of them) which makes the torque.

[mechman600]

Originally Posted by Arragonis

Which diesels have a throttle plate ? Just interested.

The VW 2.0 TDI has one to assist with EGR flow and assist with light load stoichiometric operation for the purpose of regenerating the NOx catalyst. I do not know what position the throttle plate goes to on deceleration.[/QUOTE]

Diesels have more engine braking, higher gearing may mask it somewhat. A diesel in 4th may brake as much as a petrol in 4th but the rpms would be very different with the petrol being higher. Higher rpms = more compression strokes, Diesels have more compression for each stroke.

Again, remember that on deceleration, compression is a giant spring. While higher compression may be harder to push against on the compression stroke, it forces the piston down much harder on the power stroke, thereby turning the engine and cancelling out any energy lost during the compression stroke (inefficiencies aside), so no more power is needed to rotate an engine with a higher compression ratio vs one with a lower compression ratio.

A compression brake (on larger diesels), aka "Jake Brake", solves this by opening the exhaust valve just before TDC on the compression stroke to exhaust the compression pressure to keep the giant compression spring from forcing the piston back down.

[Arragonis]

Wikipedia backs up what you are saying but experience with Diesels for me and Euro suggests not. Could it be the effect of the much heavier flywheel ?