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SteveS

Member Since: 05 Oct 2010
Location: Devon & Berkshire
Posts: 388

2009 Defender 110 Puma 2.4 USW Santorini Black

Courtesy of Wikipedia. The jolly old EGR does appear to have a use after all......

Diesel engines[edit source]

Diesel engines do not have engine braking in the above sense [petrol]. Unlike petrol engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects.

However, some alternative mechanisms which diesel engines use that replace or simulate real engine braking include:
A compression release brake, or jake brake, this is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere.
Normally during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel. (This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm.) Having lost the energy stored within the compressed air, the engine is then made to pull the piston down (which sucks new air into the cylinder), and then travel upward again, compressing the new volume of air, which will again be released to the atmosphere after having been compressed. The engine loses energy.This type of brake is banned or restricted in many locations where people live because it creates a sound loud enough to disturb the peace, including waking people at night. It is very effective however, and creates immense amounts of braking force which significantly extends friction brake life - A 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking force.[citation needed]An exhaust brake - This works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one.

A mechanism related to the exhaust brake is back-pressure from a turbocharger. In turbo diesels with variable-vane turbos, the vanes will close when the accelerator is released, which creates a back-pressure braking effect similar to an exhaust brake. Even fixed turbos, especially larger ones, will cause some back-pressure when they are below the turbo threshold (albeit not to the same extent as a variable turbo) and contribute to the braking effect.

Modern diesel engines have engine braking characteristics more akin to petrol engines. This is due to additional devices to allow them comply with emissions regulations. Two in particular cause significant engine braking:

1) Particulate filter

This device filters out soot particles before they exit the exhaust, but it creates a large obstruction in the exhaust path. This causes considerable backpressure, much more than from the turbo charger mentioned above

2) EGR (Exhaust Gas Recirculator)

This device sucks exhaust gas back into the air intake and is usually controlled by a throttle which, depending on the design, can cause a restriction effect similar to that in petrol engines.

7th Sep 2013 7:49am

ericvv

Member Since: 02 Jun 2011
Location: Near the Jet d'Eau
Posts: 5816

2009 Defender 110 Puma 2.4 SVX Station Wagon Santorini Black

Not sure I take all that.
Our Puma's surely don't have a jake brake.
My Puma 2.4 does have an EGR, not sure if this reinforces the engine braking.
The more recent Puma 2.2 has the by Wikipedia mentioned particulate filter, maybe on top of that also an EGR. (Not sure abt that).
So even if the Puma 2.4 has an EGR which supposedly helps engine braking, the Puma 2.2 has the DPF which supposedly has a similar effect.
So why does my Puma 2.4 with an engine compression of 17.5:1 have significant more engine braking effect than a Puma 2.2 which has a lower engine compression of 15.5:1 ?
And why does a Puma 2.2 with that compression have significantly more engine braking than any gasoline car which usually have a compression of 8 to 10:1, but never more than that?
I can only conclude that engine compression does have a big effect on the engine braking..... You never actually own a Defender. You merely look after it for the next generation.
http://youtu.be/yVRlSsJwD0o
https://youtu.be/vmPr3oTHndg
https://www.youtube.com/watch?v=_GtzTT9Pdl0
https://www.youtube.com/watch?v=ABqKPz28e6A
https://www.youtube.com/watch?v=rLZ49Jce_n0
https://www.youtube.com/watch?v=XvAsz_ilQYU
https://www.youtube.com/watch?v=K8tMHiX9lSw
https://m.youtube.com/watch?v=dxwjPuHIV7I
https://vimeo.com/201482507
https://www.youtube.com/watch?v=ZSixqL0iyHw

7th Sep 2013 8:04am

SteveS

Member Since: 05 Oct 2010
Location: Devon & Berkshire
Posts: 388

Done a bit more research and it seems to come down to pumping losses.....as follows. Higher compression engines appear to have more pumping losses. Looks like there are many things at play here including turbo geometry, EGR, pumping etc.

RE: pumping losses - I'm thinking the heating effect when you put your finger over the end of a bicycle pump and squeeze - that heat (energy) is lost to air (water jacket) and the compressed air when released gives back less energy, thus engine braking....more compression, more heat generated and thus could be lost to cooling system and therefore more engine braking on high compression ratio engines.....seems plausible to me. But then what do I know?

One of the better forum posts I could find below
~~~~~~~~

pumping losses: energy put into compressing the air in the cylinders heats the air up and some of this energy is lost. This means that the downward stroke as the compressed gas (but not burning fuel) expands is delivers less torque than the cylinders in compression, so there is net negative torque. This has a bigger effect in diesels because of higher compression and more air in the cylinder.

engine friction: pretty straight forward. net negative torque.

parasitic losses: oil pumps, alternators, water pumps, air con, brake servo vac pumps all require drive from the engine, so there will be net negative torque.

So yes, of course a diesel engine will engine brake off throttle.

And because of the pumping losses associated with compressing nigh on full cylinders, engine braking is normally greater in a diesel than a petrol

7th Sep 2013 9:23am

ericvv

Member Since: 02 Jun 2011
Location: Near the Jet d'Eau
Posts: 5816

Whatever you say Steve

I only know one thing, my Puma 2.4 has the best engine braking I have ever experienced with any car.

Going down a rather steep and long ramp into an underground parking in first high, it nicely does it steadily at slightly above tickover, and that without any braking at all.

Don't see many others do that.

You never actually own a Defender. You merely look after it for the next generation.
http://youtu.be/yVRlSsJwD0o
https://youtu.be/vmPr3oTHndg
https://www.youtube.com/watch?v=_GtzTT9Pdl0
https://www.youtube.com/watch?v=ABqKPz28e6A
https://www.youtube.com/watch?v=rLZ49Jce_n0
https://www.youtube.com/watch?v=XvAsz_ilQYU
https://www.youtube.com/watch?v=K8tMHiX9lSw
https://m.youtube.com/watch?v=dxwjPuHIV7I
https://vimeo.com/201482507
https://www.youtube.com/watch?v=ZSixqL0iyHw

7th Sep 2013 9:51am

limbrickh

Member Since: 05 Oct 2012
Location: Canterbury
Posts: 20

2012 Defender 110 Puma 2.2 XS CSW Santorini Black

Thanks for all the feedback! This has been really helpful.

I guess I'm slowly formulating a view on the best approach...

Engine revving high when cold = bad for long term engine wear
Heater on during descent = colder engine
Sitting on brakes down long mountain descent = bad for brake wear
Sitting on brakes for long mountain descent = increased risk for bad outcome
2.2 = less engine braking than other Defenders
Land Rover engines = can take a bit more pain than others

A gut feeling tells me getting down the mountain in one piece (rather than trying to preserve my engine for the long term) is most valid. So, taking on board all your input...

I'm going to turn my heater off
Warm up my engine before the descent
Use engine braking (High second)
Use brakes to stop speed building up
Be prepared to use a higher gear (third) a bit more readily but only judiciously.

Thanks everyone!

7th Sep 2013 11:29am

ericvv

Member Since: 02 Jun 2011
Location: Near the Jet d'Eau
Posts: 5816

Common sense prevailing

7th Sep 2013 11:31am

Birdy

Member Since: 07 Oct 2011
Location: Côte d'Azur
Posts: 855

2011 Defender 110 Puma 2.4 XS CSW Santorini Black

VeeTee,

Nice pic, wish I could upload too. Especially as in June I didn't go down in my truck but a Morgan with no hood!

Peter

7th Sep 2013 6:53pm

VeeTee

Member Since: 06 Mar 2011
Location: Somewhere
Posts: 1512

Why can't you upload? Would be nice to see the pics of your Morgan

Cheers, Vincent
1959 Polynorm 1/4 Ton Trailer, Olive Drab Green (sold)
1970 M416 Military Trailer (Camping Trailer Conversion), Epsom Green (sold)
1975 Series III 88 V6, Light Green (sadly sold)
1996 Defender 110 CSW 300 Tdi, Epsom Green (sold)
2000 Freelander 1 TD4 3-drs, Silver (sold)
2006 Freelander 1 TD4 5-drs Facelift Automatic, Tonga Green (sold)

MySite

7th Sep 2013 7:03pm

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