How to drive a diesel

[Bendolfc]

I don't have a D5 but my experience of the 2.0D is that for best economey you need to drive it where the torque is, changing up too soon and driving at low revs is just as wasteful as reving the nuts off it, also if you are chaning up too soon, you will be putting a lot of stress through the DMF which has to smooth out all the vibrations that would otherwise be felt from driving at too low revs.

[5cilinder]

Torque has nothing to do with it, petrol engines also benefith from it
Its motorfriction /divided by miles
The less motorfriction needed for a given distance the less fuel consumed wich means low revs in the highest gear possible.
if thats done on a flat highway without full throttle you dont have dmf stress or motorwear
If towing or uphill with accelleration in hot weather (temporarly) downshifting
is required to take the stress down on the dmf and crankshaft bearings
it also increases the coolingcapacity when specificly needed in those situations due to higher coolingpump rpm's wich are lineair with engine rpm's

[rippedoffagain]

I've had 2 diesel motors over the years. The first one dropped to bits (the nothing to do with the engine) and the second one, unfortunately, sort of blew up and bled to death on my front lawn.

In terms of fuel economy, pretty much what's already been said. try to keep the revs down. At the higher revs, when the turbo is also giving it rock all, economy drops to not much better than petrol. The turbo on a turbo diesel is really there to make up for the inadequacies of a diesel engine at times when you need that bit extra, like getting out of a busy junction. Best to try not to need it. Unfortunately, as my dad always says, that means adapting your driving style to be less demanding on the acceleration.

Diesel comes into its own when you're on the motorway, and the engine is barely ticking over, at lower revs than a comparable petrol engine would be running at (because diesel has more torque than petrol at the lower revs). Then you're practically running on fresh air, unless you're caning it, and then you may as well have a petrol motor again.

[copperknob]

oooof i got v50 drive stop/start on a run getting 86mpg

[Jim314]

Short trips starting with a cold engine are inefficient for a diesel because a diesel takes significantly longer time than a petrol to warm up to optimum operating temperature. A diesel generates much less waste heat than a petol operating at the same power output. Some diesels do not generate enough heat to allow low power idling for extended periods.

Using engine braking instead of the the friction service brakes does not increase fuel efficiency. However, if you anticipate braking and so take your foot off the accelerator and coast, then you will increase fuel efficiency. But do not avoid using the brake pedal if it means getting too close to the vehicle in front.

Traditionallly the combustion air for a diesel is not throttled as it is for a petrol spark ignition engine. There is no vacuum in the intake manifold and a separate vacuum pump is required to supply vacuum for various vacuum valves and the brake booster. This reduces so called pumping losses and increases fuel efficiency, but it means a diesel without a throttle provides little or no engine braking, unless an expensive special vacuum braking system is installed (called a Jake brake in the US). See http://en.wikipedia.org/wiki/Engine_braking

Volvo added a throttle to its auto diesels in later model years but it is not clear under what conditions the throttle is closed, partially or completely. Under cruising conditions the throttle is presumably wide open. It could be that the throttle is only closed when the engine is turned off and, if so, the function as designed would be to have a means of preventing the engine from running away if 'self fueling' would occur, e.g. oil from the sump passing around the piston rings.

Addition in editing
The Wikipedia entry above points out that a variable vane turbocharger (which the Volvo auto diesels have, I think) provides some engine braking by raising the 'back' pressure and creating a positive pressure difference between intake and exhaust streams. This opposes the outward motion of the pistons and so provides some engine braking.

[Jim314]

I have owned only petrol engined cars, about evenly split between manual and automatic transmissions. I like a petrol manual because of the lower cost compared to a petrol automatic or a diesel manual or automatic. I always wanted a diesel but would never choose one because of the cost premium and the dirty exhaust.

In the mountains a petrol manual is wonderful because the engine braking is sufficient to descend without riding the friction brake and all the heat of braking is exhausted out the tail pipe.

How do Volvo diesels (manual or automatic) perform in descending mountains compared to Volvo petrol engines?

I have wondered if there is a problem with heat buildup in an automatic transmission when using engine braking to descend mountains. The last mountain I descended was Pike's Peak in Colorado, USA in 2001 in my wife's 1996 850 wagon, 4-spd auto, petrol 2.4L 170 hp. I hardly touched the brake pedal going down and at the brake check station at the bottom of the mountain the technician was amazed at how low was the temperature of the discs on that 850. I wonder if the transmission fluid (ATF) was overheated or if the ATF cooler was able to remove the excess heat and keep the temperature down?

[artiedtke]

Originally Posted by Jim314

I have owned only petrol engined cars, about evenly split between manual and automatic transmissions. I like a petrol manual because of the lower cost compared to a petrol automatic or a diesel manual or automatic. I always wanted a diesel but would never choose one because of the cost premium and the dirty exhaust.

In the mountains a petrol manual is wonderful because the engine braking is sufficient to descend without riding the friction brake and all the heat of braking is exhausted out the tail pipe.

How do Volvo diesels (manual or automatic) perform in descending mountains compared to Volvo petrol engines?

I have wondered if there is a problem with heat buildup in an automatic transmission when using engine braking to descend mountains. The last mountain I descended was Pike's Peak in Colorado, USA in 2001 in my wife's 1996 850 wagon, 4-spd auto, petrol 2.4L 170 hp. I hardly touched the brake pedal going down and at the brake check station at the bottom of the mountain the technician was amazed at how low was the temperature of the discs on that 850. I wonder if the transmission fluid (ATF) was overheated or if the ATF cooler was able to remove the excess heat and keep the temperature down?

If anybody knows anything about diesel engines, they will know that engine braking is far far superior to a petrol engine.

Also the cost premium you mention ceases to exist if you go for an older car that has 'stopped' depreciating.

[Jim314]

I did not set out to provoke anyone by going to the diesel thread and criticising the diesel compared to the petrol. I greatly admire the efficiency of the diesel engine, but I don't drive enough long trips to justify the cost premium over a petrol engine. Also before the DPF was required, the diesels did emit a lot of fine particulates in the exhaust. The DPF has eliminated the particulates, but has reduced efficiency and has been the source of some operating problems.

I think fuel is priced unreasonably low in the US and this is one reason why so many Americans buy fuel guzzlers. For their part, the Europeans were willing to drive air polluting diesels to obtain great mpg.

Diesel engines do not have engine braking in the above sense. 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 subject 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.
[snip]
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.
http://en.wikipedia.org/wiki/Engine_braking

[craigv60]

@ Jim314 -- the throttle butterfly in modern diesels is used for two reasons:

1) when the engine is switched off, the butterfly closes the intake to stop the engine running on and 'shuddering' to a stop. In fact, VW call this flap the 'anti-shudder valve'

2) when the diesel particle filter is being actively regenerated, the flap partially closes to throttle the intake. This intake restriction, together with extra post-injections of fuel, helps to raise the temperature in the exhaust manifold and DPF canister to the point were the soot trapped in the DPF starts to 'coke' like a charcoal burner, and is converted into carbon dioxide and water.

Apart from these conditions, the flap is usually in its wide-open resting position.

[Jim314]

craigV60,

So in the Volvo and VW automobile diesels the 'anti-shudder' throttle is not used to implement engine braking. And if we take the Wikipedia article on engine braking as authoritative, we conclude that the engine braking in Volvo and VW diesels is accomplished by exhaust restriction of the variable vane turbocharger. Right? And it would seem that the engine managment system has no means of controlling the variable vane turbo to choose between braking and coasting.

Presumably the throttle control in the engine management system has no means of determining whether, when the driver lets off on the accelerator pedal, he wishes for coasting or wishes for engine braking. So it would appear that engine braking via the turbocharger is always implemented and coasting does not occur.

What I'm thinking about is the case where one is driving for efficiency on level ground at high speed and wants to maintain spacing from traffic ahead by feathering the accelerator pedal. In that case the absence of engine braking would yield higher efficiency, but of course at the expense of requiring more attention by the driver. If there is no engine braking, then the driver would have to let off on the accelerator earlier than if there was engine braking. The vehicle would be slowed down by aerodynamic resistance.

If all the above is true, the diesel would still have the advantage over the petrol in high speed cruising because, while powered, there is no engine retardation in the diesel because there is no vacuum in the intake manifold vacuum, but in a petrol engine cruising at part throttle there is a vacuum in the intake manifold. This vacuum is constantly retarding the engine.

IfI understand the matter correctly, this latter effect is why variable displacement (aka cylinder deactivation) is useful in the petrol engine. Cruising at constant speed requires a certain definite power which is only a small fraction of the max power rating of the engine. For example, I think my petrol V70 lightly loaded and with nothing on roof requires about 30 hp to cruise at 70 mph on level ground. This is only 18% of the max power available so the throttle on the air intake is significantly closed creating a relatively high vacuum in the intake manifold and so retarding the engine. This is the so called pumping loss of the petrol engine.

I don't think it is practical, and may not be possible, to implement cylinder inactivation in an inline engine, but suppose the V70 were powered by a V6 with cylinder inactivation. At constant 30 hp output from 3 cylinders rather than 6 cylinders the throttle must be opened up to get twice as much air in the combustion cylinders, the intake manifold vacuum is less and the pumping loss is less.