How to drive a diesel

[cbee]

Hi Chaps,

I have seen it said that in order to get the best fuel economy from a diesel you must know how to drive it; the implication being that it is not the same as driving a petrol.

I admit that I don't know. The most we have ever got is 42.5 mpg when driving long distance, or about 32 mpg round town (10 mile commute) on our 6 speed sept '04 XC70 D5.

Can anyone point me in the right direction as to how to drive the diesel compared to driving my '99 petrol V70 please?

[5cilinder]

The same as petrol: Drive in the highest gear possible but shift back when going on hills
Avoid braking on the discs ,brake on engine and anticipate on traffic.
And if you dont exceed 60 miles on the highway you come a long way.
hard accellarating is no problem on the condition dat you dont waste the energy with braking.
So entering a highway quick is no problem

[TiredGeek]

Is it the 163 or later 185 hp engine? Easy check, is the engine cover black or silver/grey?
Manual or slushbox?

If it's the 185 then you're doing about right mpg wise. If it's 185 and auto then you're doing pretty well actually.
If it's a 163, then you're about 8 mpg lower than it should be and there's some sort of issue.

As 5cilinder says, change up as soon as is possible, don't rev the nuts off it, be gentle with the throttle, and ease off rather than brake if you can.

With mine I get around 30 around town (7 mile commute) and 36 - 40 on a run (though I think I'm probably going a tad faster than is ideal for consumption) Best ever was 48 pottering around the highlands at 40mph or so.

[cbee]

Thanks for the replies.

In answer to TiredGeek the engine cover is black - 6 speed manual.

The good lady is able to coax a little more out of it than I do (on the odd occasions I am allowed to drive it) but that could be because I'm enjoying the incredible acceleration when compared to my non turbo V70!

The only thing I have read in Honest John is to use a little revs when pulling away from standstill as that helps to prevent the DMF from giving up.

The interesting thing is that when we towed a caravan it hardly made much difference to the mpg - unlike the old 740 2.3 petrol auto, 32 on a run - 15 with van on the back!

[NigeS]

Just done a pretty consistent trip to Devon and back for our hols. About 250 mile in one direction almost all dual carriage way or motorway. On the way down I was pretty steady at 75mph and got the average up to about 45mpg. On the way back I increased that cruising speed by about 15mph, the average mpg dropped quite significantly to 35mpg. Otherwise generally weekly pottering about gets about 42mpg. That's a D5 Geartronic V70, 163bhp.

[Lancee]

NigeS, probably better on the way back with the wheel bolts done up !!!!!!!!!!

[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