XC70: General Best mpg yet.

[apersson850]

Originally Posted by BigGreenThing

Overall, I think the 2ltr 5-cylinder D3 diesels appear to be a fairly frugal motor, but wondered why Volvo set the turbo at such incredibly low revs (or whether that can even be adjusted). I only have to breathe on the accelerator and the turbo seems to kick in!

Because it's a diesel. Diesels are able to produce a lot of torque already at low engine speeds, provided they have enough air flow.
Since they run with a variable mixture ratio, the fuel required is injected precisely to match the need, but there's always an overflow of air.

It's not like a petrol engine. If you rev up the turbo, and don't waste the air through a gate valve, then you also have to inject a certain amount of petrol. Otherwise the engine will stop. In the same situation, the diesel will not stop, just become weaker.

[yostumpy]

A bit OT...


Interestingly, I tried an experiment on my old Land Rover Defender, which had a Bosch VE pump. I disconnected the tube from the turbo to the fuel pump (and plugged it), that provides the vacuum/pressure to move the diaphram to allow more fuel to correspond with increased boost. The result was interesting. Folks said it wouldn't work / be horrible/ break! But It worked, acceleration to about 40-45 was still brisk, but power was down, obviously, on the flat /downhill it would still pull over 80 mph, but an uphill incline, and 60 mph was about it. I was inspired by an early eco version of the VW Golf , The 'UMWELT' version, that effectively did just as I had done. There was NO smoke out of the rear pipe (unlike normal), BUT the MPG was the same as before, in all conditions, after a lot of fill ups, so I changed it back.

[apersson850]

Yes, there are lean burn versions of petrol engines. But the general rule is a mixture of 14:1, air vs. fuel.

Petrol engines ignite by injecting a burnable mix of air and fuel into the cylinder, compressing it and then add a spark. The mixture has to have a certain relation to burn well. You can't go too far away from that and still have it work.

Diesels ignite by pulling/pushing a large amount of air into the cylinder, compressing it a lot, so much that it becomes so hot, that when you inject the fuel it combusts by itself. No spark needed. Hence any amount of fuel will burn. The more you inject, the more torque you'll get.
If you inject too much, it will emit black smoke, since not all is properly burned.

[FreshAir]

Diesel engines are at most efficient at high air to fuel ratios, which translates to light 'throttle' positions.

Diesel efficiency gets similar to petrol as you get towards the acceptable soot limits / max torque at engine rev's.

The main reason why the 2 litre is more economical than the 2.4 litre is reduced friction losses which is mainly from piston movement in the bores. Slightly offset by the longer stroke 2.4 litre being a more economical bore/stroke ratio, and being able to run at higher air to fuel ratio for given engine load. This is why the 1.6 litre diesel can be less economical than larger diesel engines as in normal use (not EU tests) as the engine is under higher relative load to provide the power for normal driving compared to the larger diesel engines driven at the same speed.

[bananarama]

We just came back from a 3 week road trip through France and Switzerland in our V70 D3 Auto, we touched all kinds of roads (tolls, N, and D); we got 50.2mpg

No idea if that's a good or bad figure...