Thread: Improving fuel efficiency v8i

Originally Posted by nsgnomad

Thanks. I thought they used less than the V8 disco, at least based on figures I have seen from mag test reviews. But yes, none of these vehicles are lightweight. Judging from the other replies the best device for economy is a careful right foot.

Since when have they been right?.The new V8 T/D cruisers are supposed to get Tdi economy,10ltrs per hundred,try 14 driven very carefully. Pat

Originally Posted by PAT303

Since when have they been right?.The new V8 T/D cruisers are supposed to get Tdi economy,10ltrs per hundred,try 14 driven very carefully. Pat

Actually, They are right Pat, 10 litres of diesel, and 4 litres of OIL per 100km

JC

Originally Posted by justinc

Actually, They are right Pat, 10 litres of diesel, and 4 litres of OIL per 100km

JC

Providing the oil doesnt disappear when the bottom end falls out as a few have been known to do! Then its 0L diesel and 5L oil/100m!!!

Originally Posted by Dougal

That isn't going to save fuel. If you've ever seen a BSFC map you'll know why.

Engines are most efficient with the throttle most of the way open (just before they hit enrichment) with lower rpm.

Going down a gear raises the rpm (more frictional losses) and results in a bigger pressure drop across the throttle (more pumping losses).

The best fuel economy is found with the tallest gears your engine can pull. Making sure the auto stays in lockup will help greatly too.

A little more complex than that with a petrol, depends on how much torque and where, engine vacuum has alot to play in this aswell, labouring an engine becomes inefficient, but revving an engine, when it could do the same work at lower revs is also

Originally Posted by rovercare

A little more complex than that with a petrol, depends on how much torque and where, engine vacuum has alot to play in this aswell, labouring an engine becomes inefficient, but revving an engine, when it could do the same work at lower revs is also

If you can find a BSFC map for these engines then you can pick exactly where's the best point to run it.
Generalising from the othe petrol BSFC maps I've found, the absolute best point is found at an rpm around 2/3-3/4 of max torque with the pedal almost buried.

Of course if you don't need that amount of power at your cruising speed, you're best to raise your gearing to drop the rpm. Basically find the power you need and follow that constand power curve across the map until you find the lowest fuel consumption.
These maps are created from measured results, so they include all the effects of valve timing, spark timing, your engines VE characteristics etc.

Here's a prius BSFC map:


Here's an audi W12 BSFC map:


Opposite ends of the spectrum, but the generalisations still hold.

These plots show one reason why diesel engines tend to be more efficient in practice - the specific fuel consumption varies a lot less with rpm and power.

They also bring into question the oft stated contention that a more powerful engine operating at low power is more economical than a smaller engine operating near maximum power - these diagrams suggest that you need to have an engine sized so that in your most common operating conditions you can use near full throttle at around half maximum rpm, the actual rpm not being all that critical but the near full throttle is.

John

Originally Posted by JDNSW

These plots show one reason why diesel engines tend to be more efficient in practice - the specific fuel consumption varies a lot less with rpm and power.

They also bring into question the oft stated contention that a more powerful engine operating at low power is more economical than a smaller engine operating near maximum power - these diagrams suggest that you need to have an engine sized so that in your most common operating conditions you can use near full throttle at around half maximum rpm, the actual rpm not being all that critical but the near full throttle is.

John

Dead right, the "big lazy engine" theory is complete bollocks. Smaller engines have less internal friction and less pumping loss. This is why car makers are turning towards smaller boosted engines to reduce fuel economy.

Here's the BSFC plot for a VW 1.9tdi. One of the most efficient engines you'll find on the road regardless of size.

Originally Posted by Dougal

That isn't going to save fuel. If you've ever seen a BSFC map you'll know why.

Engines are most efficient with the throttle most of the way open (just before they hit enrichment) with lower rpm.

Going down a gear raises the rpm (more frictional losses) and results in a bigger pressure drop across the throttle (more pumping losses).

The best fuel economy is found with the tallest gears your engine can pull. Making sure the auto stays in lockup will help greatly too.

I'm talking about not lugging the motor in too high a gear, don't hold your foot flat to the floor and having the motor lugging up a hill, drop back a gear and hold around the same revs, it does save fuel, Regards Frank.