What determines fuel consumption?

Cruise and acceleration have very different requirements. The motor has to go rich for acceleration. For cruise it can be leaned out to stoichiometric on a lambda sensor or even leaner using a wideband sensor or exhaust gas temps. The default fuel map will be rich for acceleration, when the ECU detects a steady throttle and rpm (and/or AFM/MAF output) indicating cruise it will start to use the lambda to lean the mixture and not otherwise. A diesel is set lean and has a device in the injection system that gives a rich mixture if the engine speed is lower than that demanded by the "throttle".

My mates petrol Volvo can show as low a 5-7mpg when accelerating. Much much better in cruise about 30mpg, overall average improves when he drives it, gets worse when his wife drives. All fuel injected engines should stop delivering fuel when in overrun as the engine will not fire. Most petrol engines will start injecting fuel on overrun when the revs drop below 1500rpm so they don't die as you declutch to stop. If you lift off without declutching make sure the revs are in the fuel cut off region or it will use excess fuel. Unburned fuel passing though the exhaust can posion the cat and lambda sensor.

Another mate's Picasso Diesel shows 999mpg as it goes down hill. It's really doing infinite mpg as some distance / 0 fuel = infinity.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Wow, sounds like a nice machine! Do you know where I'd be able to get one fairly cheaply, or where I'd be able to find the OBD-II protocols to make one?

I have always held the belief, perhaps naively, (it was taught me by my father who was a bus driver) that all other things being equal driving with the engine revs just above the point at which max torque is produced from the engine gives best fuel economy. When the revs fall due to additional load perhaps due to change of direction, passing vehicles, slight inclines etc the torque increases helping to maintain speed.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "PeterE" saying something like:

Upsides;

Long stroke engine ... more torque

More torque... less throttle use

Less throttle use... more economy.

Downsides;

Long stroke engine... higher piston speed

Higher piston speed... more piston/bore wear

Long stroke engines tend to have lower rev limits than short stroke ones.

As a bike-related aside; the old BSA 650 twins would quite commonly return 60mpg. I can't think of a 650 four nowadays (or since the 70s) that could do that unless ridden very gently.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "sa3214" saying something like:

I've found that to be generally true.

Doesn't fuel efficiency start with the engine itself? Vital ingredients:

Long stroke engine ... able to induct as much air as an equivalent cc short stroke engine with twice as many valves.

Only two valves per cylinder ... four valves per cylinder means double the amount of energy sapping valve springs.

Light valve springs ... the sort you can squash between two fingers, not the usual ones which require a vice to compress.

Less camshaft bearings.

Cam profile tuned to produce as much torque as possible at the least amount of revs.

As high a compression as possible.

Light pistons.

High gearing.

Bmw 525e meets all the above requirements.

But then it could be improved with modern kit, such as electrically operated valves, because a throttle-butterfly is an unneccesary restriction. Also a nice sleek body would be much better than a brick like E28 chassis!! Maybe the engine could be all aluminium.

Maybe even desmodronic valves??

So I need to drive around at 90mph?

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

As a general rule, accelerate smoothly up to about 55 mph, then once in top gear begin to back the pressure off the accelerator slightly. You'll find that the car continues to cruise at 55 mph even though you have backed your foot off a bit. Just "tickling" it like that does save some juice. Also looking far ahead and slowing smoothly to allow the lights to change to green before you have to stop for them helps. If locked in a file of traffic, don't follow the usual conga dance; allow a space to open up in front of you for several car-lengths before moving up; you can reduce the number of handbrake off/clutch/accelerate/brake/ cycles by half if you ignore the bloke behind you who's leaning on the horn. You can also do a bit of lateral thinking in the nature of "Is this journey really necessary?" (That one's a biggy.)

What's the point when so many people drive huge 4WD's short distances to the shops each of them getting about 9mpg? WELL, their day will come and with the Middle East well on it's way to the bonfire that's likely to be soon....

Des.

I had a Sierra a few years ago that had really sweet gearing and a pair of economy lights on the dashboard (factory fitted) The idea was to save fuel by keeping in between with both of the lights off by gentle use of the accelerator. Seemed like a good idea to me!

All gearboxes should be like that one was. It made driving totally different.

Des.

I'm interested in a possible 'C)': Is it not the case that the ECU only delivers stoich 14.7:1 a/f up to about 75% throttle, going up to 13:1 a/f? If this is the case then that's a 12% increase in fuel consumption /rev all other things being equal -- which they're not of course, but bare with me...

Given that the difference beween 4th an 5th gear is -say- 25%, and the fact that the engine in 4th will be nearer peak volumetric efficiency might this close the gap somewhat?

I could never fathom how I consistently got over 32mpg (UK gals!)from a

Two factors came to me a possible reasons for the astonishing economy,; the gradient and the decreased air densisy. Firstly the journey out went from 5200ft-11,00+ ft all foot flat to the floor or very nearly where it would be just about able to maintain

I've never seen any numbers that suggest the effect of altitude on fuel economy but I'd be very interested if anyone does.

In my final weeks stateside I rented a brand-new Pontiac Grand-Prix. A

The long journey I make London-Edinburgh I make on a regular journey I only get 32mpg from a piddly 2.0 167bhp Xsara although it is turning over at nearly double the rate of the Mustang at 70mph! I used to get near enough exactly the same in a 1.8 Golf GTi 16v and a 3.0 (4v) Carlton GSi. OTOH the best my Volve 2.3 Turbo (t5) ever did was 27mpg

Do turbos complicate best fuel economy practices? If you labour a turbo it produces boost which I can only imagine is a Bad Thing, so is it better to drive turbo cars in a higher gear?

I see there's a placeholder for an article on tuning turbos on your website Dave...

Thomas Hood

Out of interest - what sort of car do you have with such torque characteristics and gear ratios

(me - to cruise at or above peak torque I'd have to do 90mph.)

90mph = 3899rpm

Redline is 7200rpm so it's geared for 166mph in top just needs an extra 100bhp. T28 turbo, FMIC, exhaust (it needs a new one), chip, induction kit, fuel pump, boost control valve - about £1500 should do it. If that doesn't result in Single Digit fuel consumption then another £500 for 440cc injectors, a remap and 300ZX AFM giving 330bhp will (also needs another £500 for a clutch). About £2500 for brakes, shocks and wheels.

I've had it 4.5 years so at £3000 in 2000 and current value less than £1000 it's cost about £500/year. The bits starting with exhaust start arriving as of Xmas. I somehow think Direct Line are going to throw a fit.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Clive George wrote in uk.rec.driving on Thu, 11 Nov

It is indeed. Efficiency of the engine is Power out/Power in. Pout = Pin - Plost. The lost power is mainly going to be friction in the engine. Higher rpm = more power lost due to friction.

So by switching from 3rd to 4th, say, you'd reduce the engine speed, causing less power loss through friction in the engine. You'd still be travelling at the same speed, so the useful power output would be the same.

So it'd be higher efficiency.

I'm a fan of the early 90s 200SX...

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Peter Hill saying something like:

Of course, doesn't everyone?

Are you doing the 56mph on a motorway or on windy roads where you will be changing speed all the time?

It's perfectly possible that the load/rpm combination to cruise at 70-75 is in a far more efficient operating region of the *engine* than the combination needed for 56 mph.

The efficiency of an engine does vary quite a lot with rpm/load, easily enough to outweigh any drag difference between 56 and 70 mph.

I remember hearing something that said you actually used *less* fuel if you floored it up to your desired speed as fast as possible, rather than accelerating gently. The reason was that while flooring it the engine is working far more efficiently than it is when "tickling" it with 10% throttle at low rpm.

Of course flooring it you'll get up to 55 much sooner so can back off much sooner.

Just out of interest, what happens when the engine works at different "efficiencies"? Does it not burn all the fuel? Does some of the energy get "wasted" somehow?