What determines fuel consumption?

'Unburnt hydro-carbons' and 'carbon monoxide' are measured, and are easily lost in 'to suitable accuracy'.

More heat, less speed.

Does 'smoothly' imply 'gently' and not imply 'floored' ? (With a normally aspirated petrol engine)

"scott" wrote in message news:QkHld.57$ snipped-for-privacy@newsfe5-gui.ntli.net...

Yes the energy does get wasted, different percentages of it at different engine speeds. Some not totally accurate generalisations. At high speeds more energy is wasted down the exhaust pipe as unexpanded charge than at low speeds. Low pressure turbos (such as Saab) can extract some of this energy to increase efficiency but high pressure turbos increase back pressure and can improve power but at the expense of efficiency. Modern ECUs vary the ignition timing to suit the engine conditions and in some cases the valve timing can also be varied (Honda & Jaguar for example) to minimise these losses. Losses within the engine vary with engine speed. There are significant losses due to windage (that is air resistance on all the bits going round, inside as well as outside the engine), aerodynamic pumping losses on the three useless strokes and shearing losses within the lubricant - all generally increasing with speed. I was taught that, in general, the most efficient speed to run an engine was at its' peak torque revs and theoretically you gear the car to run at the required crusing speed at its peak torque point to achieve the best fuel economy. Normally this "overgears" the engine and reduces the achievable top speed - hence old-fashioned cars with overdrives were faster in top gear than overdrive (my old Triumph Stag for example). However aerodynamic drag on the car body increases enormously with speed - I believe it is a third power law which is why normal saloons can do 100mph on 100bhp but a supercar needs many hundreds of bhp to do 200mph.

All these variables make it very difficult to predict the exact conditions for optimum miles per gallon but I would think as slowly as possible in top gear but not too far back down the torque curve from the peak torque point. In most cars I would expect this to be between approximately 25mph and

Andrew Graham

Sir Stanley Hooker of Rolls Royce once remarked that a 4-stroke engine has one stroke to produce power and three to wear the engine out.

Well smooth is quick! :p

I guess it depends on how you read it! Going downhill is probably going to get better (lower) consumption.

The message from "scott" contains these words:

There is a grain of truth in that - engines are more efficient at higher compressions, and of course a petrol engine at low throttle has a very low /effective/ compression ration. However, the richening of the mixture which most ECUs provide at full throttle may well more than outweigh this.

Diesels, of course, are a different matter entirely.

Ding!

The answer was staring me right in the face.

The difference in consumption during my regular commute is not significant if I vary the maximum calculated load from 80% to around 100%. But it'll reach a calculated load of 100% with only around 45% Throttle over around

Limit myself to around 45% Throttle (up to around 95% Calculated Load) and I get better (lower) consumption than if I use higher throttle settings, but it's possible to maintain good progress without enriching the fuel mixture.

Apparently that is correct, though I dont know about "floored" - I read somewhere its more "strong acceloration" rather than max acceloration, changing gear just after the particular engines torque peak; NOT at the red line. i.e. yoou are running the engine at around its more efficient pumping speed.

Tim..

Nov 2004 18:34:33 -0000:

Is it really that simple? For the same power, at lower rpm, the forces on the bearings will be higher. Friction is related to force, so there's definitely a source of higher friction at the lower speeds.

Things are a lot more complicated than you're making out - for a given power output, plotting efficiency against rpm is most definitely not a straight line tending ever downwards. There will be a maximum, and knowing this could determine which gear you want to use. (I say could, because other factors, eg the ability to accelerate may be important).

clive

The message from "DervMan" contains these words:

Put a wedge under the gas pedal? I heard of instructors doing that - which seemed stupid to me.

The message from "Clive George" contains these words:

Though, of course, power lost is proportional to the friction muliplied by the distance over which that friction operates. At low revs the piston rings, for example, will be covering much shorter distances per unit time than at high revs.

Quite possibly.

A 12bHp motorcycle can do 60-70, a 24bHp can do 75-90 etc...

In news: snipped-for-privacy@paul.vlaad.co.uk, Paul Cummins decided to enlighten our sheltered souls with a rant as follows

Citroen 2CV, 29 bhp, 75ish mph....

No, any F.I post 92 engine remains at Stoic at any given throttle position and may do upto WOT (depending on management programming) though unless its quite a late design they usually switch to an-engine preserving rich-ish mixture after about 90% throttle. So best fuel efficency has to be when the engines pumping losses are lowest and effective compression ratio is highest- which is at max torque rpm figure.

turbo's are a different matter mind- again depends on sofotware, and whether its a shopping cart or performance car.

The best mpg would in theory would be with a car with a decent low loss lockup-able CVT transmission, (honda Jazz) probably a very low pressure turbo, with twin plugs running on the atkinson cycle (toyota prius) and with BMW's throttleless valvetronic.

Tim..

New Fiat Panda, 60 bhp, 105mph

Diversion 600, 60bhp, 110 mph

Citroen BX TRD, 65 bhp, 110 mph

Kawasaki GT750, 80 bhp, 125 mph

So yes, there does seem to be a limiting increase in return per horse.

Yes, driven all and own all of them.

Well AFAIK the drag force increases with speed squared (roughly), but the power required to keep a constant speed increases with drag cubed (because power = speed * force)

I have several graphs showing the efficiency of engines against rpm and torque, but it would be quite interesting to combine these with a speed cubed rule to work out the mpg figure at every point on the graph. Haven't got time at the moment to do this though.

Hehe, that's why my 2-stroke in my model car can rev up to 36krpm and is oh so efficient ... NOT!!! :-)

At low rpms the power lost due to inefficient burning (or other non-friction things) outweight the friction losses.

As Clive said, the graph of efficiency against rpm is certainly not a straight line. It has a peak at a given rpm, and the rpm of this peak will depend on what power you want. (normally higher power = higher rpm that is most efficient).

FWIW one of the graphs I have shows that the maximum efficiency of this engine occurs at about 2800 rpm and with about 85% of the maximum torque at that rpm. The efficiency at this point is about 245 g/kWhr. If you follow the line of constant power down to lower rpms, the fuel usage increases to about 300 g/kWhr, and if you follow it to higher rpms it gets up to about the same. For comparison, at 1000rpm using a small amount of throttle the efficiency is around 500 g/kWhr, almost double the fuel usage.

If anyone wants the whole graph I can email it to you, unfortunately I don't have any web space to host it on atm.

Ooh, please.

If you replace xxxx-x with evil-c you should find me.

cheers, clive

Which one? 1.7, 1.9 or 1.7 TD? BHP is that for a 1.9, and I thought that topped out just under 100.

cheers, clive