Driving style to increase MPG

barura

I noticed a change in my MPG while driving on motorways/dual carriageways. It increased even though the RPM was around 4.2k on my little 999cc engine. From about 42~mpg To the upper 40's. Instead of 20eur getting 125~miles, I was getting upper 140, 150miles per 20euro.

So, to see how far I could get 20eur of petrol to get me, I've decided to change my driving style. From getting to 5th and keeping the revs in around 2.1k @ 60km/h, I'm sticking in 4th and the revs around 2.5k at the same speed. This is due to a lot of urban driving that I do so generally I don't go above 60/80km/h.

Now the reason for this, I figured, is that the car has better "pull" in these ranges 2.5krpm and up. The torque is a lot easier to access and as such the engine actually works more efficiently, even if the revs are higher.

I'm currently looking at 100miles so far and judging by the pin on the fuel gauge, I'm expecting another 40miles from this 20euro.

Obviously, I will do this the proper way by brimming my tank when I have funds, but I don't really have 70euro to spend doing that at the moment. :P

The thing I'm also trying to prove (hopefully) is that contrary to the "keep revs low and you will save on fuel" is generally on cars where the toque/power is from the low revs, such as higher capacity cars. I'll report back with my findings after a few more refills, but this is something that I think people with smaller engines might be under a misconception of. Anything that can save money is a good thing, right?

-Corkie-

Good idea but all that concentrating on mpg will drive you potty...

CiniO

I've got very similar observations to yours.
Generally in any petrol engine, I wouldn't really drive under 2000 rpm.
Usually I try to drive at range of 2500 - 3000 and I find it the most economical in most petrol engines.

SilverBell

barura wrote: »

I noticed a change in my MPG while driving on motorways/dual carriageways. It increased even though the RPM was around 4.2k on my little 999cc engine. From about 42~mpg To the upper 40's. Instead of 20eur getting 125~miles, I was getting upper 140, 150miles per 20euro.

So, to see how far I could get 20eur of petrol to get me, I've decided to change my driving style. From getting to 5th and keeping the revs in around 2.1k @ 60km/h, I'm sticking in 4th and the revs around 2.5k at the same speed. This is due to a lot of urban driving that I do so generally I don't go above 60/80km/h.

Now the reason for this, I figured, is that the car has better "pull" in these ranges 2.5krpm and up. The torque is a lot easier to access and as such the engine actually works more efficiently, even if the revs are higher.

I'm currently looking at 100miles so far and judging by the pin on the fuel gauge, I'm expecting another 40miles from this 20euro.

Obviously, I will do this the proper way by brimming my tank when I have funds, but I don't really have 70euro to spend doing that at the moment. :P

The thing I'm also trying to prove (hopefully) is that contrary to the "keep revs low and you will save on fuel" is generally on cars where the toque/power is from the low revs, such as higher capacity cars. I'll report back with my findings after a few more refills, but this is something that I think people with smaller engines might be under a misconception of. Anything that can save money is a good thing, right?

Interesting experiment. Normally engines are more efficient under load. I would think you might get worse mpg with what you propose but it'll all come out in the figures. Best of luck and report back here.
G

x in the city

-Corkie- wrote: »

Good idea but all that concentrating on mpg will drive you potty...

yes lol

barura

-Corkie- wrote: »

Good idea but all that concentrating on mpg will drive you potty...

Nah it won't, it's driving in 4th as opposed to 5th. It's not like it plagues my mind at night!

CiniO wrote: »

I've got very similar observations to yours.
Generally in any petrol engine, I wouldn't really drive under 2000 rpm.
Usually I try to drive at range of 2500 - 3000 and I find it the most economical in most petrol engines.

You posted that you had a 1.2 fiat in a thread, right? Mate of mine drives one and found similar as well.

dgt

CiniO wrote: »

I've got very similar observations to yours.
Generally in any petrol engine, I wouldn't really drive under 2000 rpm.
Usually I try to drive at range of 2500 - 3000 and I find it the most economical in most petrol engines.

barura wrote: »

You posted that you had a 1.2 fiat in a thread, right? Mate of mine drives one and found similar as well.

That would be me

Our standard 1.2 does 48mpg on an economy run while mine does 51mpg if I drive it very handy (that would drive me mad though, it averages about 44mpg normally) as the cat is removed I have a lot more low down grunt than normal, hence I can drive 50kmh in 6th gear @1400 rpm I also find the car does better mpgs at 100kmh as opposed to 80kmh

Driving at a constant speed for prolonged periods increases mpgs

BrianD3

I have noticed this but the car needed to be close to labouring before I noticed lower revs causing economy to disimprove. I wouldn't have thought that a 1.0 litre NA petrol car would be labouring or close to it at 2k revs.

IIRC the 1.5 DCi Megane that I sometimes drive gives better economy in 4th at 50 km/h than it does in 5th at 50 km/h - but the car is on the verge of labouring in the higher gear. So I don't use 5th below 60 km/h

My 1.6 Laguna petrol is better that the 1.5 DCi at low revs, it is happy at 55 km/h in 5th and I often drive at <2000 rpm. My average for the last 10,000 miles is 44 mpg which I think works out at ~135 miles for 20 quid at current prices.

Notwithstanding the comments above about the 1.5 DCi, it is way more economical than 44 mpg.

In general I find the key to economical driving is reading the road, braking as little as possible, lifting off the throttle way in advance if you need to stop or slow down and utilising DFCO (deceleration fuel cut off)

barura

It's not labouring, but it's not as pokey. 1.5k in 5th and it's pretty unhappy.

And I do lot of stop start motoring, so it's also got that going for it as well. Though I would expect a small engined car to be more economical than a bigger one in those conditions.

CiniO

barura wrote: »

Nah it won't, it's driving in 4th as opposed to 5th. It's not like it plagues my mind at night!



You posted that you had a 1.2 fiat in a thread, right? Mate of mine drives one and found similar as well.

Fiat bravo 1.2 yes definitely.
But I have a mazda 6 2.0 as well and observations are similar.
Onboard computer shows tiny bit better MPG on 4th gear at 2500rpm than at 5th at 2000rpm (both at the same speed).

But generally any petrol engine I drove was a bit like that.
As I said my golden rule is to never drive under 2000rpm any petrol engine. Trying to keep it between 2500 and 3000 seems to be reasonable. When climbin uphill - more rpm depending on gradient of the hill.
Obviously when keeping revs at 5000rpm, MPG will go down.
And as well all engines vary, and have max torque and max power at different rpm's but general rule to keep it above 2000rpm works for me anyway.

barura

Thanks for that! That's good to hear.

I'll be sure to post my results.

CiniO

dgt wrote: »

That would be me

Our standard 1.2 does 48mpg on an economy run while mine does 51mpg if I drive it very handy (that would drive me mad though, it averages about 44mpg normally) as the cat is removed I have a lot more low down grunt than normal, hence I can drive 50kmh in 6th gear @1400 rpm I also find the car does better mpgs at 100kmh as opposed to 80kmh

Driving at a constant speed for prolonged periods increases mpgs

Assuming you've got 6 gears it probably must be stilo or punto.

Anyway - mine bravo 1.2 does about 50mpg when driven very very handy. (at constant speed in 5th gear at about 2500 - 3000 rpm). Speeds about 85 - 100km/h.

It was bit better on 175/65 tyres, but since I swapped for 185/60, 50mpg is max. Handling though is better.

Normally though, I floor it as much as I can, and get something like 38mpg, which I still think is good, as I barely go lower than 4000rpm then;)

BostonB

Braking less often will help too. Read the traffic well in advance so you don't have to use the brakes or accelerate. Lighten the car, drive with half tank of fuel etc. Don't carry loads of stuff in the boot etc.

Personally I wouldn't have the patience.

dgt

CiniO wrote: »

Assuming you've got 6 gears it probably must be stilo or punto.

Anyway - mine bravo 1.2 does about 50mpg when driven very very handy. (at constant speed in 5th gear at about 2500 - 3000 rpm). Speeds about 85 - 100km/h.

It was bit better on 175/65 tyres, but since I swapped for 185/60, 50mpg is max. Handling though is better.

Normally though, I floor it as much as I can, and get something like 38mpg, which I still think is good, as I barely go lower than 4000rpm then;)

Nope its a Bravo 1.2 with a 6 speed fitted from a Punto I've lost full lock on the right as a direct result of that. I found the extra gear leaves the ratios closer, much easier on petrol town driving

Its on 205/40R17s and while this has blunted the performance slightly it does leave it easier on the long run. I don't recommend 205/40s on a 5 speed box mind you...

shedweller

BostonB wrote: »

Braking less often will help too. Read the traffic well in advance so you don't have to use the brakes or accelerate. Lighten the car, drive with half tank of fuel etc. Don't carry loads of stuff in the boot etc.

Personally I wouldn't have the patience.

Agreed on the patience. There comes a speed where your head just gets done in!
I've experimented with weight reduction and though it may work it is not really measurable. You'd need exactly the same weather conditions for a whole tank and exactly the same trips to come to any conclusion.

Overall though, i can't see this topic being too popular here.

Save your money elsewhere, but not on the roads!:rolleyes:

barura

shedweller wrote: »

Agreed on the patience. There comes a speed where your head just gets done in!
I've experimented with weight reduction and though it may work it is not really measurable. You'd need exactly the same weather conditions for a whole tank and exactly the same trips to come to any conclusion.

Overall though, i can't see this topic being too popular here.

Save your money elsewhere, but not on the roads!:rolleyes:

What makes you think I'm not saving my money elsewhere? I'm pretty bloomin' broke! If watching how I drive by being in 4th rather than 5th for going about my business in 60km/h area's where I do most of my driving, leads to an extra 15 miles per 20euro, then after 6 weeks that's 20euro I wouldn't have had otherwise. By the end of the year that's 200euro, which would pay my tax for the car. So doing something as simple as this will save me a bit of money in the bigger scheme of things.

And this is just with me doing 120 miles~ a week, what if it went up to 300 for a commute or if I started couriering? It's all useful data at the end of it. At least I'm trying to show people that higher MPG is possible without compromising performance. In fact, if you're in the power band of the engine, it increases performance while increasing efficiency, or at least, that is what I appear to be experiencing.

But you're right, my topic won't be popular here. I should have just posted a "What diesel should I buy for my 5 mile commute a day in tax band a" seeing as they're so popular around here.

dougie-lampkin

Better economy comes from knowing your BSFC, not the Irish mentality of going straight from 1st to 5th as quickly as possible. BSFC is a true measure of your engine's efficiency.

On small bore petrols, the BSFC tends to be further up the rev range, compared to diesels or large bore petrols. Here's an example from a 1.0L Suzuki Swift:



The idea is that your engine is more efficient the lower the BSFC reading, so you should try and spend as much of your time in the 250 island as possible. Here it goes up to 3500 RPM, far higher than you'd think would be economical. If you were to shift at ~4000 with this engine, you'd be just past peak torque, so after the gear change you'd be back in the 250 island, and just below peak torque, which is about as economical as possible short of fitting a CVT.

Of course, the idea behind keeping near peak torque is that you should only be gently tipping the throttle at all times, not flooring it up through the gears. If you can find a routine and stick with it, it isn't as frustrating as you'd imagine.

I'd imagine your 999cc would be close enough to the Suzuki map, it's worth a try and see if your economy increases. You really would want a BSFC map for your particular engine to get the benefit of this driving style however.

Voodoomelon

Thats some interesting stuff. Is there an online library for all cars per chance?

Mr Benevolent

Very interesting. My Corsa matches that BSFC chart almost exactly, which matches driving in 4th or even 3rd instead of 5th. 3000rpm in 3rd is noisy in a Corsa B.

One thing though, how do you control torque on the chart? For example, on that chart best SFC is between 2500-3500rpm @ 55Nm torque, yet at 2800 it's giving @77Nm anyway. How do I maintain 55Nm? Or am I being stupid?

CiniO

Confab wrote: »

Very interesting. My Corsa matches that BSFC chart almost exactly, which matches driving in 4th or even 3rd instead of 5th. 3000rpm in 3rd is noisy in a Corsa B.

One thing though, how do you control torque on the chart? For example, on that chart best SFC is between 2500-3500rpm @ 55Nm torque, yet at 2800 it's giving @77Nm anyway. How do I maintain 55Nm? Or am I being stupid?

I think that whole case is way more complicated than dougie-lampkin described it.

About controlling a torque it just depends on how much you press your accelerator pedal.
Like if your revs are at 2800rpm and you press the foot to the floor, you will get the max torque possible which in this example is 77Nm.
If you press it gently just to maintain a steady speed, a torque will be much smaller.
If you release the pedal, and let engine slow down, then the torque will be actually negative (like minus 20Nm or sth).


The problem with a chart is, that those islands there show fuel consumption in g/KWh (grams per kilo-watt-hour). KWh is an unit of energy. While engine speed (revolutions) multiplied by torque is power, which is different thing to energy.


Therefore it looks like, that most economical would be revolutions between 2500 and 3500 at torque around 55Nm , with consumption of 250 g/KWh or lower. While whatever torque you are using at say 1500rpm, consumption would be bigger.
But it only looks like that - it's not true.

Problem here lies in fact, that no resistance is taken under consideration (rolling resistance, air resistance, etc).

Looking at the chart if you on certain gear (say 5th) at 3000rpm using a torque of 60Nm to keep you going at steady speed (to work against resistance). Your fuel consumption would be about 260 g/KWh.

Now if you slow down by half on the same gear to 1500rpm. You will need significently smaller torque to keep you going, as resistance will fall gradually (say 30Nm will do). So now your fuel consumption per KWh has risen from 260 to about 400. But considering your engine speed has decresed x2, and your torque has decresed x2, then your power has decreased 2x2 = 4. Power is 4 times less. Therefore you are traveling twice slower with 4 times smaller power, so you use 2x less energy.

So assuming to travel certain distance (few miles) you need (lets guess) 10KWh of energy in first case (at 3000rpm), so your consumption will be 260*10 = 2600g.
In second case (1500rpm) to travel the same distance you will need only 5KWh, so even consumption per KWh is bigger (400), but in total 5*400=2000g so you will use less fuel to travel the same distance.

Of course all my figures were just as an example, and has nothing to do with reality.
But that's the principle, and that actually proves that from this chart we generally know nothing.

Only what the chart tells us, is at what engine speed and torque this particular engine is the most efficient. If you wanted to run a f.e. electricity generator by this engine, that central island on the chart would be the perfect conditions to gain biggest amount of electricity for lowest amount of fuel.

But in car, there's so many other conditions (like gear ratios, air resistance, etc), that generally speaking we know nothing from above chart.

Dardania

That BSFC info is a very powerful way of optimising your driving style (even though it doesn't take into account gearing & wind resistance)

Is the BSFC graph something that manufacturers should give with their cars?

barura

Woo! Science! I'll be filling her up tomorrow. After doing it the way I am currently doing it for the next 4 refills, I'll know for sure if it works out better. I'm sure a whole thesis could be done on this, but I don't really have the time to invest in the investigation of this, though that data is definitely interesting...

BostonB

I guess you can only work it out by doing some of your own testing, based on your own usage.

Dardania

use something like spritmonitor.de to track your consumption, and keep notes...

dougie-lampkin

Confab wrote: »

Very interesting. My Corsa matches that BSFC chart almost exactly, which matches driving in 4th or even 3rd instead of 5th. 3000rpm in 3rd is noisy in a Corsa B.

One thing though, how do you control torque on the chart? For example, on that chart best SFC is between 2500-3500rpm @ 55Nm torque, yet at 2800 it's giving @77Nm anyway. How do I maintain 55Nm? Or am I being stupid?

The problem with small cars as you say, is the noise and vibration of high revs. You would also look like a bit of a plonker driving close to 4000 RPM in a 1 litre.

Torque is treated as separate from the BSFC figures from my understanding, and the line with the x plots is the engine's torque curve. You need to find a balance between max torque (usually the point where the least throttle is required to maintain a steady RPM) and minimum BSFC.

As CiniO says, there are way too many variables to take into account out on the road. Especially with the artificial gizmos modern engines have. Another tip is to get a vacuum gauge. By experimenting with gear and throttle combinations, you can find the point where the least amount of fuel is being sucked into the engine. It's not as technical as BSFC maps, but it's a lot easier to use on the fly.

BostonB

dougie-lampkin wrote: »

The problem with small cars as you say, is the noise and vibration of high revs. You would also look like a bit of a plonker driving close to 4000 RPM in a 1 litre. ...

From memory 4k isn't that much in 4cyl 16v 1L. Redline is about 6k or something, and its pretty smooth 16v. A bit of noise sure. I've heard the newer 3cyl though are a lot worse for noise and vibration. Still reving the nuts of something, isn't usually the way to save fuel.

Mr Benevolent

BostonB wrote: »

From memory 4k isn't that much in 4cyl 16v 1L. Redline is about 6k or something, and its pretty smooth 16v. A bit of noise sure. I've heard the newer 3cyl though are a lot worse for noise and vibration. Still reving the nuts of something, isn't usually the way to save fuel.

All 3cyls are horrible, but the Yamaha designed Corsa 3 cyl is truly horrible to hear. Vibration is ridiculous and it's as harsh as prison love.

barura

Filled her up there. Go 141miles on that tank. Popped 20euro in again, lets see how this goes. Driving was mainly local stuff, hops of about 10-20miles at most.

CiniO

Confab wrote: »

All 3cyls are horrible, but the Yamaha designed Corsa 3 cyl is truly horrible to hear. Vibration is ridiculous and it's as harsh as prison love.

My father has a corsa 1.0 3cyls engine, and I must say that engine sound and level of vibration seems to be great comparing to his previous car which was Cinquecento 700cc 2 cylinder

barura

Isn't the recent reincarnation of that engine the twin air?

CiniO

barura wrote: »

Isn't the recent reincarnation of that engine the twin air?

I strongly doubt the new fiat 2 cylinder twin air has anything to do with that old engine.
The one in Cinquecento 700cc was a little bit modified version of 650cc engine from fiat 126, which was made from mid 70's

conzy

CiniO wrote: »

About controlling a torque it just depends on how much you press your accelerator pedal.
Like if your revs are at 2800rpm and you press the foot to the floor, you will get the max torque possible which in this example is 77Nm.
If you press it gently just to maintain a steady speed, a torque will be much smaller.
If you release the pedal, and let engine slow down, then the torque will be actually negative (like minus 20Nm or sth).

Most of your post seems like good info but im not sure about this bit. At 4000rpm the engine generates the same amount of torque regardless of how quickly the accelerator was pressed. And if you are doing 1200rpm and you put the accelerator flat to the floor you don't magically get the peak torque of the engine. That doest come until much higher in the rev range. Otherwise there would be no need for gearboxes!

BostonB

How can an engine produce negative torque?

CiniO

conzymaher wrote: »

Most of your post seems like good info but im not sure about this bit. At 4000rpm the engine generates the same amount of torque regardless of how quickly the accelerator was pressed.

Of course not.
The amount of torque on the chart is the maximum torque at given revolutions. That's maximum what engine can do. To get it, you have to press the pedal to the floor. Otherwise torque will be smaller.

Hence, that torque is the same for rotary motion, as force for straight line motion. Because engine shaft rotates, it generates torque, not force. But then this torque is converted to force because of the car wheels contact with surface.

Anyway - if what you are saying that engine is producing a set amount of torque at given revolution, then engine would be accelerating all the time. Doesn't make sense.

Assume you are driving at 60km/h. Your car need to be pushed with certain force, to oppose resistance (air resistance, traction resistance, etc).
To keep a car at steady speed, resistance force must equal pushing force.
That means that to keep car at steady speed, you need exact amount of force. So to achieve it, you need exact amount torque. And that's exactly what you are doing with your accelerator pedal. By pressing it, you increse the amount of torque the engine is producing. Maximum what you can achieve, is max torque for given revoultions, which you can see on the chart.

And if you are doing 1200rpm and you put the accelerator flat to the floor you don't magically get the peak torque of the engine.

Of course you don't.
If you floor it at 1200rpm, all you get is a maximum torque for 1200rpm according to the chart.

That doest come until much higher in the rev range. Otherwise there would be no need for gearboxes!

True.
You need gearboxes, as engine can reach it's maximum torque only at narrow range of revolutions.
But chart only show maximum torque for any revoultions.
It's up to driver's operating the accelerator pedal to adjust amount of torque engine is producing.

BostonB wrote: »

How can an engine produce negative torque?

Of course it can.
Or in other words it produces torque in opposite direction.
When you release you foot of the accelerator, then your engine is forcing wheels to slow down. For it you need a torque in opposite direction.

Dardania

BostonB wrote: »

How can an engine produce negative torque?

When the car is going faster than the engine (through the gears) are - its negative relative to your speed.

I have to say, sitting in traffic today I'm having second thoughts on a Cinios point that BSFC isn't as useful as thought because of variables down the line like gearing and wind resistance-they are variables you don't really have control over, so they're irrelevant...

Wind resistance is probably linear (or at least mostly so with speed) and the gearing is only related to the speed you want to go...

What would be interesting (and ill.post when I.find one) would be the chart for a variable valve timed engine...

SilverBell

Dardania wrote: »

I have to say, sitting in traffic today I'm having second thoughts on a Cinios point that BSFC isn't as useful as thought because of variables down the line like gearing and wind resistance-they are variables you don't really have control over, so they're irrelevant...

Wind resistance is probably linear (or at least mostly so with speed) and the gearing is only related to the speed you want to go...

Hi Dardania,
I'll try to answer two points there .
1 BSFC....Thats why no manufacturer claims a certain fuel consumption without backing it up by giving the speeds they tested it at.
Loads of people say, ah jaysus no one travels at a constant speed, ah jaysus, no two journeys are the same, but an engineer has to start somewhere to give a comparison. So while some factors might be out of your control, they are not irrelevant.

An engine is usually designed to tally with the gearing and intended use of the car, so that it will cruise economically in its best BSFC region at a reasonable speed, say 50 or 60mph depending on the vehicle.

2 Wind resistance..... is proportional to the square of speed.
Cheers G

CiniO

Dardania wrote: »

When the car is going faster than the engine (through the gears) are - its negative relative to your speed.

The easiest way to say it would be something like that:
if you press accelerator and your car accelerates - torque is positive.
if you release accelerator and your car decelerates - torque is negative.

I have to say, sitting in traffic today I'm having second thoughts on a Cinios point that BSFC isn't as useful as thought because of variables down the line like gearing and wind resistance-they are variables you don't really have control over, so they're irrelevant...

They are relevant very much I'd say.

Wind resistance is probably linear (or at least mostly so with speed)

Wind resistance is squarely dependent on speed.
Like at 100km/h your wind resistance would be 4x bigger than at 50km/h.

and the gearing is only related to the speed you want to go...

The problem is that according to chart, engine is most economical only in certain revolution and under certain torque.

Maybe I wrongly said that above chart is useless.
It's useless on it's own. We need much more information to calculate it properly.

According to the chart, you need engine running at certain revolution and producing certain torque. You would need to calculate what speeds your vehicle has in all gears, calculate all resistance for such speeds, and try to match the resistance with optimum produced torque. Then engine would be running the most efficiently.

But then again it still doesn't mean that would be the best way to save fuel, as by lowering speed even a small bit, you are lowering resistance significently (hence square relation of speed to wind resistance).
Then it might come up, that even engine runs at less efficient conditions (lower revs, or improper torque), and uses more fuel per produced KWh, but difference in amount of KWh needed to run the car at lower speed, is even bigger than difference in engine efficiency, and in the end, you need less fuel to travel certain distance.

My general point is that chart shows exactly the conditions when engine is running most efficiently.
But it isn't really linked that much with condition when car is running most efficiently.

SilverBell

CiniO wrote: »

My general point is that chart shows exactly the conditions when engine is running most efficiently.
But it isn't really linked that much with condition when car is running most efficiently.

Surely when the engine is running most efficiently, the car is then running most efficiently?

Designers try to match the loading conditions (drag, speed, rolling resistance), They can get all these parameters for a vehicle from CFD, models, or real tests. Then you match that to a torque and engine rpm so that the vehicle will then coast at a certain speed, when the engine is in its optimum area of the BSFC map.

Dardania

I agree with your point Cinios on the wind resistance, and that the faster one goes, the more resistant it will be - it's fun admitting you're wrong I've also found this page:

http://autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_110216/article.html & this page: http://www.camaros.net/forums/showpost.php?p=1381472&postcount=72

which seems to describe how your BSFC "sweet spot" changes depending upon the engine load, which corroborates your point that as external factors such as wind resistance, and the gearing ratios come into effect, the less important the full load BSFC chart plays in isolation...

If nothing more, once one has committed to driving at a particular speed (give or take 10 kmh) with certain fixed gearing ratios, you can optimise the speed to the engine (if you have a way of reading what the engine load is)

I wonder is that taken into account when expensive German saloons are brought out with 8 gear auto boxes, to match engine to gearing with some algorithms?

Shane732

Ha, you're getting 42 MPG!! I wouldn't be too worried about increasing it.

I'm getting half of what you're getting!!

SilverBell

CiniO wrote: »

Therefore it looks like, that most economical would be revolutions between 2500 and 3500 at torque around 55Nm , with consumption of 250 g/KWh or lower. While whatever torque you are using at say 1500rpm, consumption would be bigger.
But it only looks like that - it's not true.

Problem here lies in fact, that no resistance is taken under consideration (rolling resistance, air resistance, etc).

Looking at the chart if you on certain gear (say 5th) at 3000rpm using a torque of 60Nm to keep you going at steady speed (to work against resistance). Your fuel consumption would be about 260 g/KWh.

Now if you slow down by half on the same gear to 1500rpm. You will need significently smaller torque to keep you going, as resistance will fall gradually (say 30Nm will do). So now your fuel consumption per KWh has risen from 260 to about 400. But considering your engine speed has decresed x2, and your torque has decresed x2, then your power has decreased 2x2 = 4. Power is 4 times less. Therefore you are traveling twice slower with 4 times smaller power, so you use 2x less energy.

So assuming to travel certain distance (few miles) you need (lets guess) 10KWh of energy in first case (at 3000rpm), so your consumption will be 260*10 = 2600g.
In second case (1500rpm) to travel the same distance you will need only 5KWh, so even consumption per KWh is bigger (400), but in total 5*400=2000g so you will use less fuel to travel the same distance.

Hi Cinio,

I think you might have forgotten to take time into account. If you half the speed, you double the time. Therefore you will also need the same energy to cover the same distance?
G

celeniu

Lads,
All that science is great but one sure way of increasing MPG's is to stick the car into neutral when going down hills(even slight hills as the cars momentum will carry you)and let gravity work its magic.I estimate better mpg's of between 10% and 20%.Try it for a week and see.

CiniO

celeniu wrote: »

Lads,
All that science is great but one sure way of increasing MPG's is to stick the car into neutral when going down hills(even slight hills as the cars momentum will carry you)and let gravity work its magic.I estimate better mpg's of between 10% and 20%.Try it for a week and see.

I think you forgot about one detail.
If you stick into neutral, your engine will be idling and still need some fuel to keep it at low rev's.
If you leave it on gear while going down the hill, then engine is rotated by the force coming from the wheels, and no fuel is used at all. Injectors actually stop injecting any fuel.


I can't see then, where are you saving?

CiniO

SilverBell wrote: »

Hi Cinio,

I think you might have forgotten to take time into account. If you half the speed, you double the time. Therefore you will also need the same energy to cover the same distance?
G

Hmm. Not really.

Let's take a look at really basic physics.

W = F*d
(WORK = FORCE * DISPLACEMENT)

As it was said already by few people in this thread, wind resistance is squarely dependent on speed. Considering that at higher speeds wind resistance is the major resistance factor, we can assume that resistance at 100km/h will be 4x bigger than at 50km/h.
Force needed to keep car at steady speed is equal to resistance at that speed.
Therefore at 100km/h force needed to run the car is 4x bigger than at 50km/h.

Displacement is just a distance traveled, so at both speeds it's the same.

So generally speaking if force is 4x bigger, then work is 4x bigger as well.

To do 4x bigger work, you need 4x more energy.

So actually I was wrong before saying about energy x2, as to run the same distance at double speed, you need 4x more energy.

To be precise it might not be exactly 4x as this was only assuming all resistance is squarely dependent on speed. In reality it's only wind resistance. Other kind of resistance like rolling friction is generally linear to speed. Anyway wind resistance plays significent role at higher speed, so we can assume it's somethine between 3x to 4x more energy to maintain 2x bigger speed on the same distance.

NinjaK

what is an average mpg for a petrol car doing city/suburban driving, no motorway driving.

CiniO

NinjaK wrote: »

what is an average mpg for a petrol car doing city/suburban driving, no motorway driving.

What is the average number of pints a person drinks when going to pub?
If you know the result, multiply it by 7 and you will get the average MPG.

NinjaK

CiniO wrote: »

What is the average number of pints a person drinks when going to pub?
If you know the result, multiply it by 7 and you will get the average MPG.

:rolleyes:
im sure there is a rough number

CiniO

NinjaK wrote: »

:rolleyes:
im sure there is a rough number

I'm sure there isn't.

With prius you can theoretically go for as much as 70mpg, while in 4.7litre Jeep you might struggle to reach 15mpg.

NinjaK

CiniO wrote: »

I'm sure there isn't.

With prius you can theoretically go for as much as 70mpg, while in 4.7litre Jeep you might struggle to reach 15mpg.

average car, not those extremes

SilverBell

CiniO wrote: »

As it was said already by few people in this thread, wind resistance is squarely dependent on speed.

yes...I already said that:D

CiniO wrote: »

So generally speaking if force is 4x bigger, then work is 4x bigger as well.

sorry, you are right.... must have got really mixed up reading your first post.:o

SilverBell

celeniu wrote: »

Lads,
All that science is great but one sure way of increasing MPG's is to stick the car into neutral when going down hills(even slight hills as the cars momentum will carry you)and let gravity work its magic.I estimate better mpg's of between 10% and 20%.Try it for a week and see.

As Cinio has said, quite the opposite!
most modern cars can sense when the throttle is at idle, and the engine rpm is greater than idle, so fuel delivery is stopped.