Fuel efficient driving.

Kilty_2

Yeah the salesman said itd take a couple of thousand miles to start returning optimal fuel economy - it's not that bad at the moment though, done 180 miles to about half a tank (accurate measurement there :rotfl:)

cyclonebri1

Wig wrote: »

Strange how people say something as though it is a fact without any evidence to back up their assertion.

And, I am IMO in perfect control of my car when I do this, thanks very much, never had a problem with it. All the nannies can say what they like, in response to this post, fact is, I just slip out the clutch and I'm in gear again if I ever need to be. I use the brakes If going down a steep hill to stop the car reaching a silly speed. No danger, no nannies, no problem, no discussion - in other words, you have your opinion, I have mine & I won't respond any further to any comments on the "not being in control" myth.


BTW Thanks Tom for a great post.

I totally agree, I never could understand the blurb that said "if you were in neutral, then you weren't in control".

Brakes work regardless of gears/speed. Try as I may I can't figure an emergency condition where I could accelarate quicker than if I took that condition into account and gear selected accordingly???

But I am assuming a modicom of driving ability that is sadly lacking at present.;)

bengalknights

Limey wrote: »

None of the above instructions apply when driving my everyday runabout. I've tried and no matter what it gets a stirling 22mpg.





At least you never have to slow down as everything gets out of your way. :eek:

You could always try and convert it to LPG wont improve mpg but cost of driving will decrease.

optimus_primera

DirectDebacle wrote: »

And the dangerous junk bolted to the front will ensure maximum damage to a pedestrian who can't get out of your way. Good way to have negated 25 years of vehicle safety research and development though. Nice one.

Yeah, cos getting hit by a flat fonted 2 ton vehicle without a bull bar will be much safer .........

Fridge3

tomstickland wrote: »

INTRODUCTION
With fuel prices where they are I thought I'd finally get round to writing up some thoughts that I've had regarding fuel efficient driving. I've travelled around 3 x 550 mile tank loads whilst thinking about this. So this is the result of around 20 hours of thought.

I was also inspired by an article in the Guardian where an instructor obtained far better lap times than a journalist at the same time as obtaining higher mpg. The reasons for this are clear with a bit of thought.

RATE OF ACCELERATION - WHO CARES?
Often when I read articles or comments about fuel efficient driving then people talk about "gentle acceleration". My opinion is that, within reason, rate of acceleration is irrelevant to obtaining good mpg.

The area to look for gains is in the area of deceleration. Again, there are sound reasons for this.

ACCELERATION, CRUISING, DECELERATION
Consider a vehicle moving from rest up to a speed, travelling on a flat road at that speed for some time and then slowing down. The acceleration phase will require the largest forces to act on the vehicle and consequently will require the most power output from the engine and will consume fuel at the highest rate. The constant speed part will only require forces to overcome rolling resistance and aerodynamic drag. This will require some energy input from the engine and a lower and constant rate of fuel usage. The slowing down part will require virtually no fuel - maybe none if engine braking is used.

If there were no resistive forces due to rolling resistance and aerodynamic drag then the engine could be switched off once the acceleration had finished and then the vehicle would roll all of the way to the destination. Fuel economy would be very high. The longer the journey then the higher it would be. Obviously, in practice the drag forces cause vehicles to slow, so the engine burns fuel to generate the forces to balance the drag out. These forces scale with speed squared, so fuel economy should drop off rapidly as cruising speeds are increased.

Now consider the accelerator pedal position during the example. It will be pressed most during the acceleration phase, a lot less so during the constant speed section and not at all during breaking.

WHY I REALLY DON'T CARE ABOUT ACCELERATION
So why is rate of acceleration unimportant? Because a higher rate of acceleration is balanced out by an equivalently shorter time of acceleration. Twice as much acceleration for half the time - it balances out. If a driver wishes to accelerate from zero to, say, 60mph, then the kinetic energy put into the vehicle is the same regardless of rate of acceleration. Taking 20s to reach 60 with the throttle hardly pressed is no better than 10s with twice as much throttle.

WHAT IS MOST EFFICIENT RATE OF ACCELERATION?
Drive train losses will scale with torque levels and engines will run rich at maximum throttle, so there will be a loss of efficiency at maximum throttle. However, engines run more efficiently at 50-80% loading, so quite vigourous acceleration is not a problem. A vehicle that accelerates quickly will have a slightly higher average speed over the distance covered, but this effect will be tiny when the distance is a typical value.

See hypermiling for discussion of this.

DRIVER DECISIONS
It is assumed that a driver has selected a speed at which they wish to travel, so it is a foregone conclusion that they will use a certain amount of fuel to turn into the kinetic energy that the vehicle will possess when moving at that speed.

Also locked into that choice is the resistance forces to be overcome when moving at that speed and hence the rate of fuel usage during the cruising part.

LOWER CRUISE SPEED WILL SAVE FUEL
If the driver decided to lower their cruising speeds then they could improve their mpg. This effect will work down to quite low speeds. At very low speeds the engine will not be loaded hard enough and its efficiency will fall off. So the most efficient cruise speed is going to be somewhat lower than the 56mph in 4th or 5th that most manufacturers design for - it will possibly by in 2nd or 3rd gear at the optimum engine rpm (somewhere around 2-3K). I've got some engine efficiency maps that will show this.

HOWEVER
Personally I don't want to travel more slowly. I've got somewhere to go and a certain amount of time to do it. I want to make sure that I get the best mpg for the speeds that I want to cruise at.

The only area of choice left in this example is in when to start slowing down.

MOMENTUM
A vehicle travelling at a certain speed has enough energy in it to allow it to travel on for a distance with no more engine input. In crudest terms, and against good driving practice, the engine could be turned off and the vehicle would roll on, with resistance forces gradually using up that energy and lowering the speed of the vehicle until it eventually stops.

The most efficient system (against good driving practice) would turn the engine off at the point where the vehicle could then roll all of the way to the destination.

In the real world anticipating a stop event and starting the slowing in good time, using gradual engine braking is the most obvious area where energy wastage can be minmised.

BRAKING IS BAD
All braking is clearly throwing away energy. It turns the kinetic energy of the moving vehicle into heat. That energy came from the engine in the first place. The total amount of braking done is a measure of how much energy is being wasted.

Gradual slowing by coming off the throttle in the cruising gear and then gradually moving down the gears as the rpm falls too low is the method that is being advocated. Most engine management will cut the fuel to the engine under these circumstances.

Extreme engine braking is still braking. The higher the rate of deceleration then the more wasteful any slowing method is, regardless of whether it's engine braking, the brakes themselves or a parachute out the back of the vehicle. None of those methods will ever reclaim the energy that's being chucked away.

COASTING VERSUS ENGINE BRAKING
Coasting is bad driving practice, so this should not be discussed!
However, from a point of view of the physics of it I suspect that coasting with the engine running would actually give the best fuel efficiency.

A FUEL EFFICIENT DRIVER
A fuel efficient driver should concentrate more on planning ahead to avoid braking than thinking about acceleration rates.

At all times such a driver is looking well ahead, planning and anticipating. Their aim is to keep moving at all costs and avoid braking. This means commencing the slowing operation at a carefully judged time and position. The amount of braking on a journey is a measure of how badly and inefficently the driver is operating. Vigourous acceleration is fine, as long as the speed that is gained is used to cover distance, not thrown away in braking.

SOME SITUATIONS

Stuck behind a bungee driver who moves at 45mph on an open A road, brakes hard for every corner and then accelerates gradually back to 45mph

Most efficient solution: build a buffer zone and travel at an average speed that catches them at the corners but allows them to pull away on the straights.

Next most efficient solution: accelerate hard to overtake them. This isn't too bad in efficiency terms providing that the speed gained is all used to cover ground. If after overtaking hard braking is needed then it's wasteful.

Least efficient solution: Follow them closely also braking for every corner. This is inefficent because the unneccessary braking throws away energy.
Approaching a bottleneck, vehicle approach from the other direction but still space to get through if done promptly

Coming to a halt will be more wasteful than accelerating hard and getting past the bottleneck. All of this assumes that to do so isn't considered too risky.

Following a driver who stops at junctions even when they are clear.

Allow a bigger gap to build up and then roll over the junction at a safe speed. This can often be 15-20mph.

Twisty road.
Plan ahead so corners can be taken without braking. This is why the instructor did better than the journalist in the Guardian article. Journalist was not a regular driver and braked for the corners and didn't carry enough speed round them, then accelerated out the other side. Instructor carried the speed round the corners.

Instructor could have obtained even better mpg by doing longer lap times. As it was, they were efficient enough to obtain better mpg than journalist with short lap times.

TRIALS
I've been doing some trials to see how much difference only changing braking behaviour can make.

A plot of my speed profile on a road would be show reasonably hard acceleration, then a very smooth speed profile with no hard braking. It's actually quite a fun challenge and nothing spoils it more than occasional occurances like people braking hard and turning off or traffic lights turning red when you are about to roll over them.

RESULTS
Results In a 1.9 TDi Diesel Golf. Motorways (70-90plus), A roads (60 and overtaking), urban areas (at posted limits), rural roads etc

-previous worst mpg: 42mpg. Extended period on motorway at 90mph plus, purposefully fuel inefficient driving on A roads.
-previous typical mpg: 45mpg
-previous best mpg: 51mpg (low speeds (60ish) for extended periods on motorway)

Results on 3 tanks where only braking and slowing has changed:
49mpg
51mpg
51mpg

Percentage improvement for 51/46 mpg: 10%.
I'm still driving at the same cruise speeds as before, I'm still overtaking slow traffic, I'm still doing a few bits at 90mph plus or longer periods at 80mph indicated. If I lowered those cruise speeds then I'd expect to do better.

CONCLUSION

I've said the same thing several times here. Most of it is pretty obvious with a bit of thought. It's basic good practice. But I've read loads of irrelevant advice on efficient driving and thought it would be good to look at this.

SUMMARY
Things that I would classify as unimportant:
-gentle acceleration
-removing items from boot of car
-only half filling tank with fuel to save weight

Things that are important:
-lowering cruise speeds
-planning ahead to avoid unnecessary braking
-tyre pressure
-tracking (wheel alignment)
-roof racks, open windows
-significant weight loss, but hardly practicable for most vehicles. Most significant for stop - start since weight only really affects the acceleration fuel consumption.
-small cross sectional area vehicles (aerodynamic drag). Most important if long periods at high cruise speeds.

https://forums.moneysavingexpert.com/discussion/674625

skiddlydiddly

cyclonebri1 wrote: »

I totally agree, I never could understand the blurb that said "if you were in neutral, then you weren't in control".

Brakes work regardless of gears/speed. Try as I may I can't figure an emergency condition where I could accelarate quicker than if I took that condition into account and gear selected accordingly???

But I am assuming a modicom of driving ability that is sadly lacking at present.;)

Car coming towards you, another car suddenly pulls out in front of you.You can't brake enough to avoid hitting it up the ar** but putting your foot down going an exta 5mph would let you drive around on the other side of the road and still miss the oncoming car.Having to dip the clutch and pick a gear when you have no reference point takes time.

Thats one off the top of my head .

AndyGuil

Take your foot off the throttle and use engine braking. The injectors will be turned off and no fuel will enter the engine until the rpm reaches tickover. It is good when you need to reduce your speed or control it going down a hill. You can rest your foot lightly on the brake pedal if you need to warn drivers behind you are slowing down.

marvic31

Anybody else confused? :huh:

Altarf

tomstickland wrote: »

Personally I don't want to travel more slowly. I've got somewhere to go and a certain amount of time to do it. I want to make sure that I get the best mpg for the speeds that I want to cruise at.

Up until about two years ago, I commuted about 130 miles a day by car, and I used different routes to work and home due to the different traffic conditions.

On the route home, which was mostly dual carriageway A roads with some motorway, I found that it was irrelevant whether I did 60mph or 75mph, you got home at almost exactly the same time. The reason being was that there were a number of 'bottlenecks' (junctions, roundabouts, etc) and if you did 75mph, you just arrived at it sooner.

It was always quite amusing to pootle along at 60mph, and catch up three, four or even five times on my home with cars that had zoomed past, and even 'overtake' them as they never seemed to work out that the queuing traffic in the slow lane leading to the roundabouts usually moved faster than the queuing traffic in the fast lane.

So slowing down made no difference to journey time, but did decrease fuel consumption.

Now I only drive 3 miles to the railway station, so instead of getting 45-48mpg, I only get 30-35mpg, and as low as 20-25mpg in the winter.

cyclonebri1

skiddlydiddly wrote: »

Car coming towards you, another car suddenly pulls out in front of you.You can't brake enough to avoid hitting it up the ar** but putting your foot down going an exta 5mph would let you drive around on the other side of the road and still miss the oncoming car.Having to dip the clutch and pick a gear when you have no reference point takes time.

Thats one off the top of my head .

Ah but assuming you were driving normally, you would likely not be in the exact gear to give you maximum accelaration at that exact instant.;);). Plus in that scenario if there was an oncoming car I would "go for" the car that had pulled out in front of me rather risk a head on collision.

In spite of having had fast cars in the past the argument of driving out of trouble has always seemed flawed to me. But then again we all drive differently.