Fuel efficient driving.

DirectDebacle

cyclonebri1 wrote: »

Sorry mate, but you still don't get the issue. If there is time to consider pulling out, getting in the correct gear, avoiding the car in front and avoiding the oncoming car, (after checking for any), and then checking behind to ensure someone hasn't pulled out to avoid the possible collision you are about to be involved in, then there is time to stop

I beg to differ. This is skiddlys scenario:

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 .

Now in all fairness it is his hypothetical example and when he invented it he built into it the only appropriate action to take.;)

Fair enough.

You have put forward a different hypothesis which is yours and you own exclusive rights to. In your scenario stopping is possible.

I reckon a no score draw is a fair result for something that never really happened.:D

My view is that if you are driving at the correct speed, in the correct gear and on the correct side of the road, then that is a good position to be in under most circumstances.

cyclonebri1

DirectDebacle wrote: »

I beg to differ. This is skiddlys scenario:



Now in all fairness it is his hypothetical example and when he invented it he built into it the only appropriate action to take.;)

Fair enough.

You have put forward a different hypothesis which is yours and you own exclusive rights to. In your scenario stopping is possible.

I reckon a no score draw is a fair result for something that never really happened.:D

My view is that if you are driving at the correct speed, in the correct gear and on the correct side of the road, then that is a good position to be in under most circumstances.

Ok fair comment, but I will say in an emergency situation, you do have only 2 choices. Acceleration does not even come into it.

In a split second you decide to either brake or swerve.
They are the only 2 options.
Overtaking would never be an option, there is simply not enough time to consider a third.

Harry_Flashman

I have to admit coasting an awful lot. Almost always decelerate out of gear. It does help me beat manufacturers quoted MPGs though - on every car I've owned.

tomstickland

Hi, sorry, short on time so haven't read any of the recent posts.

Someone asked what age car. It's a 1997 Golf 1.9 TDi AHU engine. It's got 190,000 miles on it and does chuck a bit of smoke sometimes when under acceleartion which is wasted fuel.

Considering that I peaked at 90mph for half an hour and the nature of my commute then I think 51mpg is pretty good. The book figure is 50mpg (I'll check).

I worked out that my journey to work is 12 miles long and has 12 potential stop points on it (crossroads, T junctions etc). It's also hilly and twisty.

This week I've lowered my cruising speeds by 10% everywhere in order to try and make a 20% improvement in mpg. I've got to 350 miles at half tank marker. Past experience has shown that the 3/4 tank point plus about 12% gives the mpg. I'm looking at about 520miles by the 3/4 point, which would be 59mpg.

Several of the posts I did look at earlier in the week:
-carburettors have acceleration enrichment. Yes. Fuel injected engines will also do this. So more petrol goes on under acceleration. Yes. That's not a problem as long as that fuel is used to produce useful work.

The same applies to the drag car comments. Yes, engines use more fuel per second when loaded hard. The important point is how much work is being done per unit of fuel. It will be better under a decent load (50-70%) than at low loads.

So more fuel is going in per unit time, but it's producing more work per unit of fuel.



A FEW MORE THOUGHTS

Looking at a bsfc graph suggests that lowish rpm (1,500 to 2,500rpm ) and mid range loads is best.

This suggests that best practice is to use mid range accelerator pedal position (certainly not very light throttle) and to change gear smartly. Not rev it hard.

Cruising will generally not load the engine enough for peak efficiency, but because the loads are quite low then the actual fuel consumption will be quite low. At low speeds the power requirement will be low but the engine will be in an inefficient load zone. Increasing the speed will increase the efficiency and increase the power demand (lower mpg), until there is some speed where the two combine to give the best result.

Matching the engine output to the cruise speed needs to be considered. A small engine and low speeds would allow a low power requirement and the engine to be operating at good efficiency. Hence a small engine and low speeds would give the best mpg. A small engine at high speeds will be working outside it's most efficient load range.

Hence why a small engines are not always best for motorway use.

A very large engine will not be loaded enough at moderate speeds, which is inefficient. It will need high speeds to be loaded properly, but then will require a lot of power to overcome drag, which will give poor mpg.

A rather perverse output of the simple model being used is that it would be more efficient to have short bursts of acceleration followed by the vehicle gradually slowing rather than constant speed operation. So the engine would spend short periods of time operating at good efficiency rather than an extended period off optimum.

I'll find some bsfc plots to illustrate this.

tomstickland

Engine braking
Engine off and coasting would give the most distance travelled per starting speed. Engine braking in a high gear is next. Extreme engine braking by selecting a low gear is as wasteful as using the brakes.

My statement is "the slowest rate of deceleration is best".

In practice this would mean using a high gear until the engine rpm reaches the idle speed and then selecting the next gear down.

Regarding coasting, it's bad practice.
However, coasting with the engine off would be the most efficient.
Coasting with engine on versus engine braking is hard to decide.
The idling engine needs fuel to tick over - to turn the engine against frictional losses, but then you're not taking this energy from the moving vehicle. If the car is slowed using engine braking then this energy is not taken from the fuel but from the momentum of the vehicle.

To a first approximation the two things cancel out. ie: What you save in fuel by using engine braking is taken from the vehicle's kinetic energy.

tomstickland

skiddlydiddly wrote: »

Also, you are right about the speed difference over my example of a 1/4 mile but are forgetting that if say I'm doing 110mph at the 1/4 mile mark, if I slowly accelerated up to 110mph I would cover considerably more distance, easily
triple or quadruple so is more efficient.

You are not comparing like for like in terms of distance travelled. You have to allow the faster car to then cruise to the point where the slow car reached the target speed.

If your car has a trip computer with live mpg figures just watch how it drops when you boot it(obviously these aren't 100% accurate but give you an idea).

The instantaneous fuel consumption will be lower the more you press the accelerator pedal. That is true.

Let's say you do a moderate acceleration run and reach 60mph by the end of the drag strip.

Then you do a gentle acceleration run are only doing 30mph by the end of the strip and then have to travel on for another mile to reach 60mph.

Now go back and do the first run again. Once you've reached the end of the drag strip you have to continue for a mile at low throttle to maintain 60mph to the finish point.

Now compare the fuel used between the gentle run and the harder run followed by crusing.

During the slow acceleration run a fuel usage computer would show a lower figure than the moderate acceleration run did during its acceleration portion. But the acceleration portion was followed by a mile of much lower consumption as the engine was only having to work against drag forces to hold a steady speed.

Less fuel would be used at moderate acceleration compared with gentle acceleration.

The higher rate of acceleration does increase the average speed of the vehicle over the distance covered, so, yes it does lower the mpg slightly, but the gains in operating the engine in an efficienct region will outweight that. It will cause the optimum rate of acceleration to be bit lower than the peak efficiency point though.

tomstickland

AndyGuil wrote: »

Acceleration does affect the economy of the car. There is more than just kinetic energy, driving efficiently means around 30-35% of the fuel consumed is transferred to kinetic energy. The rest of the energy is heat and noise. Accelerating hard will increase the amount of heat the engine produces considerably, the higher RPM means there is more friction within the engine and gearbox and there is a much higher amount of noise. Heat energy is the highest energy loss in the process.

Avoid high rpm due to frictional losses in the engine (turned into heat as you say).
I didn't say hard acceleration.
I said acceleration that loads the engine decently.
Not very gentle acceleration, which creates the illusion of being efficient because an mpg gauge shows a low figure and it seems logical that "not much fuel is being used", but it goes on for a longer time and is an inefficient operating zone.

tomstickland

A lot of good points in this discussion.

I didn't word some things in the original post how I'd do it now.

You will have to prove it to convince me. The kinetic energy put into the vehicle is the same for each example but the engine will be requires to produce twice the force to accelerate in half the time. Your example only works out if the engine efficiency doesn't vary with power output or rpm.

Agreed - I very much think acceleration rate is important. With a lack of detailed information for every vehicle I would say "it's definitely not maximum throttle and it's definitely not very little throttle. It's somewhere in the middle."


Coasting with engine off obviously is the best.
See my update on coasting above for engine on situations.

I think I'll make a new master version based on the original plus amendments.

tomstickland

I really ought to go to bed.

Reading my original post, I actually disagree with some of it now! I was trying too hard to be provocative.

I'm going to write a new version next week based on the comments and feedback.

jeferey

Hi Tom - great post :T

tomstickland wrote: »

Engine braking
Engine off and coasting would give the most distance travelled per starting speed.
Engine braking in a high gear is next. Extreme engine braking by selecting a low gear is as wasteful as using the brakes.

Agree although sometimes extreme engine braking is useful to slow down quicker if some k**b pulls out on you.

tomstickland wrote: »

My statement is "the slowest rate of deceleration is best".
In practice this would mean using a high gear until the engine rpm reaches the idle speed and then selecting the next gear down.

Agree - or even two or three gears depending on the speed.

tomstickland wrote: »

Regarding coasting, it's bad practice.
However, coasting with the engine off would be the most efficient.
Coasting with engine on versus engine braking is hard to decide.
The idling engine needs fuel to tick over - to turn the engine against frictional losses, but then you're not taking this energy from the moving vehicle. If the car is slowed using engine braking then this energy is not taken from the fuel but from the momentum of the vehicle.
To a first approximation the two things cancel out. ie: What you save in fuel by using engine braking is taken from the vehicle's kinetic energy.

In my experience (3 years hypermiling), engine braking in high gear gives me the best mpg.
I used to coast a lot more than I do now as I didn't fully understand how you use little or no fuel when engine braking as long as the revs are slightly above tickover.
I only coast now occasionally when I know engine braking will slow me down too much, i.e. on a short steep slope followed by a long slight downhill slope.
I get an average of over 50mpg (manufacturer's figures for my car are 44 mpg combined) using a lot of the techniques you have discussed on this thread.
I also coast with the engine off up to traffic lights if I know they are not going to change for a few minutes and I can time it to get there with hardly any braking without p'ing off any driver behind.

tomstickland wrote: »

A rather perverse output of the simple model being used is that it would be more efficient to have short bursts of acceleration followed by the vehicle gradually slowing rather than constant speed operation. So the engine would spend short periods of time operating at good efficiency rather than an extended period off optimum.

Again, this is correct, it is known an pulse and glide - it is easier for hybrid owners to do this but I sometimes practice this is in a slightly different different form in my non-hybrid diesel (I don't switch the engine off :eek:). On the motorway while I am drafting behind a lorry (at a safe distance of course), if I get to a downhill section, I accelerate to get a bit closer to the lorry and then take my foot off the accelerator until I have dropped back a bit then repeat. This makes a bigger difference to my mpg than just travelling at a constant speed which seems to confirm your thoughts Tom. Can't always be @rsed to do it though .

tomstickland wrote: »

This week I've lowered my cruising speeds by 10% everywhere in order to try and make a 20% improvement in mpg.

Many studies have shown that any speed above 60mph starts to seriously dent mpg due the greatly increased drag.

This graph is for a 1986 Volkswagen Golf GTI.
Interesting to note the best mpg was obtained in the low 40's mph but as previously stated it varies from car to car.
Please let us know your mpg figures after your speed reduction.