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Fuel efficient driving.
tomstickland
Posts: 19,538 Forumite
Now edited to update on 16th March 2011
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.
The three main stages of a driving cycle - 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 will 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.
DRIVER DECISIONS
During acceleration then the driver can choose the rpm operating range by deciding when to change gear and the engine load by how hard the accelerator pedal is pressed.
During constant speed cruising they select what speed to cruise at and what gear to use.
During deceleration the driver chooses when to start decelerating and at what rate to do that by choice of gears and use of brakes.
People tend to concentrate on acceleration because it clearly uses fuel. Most people appreciate the cruising speed is important. However, the deceleration part is one the most important in terms of making the most efficient use of fuel.
CHOOSING THE ENGINE RPM AND LOAD FOR EFFICIENCY
The efficiency of an engine depends on the rpm and the torque output it is generating. High rpm causes large frictional losses, low rpm has poor volumetric efficiciency (how much air is sucked in per rev versus the actual swept volume of the pistons).
Very high loads usually mean inefficient rich fuelling, but low loads mean throttling petrol engines (inefficient) and a diluted cylinder charge.
Here's a bsfc plot from Wikipedia.
This shows the fuel consumption per unit of power output for a range of engine rpm and load levels. The a low contour means less fuel per unit of work done.
The x axis is rpm ( min to the power -1 means 1/min i.e. number of events per minute, aka rpm)
The y axis is mean effective pressure measure in units of bar (1 atmosphere is approximately 1 bar). This is the torque divided by the cubic capacity. Consider it to be torque or throttle angle.
This particular engine makes peak efficiency at around 2,200rpm and 15/18ths of peak torque. That's quite startling really - it makes peak efficiency at 83% of peak torque output at 2,200rpm.
More maps here: http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps#Volkswagen_2.0L_5_cyl_diesel
Note that peaks are generally at around 2,000rpm and quite high load levels.
ACCELERATION PHASE - GENTLE IS INEFFICIENT
Often when I read articles or comments about fuel efficient driving then people talk about "gentle acceleration".
Most people understand that engine efficiency is not good at high engine loads, but fail to appreciate that it's also poor at low loads. The optimum rate of acceleration actually loads the engine somewhere in the upper end of its range.
When accelerating then the driver can choose rpm and load level. The rpm is what is reported by the rpm dial in the car and changing gear up will cause the rpm to drop. The load level is selected by the driver's choice of accelerator pedal position.
The most efficient acceleration will keep the operating point near the peak bsfc. For the engine with the bsfc plot shown above that means operating equally both sides of 2,200rpm and keeping the load at around 80% of maximum. ie: Short bursts of quite hard acceleration with a central rpm of 2,200rpm.
The most efficient acceleration strategy is a purposeful burst of moderate to high acceleration followed by a gear shift at relatively low rpm.
The instantaneous fuel consumption rate during high acceleration will be higher than during gentle acceleration. The total amount of fuel consumed will be the rate multiplied by the time spent accelerating. A short burst at a high rate will use less fuel than a long period at a low rate, providing that the engine is being operated with a more efficient load and speed combination during the higher rate.
The most important area to look for gains is in deceleration.
CRUISING SPEED
When cruising then the driver can choose a road speed and a gear. The most efficiency combination is one that occurs as close to the peak efficiency island as possible.
For a particular gear than increasing the road speed will increase the rpm and the load. The load will increase with speed squared. So there's a slightly curved line on the bsfc plot, getting steeper as it moves to the right. If the driver moves up a gear than the rpm will fall off and the load will increase. Hence another line can be drawn on the bsfc plot for this next gear. It will be higher than the first line. This can continue through each higher gear to give a set of results for each rpm point.
The best combination of speed and gear will be that which passes as closely as possible to the bsfc peak.
In practice most people don't have the actual bsfc plot for their engine, so they have to make some basic assumptions. Sensible choices would be to keep the rpm at around 2,000rpm (1,500 to 2,500) and the load at least somewhere in the middle.
LOWER CRUISE SPEED WILL SAVE FUEL - WITHIN REASON
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.
CRUISE SPEED AND ENGINE SIZE
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.
This is 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.
-update: hypermilers call this "pulse and glide" and believe it to work.
DRAG
When cruising at a constant speed then all of the engine output is being used to overcome drag and rolling resistance losses. The drag factor is more important than rolling resistance. Lowering the drag by being "sucked along" behind a larger vehicle is a basic concept that will work. There is much debate about the safety considerations of this and, personally, it's not something that I have much interest in doing.
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 to idle spees is the method that is being advocated. Most engine management will cut the fuel to the engine (or to a minimum) 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 with the engine off would give the most distance travelled per starting speed.
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.
A FUEL EFFICIENT DRIVER
A fuel efficient driver should concentrate on planning ahead to avoid braking. This is an essential to making the best use of fuel that has already been used.
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.
COMPROMISE BETWEEN SAFETY, EFFICIENCY AND OTHER TRAFFIC
Techniques for the most efficient use of fuel might annoy or frustrate other drivers and in some cases can be dangerous. The driver has to make a compromise between these factors. Luckily it's already been shown that sluggish acceleration is inefficient so there's no need to annoy people by holding them up when pulling away. The slowing aspect can annoy other drivers, so adjust to the conditions slightly.
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 were 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.
Results as above and with 10% reduction in cruise speeds. Results so far suggest 55-60mpg.
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
-idling at traffic lights. I did some calculations on this and the effect was small unless a lot of time is spent doing it.
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.
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.
The three main stages of a driving cycle - 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 will 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.
DRIVER DECISIONS
During acceleration then the driver can choose the rpm operating range by deciding when to change gear and the engine load by how hard the accelerator pedal is pressed.
During constant speed cruising they select what speed to cruise at and what gear to use.
During deceleration the driver chooses when to start decelerating and at what rate to do that by choice of gears and use of brakes.
People tend to concentrate on acceleration because it clearly uses fuel. Most people appreciate the cruising speed is important. However, the deceleration part is one the most important in terms of making the most efficient use of fuel.
CHOOSING THE ENGINE RPM AND LOAD FOR EFFICIENCY
The efficiency of an engine depends on the rpm and the torque output it is generating. High rpm causes large frictional losses, low rpm has poor volumetric efficiciency (how much air is sucked in per rev versus the actual swept volume of the pistons).
Very high loads usually mean inefficient rich fuelling, but low loads mean throttling petrol engines (inefficient) and a diluted cylinder charge.
Here's a bsfc plot from Wikipedia.
This shows the fuel consumption per unit of power output for a range of engine rpm and load levels. The a low contour means less fuel per unit of work done.
The x axis is rpm ( min to the power -1 means 1/min i.e. number of events per minute, aka rpm)
The y axis is mean effective pressure measure in units of bar (1 atmosphere is approximately 1 bar). This is the torque divided by the cubic capacity. Consider it to be torque or throttle angle.
This particular engine makes peak efficiency at around 2,200rpm and 15/18ths of peak torque. That's quite startling really - it makes peak efficiency at 83% of peak torque output at 2,200rpm.
More maps here: http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps#Volkswagen_2.0L_5_cyl_diesel
Note that peaks are generally at around 2,000rpm and quite high load levels.
ACCELERATION PHASE - GENTLE IS INEFFICIENT
Often when I read articles or comments about fuel efficient driving then people talk about "gentle acceleration".
Most people understand that engine efficiency is not good at high engine loads, but fail to appreciate that it's also poor at low loads. The optimum rate of acceleration actually loads the engine somewhere in the upper end of its range.
When accelerating then the driver can choose rpm and load level. The rpm is what is reported by the rpm dial in the car and changing gear up will cause the rpm to drop. The load level is selected by the driver's choice of accelerator pedal position.
The most efficient acceleration will keep the operating point near the peak bsfc. For the engine with the bsfc plot shown above that means operating equally both sides of 2,200rpm and keeping the load at around 80% of maximum. ie: Short bursts of quite hard acceleration with a central rpm of 2,200rpm.
The most efficient acceleration strategy is a purposeful burst of moderate to high acceleration followed by a gear shift at relatively low rpm.
The instantaneous fuel consumption rate during high acceleration will be higher than during gentle acceleration. The total amount of fuel consumed will be the rate multiplied by the time spent accelerating. A short burst at a high rate will use less fuel than a long period at a low rate, providing that the engine is being operated with a more efficient load and speed combination during the higher rate.
The most important area to look for gains is in deceleration.
CRUISING SPEED
When cruising then the driver can choose a road speed and a gear. The most efficiency combination is one that occurs as close to the peak efficiency island as possible.
For a particular gear than increasing the road speed will increase the rpm and the load. The load will increase with speed squared. So there's a slightly curved line on the bsfc plot, getting steeper as it moves to the right. If the driver moves up a gear than the rpm will fall off and the load will increase. Hence another line can be drawn on the bsfc plot for this next gear. It will be higher than the first line. This can continue through each higher gear to give a set of results for each rpm point.
The best combination of speed and gear will be that which passes as closely as possible to the bsfc peak.
In practice most people don't have the actual bsfc plot for their engine, so they have to make some basic assumptions. Sensible choices would be to keep the rpm at around 2,000rpm (1,500 to 2,500) and the load at least somewhere in the middle.
LOWER CRUISE SPEED WILL SAVE FUEL - WITHIN REASON
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.
CRUISE SPEED AND ENGINE SIZE
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.
This is 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.
-update: hypermilers call this "pulse and glide" and believe it to work.
DRAG
When cruising at a constant speed then all of the engine output is being used to overcome drag and rolling resistance losses. The drag factor is more important than rolling resistance. Lowering the drag by being "sucked along" behind a larger vehicle is a basic concept that will work. There is much debate about the safety considerations of this and, personally, it's not something that I have much interest in doing.
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 to idle spees is the method that is being advocated. Most engine management will cut the fuel to the engine (or to a minimum) 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 with the engine off would give the most distance travelled per starting speed.
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.
A FUEL EFFICIENT DRIVER
A fuel efficient driver should concentrate on planning ahead to avoid braking. This is an essential to making the best use of fuel that has already been used.
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.
COMPROMISE BETWEEN SAFETY, EFFICIENCY AND OTHER TRAFFIC
Techniques for the most efficient use of fuel might annoy or frustrate other drivers and in some cases can be dangerous. The driver has to make a compromise between these factors. Luckily it's already been shown that sluggish acceleration is inefficient so there's no need to annoy people by holding them up when pulling away. The slowing aspect can annoy other drivers, so adjust to the conditions slightly.
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 were 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.
Results as above and with 10% reduction in cruise speeds. Results so far suggest 55-60mpg.
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
-idling at traffic lights. I did some calculations on this and the effect was small unless a lot of time is spent doing it.
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.
Happy chappy
0
Comments
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when i come up to junctions i press the clutch and freewheel so I dont have to break...weeeee but someone said yuor not in full control of the car“Life isn't about finding yourself. Life is about creating yourself.”
― George Bernard Shaw0 -
Well i was under the impression that using gears to slow down used less petrol than braking0
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I've edited the post because I could see how what I wrote was badly worded.
Now says:
The higher the rate of deceleration then the more wasteful it is, regardless of whether it's engine braking, the brakes or a parachute out the back of the vehicle. None of those methods will ever reclaim the energy that's being chucked away.
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.Happy chappy0 -
What sort of Golf is that?
I've had two Mk4 diesel Golfs. First a 90bhp TDi that did 60 mpg consistently over thousands of miles in mixed motoring. Second a 150 bhp GTi that did 55-60 mpg even when severely pressed and over about 25k miles from new.0 -
tomstickland wrote: »
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.
.
Since when has driving at 70/90mph been " good practice?0 -
Some things i've noticed whilst trying to improve mpg
Corners tend to kill speed quite dramatically and therefore mpg. Taking more of a racing line through corners (within the limits of the law) helps.
When leaving the motorway at a slip road with traffic lights at the end, I sometimes find it better to brake and slow early, taking longer to reach the traffic lights, by which time the lights have changed allowing me not to have to stop and retaining some momentum.
Does anyone know at what sorts of rpm the engine cuts the fuel when decelerating?
Interestingly although the drag will vary as the square of the velocity, the power required to overcome the drag varys as the cube of the velocity. Which is one of the main reasons why land speed records are so hard to beat.0 -
tomstickland wrote: »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.
Your wrong...... Not by much, but you are wrong.
When you put your foot down, the engine management system dumps a load of extra fuel into the inlet manifold.
If you accelerate slowly you can avoid this happening.... But you have to accelerate EXTREMELY slowly to do this.
Going back in time..... With a carburettor driven engine, there's a valve in the carb which pumps more or less fuel into the inlet depending on how fast you push down on the pedal, the harder you push, the more fuel it dumps for the initial "kick" of acceleration. This is why on these older engine's, smooth acceleration is vital, too much fuel and the engine will misfire and could even stall.
On the modern engine, the engine management system handles all of this for you, but the principle remains the same.... An extra "kick" is always needed for that initial burst of acceleration.“I may not agree with you, but I will defend to the death your right to make an a** of yourself.”
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Does anyone know at what sorts of rpm the engine cuts the fuel when decelerating?
Modern engine management systems cut the fuel flow whenever it is not required, i.e. any rpm above slightly more than idling speed in a given gear.
Some cars have a driver info display that will show current consumption (besides average consumption). You can observe this displaying "zero" as soon as you lift your foot completely off the accelerator.0 -
On motorway, just follow a lorry.Happiness is buying an item and then not checking its price after a month to discover it was reduced further.0
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On motorway, just follow a lorry.
I agree It stops you unintentionally drifting up to a higher speed.
The best position for mpg is right behind the lorry where the low pressure caused by the lorry is sucking you along. Unfortunately this is not possible as the distances between the car and lorry are too close.0
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