I-S
29-11-2006, 08:24 PM
By Popular Demand (TM), a kieranesque piece on driving techniques to save fuel without wasting time.
Fuel consumption is a very complex topic because there's so many variables involved. The weather, the exact fuel quality, when your car was last serviced, traffic conditions, temperature, tyre pressures and the biggest variable of all... the driver.
Assuming that your car is well maintained and correct tyre pressures all round, regularly checked, etc, then driving technique is the single biggest thing you can do to improve fuel consumption. There are various different things that can be applied... some of them are about how to drive most efficiently within certain situations and others are about how to take best advantage of the mechanical system that is the car.
Let's look at how the car works first.
1. Brakes
Hmmm... brakes stop the car, how do they help improve fuel consumption? Answer is that they don't. They're the single biggest waste of energy on your car that you have any control over. We all know how brakes work... pads press on disc, get hot, stop car. The car's motion is converted into heat. The engine and drivetrain are about 25% efficient at converting the energy in the fuel into motion of the car. The rest is converted (remembering from science at school about conservation of energy... it can not be created or destroyed, merely changed from one form to another. In this case it starts as chemical energy) into heat. The brakes are 100% efficient at converting the motion of the car into heat (not all of this heat is in the brakes themselves, however). HEAT IS WASTE ENERGY!
So, that's the physics lesson. The bearing on fuel consumption is that any energy put into the brakes was originally 4 times as much fuel, and that's a lot of energy. A quick back of an envelope calculation says that at about 95km/h (58mph or so) your car has 0.5MJ of energy. Slamming the brakes on has wasted at least 2MJ of fuel (in pure energetic terms... in reality, with getting back up to speed again and loss of momentum, it's quite a bit more). 1 litre of fuel contains about 33MJ of energy. Thus if you can avoid braking, you can maintain momentum and avoid wasting energy. Of course, if a child runs out in the road, or the guy in front slams on his brakes, you stop, but there are a great many times that people use their brakes when they don't need to. That will come under the later section...
2. Gears
First of all the "keep it in as high a gear as possible to save fuel" that most people believe is a myth. IT IS WRONG. For example, assuming a 5-speed manual, many people will use 4th gear at 30mph, giving around 1500-1750rpm. At this speed most engines (and the VR4 may be somewhat excepted) don't have a great deal of torque on hand. When traffic slows to 25mph the revs drop accordingly (1250-1500rpm now), and then the traffic clears. Prod the throttle and nothing happens. You have to press hard on the throttle for the engine to slowly pull the car back to 30mph. Few revs, yes, but large throttle excursions mean lots of fuel being put in the engine. In third gear you might have 2000-2250rpm, and the engine will require much less throttle to get the car up to speed again more quickly. The total amount of fuel put into the engine does not depend on revs, but on a product of revs and throttle.
Use the right gear. 3rd in 30. Revs aren't your enemy
3. Engine braking
Engine braking is generally a good thing. GEAR braking is a very very bad thing. What's the difference? Engine braking is where you are in a gear and take your foot off the throttle and the car slows down. Gear braking is where you change to a lower gear, bring the clutch up with wildly mismatched input and output speeds in the gearbox (eg engine at 1000rpm and output at 2000rpm), and the car suddenly slows down as the momentum of the car spins up the engine. Gear braking can easily be replaced by proper engine braking by rev-matching the input and output before engaging the clutch (modern synchro means you don't need to double declutch. You can if you want though), then removing throttle pressure and letting the car slow down that way.
Rev-match downshifts and let the car slow in gear
4. How the engine uses fuel
So now we're using a lower gear in town... more acceleration available, more engine braking available. That's good. Now by leaving more of a gap to the car in front and backing off the throttle when you see an obstruction (queue, traffic lights, zebra crossing, etc) ahead of them, you don't need to use the brakes to slow down. The car will slow down by engine braking, and you can easily still be rolling at 10-15mph by the time the obstruction clears and you can carry on. There's two big savings of fuel here... the obvious one is the "keep rolling" means you're carrying momentum, which is energy (ok, it's proportional to). The less obvious one is down to an interesting foible of fuel-injection. When rolling along on a flat surface or down a hill, with the car in gear there is NO FUEL being injected into the engine. NONE. Why would there be? The car's motion is keeping the engine turning, and the throttle is closed. This means that if you're driving down a long hill and are tempted to put the car into neutral (or coast on the clutch) to "save fuel", you'll actually waste it; in neutral the car must inject fuel to keep the engine turning over, rather than letting the motion of the car do that. If you let the car slow down in gear, you will actually feel when the ecu starts injecting fuel again when you get to idle rpm. Coming back to the town-driving example, if you are at 30mph in 3rd, see an obstruction, let off throttle, slow to 15mph, obstruction clears, carry on, you have used no fuel in the time that you slowed down, and carry more speed so less is required to get back to 30mph.
Never coast (neutral or clutch depressed). Leave space so you don't need to use brakes
5. The open road
Ahhh, that lovely NSL sign. Acceleration is not the enemy of fuel consumption... get up to speed quickly. Go on, you know you want to. Put it like this... say you have a section of straight roal at NSL from a 30 limit, and then a corner you know you can take at 45 safely. If you use firm acceleration in 3rd to go from 30-60mph, then change to 5th, light throttle to maintain speed, then rev-match downshift to 4th and let the car slow by itself to 45 and then into the corner, you will actually use less than or a similar amount of fuel to going 30-40mph in 3rd, 40-55 in 4th, 55-60 in 5th, no time to maintain speed, brake, downshift. The former is certainly more fun. The car is at its most efficient at around 55mph.
Don't be afraid to get up to speed quickly. Use the right gears for acceleration and for maintaining speed, and don't be afraid to miss a gear out (eg 3-5 upshift).
6. Motorway
From the above, you can probably guess that joining a motorway can be done similarly briskly. go to 50 in 3rd, then 4th to 70 and while you join, once you're in lane and at speed, then take 5th. The other thing with motorways is when you get a medium density of traffic (the M6 north of birmingham northbound on a sunday afternoon is ALWAYS like this). Lane 3 will be full of the suicide squad... a continuous train of cars all within a car length of one another, brake lights twinkling every few seconds. Lane 2 will probably have nice, open areas and you can get a proper gap to the person in front, and in 10 minutes time that car that was next to you in lane 3 will still be in sight... is a couple of hundred yards over 10 minutes worth the extra fuel, stress and danger?
Leave a bigger gap so you don't need to brake. Think about whether minute gains in distance travelled are worth additional accel/brake routines
7. Cruise control
As per the other thread... If you're on a flat, straight road that goes on for miles and miles then by all means, use it. The thing about cruise control is that, just like an autobox, it can not PREDICT. It can only REACT. The reason that in this country cruise control will tend to be less efficient is that we don't really have long straight flat roads, and on a hill cruise just ends up dumping lots of fuel to try to maintain 70 up a hill when the driver could let the car slow to 65 for a lot less fuel used. Additionally, our roads are generally rather busy. We've all seen BMW man flying down the outside lane on cruise, coming up behind someone and slamming on the brakes to cancel it and not crash into them. If you use the "cancel" command on the cruise then it won't be so bad, but in general you should be able to better it manually.
Think that will do for starters...
Fuel consumption is a very complex topic because there's so many variables involved. The weather, the exact fuel quality, when your car was last serviced, traffic conditions, temperature, tyre pressures and the biggest variable of all... the driver.
Assuming that your car is well maintained and correct tyre pressures all round, regularly checked, etc, then driving technique is the single biggest thing you can do to improve fuel consumption. There are various different things that can be applied... some of them are about how to drive most efficiently within certain situations and others are about how to take best advantage of the mechanical system that is the car.
Let's look at how the car works first.
1. Brakes
Hmmm... brakes stop the car, how do they help improve fuel consumption? Answer is that they don't. They're the single biggest waste of energy on your car that you have any control over. We all know how brakes work... pads press on disc, get hot, stop car. The car's motion is converted into heat. The engine and drivetrain are about 25% efficient at converting the energy in the fuel into motion of the car. The rest is converted (remembering from science at school about conservation of energy... it can not be created or destroyed, merely changed from one form to another. In this case it starts as chemical energy) into heat. The brakes are 100% efficient at converting the motion of the car into heat (not all of this heat is in the brakes themselves, however). HEAT IS WASTE ENERGY!
So, that's the physics lesson. The bearing on fuel consumption is that any energy put into the brakes was originally 4 times as much fuel, and that's a lot of energy. A quick back of an envelope calculation says that at about 95km/h (58mph or so) your car has 0.5MJ of energy. Slamming the brakes on has wasted at least 2MJ of fuel (in pure energetic terms... in reality, with getting back up to speed again and loss of momentum, it's quite a bit more). 1 litre of fuel contains about 33MJ of energy. Thus if you can avoid braking, you can maintain momentum and avoid wasting energy. Of course, if a child runs out in the road, or the guy in front slams on his brakes, you stop, but there are a great many times that people use their brakes when they don't need to. That will come under the later section...
2. Gears
First of all the "keep it in as high a gear as possible to save fuel" that most people believe is a myth. IT IS WRONG. For example, assuming a 5-speed manual, many people will use 4th gear at 30mph, giving around 1500-1750rpm. At this speed most engines (and the VR4 may be somewhat excepted) don't have a great deal of torque on hand. When traffic slows to 25mph the revs drop accordingly (1250-1500rpm now), and then the traffic clears. Prod the throttle and nothing happens. You have to press hard on the throttle for the engine to slowly pull the car back to 30mph. Few revs, yes, but large throttle excursions mean lots of fuel being put in the engine. In third gear you might have 2000-2250rpm, and the engine will require much less throttle to get the car up to speed again more quickly. The total amount of fuel put into the engine does not depend on revs, but on a product of revs and throttle.
Use the right gear. 3rd in 30. Revs aren't your enemy
3. Engine braking
Engine braking is generally a good thing. GEAR braking is a very very bad thing. What's the difference? Engine braking is where you are in a gear and take your foot off the throttle and the car slows down. Gear braking is where you change to a lower gear, bring the clutch up with wildly mismatched input and output speeds in the gearbox (eg engine at 1000rpm and output at 2000rpm), and the car suddenly slows down as the momentum of the car spins up the engine. Gear braking can easily be replaced by proper engine braking by rev-matching the input and output before engaging the clutch (modern synchro means you don't need to double declutch. You can if you want though), then removing throttle pressure and letting the car slow down that way.
Rev-match downshifts and let the car slow in gear
4. How the engine uses fuel
So now we're using a lower gear in town... more acceleration available, more engine braking available. That's good. Now by leaving more of a gap to the car in front and backing off the throttle when you see an obstruction (queue, traffic lights, zebra crossing, etc) ahead of them, you don't need to use the brakes to slow down. The car will slow down by engine braking, and you can easily still be rolling at 10-15mph by the time the obstruction clears and you can carry on. There's two big savings of fuel here... the obvious one is the "keep rolling" means you're carrying momentum, which is energy (ok, it's proportional to). The less obvious one is down to an interesting foible of fuel-injection. When rolling along on a flat surface or down a hill, with the car in gear there is NO FUEL being injected into the engine. NONE. Why would there be? The car's motion is keeping the engine turning, and the throttle is closed. This means that if you're driving down a long hill and are tempted to put the car into neutral (or coast on the clutch) to "save fuel", you'll actually waste it; in neutral the car must inject fuel to keep the engine turning over, rather than letting the motion of the car do that. If you let the car slow down in gear, you will actually feel when the ecu starts injecting fuel again when you get to idle rpm. Coming back to the town-driving example, if you are at 30mph in 3rd, see an obstruction, let off throttle, slow to 15mph, obstruction clears, carry on, you have used no fuel in the time that you slowed down, and carry more speed so less is required to get back to 30mph.
Never coast (neutral or clutch depressed). Leave space so you don't need to use brakes
5. The open road
Ahhh, that lovely NSL sign. Acceleration is not the enemy of fuel consumption... get up to speed quickly. Go on, you know you want to. Put it like this... say you have a section of straight roal at NSL from a 30 limit, and then a corner you know you can take at 45 safely. If you use firm acceleration in 3rd to go from 30-60mph, then change to 5th, light throttle to maintain speed, then rev-match downshift to 4th and let the car slow by itself to 45 and then into the corner, you will actually use less than or a similar amount of fuel to going 30-40mph in 3rd, 40-55 in 4th, 55-60 in 5th, no time to maintain speed, brake, downshift. The former is certainly more fun. The car is at its most efficient at around 55mph.
Don't be afraid to get up to speed quickly. Use the right gears for acceleration and for maintaining speed, and don't be afraid to miss a gear out (eg 3-5 upshift).
6. Motorway
From the above, you can probably guess that joining a motorway can be done similarly briskly. go to 50 in 3rd, then 4th to 70 and while you join, once you're in lane and at speed, then take 5th. The other thing with motorways is when you get a medium density of traffic (the M6 north of birmingham northbound on a sunday afternoon is ALWAYS like this). Lane 3 will be full of the suicide squad... a continuous train of cars all within a car length of one another, brake lights twinkling every few seconds. Lane 2 will probably have nice, open areas and you can get a proper gap to the person in front, and in 10 minutes time that car that was next to you in lane 3 will still be in sight... is a couple of hundred yards over 10 minutes worth the extra fuel, stress and danger?
Leave a bigger gap so you don't need to brake. Think about whether minute gains in distance travelled are worth additional accel/brake routines
7. Cruise control
As per the other thread... If you're on a flat, straight road that goes on for miles and miles then by all means, use it. The thing about cruise control is that, just like an autobox, it can not PREDICT. It can only REACT. The reason that in this country cruise control will tend to be less efficient is that we don't really have long straight flat roads, and on a hill cruise just ends up dumping lots of fuel to try to maintain 70 up a hill when the driver could let the car slow to 65 for a lot less fuel used. Additionally, our roads are generally rather busy. We've all seen BMW man flying down the outside lane on cruise, coming up behind someone and slamming on the brakes to cancel it and not crash into them. If you use the "cancel" command on the cruise then it won't be so bad, but in general you should be able to better it manually.
Think that will do for starters...