Driving Economically.

almillar

Great post and worth a read for all drivers.
To pick up on mikey's point about overtaking, you should do it as fast as possible. Plan it of course, but don't dither when you're driving along the wrong side of the road, this is not the time to be thinking about fuel economy!
I have to disagree with part 10 of the original post though - DON'T slow down for the slip road when you see the 300m (3 diagnonal bars) sign. The motorway is pretty much designed for everyone to do 60 or 70 on. It therefore doesn't have any junctions on it, to keep everyone at a constant speed. That's where the sliproads (on AND off) come in - these are where you accelerate ONTO the motorway and decelerate OFF the motorway. I guess if you're on the motorway on your own you can do whatever you want in a rush, but certainly in traffic, slowing down to save some fuel is selfish and irresponsible and will cause a concertina effect on drivers behind.
Hold your speed until you are on the sliproad, THEN slow down.
And finally just a general point on the engine braking thing - you'll be slowing down without your brake lights showing - always look behind you and if there's someone there use the brake pedal lightly too, so that you're INDICATING that you're slowing down. Getting rear ended and not being able to drive your car will save plenty of fuel!

Delivery_Driver

Notmyrealname wrote: »

It isn't. Aerodynamic drag increases by a square of the value as the speed increases. RR mechanical drag is a constant.

On the course I did, we were shown a graph of wind resistance vs speed for that lorry. Basically very little until about 53MPH then it increased exponentially.

I know aerodynamic drag is exponential.The drag of a RR can also be adjusted but not really the same thing.I meant for the sake of this example let them cancel each other out as a RR run is typically in 4th so in my car is at speeds up to 120 odd mph so a fair bit of wind resistance if it were on the road.
I don't want to get too hung up on the point I disagree on as the rest of the post is sound advice.
I have also done an extended version of this course, based on the police roadcraft one and I think the bit about accellerating quickly is more to get you past and less time exposed to danger on the wrong side of the road than actual fuel saving.

Notmyrealname

almillar wrote: »

I have to disagree with part 10 of the original post though - DON'T slow down for the slip road when you see the 300m (3 diagnonal bars) sign. The motorway is pretty much designed for everyone to do 60 or 70 on.

Most traffic in L1 is doing around 55-60MPH as many motorway users seem to think that is the lorry lane. You can back off at the 300m mark with no problem at all and if you were doing 70MPH when you reached it you'd still be doing over 60MPH in most cases as you exited.

Notmyrealname

Delivery_Driver wrote: »

I have also done an extended version of this course, based on the police roadcraft one and I think the bit about accellerating quickly is more to get you past and less time exposed to danger on the wrong side of the road than actual fuel saving.

The course I did was as a lorry driver for a parcel company. You don't do much overtaking in an artic

derboy_2

EVERY TIME YOU USE THE BRAKES YOU'RE BURNING POUND NOTES.

This is not true! Let me explain. Speeding up again to a particular speed uses fuel (pound notes). Slowing down does not incur any cost.
So with that in mind here is an example. You are approaching a 30mph speed limit and need to slow down.
1-The most efficient way to do this is to lift off the throttle in the highest usable gear, and let the speed come down. -This requires you to have enough road to do this.
2-If you do not have enough road to use option 1 you need to slow the car down more. Using 'engine braking' and going down through the gears you do not use any fuel but put increased wear on the engine.
3-If you come off the throttle as in option 1 then use the brakes at the end to scrub off the last bit of speed down to 30mph. This also uses no more fuel than option 2. BUT IT IS SAFER!

When you are actually trying to slow the car down and you dont have enough distance to use option 1, use the brakes! It is much more controllable than option 2 and uses no fuel.

So many times I've seen people use engine braking to slow down, and because they think using the brakes costs money allow the gap to get dangerously close to the car in front. Engine braking is not a good way to slow the car down in normal driving. Allowing the car to get to the right speed by getting off the throttle is good, provided you dont need to 'slow' the car.

Joe_Horner

derboy wrote: »

EVERY TIME YOU USE THE BRAKES YOU'RE BURNING POUND NOTES.

This is not true! Let me explain. Speeding up again to a particular speed uses fuel (pound notes). Slowing down does not incur any cost.
So with that in mind here is an example. You are approaching a 30mph speed limit and need to slow down.
1-The most efficient way to do this is to lift off the throttle in the highest usable gear, and let the speed come down. -This requires you to have enough road to do this.
2-If you do not have enough road to use option 1 you need to slow the car down more. Using 'engine braking' and going down through the gears you do not use any fuel but put increased wear on the engine.
3-If you come off the throttle as in option 1 then use the brakes at the end to scrub off the last bit of speed down to 30mph. This also uses no more fuel than option 2. BUT IT IS SAFER!

When you are actually trying to slow the car down and you dont have enough distance to use option 1, use the brakes! It is much more controllable than option 2 and uses no fuel.

So many times I've seen people use engine braking to slow down, and because they think using the brakes costs money allow the gap to get dangerously close to the car in front. Engine braking is not a good way to slow the car down in normal driving. Allowing the car to get to the right speed by getting off the throttle is good, provided you dont need to 'slow' the car.

The main point is that, by thinking ahead, you can usually come very close to using "option 1". This is both more economic AND safer because you have to start thinking ahead to do it.

eta: slowing down doesn't incur any direct cost but turning useful kinetic energy (from your speed) into waste heat at your brakes "wastes" some of the fuel you've used to gain that speed in the first place.

Delivery_Driver

derboy wrote: »

EVERY TIME YOU USE THE BRAKES YOU'RE BURNING POUND NOTES.

This is not true! Let me explain. Speeding up again to a particular speed uses fuel (pound notes). Slowing down does not incur any cost.
So with that in mind here is an example. You are approaching a 30mph speed limit and need to slow down.
1-The most efficient way to do this is to lift off the throttle in the highest usable gear, and let the speed come down. -This requires you to have enough road to do this.
2-If you do not have enough road to use option 1 you need to slow the car down more. Using 'engine braking' and going down through the gears you do not use any fuel but put increased wear on the engine.
3-If you come off the throttle as in option 1 then use the brakes at the end to scrub off the last bit of speed down to 30mph. This also uses no more fuel than option 2. BUT IT IS SAFER!

When you are actually trying to slow the car down and you dont have enough distance to use option 1, use the brakes! It is much more controllable than option 2 and uses no fuel.

So many times I've seen people use engine braking to slow down, and because they think using the brakes costs money allow the gap to get dangerously close to the car in front. Engine braking is not a good way to slow the car down in normal driving. Allowing the car to get to the right speed by getting off the throttle is good, provided you dont need to 'slow' the car.

Look and plan well ahead and you'll generally have plenty of time to do it as the OP says.On roads I am familiar with I can be hitting the new speed limit exactly as I pass through it without touching the brakes as I remember little details where I need to be lifting off, whether its a bush, pothole, grate..whatever.
He is not suggesting that you don't brake when you need to, just that its more efficient not to need to in the 1st place.

mikey72

almillar wrote: »

.............To pick up on mikey's point about overtaking, you should do it as fast as possible. Plan it of course, but don't dither when you're driving along the wrong side of the road, this is not the time to be thinking about fuel economy!.................

Notmyrealname wrote: »

The course I did was as a lorry driver for a parcel company. You don't do much overtaking in an artic

So if you assume most overtaking is on dual carriageways, or motorways, rather than pulling out onto the wrong side of the road, I'm still not convinced full throttle foot to the floor will be the most economical.
Even in my car, with ecu controlled fuel injection, it's not always the case I have to floor it to get round safely, if it's a long straight road.

Joe_Horner

mikey72 wrote: »

... I'm still not convinced full throttle foot to the floor will be the most economical.

Just throttle to the floor probably won't be - you also have to keep in an economic rev range around peak torque.

Here's a fairly typical torque curve - it's the shape that matters, not the figures btw:



This engine will be producing over 95% of it's maximum torque between about 2000 and 4750 RPM. If you change early so the revs in the next gear drop to 1500, or late so you rev to 5500 in the low gear, you'll be down to 83% maximum either before or after the change.

Torque is (almost) a direct measure of an engine's efficiency for any given design, so those torque "% of max" figures can be taken as good approximations of efficiency "% of max".

Full throttle is also inherently the most efficient way to run an engine because it breathes best when there's the least restriction in the intake (again, think of the effect a clogged air filter has).

So, accelerating with full throttle while staying in a suitable rev range will give best efficiency on both counts.

How close you can get to doing it in practice depends on the car - how "peaky" or "flat" its particular torque curve is and how suitable the gearing is to stay in a good range of engine speeds.

Admittedly, the likely saving from this is nothing like the saving available from not throwing energy away as heat by braking late and hard!

Delivery_Driver

What happened to torque and power crossing at 5252 rpm in that graph?I'm not nitpicking, just wondering if there is some reason for it?