A few questions regarding fuel consumption

Chickabiddybex

I can be specific if it helps. I live almost at the top of a huge hill. I go down it every morning to go to work.



I join the road from a side street from stopped and I usually reach 30mph when I'm just at the top of the hill.

(By the way don't tell me to brake with my gears I've already weighed up pros v cons of that style of driving)

WTFH

Chickabiddybex wrote: »

I can be specific if it helps. I live almost at the top of a huge hill. I go down it every morning to go to work.
I join the road from a side street from stopped and I usually reach 30mph when I'm just at the top of the hill.

How long is the hill (distance from top to the lights)?

Chickabiddybex wrote: »

(By the way don't tell me to brake with my gears I've already weighed up pros v cons of that style of driving)

And what are the pros and cons in your opinion?

Personally, I'm a lazy driver. If I don't need to have my foot on the clutch and brake when trying to maintain a speed, I won't. I'll try to select an appropriate gear and then my right foot is ready to go to accelerator or brake if required.
If you're using the clutch because the engine would be revving too high, then change up.
If you're using it because the engine would be revving too low, then change down.
That way, you're always ready to accelerate or brake if something suddenly happens around you.

Bongles

Chickabiddybex wrote: »

I can be specific if it helps. I live almost at the top of a huge hill. I go down it every morning to go to work.



I join the road from a side street from stopped and I usually reach 30mph when I'm just at the top of the hill.

(By the way don't tell me to brake with my gears I've already weighed up pros v cons of that style of driving)

Stay in gear down the hill, controlling your speed with the brakes.

At the lights, wait in neutral with the handbrake on unless you are at the front of the queue and you think the lights are about to change, in which case you would be in 1st gear with the clutch down, handbrake still on, preparing to go on green if it's clear. Keep your foot off the brakes while you're waiting unless you're at the back of the queue feeling vulnerable and you'd like your brake lights on to make yourself more obvious to the next person barrelling down the hill into the back of you.

Simples

Weird_Nev

It's a very hard thing to explain through typing. Practical demonstration really is the best way. However, some pointers:

1) When the car is moving, unless you're actually changing gear the clutch should be engaged. That is to say engine should be connected the road unless it is absolutely necessary not to be, I.e. you are changing gear, or coming to a complete stop.

2) You shouldn't "brake with your gears" I.e. you shouldn't change from 4th to 3rd and let the engine slow the car instead of the brakes, however you SHOULD allow the engine to regulate the cars speed. Going downhill this may mean remaining in a relatively low gear, 3rd say, and allowing the compression of the engine to "brake" the car. This prevents heat build up in the brakes in long descents. You certainly should not be "clutch out and braking" down hill, under any circumstances, unless you're in the final stages of coming to a complete stop.

3) Anything longer than a momentary stop should result in you putting the handbrake on, then gearstick to neutral. I assume you have some view of the traffic lights as you approach them? You can eitehr moderate your speed on approach so that you never actually stop and they change for you, or else if you know you are going to stop, then you should have the car "at rest", that is requiring NO input from the driver to remain stationary.

Beyond that, It's too specific/complex to explain the nuances of by typing. I'd be here all night.

I'd just like to add that Fuel Economy should not be the primary concern that affects your driving style. Safety first, at all times. Then Smoothness, efficiency and dare I say it, speed. But safety first. Reduced fuel use will be a product of a smooth, considered and mechanically sympathetic driving style.

Ultrasonic

As others have said, stay in gear driving down the hill, this will use little fuel and save wear on the brakes. Ideally try to slow down such that you don't completely stop at the traffic lights, but if you do I'd put the car in neutral to save wear on the clutch mechanism (thrust bearing I think?).

Weird_Nev wrote: »

Reduced fuel use will be a product of a smooth, considered and mechanically sympathetic driving style.

I echo your comments about not compromising safety, but if absolute optimal fuel consumption is the aim then the above statement isn't actually true. I say this since fast (high load) acceleration is more efficient than slow/smooth acceleration (see BSFC plot above), provided that this is not unnecessary acceleration. Accelerating hard to then brake obviously wastes fuel. High load acceleration will though not be the most mechanically sympathetic (though to be clear I am not suggesting lugging the engine!)

reeac

Weird_Nev wrote: »

It's a very hard thing to explain through typing. Practical demonstration really is the best way. However, some pointer

2) You shouldn't "brake with your gears" I.e. you shouldn't change from 4th to 3rd and let the engine slow the car instead of the brakes, however you SHOULD allow the engine to regulate the cars speed. Going downhill this may mean remaining in a relatively low gear, 3rd say, and allowing the compression of the engine to "brake" the car.

I don't understand. The procedure that you describe sounds exactly like braking with your gears and is what I normally do both with manual and auto gearboxes. Are you recommending using engine braking only to maintain a constant speed downhill having first reduced to that speed by using the brakes? If so, why?

Weird_Nev

Ultrasonic wrote: »

I echo your comments about not compromising safety, but if absolute optimal fuel consumption is the aim then the above statement isn't actually true. I say this since fast (high load) acceleration is more efficient than slow/smooth acceleration (see BSFC plot above), provided that this is not unnecessary acceleration. Accelerating hard to then brake obviously wastes fuel. High load acceleration will though not be the most mechanically sympathetic (though to be clear I am not suggesting lugging the engine!)

No, you're saying exactly what I'm saying. You can accelerate rapidly, but in a smooth and progressive style. You can hold a gear and rev it out and do this in a mechanically sensitive and smooth way. It just takes a bit of finesse. We're honestly on the same hymn sheet on this one.

reeac wrote: »

I don't understand. The procedure that you describe sounds exactly like braking with your gears and is what I normally do both with manual and auto gearboxes?

Envisage this situation:
You are leaving the motorway travelling at 70Mph in 5th. You intend to join a slip way and approach a roundabout.
You can:
1) Without using the brake, change to 3rd and allow the increase in engine speed to decrease the vehicle speed. If you're a real mechanical sadist you won't even match the engine speed to road speed first and you'll dump a load of kinetic energy from the speed of the car into increased rotational speed in the flywheel and engine components via your poor clutch. Then then you allow the increased engine braking of an engine at 5,000rpm or whatever slow the car more rapidly than it would slow in 5th at 2,000rpm. This would be the WRONG thing to do and is what I mean by "slowing the car with the gears".
2) Use the brakes to bring the vehicle speed smoothly down to a sensible approach speed for the roundabout, remaining in 5th. On seeing the roundabout and making your assessment as to whether you can enter it safely, or must come to a stop, you then either block change to 2nd or perhaps 3rd, whatever is appropriate to negotiate the roundabout, or else bring the car to a halt and put the car in 1st ready to move off when the opportunity comes.

There's a middle ground of course where people brake a bit, change down, brake a bit more, change down, or perhaps use a little bit of gear braking in each gear, or some combination. Why? What does that achieve apart form more unnecessary gear changes, more confusion, more chance of a botched gear change, and a jerky and mechanically unsympathetic journey.

Honestly, people start doing all manner of bizarre things when they think they're a "skilled" driver. You get repeated downshifts as the vehicle decelerates, you get left foot braking and heel and toeing when it's completely unnecessary. All you need to do is slow down, and change gear once, in that order.
Brakes to slow, gears to go.

reeac wrote: »

Are you recommending using engine braking only to maintain a constant speed downhill having first reduced to that speed by using the brakes? If so, why?

Yes I am.
There are a couple of really good long hills near me. I'm talking hills with runaway truck gravel traps at three points along them and take ten minutes to descend. If you simply let your car roll along in 5th, You'll rapidly exceed the 60kph limit. If you brake repeatedly to maintain the 60kph limit, you'll rapidly cook your brakes.

So, as the hill starts, you ensure the car is doing 60Kph, braking if necessary (you come in from a 100kph limit). Then, as my car is an auto, I have to put it into manual hold mode and select 3rd gear. The compression of the engine holds the car at that speed the car without braking at all. That's all there is to it. It's not hard. Sure, the engine revs away merrily at 4000 rpm, but that's what it takes to slow 2.2 tonnes of mass against gravity. The engine is good to 7000rpm, and no damage is done. If I NEED to brake hard for some reason, I'm not doing that on brakes that are already in the upper reaches of their thermal capacity.

Similarly, on a shorter hill I might consider remaining in a gear lower than I otherwise might be, and allowing the engine to do the bulk of the work in slowing the car since it is turning faster, compressing more air, and spending more energy for me without me having to convert it to heat in the brakes.

Ultrasonic

Weird_Nev wrote: »

No, you're saying exactly what I'm saying. You can accelerate rapidly, but in a smooth and progressive style. You can hold a gear and rev it out and do this in a mechanically sensitive and smooth way. It just takes a bit of finesse. We're honestly on the same hymn sheet on this one.

Sorry for misunderstanding, glad we're in agreement . So many people take 'smooth' to mean accelerating like a snail that I've just got used to trying to correct this...

We're also in total agreement re. braking with gears. I remember my driving instructor commenting once that it is rather cheaper to replace brake pads than a gearbox. Staying in the gear you're in and slowing down is fine though, and your case 2) is exactly what I do.

Weird_Nev

Ultrasonic wrote: »

Sorry for misunderstanding, glad we're in agreement . So many people take 'smooth' to mean accelerating like a snail that I've just got used to trying to correct this...

I'm an ex-impreza driver, and BMWist, and lots of other silly, turbo'd and fun cars. Current car is a 3.8 V6 Pajero Auto (We 4x4, drive along beaches and generally tool about in it) I wouldn't have accepted those fuel bills if you didn't get something back for it. Like 0-60 in 4.8 Seconds in the case of the Prodrive Pretzle.

WTFH

I'd rather have smooth and fast than jerky and slow (we're talking about driving, right?)