A few questions regarding fuel consumption

Joe_Horner

Weird_Nev wrote: »

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.

In terms of economy, slowing with the gears has large benefits, especially in modern cars where fuel is shut off completely on the over-run.

There's a further alternative to the two you gave in the "leaving a motorway" scenario which is mechanically sympathetic and saves fuel. Whether or not it's appropriate in any given case depends a little on whether you're leaving alone or have someone following you. As a driver, you should be able to assess that sort of thing as you go rather than blindly doing things by rote:

As you leave the main carriageway, ease right off the throttle and allow your speed to drop. As your speed drops to a point suitable for a couple of gears lower, change down correctly (which is not unkind to the car) and allow more speed to bleed off. Use brakes to come to a final stop if required.

Most slipways are easily long enough to take that approach, especially ones that take you up to a flyover rather than down to an underpass.

The fuel savings can be surprisingly big:

Your 2.2 tonne car travelling at 70 MPH has about 1.9MJ of kinetic energy which has to be got rid of. Given typical petrol energy density of 36MJ per litre and typical engine thermal efficiency of around 33%, that kinetic energy represents about 1/6 litre (about 26 pence) of fuel that you've burnt, assuming perfect mechanical and aerodynamic efficiency for simplicity).

Taking the two extreme scenarios:

If you brake as hard as possible to remove that energy, that's 26 pence that you've just turned to heat in your brakes (ok, maybe only 20p or so allowing for the speed you'd lose anyway from drag etc)

If you allow the car to entirely coast to a stop in its own time, that's extra distance that you've travelled for your 26p.

In practice, you'll be doing something between those extremes, but the closer you can get to coasting the speed off the better for fue economy.

Now, it may only be 20p saved in the example above but, if you multiply it by every time you need to stop or slow down, it adds up to a large part of the fuel you use.

Using your brakes is by far the biggest killer of fuel economy because it turns fuel directly to waste heat, but avoiding them (for most people) means learning a whole new approach to driving - starting with actually thinking about what you're doing and leaving appropriate gaps!

Ultrasonic

Joe_Horner wrote: »

As you leave the main carriageway, ease right off the throttle and allow your speed to drop. As your speed drops to a point suitable for a couple of gears lower, change down correctly (which is not unkind to the car) and allow more speed to bleed off.

I'm not really sure why that would be better than staying in top gear for longer (which is what I do)? The only point at which I can see an arguement for dropping a gear is when revs drop so low that fuel needs to be injected to maintain idle revs, but this is pretty low speed in my car. Staying in top gear for longer will minimise engine braking and so allow the car to travel with no fuel used over a greater distance.

Weird_Nev

Yeah, thanks for that Joe.
I can drive with a modicum of anticipation and forethought. After 6 weeks full time advanced driver training to advanced Police driving status, and 7 years experience driving to that level in the police in central London (without accident) you'd kind of hope so, really.

Typically, I might roll of the throttle, lose speed through engine braking for perhaps 75% of the distance and 50% of the speed, then brake for the last 25% of the distance to lose the last 50% of the speed.

It's too dependent on circumstances to be any more specific. I will not sacrifice safety for fuel economy, and I will not fail to make progress for the sake of a few pence.

You have to sit along side someone driving to get any sense of what makes them tick, their level of ability and where they might need improvement. that's why these internet discussions so frequently descend into sniping. It's impossible to put into words. Even videos can be misinterpreted.

I'm not a brilliant driver. far from it. I know where my weaknesses lie and I often make mistakes. But I am at least aware of those mistakes, and seek to improve.

Joe_Horner

Ultrasonic wrote: »

I'm not really sure why that would be better than staying in top gear for longer (which is what I do)? The only point at which I can see an arguement for dropping a gear is when revs drop so low that fuel needs to be injected to maintain idle revs, but this is pretty low speed in my car. Staying in top gear for longer will minimise engine braking and so allow the car to travel with no fuel used over a greater distance.

Changing down when the car's at a comfortable speed reduces brake wear without any appreciable increase in engine / clutch / gearbox wear and keeps the car more responsive (by the time 3rd is appropriate 5th will be lugging if you try to accelerate in most cars). That's useful if the junction / roundabout you're approaching is clear.

Also most cars don't keep fuel cut off until you reach idle - they tend to start delivering again at around 1200 - 1500 RPM to prevent stalling on rapid stops. That can easily be 45 - 50mph in top depending on your gearing, so coasting in top below that will start to use fuel again.

Weird_Nev wrote: »

Yeah, thanks for that Joe.
I can drive with a modicum of anticipation and forethought. After 6 weeks full time advanced driver training to advanced Police driving status, and 7 years experience driving to that level in the police in central London (without accident) you'd kind of hope so, really.

Typically, I might roll of the throttle, lose speed through engine braking for perhaps 75% of the distance and 50% of the speed, then brake for the last 25% of the distance to lose the last 50% of the speed.

It's too dependent on circumstances to be any more specific. I will not sacrifice safety for fuel economy, and I will not fail to make progress for the sake of a few pence.

You have to sit along side someone driving to get any sense of what makes them tick, their level of ability and where they might need improvement. that's why these internet discussions so frequently descend into sniping. It's impossible to put into words. Even videos can be misinterpreted.

I'm not a brilliant driver. far from it. I know where my weaknesses lie and I often make mistakes. But I am at least aware of those mistakes, and seek to improve.

As you say, it's hard to judge what people are doing / describing on a forum rather than sitting next to you at the wheel and circumstances are different every time. What you've just described sounds close to what I was attempting to put across anyway - it'd be silly to quibble over a few % of speed / distance based on words on a forum!

But, if your observations of other cars are anything like mine, you must agree that we seem to be in the minority with a fairly large majority of drivers coming off and either braking hard then crawling down the slip or keeping power on and then braking all their speed away as late as possible. These are the people I suggested would have to learn a whole new approach

Ultrasonic

Joe_Horner wrote: »

Changing down when the car's at a comfortable speed reduces brake wear without any appreciable increase in engine / clutch / gearbox wear and keeps the car more responsive (by the time 3rd is appropriate 5th will be lugging if you try to accelerate in most cars). That's useful if the junction / roundabout you're approaching is clear.

Oh I'd certainly be in gear at the point I'm at the roundabout to be able to drive across if clear, I thought you were describing doing so rather earlier. I would personally however never change down a gear to help braking, I'd rather wear out brake pads than gears. (Actually, I say never, I would going down a long hill now that I think about it, but not for the situations we were talking about.)

Also most cars don't keep fuel cut off until you reach idle - they tend to start delivering again at around 1200 - 1500 RPM to prevent stalling on rapid stops. That can easily be 45 - 50mph in top depending on your gearing, so coasting in top below that will start to use fuel again.

Are those petrol rev ranges? My diesel doesn't inject fuel until pretty close to if not at idle, perhaps at 800-900 rpm, which is just under 30 mph in 5th gear*. (I'm judging the no fuel used by the car trip computer and what is reported via OBDII to a Scangauge). I must admit I'm surprised (and actually a little sceptical) about cars using fuel when slowing down in gear at 45 mph+. Although perhaps you are talking about cars with 6 forward gears?

*Edit: what actually seems to happen if I do hit the point where I guess the engine decides to inject fuel again is I find the engine is idling fine, but the rev needle is showing lower than it's normal idle revs, perhaps sitting at 700 rpm rather than 800 rpm. I've only done this a couple of times, and noticed at the point I'm stationary and in neutral. I tend to then gently push the accelerator to get the rev needle back up, but all would probably be fine if I just waited till I need to move off.

Joe_Horner

Ultrasonic wrote: »

Oh I'd certainly be in gear at the point I'm at the roundabout to be able to drive across if clear, I thought you were describing doing so rather earlier. I would personally however never change down a gear to help braking, I'd rather wear out brake pads than gears. (Actually, I say never, I would going down a long hill now that I think about it, but not for the situations we were talking about.)

Are those petrol rev ranges? My diesel doesn't inject fuel until pretty close to idle, perhaps at 900 rpm, which is just under 30 mph in 5th gear. (I'm judging the no fuel used by the car trip computer and what is reported via OBDII to a Scangauge). I must admit I'm surprised (and actually a little sceptical) about cars using fuel when slowing down in gear at 45 mph+.

Yeah, sorry. Bear in mind that petrols tend to have higher idles and are more prone to stalling if they drop below than diesels because their torque curves drop off so much faster at low revs - a good diesel can recover from almost standstill!

As for the rest, guess it's another case of "not being there"

Ultrasonic

Joe_Horner wrote: »

As for the rest, guess it's another case of "not being there"

Indeed, I'm with you and Weird Nev there, it is very difficult to talk about driving. I also don't consider myself any sort of driving expert BTW!

Joe_Horner

Ultrasonic wrote: »

Indeed, I'm with you and Weird Nev there, it is very difficult to talk about driving. I also don't consider myself any sort of driving expert BTW!

Nor me, but I do manage to get a fairly consistent 25% better economy than book figures out of anything I drive without holding people up or causing accidents (as far as I know :P )

That's not by being a good driver, btw, just by being a skinflint

reeac

Joe_Horner wrote: »

Nor me, but I do manage to get a fairly consistent 25% better economy than book figures out of anything I drive without holding people up or causing accidents (as far as I know :P )

That's not by being a good driver, btw, just by being a skinflint

Just a final comment. I do a particular 10 mile rural road trip at least once a week and in my Jazz I get 53 mpg. with normal driving but if I do it without using the brakes at all [and it's quite possible to do that if traffic is thin] I get 70 mpg. The trip includes two 30 mph. villages and some ups and downs so quite a bit of engine braking.

Bongles

reeac wrote: »

Just a final comment. I do a particular 10 mile rural road trip at least once a week and in my Jazz I get 53 mpg. with normal driving but if I do it without using the brakes at all [and it's quite possible to do that if traffic is thin] I get 70 mpg. The trip includes two 30 mph. villages and some ups and downs so quite a bit of engine braking.

How much longer does it take you? Not much I expect - but if nothing else, if your driving was otherwise identical on the two runs, every time you lift off early on the 70mpg run to avoid the brakes will cost you a little bit of time compared to the 53mpg run where you're prepared to accept the brakes.

There's another aspect to all this and that is how much you value your own time. The time you save on one 10 mile trip might be pretty minimal, but then so is the cost saving over one trip (I make it about 29p from your figures assuming £1.40 a litre). But add them up over one trip per week and all the rest of the driving you do and the differences - time and cost - can become significant.

I was bored so I did some sums. One trip a week - 52 trips a year - will save you just over £15. If you value your time at minimum wage than as long as the total time penalty is less than about 2.4 hours over the 52 trips you're quids in. That equates to an average speed difference of 6 or 7 mph. I doubt you're noticing that much difference between your 53mpg and your 70mpg. Value your time at £15 an hour and you only need to be a couple mph faster on average for you to be better off with your 53mpg runs, which isn't implausible.

Each individual will, of course, have their own idea of what their time's worth. (As well as other, independent, factors like getting pleasure from improving mpg or getting pleasure from improving average cross country speed).