fuel economy

greenman7

Originally posted by Ultrasonic
1) You haven't 'perfected the pulse and glide technique', you're doing in wrong. If you want to pulse and glide properly you should be coasting in neutral, not in gear. You want to avoid engine braking.

Pulse and glide includes coasting in neutral, coasting with the engine off and coasting in gear with the engine. The reason I use engine braking is that I am consuming less fuel than in neutral. Engine braking is a well known technique for reducing speed when anticipating the road ahead for traffic lights, bends, junctions etc.

2) If passengers don't notice you regularly accelerating from 57 mph to 65 mph then you aren't accelerating hard enough during the 'pulse' phase either.

The reason they do not notice is that I drive smoothly, I am not constantly braking and changing gears.

3) It is extremely rare that anyone could practically employ the P & G technique on UK roads without seriously aggravating other road users.

P & G is one of many techniques employed by hypermilers in the Uk, I do not think I am the only person using them. A high awareness of all other road users including cars behind is essential. I tend to drive on motorways at night when there is less traffic.

Ultrasonic

cyclonebri1 wrote: »

Below that figure, the point where the turbo kicks in, all efficiency is lost and yes, you are wasting fuel.

Would you care to comment on why BSFC plots like this one (or the one directly below it) for turbo-diesels don't appear to show any particular rpm below which engine efficiency dramatically decreases? As I'm sure is obvious to you I am not an expert, but to my (possibly incorrect) understanding this fact would seem to contradict your view.

One thing I do acknowledge is that BSFC measurments are steady state, and so don't take into account the energy taken to get the turbo to spin up when accelerating. But at the point we are talking about driving at a constant speed I'd have thought they gave the full story?

Ultrasonic

greenman7 wrote: »

The reason I use engine braking is that I am consuming less fuel than in neutral.

Yes you will use less fuel than in neutral but you will glide for a much shorter distance.

greenman7 wrote:

Engine braking is a well known technique for reducing speed when anticipating the road ahead for traffic lights, bends, junctions etc.

Yes, absolutely, I do it all the time. As you say it is useful when you need to reduce speed. It does not however make sense for P & G.

greenman7 wrote:

The reason they do not notice is that I drive smoothly, I am not constantly braking and changing gears.

Please read my post again - you've completely missed my point. The sentence immediately following the one you quoted is critical.

greenman7 wrote:

I tend to drive on motorways at night when there is less traffic.

That I'll grant you is one time that I would see P & G being viable.

LincolnshireYokel

You need to know the torque peak of the engine. Always try and maintain the engine speed at the torque peak, you cannot possible get the fuel efficiency any better. I coast down hills, and i try and change gear without using the clutch, which means you have to develop an intimate knowledge of the relationship between road speed, engine speed and gear ratios.

Which ofc you had to do before synchromesh gears.

forgotmyname

Ultrasonic wrote: »

Must admit I can't follow the trip data you've given above. It sounds like it took you 39.70 L to drive 14 miles, plus however far four trips to town is. Unless you live a very long way from town I must be missing something!

Gosh your fussy.

Its not my fault i have not put all the data i was thinking about. I thought about it and you should have known that

Err why changed my post? It doesn make sense to me either lol.

Start again.... Its Diesel.

Filled up drove less than 1mile home, Drove approx 10 miles to caravan storage. Hooked up. Ignore 2 very short short journeys lol.
Drove 4 miles into town the next day. Filled up on the way out.
322.2 miles covered. 39.7 litres.

Hopefully this post makes sense

Just to clarify, are you saying that 80 mph is the absolute most efficient speed to travel at in your car

Ahh NO.. lol in 6th gear. its possibly the most efficient speed/revs

In 4th gear at 38mph it will virtually drive itself. I can get 70+mpg given a clear road. But again 38mph in 5th gives worse consumption.

Do you hate me yet? LOL

greenman7

Originally posted by Ultrasonic
Yes you will use less fuel than in neutral but you will glide for a much shorter distance.

Of course this is true and it opens up a whole new debate that I have seen many times. I suppose my latest running average of 69.9mpg leads me to believe that gliding in gear is working for me - as that is approximately 5mpg more than I was getting before I started using this technique a lot of the time.

Yes, absolutely, I do it all the time. As you say it is useful when you need to reduce speed. It does not however make sense for P & G.

You might not think it makes sense for P & G but the technique is documented as one for P & G - just google it.

Please read my post again - you've completely missed my point. The sentence immediately following the one you quoted is critical.

I agree that the acceleration needs to be quick or it does not work, I think it is working for me at my rate of acceleration. I do have passengers a lot of the day and nobody has ever said I am slow or commented on my speed variation!

To get back to the subject of fuel economy P & G is one of many hypermiling techniques anyone can use to get more mpg. Imagine if all drivers were to decrease their fuel used (using some of these techniques) by say 10% the amount of pollution cut down and less imports of fuel.

cyclonebri1

Ultrasonic wrote: »

Would you care to comment on why BSFC plots like this one (or the one directly below it) for turbo-diesels don't appear to show any particular rpm below which engine efficiency dramatically decreases? As I'm sure is obvious to you I am not an expert, but to my (possibly incorrect) understanding this fact would seem to contradict your view.

I'm no expert either, but from my experience, with my turbo diesel cars, not something I've read in a book, there are definite fuel savings to be had by keeping at least inside the turbo operating range. And probably, I'm not sure, as drag in higher gears comes into this, by keeping at the point of maximum torque.

You are talking theory, I'm simply saying what I have found in
practice, simply driving 1 tells you the point at which acceptable performance disappears out the window, and that's about 1500 rpm in the cars I've had.

No argument needed.;)

Johnmcl7

cyclonebri1 wrote: »

I'm no expert either, but from my experience, with my turbo diesel cars, not something I've read in a book, there are definite fuel savings to be had by keeping at least inside the turbo operating range. And probably, I'm not sure, as drag in higher gears comes into this, by keeping at the point of maximum torque.

I find exactly the same in practice, I originally used to drive avoiding the turbocharger assuming that is where the engine would be most efficient which the trip computer certainly liked but then found driving at the lower end of the turbocharger to be more efficient.

John