fuel economy

Mikey_Moos

Turning off the engine on a petrol car whilst at lights and queues will save you a fortune... Diesel's dont really like the old on/off technique though...

That and thinking 100m down the road... if its a single road with lots of turn-offs, chances are someone is going to be turning; so keep your distance from queues and use the engine to slow yourself down!

cyclonebri1

hareng wrote: »

I too have noticed what i could describe as remarkable fuel consumption by doing exactly that. I found it out by knocking down two gears and holding speed at that in 4th, turbo ready to go, mean while watching the instant economy! To be conclusive i would have to do more long term testing.
I firmly believe it could be the mapping of the ECU and the weight, gearing of the car, mines standard and i do own VAS with plug in OBD readers.

Personally i would never ever buy another car without a trip computer, i drive on it 90% of the time. Soon ease off when see 3mpg.

With so many modern cars turbo charged you have kinda hit the nail on the head there:T

I would qualify though, you have to be just inside the turbos operating range, not ready to go but already going, albeit at low revs. About 1500rpm in the case of a modern diesel.

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

In answer to the above post, I read that 20 secs stationary is the point at which turning off the engine is the best idea, ok level crossings, but otherwise you'ld need to be psychic to decide

neilmcl

Mikey_Moos wrote: »

Turning off the engine on a petrol car whilst at lights and queues will save you a fortune... Diesel's dont really like the old on/off technique though...

Really...funny then that a lot of the leading manufacturer's diesels have start/stop technology.

forgotmyname

Trebor16 wrote: »

You say your figures are not random, but you haven't answered my question about what testing you have carried out to determine if your claims that your car uses less fuel at 80mph than at 70mphin sixth gear are indeed correct.

Testing i have carried out are runs on the same stretch of road under the same conditions with my OBDII reader plugged in.

My trip which although not 100% accurate, Is consistent.

I will get the same results time after time.

I could log the fuel flow for you, But without knowing the traffic conditions and driving style you wouldnt believe me anyway.

I know what your saying, Speed = Drag.
But its the gearing and engines torque. Obviously in my car 80mph is probably where its most efficient.

85mph and it could be going back down, Dont know never tried and dont intend to.

If i change upto 6th gear it drops the revs by 500rpm. At 50mph this is below where the turbo starts working.
I could give you the boost pressures also ro prove this.

Ultrasonic

forgotmyname wrote: »

2004 Mondeo Estate 130bhp 6 speed.

What's the engine? Petrol or diesel?

forgotmyname wrote:

My brim to brim tests are mostly short journey's so those figures are probably no good to you.

My main thought is that short distance brim to brim tests are likely to be less accurate than for longer runs, since a small change in the amount of fuel added to brim the tank will make a larger percentage difference to the calculated mpg. (Depending how you fill your tank this could be due to when the pump switches off, or when you judge it to be brimmed by eye.)

forgotmyname wrote:

Ideally what i need to do is brim in at a pump do the journey in one gear, Fill at the same pump and do the journey in the other gear.
Ive not done that.

Not surprised you haven't done that . I don't think you'd need to be quite that drastic though. If you have a couple of longer distance motorway trips you could do one at 80 mph and one at 60 mph max. motorway speed. If you used the brim to brim method to calculate the fuel economy for each trip that would give you a pretty good comparison. (Obviously ideally you'd want to do exactly the same trip in identical weather and traffic conditions, but between those two speeds the economy difference would likely be large enough to dominate I'd have thought.)

Ultrasonic

forgotmyname wrote: »

OK checked my outbound figures..
I filled up. Drove home less than 1 mile.
Next morning i drove to the caravan storage approx 10 miles.
Couple of short journeys at the other end, 3 journeys of less than a mile.

Next day 4 times into town and parked up.

Filled up on the way out. 39.70 litres. Is that really 39MPG ?

Or has someone slipped something into my tea?

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!

forgotmyname wrote:

And i never used 6th at all when towing. Left it in 5th.

Towing is likely to be a rather special case than the main discussion of this thread I'd have thought, since torque is likely to be more of an issue?

Ultrasonic

greenman7 wrote: »

I think I have now perfected the 'pulse and glide' technique to increase my mpg. I tend to use it a lot of the time even though passengers in my car are not aware of it. For example on a motorway I will accelerate to 65+mph (pulse) then take my foot off the accelerator and allow engine braking to say 57mph (glide), then accelerate again and engine brake again.

Three thoughts:

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. (Some idiots would tell you to turn the engine off during the glide phase, but don't whatever you do, do this.)

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 point is to accelerate at high load to operate the engine in a region of optimal efficiency (power generated per amount of fuel used).

3) It is extremely rare that anyone could practically employ the P & G technique on UK roads without seriously aggravating other road users. I have never even tried it for this reason.

Ultrasonic

Trebor16 wrote: »

Aerodynamics will also play a part but it doesn't matter ultimately how aerodynamic the car is as drag will still effect the fuel efficiency of the more slippery cars. The drag coefficient is not the only factor as the frontal area of the car will also affect things.

The point I was making was that how much the fuel economy varies with speed will depend on the relative contribution of aerodynamic drag to the total resistive force that has to be overcome to maitain a given speed. This will vary between cars. It may well be that the variation is small though, I have no specific knowledge of this.

Trebor16 wrote:

A brim to brim figure is not going to help us here. What we need is proper tests conducted at 70mph and 80mph, carried out in a variety of conditions, to prove or disprove the claim.

I think brim to brim tests are the most meaningful that any private individual can sensilby carry out, and as I posted above can certainly be of value. Although obviously I acknowledge that something like this test provides better data.

Ultrasonic

Mikey_Moos wrote: »

Turning off the engine on a petrol car whilst at lights and queues will save you a fortune...

It's hardly going to save fortune for most drivers, we're talking pretty modest savings. And personally I wouldn't risk the possibility of causing a starter motor to fail prematurely as a result of being used far more than intended when it was chosen for the car.

Ultrasonic

forgotmyname wrote: »

I know what your saying, Speed = Drag.
But its the gearing and engines torque. Obviously in my car 80mph is probably where its most efficient.

Just to clarify, are you saying that 80 mph is the absolute most efficient speed to travel at in your car, more efficient than say 40 mph in whatever gear is appropriate? I flat out don't believe there is any way that could be the case. Debating whether your car might be more fuel efficient in 6th gear at 80 mph or 70 mph is something I'm more open to consider.