Author Topic: The Mazda 3 Fuel Economy Thread  (Read 268594 times)

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Offline moozmooz

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Re: The Mazda 3 Fuel Economy Thread
« Reply #795 on: March 24, 2018, 12:43:36 AM »
Difficult to find corners in underground fuel tanks as they're normally cylindrical cross section with domed ends, so I'd take that theory with a huge pinch of salt.

The temperature/volume idea's also a bit of a non starter. If you look at the volumetric thermal coefficient of expansion for petrol, it's tiny, so you're not going to see any meaningful change across the normal operating temperature range. So the density won't vary much meaning that the mass of fuel you're buying, and therefore the amount of energy you're buying, is as near as damn it constant.

Interestingly, petrol and diesel are about the only fuels I can think of where you're buying blind, as there's no indication of the energy content or calorific value, and the fuel companies aren't keen to tell you either. They'll only quote you over a fairly wide range.

This is one of the things that raises huge doubts about the mileage benefits claimed by some, but, interestingly, not the fuel companies. To get more miles out, you'd need to be putting more energy in - ie higher CV. The only apparent difference is the additive package.

Offline StevenRB45

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Re: The Mazda 3 Fuel Economy Thread
« Reply #796 on: March 25, 2018, 08:59:45 AM »

Interestingly, petrol and diesel are about the only fuels I can think of where you're buying blind, as there's no indication of the energy content or calorific value, and the fuel companies aren't keen to tell you either. They'll only quote you over a fairly wide range.

This is one of the things that raises huge doubts about the mileage benefits claimed by some, but, interestingly, not the fuel companies. To get more miles out, you'd need to be putting more energy in - ie higher CV. The only apparent difference is the additive package.

Surely it's not just the potential energy that matters but how much of it can be released, and how it is released. Calorific  value is determined by burning something in steady state conditions. A combustion engine does not generally operate in steady state conditions, there's a wide range of variables in the process.

Diesel has a lower CV than petrol...so all diesels get worse economy? No because of the energy density. I get what your driving at but the conditions of combustion are also important in determining efficiency as much as "if we burn this on a bench X is released".
« Last Edit: March 25, 2018, 09:06:44 AM by StevenRB45 »

Offline Engineer Andy

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Re: The Mazda 3 Fuel Economy Thread
« Reply #797 on: March 25, 2018, 11:51:25 AM »
I'd read it was a simple as pumps measure volume not quantity as such. Hot fuel = more volume and less for your money, cold fuel more dense and more for your money.

However it's a going beyond the far end of a fart to test that for me... ;)

I suspect its a combination of both reasons as they seem equally plausable to me.  That said, I'd rather not have to make a special trip to the local filling station at night just to get the odd percentage point more in fuel/less lost by evaporating, as its likely it'll cost me far more in fuel to get there and back (also the car will be cold and running on a rich mixture for the duration), as well as my local filling stations are far more expensive (3-5p a litre, sometimes more) than the cheapest I normally use, which I've always used on the way/from work, relatives homes or going shopping in a nearby town 10-15 miles away.

I bet it makes just as much difference as to when the tanker fills up the underground tanks - I'm sure you can image how warm the fuel in the tanker gets if your filling station was the last to get its delivery on a hot, sunny day...

Offline moozmooz

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Re: The Mazda 3 Fuel Economy Thread
« Reply #798 on: March 26, 2018, 01:36:18 AM »
You can get some information from BS EN 228 : 2008 for petrol and BS EN 590 for diesel.
The differences in quoted CV (taken form ESSO technical bulletins) - 47.48 MJ/kg for 95 RON, 47.46 MJ/kg for 97 RON, and 46.0 MJ/kg for diesel - clearly don't reflect the differences in MPG you'd expect between petrol and diesel, but then you wouldn't expect it to as they're totally different fuels that have markedly different combustion characteristics.

The CV used to be measured by burning, in the case of gas, in contraptions such as Cutler - Hammer calorimeters, which measured the temperature rise in a water bath achieved by burning a precisely measured volume of gas. Large cumbersome objects that were susceptible to draughts, variation in water quality, dirt, atmospheric pressure to name but a few.
It's virtually standard practice now for CV to be determined by analysis and calculation based on CV of individual components, giving more reliable and consistent absolute and comparative measurements.

As to real world engines burning petrol, the biggest variables involved are between types of engines and use. The way the fuel is burnt is largely out of the hands of the fuel manufacturer. They provide a 'one size fits all' product.
For example, looking at my 2 Mazdas, Gen 1 and Gen 3, Gen 1 is faster revving, shorter stroke, lower compression ratio, so what's happening during the combustion stroke is drastically different between the two.

Which all takes us back to why fuel companies are extremely careful about making claims as to what benefits you may see.

You'll doubtless have seen the adverts about Lidl wine being made on an oil rig. You'd probably (I hope) accept that it's a ridiculous scenario thought up by an overpaid twit in an advertising agency, but there are plenty of people out there who still believe supermarket fuel is made in a dodgy, but extremely well hidden, back street refinery. Maybe the advertising man's not such a twit after all!

The reality is that all fuels come from a relatively small, and getting smaller, number of refineries. The only differences at the pumps are the detergent packages, branding and price. You pay your money and you take your choice.

Offline StevenRB45

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Re: The Mazda 3 Fuel Economy Thread
« Reply #799 on: March 26, 2018, 01:34:43 PM »
I guess the question would be do you expect the car to run the same on 92 as it does on 95? And if you don't why you wouldn't expect a similar difference between 95 and 98 in an engine that can detect differences in knock and adjust accordingly?

Obviously the car is not set up to run 92..but by your logic it should. It's all just a different addictive mix after all.
« Last Edit: March 26, 2018, 02:41:09 PM by StevenRB45 »

Offline moozmooz

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Re: The Mazda 3 Fuel Economy Thread
« Reply #800 on: March 26, 2018, 04:21:59 PM »
92? 98? And what's this addictive mix? Have you been sniffing your fuel tank? :)


Offline StevenRB45

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Re: The Mazda 3 Fuel Economy Thread
« Reply #801 on: March 26, 2018, 04:30:35 PM »
I need to sack my proof reader..

Ah hem, additive and I'm referring to the RON numbers.

Offline Alfisto

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Re: The Mazda 3 Fuel Economy Thread
« Reply #802 on: March 31, 2018, 09:22:25 AM »
I have a 2016 2 litre 120ps with iStop. I noticed on here a few days ago that someone has turned the iStop off and achieved better fuel economy. Is this usual.

I've done about 15500 in mine and the fuel economy meter says it has saved 125 miles, approximately 8%. does this seem reasonable compared to others.

I do pretty general type driving, mixed town and long distance and long and short journeys.

Offline Engineer Andy

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Re: The Mazda 3 Fuel Economy Thread
« Reply #803 on: April 07, 2018, 08:52:49 PM »
I have a 2016 2 litre 120ps with iStop. I noticed on here a few days ago that someone has turned the iStop off and achieved better fuel economy. Is this usual.

I've done about 15500 in mine and the fuel economy meter says it has saved 125 miles, approximately 8%. does this seem reasonable compared to others.

I do pretty general type driving, mixed town and long distance and long and short journeys.

I wouldn't be surprised if the iStop works far better when you're making a few, but lengthy stops in traffic, rather than lots of short stops where it just engages for a few seconds before restarting the engine.  Once the system is depleted, it then takes battery power instead (I think) to recharge itself, which will need itself to be topped up by using more fuel to recharge it via the alternator.  Not particular sure, but I do recall reading something like this before, here or elsewhere.  I'm sure someone with better knowledge of the system can either confirm this or correct me.

Offline Engineer Andy

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Re: The Mazda 3 Fuel Economy Thread
« Reply #804 on: April 07, 2018, 09:02:13 PM »
I guess the question would be do you expect the car to run the same on 92 as it does on 95? And if you don't why you wouldn't expect a similar difference between 95 and 98 in an engine that can detect differences in knock and adjust accordingly?

Obviously the car is not set up to run 92..but by your logic it should. It's all just a different addictive mix after all.

I think the 3 can run on lower octane fuels, as this often occurs in poorer countries where standard 95Ron octane fuel is (to the locals) very expensive.  Whether this is due to a factory setting change for these countries markets or all the models have knock sensors, I'm not sure.  My manual says its designed primarily for 95Ron (as its measured in the UK).  I may still email Mazda to ask (my dealership didn't really know) whether the mk1 or 2 has one fitted (I'm sure the MPS and mk3 does).

My car's small, but notceable increase in mpg since I've been using 97/98 Ron fuel (mainly Shell, but my last fill-up was Sainsbury's 97Ron) may have been due to the Shell fuel and the Redex cleaning the injectors, though my mainly longer distance driving (almost no short trips throughout its life) at 60-70 mph means the injectors shouldn't be in that bad a nick anyway, even after 12 years use.

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Re: The Mazda 3 Fuel Economy Thread
« Reply #805 on: April 07, 2018, 11:19:50 PM »
The 1.6 does have a knock sensor. I imagine they alter the map based on the original point of sale, if it's being sold in the UK it doesn't need to run on less than 95, if it's being sold in Egypt it'll need to run on 92..I would imagine the base timing on both would be set differently while the degree of adjustment available to the management system remains the same (base timing + or - so many degrees).

Offline mnevis

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Re: The Mazda 3 Fuel Economy Thread
« Reply #806 on: April 23, 2018, 11:10:40 AM »
480 miles per tankful.  Average 40 mpg.   Almost exactly 120 miles per quarter tankful according to the fuel gauge.  Mixed driving.  Narrow rural country roads.  Fast 'A' roads single and dual-carriageway.  Town driving but not 'driving across Manchester in rush hour' type of towns.  Very little motorway driving.   2005 1.6 Petrol Sakata.

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Re: The Mazda 3 Fuel Economy Thread
« Reply #807 on: April 27, 2018, 02:33:48 PM »
480 miles per tankful.  Average 40 mpg.   Almost exactly 120 miles per quarter tankful according to the fuel gauge.  Mixed driving.  Narrow rural country roads.  Fast 'A' roads single and dual-carriageway.  Town driving but not 'driving across Manchester in rush hour' type of towns.  Very little motorway driving.   2005 1.6 Petrol Sakata.

Sounds about right  - very similar to what I get on my 1.6 petrol TS2.  It'll be interesting to see what mine changes to when I:
  • Get new 15in alloys fitted in conjunction with going from 16in and from 205/55 R16 Dunlop SP Sport Fastresponse tyres to 195/65 R15 Michelin CrossClimate+ ones (will be fitted in 9th May);
    I'm going to try and clean out my car's EGR valve without taking it out;
  • Also gonna try to clean the throttle body and the MAF sensor (using the correct sprays) - the latter I'm not so sure if it has to have the battery disconnected first.  I'm not exactly a mechanic so we'll see after reading up on what to do.

I've already been running (admitedly on not much driving in the last year, but still on long runs) it on V-Power or supermarket 97Ron petrol and used a 4-shot bottle of redex to (hopefully) clean the injectors and valves (to a degree), so it'll be interesting (assuing I do everything correctly!) what the result is.  I'm not changing the wheels and tyres to get better mpg (they should do to a degree) but more for improving ride comfort whilst getting some winter abilities from the CCs.

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Re: The Mazda 3 Fuel Economy Thread
« Reply #807 on: April 27, 2018, 02:33:48 PM »