Fuel Economy 1.33 Vvti

[avensis57tr]

Well fingers crossed the fuel economy seems to be improving nicely has been gradually rising now on 4500 miles to around 43 mpg in normal use which is not bad considering it is driven around Sheffield in rush hour and matches the diesel Avensis my mum used to have, and getting close to what my 1.2 TSI Fabia is managing now that its run in (47 ish mpg). Plus my parents took it for a run to London and back and tip computer (which I know can be vary a bit from reality) said that it managed 49mpg which is really good considering most of that will have been at 70mph +. Just a shame that we got it before the T-spirit came back with the climate control and that it doesn't have cruise but maybe the next generation yaris will.

[fred67890]

I have a new Yaris 1.33 and driving fast (speeding fine just arrived ) it is showing 51 mpg combined and 55 on the motor way . it is semi auto transmission which keeps the revs just over 2000 useing 6 gears

[gloucester]

According to the trip computer mine is doing 48 mpg. Car is brand new so that may change. That was a strange post about cylinder wear and ridges and the one about wear between pistons and crank. Not mechanics!

[alfiejts]

Have we any mechanical engineers on?

My background is engineering so I can follow the theory but don't have the detiled technical knowledge to explain further.

As I understand it...

The piston has "piston rings" around it that form the seal between the piston and the cylinder bore.

They need to "bed in" together to form the best seal. If the seal isn't the best, the engine won't produce the most power because some compression pressure will leak past the piston rings and also the car may burn some oil as oil can get sucked past the rings from the cylinder bore into the compression chmber on the intake cycle.

If you drive the engine too hard, to early, then you wear the piston rings prematurely so they can't make the best seal, but equally if you don't drive the engine hard enough, then the cylinder bore surface can "glaze over" and harden slightly - which means that the piston rings cant bed in and get the best seal.

So the answer is to drive the car "normall". Don't thrash the watsits off it, but equally do make use of the higher rev range for short periods when you're not straining the engine, to allow the piston rings and cylinder bores to bed in properly.

Equally, centrefugal force acts on the piston which sits on the end of the "con rod". At higher revs, there's more centrefugal force and the con rod gets stretched by a very tiny amount.

We're talking microns, not mm, but it is a fact that the con rod stretches a tiny amount.

So if drive the engine only at low revs when you're running the engine in, the piston rings and cylinder bore will "bed in" through the length of the cylinder bore but the rings will never reach the last few microns of travel at the top of the cylinder bore that they will then reach when you drive at max revs - when the forces stretch the con rods to the highest level. So you'll then get the piston rings tapping againt the ridge at the top of the cylinder bore at the end of each upward stroke.

And if you have ever dismantled an engine after a few years of use you can see the wear on the cylinder bore and the ridge formed at the top where the pistonn travel stopped. In olden days, getting a "rebore" and fitting replacement oversized pistons was a common thing to fix an engine whose cylinder bore had worn so much that it wasn't sealing properly.

So that's my attempt at explaining mechanical theory.

Simplistically - when you get a new engine, you want to drive it fairly normally, using max revs now and then, but without straining it under load of thrashing it too much - to allow all the components to properly bed in together.

http://coxengineering.co.uk/bore.aspx

There's a whole book been written on this subject...

http://www.informaworld.com/smpp/content~db=all~content=a781084208

So in a nutshell - different cars with the same engine may well get different fuel consumption becasue of how they've been run in and therefore how pefect the compression seal is inside the cylinder bore and how much oil is retained on the cylinder bore to reduce friction....

[mrpj]

"Have we any mechanical engineers on?"

Retired Marine Engineer (motor) any use alfie? (40 years experience of large, medium and small Diesels, last 30 as Chief Engineer)

When we used to "run in" a newly ringed and de-glazed cylinder on large or medium marine Diesel engines or a full overhaul on smaller engines it was proper practice to vary the amount of power to that cylinder or engine - so a few hours at low power then increase the power for a few more hours and so on until the prescribed bedding in period was achieved. This was done by adjusting the delivery of the high pressure j e r k (some silly filter didn't like this word!) pump for that particular cylinder. With more fuel injected the combustion pressure increases accordingly. This extra pressure exerted a force to the piston side of the rings and forced them into increased contact with the cylinder walls thus assisting bedding in. If this wasn't done then the cylinder would again "glaze" and the proper sealing of the rings would not be achieved.

So yes, the same applies to a car engine, if the power is not varied then correct bedding in of the rings is less likely. Even a car's instruction manual will advise varying the speed during running in and not to maintain the same speed for long periods. That'll be the reasoning behind that.

[gloucester]

I don't think people should be too concerned about running in new engines. Just try and drive sensibly. Cars are subjected to so many different driving conditions nowadays on most journeys they will run themselves in. Another problem checking MPG the old way can occur when converting litres to gallons. Unless you have the correct conversion it will never be right. 4.54 litres to the Imperial gallon.

[LCPete]

I have had mine for about 4 months from new

I have recently worked out my mpg, running round locally I get 51mpg and on a run I got 57 which is really good

when I first got the car I was only getting low 40's which was very disapointing as my old shape D4D would always do high 60's

I nearly took it back but my mate at work said it was because it was running in and also it takes long to warm up in the winter

as for the running in thing I always just drive normally but never rev an engine until it gets to temp

pete

[Adam786]

I have had mine for about 4 months from new

I have recently worked out my mpg, running round locally I get 51mpg and on a run I got 57 which is really good

when I first got the car I was only getting low 40's which was very disapointing as my old shape D4D would always do high 60's

I nearly took it back but my mate at work said it was because it was running in and also it takes long to warm up in the winter

as for the running in thing I always just drive normally but never rev an engine until it gets to temp

pete

You have a 1.4 D4D this post is on 1.33 VVTI engines :) obviously your car is going to do that.. No offense :)

[t-spiritpete]

I have had mine for about 4 months from new

I have recently worked out my mpg, running round locally I get 51mpg and on a run I got 57 which is really good

when I first got the car I was only getting low 40's which was very disapointing as my old shape D4D would always do high 60's

I nearly took it back but my mate at work said it was because it was running in and also it takes long to warm up in the winter

as for the running in thing I always just drive normally but never rev an engine until it gets to temp

pete

You have a 1.4 D4D this post is on 1.33 VVTI engines :) obviously your car is going to do that.. No offense :)

Actually I think he says he used to have a D4D and has had the his 1.33vvti for 4 months ... no offence ;).

Pete.

[Adam786]

I have had mine for about 4 months from new

I have recently worked out my mpg, running round locally I get 51mpg and on a run I got 57 which is really good

when I first got the car I was only getting low 40's which was very disapointing as my old shape D4D would always do high 60's

I nearly took it back but my mate at work said it was because it was running in and also it takes long to warm up in the winter

as for the running in thing I always just drive normally but never rev an engine until it gets to temp

pete

You have a 1.4 D4D this post is on 1.33 VVTI engines :) obviously your car is going to do that.. No offense :)

Actually I think he says he used to have a D4D and has had the his 1.33vvti for 4 months ... no offence ;).

Pete.

Lol my bad! I completely misread that! Sorry :P

[gloucester]

Just done a 180 mile round trip. Mixed driving conditions. Trip computer says 53 mpg. Not done a thousand miles yet though! Will update in future. Regards. 1.33vvti.

[LCPete]

I have had mine for about 4 months from new

I have recently worked out my mpg, running round locally I get 51mpg and on a run I got 57 which is really good

when I first got the car I was only getting low 40's which was very disapointing as my old shape D4D would always do high 60's

I nearly took it back but my mate at work said it was because it was running in and also it takes long to warm up in the winter

as for the running in thing I always just drive normally but never rev an engine until it gets to temp

pete

You have a 1.4 D4D this post is on 1.33 VVTI engines :) obviously your car is going to do that.. No offense :)

Actually I think he says he used to have a D4D and has had the his 1.33vvti for 4 months ... no offence ;).

Pete.

Lol my bad! I completely misread that! Sorry :P

No worries I have updated my sig !