Understand the MPG (maybe help/maybe not)

[CrazyInWeston]

Hi all, I am in need of understanding a few things, but first a little context.

I am in the belief that a small engine in the city with stop start traffic is more economical than a bigger engine but the reverse is true when needing to use the Motorway (Interstate) for long distances because of higher speeds and thus a bigger engine is doing less work, is this true?

Well anyway, my car is odd. I've got a 2005 1.6ltr special Ford Engine (you know the one with the Ti-VCT, I know they ended up being these engines by 2008 but for 2005, rare) What I want to know, is why my MPG goes up when I rev the beans off the engine, surely this is counter intuitive?

Last year I kept my revs low, changed gear low, my MPG went down to 27MPG... This year, I keep the revs high, not changing gear until I hit 4000RPM and my MPG is over 32. I simply just dont understand, I thought higher revs meant using more fuel? Or am I an idiot?

[DickP]

Hi

Engines have parts in their rev range that they are most efficient. Low revs does not equate to low fuel use.

Think what it's like on a bicycle if you use a gear that's too tall for a given speed.

[CrazyInWeston]

1 hour ago, DickP said:

Hi

Engines have parts in their rev range that they are most efficient. Low revs does not equate to low fuel use.

Think what it's like on a bicycle if you use a gear that's too tall for a given speed.

Huh, nice analogy, interesting when you put it that way.

[CrazyInWeston]

Im so sorry to bump the thread, but wondered what I put in OP actually true regarding engine sizes and Motorway miles? Or does it all work the same depending HOW you drive your 1.1ltr vs a 2.0ltr on the motorway, I mean what DickP put made a bit of sense, right revs, less fuel because wrong revs at either too high or engine LUGGING I believe he was on about made sense, but what about a 1.1ltr doing well over 4000rpm at 70mph vs a 2.0ltr doing 2,500rpm on the Motorway?

[TomsFocus]

Cubic capacity has very little to do with fuel economy nowadays.

Direct, stratified injection can make a huge 2.0 petrol use almost no fuel while cruising at 70mph.  And the extra air charge from a high boost turbo 1.0 engine can make it use a lot of fuel.

Back in the olden days on a simple engine with old technology you would be right but not any more.  Cars these days also mostly have 6 speed gearboxes and wheels with a large rolling radius which help to keep cruising economy up and emissions down.

[ScaniaPBman]

And just to add another thing to think about on the fuel consumption front, the catalyst can overheat on certain combinations of engine design and power output.

When this is established at the development stage of engine testing, extra fuel is programmed in and dispensed, not to give more power, but to go straight through as a very rich mixture and actually cool the catalyst. The catalyst cannot burn all the extra hydrocarbons because there is not enough spare oxygen to do the job.

For more detailed info do a search for 'catalyst overheat protection'

I have no idea if catalyst overheat protection is used on any Focus petrol engines but it is not the sort of thing that they are likely to boast about.

ScaniaPBman.