n/a mpg

[Ian C]

Hmm, how so? I'm approaching this from a common sense viewpoint but I'm ready to be taught different by someone who designs cylinder heads for a living

 

I'm saying that, under a steady cruise condition, say maintaining 56mph, a decat free flow exhaust car would use a bit less fuel than a catted stock exhaust one as it has more pumping losses.

 

Otherwise it's like saying stick a spud up your exhaust and improve your mpg, surely?

 

-Ian

 

PS even I can't read that PDF you linked to

[Ian C]

You know your stuff Ian! Do you think that applies to NA engines as well?

 

Hold that judgement while Digsy shoots me down Deffo applies to NAs as well, whatever the result is!

 

-Ian

[Digsy]

Hmm, how so? I'm approaching this from a common sense viewpoint but I'm ready to be taught different by someone who designs cylinder heads for a living

 

I'm saying that, under a steady cruise condition, say maintaining 56mph, a decat free flow exhaust car would use a bit less fuel than a catted stock exhaust one as it has more pumping losses.

 

Otherwise it's like saying stick a spud up your exhaust and improve your mpg, surely?

 

-Ian

 

PS even I can't read that PDF you linked to

PDF link works for me... Maye its still in my cache or something?

 

I was looking at the pressure versus cylinder volume (the pressure variations as the cylinder moves through the four strokes) and, after an initial drop at EVO (exhaust valve opening) the exhaust stroke was an almost horizontal line from there to IVO (intake valve opening). My drive was that if the losses from the exhaust stroke are practically zero, there is less than practically zero to be gained from improving it.

 

Mind you, this was in the Bosch Book and therefore a highly generalised diagram with no info on assumptions made about back pressure, engine speed, etc. When I went in search of a more "real world" diagram I couldn't find one but instead I found a lot of written stuff saying that backpressure does have an effect on mpg.

 

The PDF contains a table showing % increase in fuel efficiency versus % change in engine parameters. In there, a 1% decrease in total pumping losses (not just the exhaust stroke) gives a quarter % increase in fuel efficiency. There are also some words saying that pumping losses throughout the cycle are significant.

 

I reckon this ties up with any gains from pumping losses being swamped by higher boosting in the turbo car, which will require more fuel to maintain the AFR.

 

...and here's that .pdf as an attachment.

912513.pdf

[Digsy]

This is similar to the idealised diagram I was looking at. Note no pressure rise in the cylinder from EVO to EVC.

[Ian C]

Oh no, I meant the attachment was too techie and tedious for me to read, not that I couldn't open it Saying that I've read it now lol

 

I hear what you are saying about increased boost levels requiring scads of fuel, I'm 100% agreeing with that, but I'm saying that in a steady state situation between two cars... Oh, I'll just copy and paste it

 

"I'm saying that, under a steady cruise condition, say maintaining 56mph, a decat free flow exhaust car would use a bit less fuel than a catted stock exhaust one as it has more pumping losses."

 

I've just noticed that's badly worded at the end, it should say "as the catted one has more pumping losses". So, *exactly* the same road conditions, engine load, etc. but one is decatted and big-bore. Which uses the most fuel?

 

-Ian

[Ian C]

This is similar to the idealised diagram I was looking at. Note no pressure rise in the cylinder from EVO to EVC.

 

Yeah, but note in the text:

"The diagram shows an "ideal" pressure-cylinder volume curve"

 

And

"In the "real" cycle, inlet and exhaust flow restrictions reduce and increase these pressures, respectively"

 

....

 

-Ian

[tbourner]

The PDF contains a table showing % increase in fuel efficiency versus % change in engine parameters. In there, a 1% decrease in total pumping losses (not just the exhaust stroke) gives a quarter % increase in fuel efficiency. There are also some words saying that pumping losses throughout the cycle are significant.

 

 

Weren't you saying the complete opposite of this when I suggested the turbo draw and subsequent reduction of pumping losses on ALL strokes of the engine means that there's not a lot of difference between NA and turbo fuel consumption?

 

I know what you said about compression ratios still stands (or was that someone else said that? [steve W2?]), but I'm pretty sure you shot down my idea that pumping losses were insignificant.

[Digsy]

Yeah, but note in the text:

"The diagram shows an "ideal" pressure-cylinder volume curve"

 

And

"In the "real" cycle, inlet and exhaust flow restrictions reduce and increase these pressures, respectively"

 

....

 

-Ian

OK, Ian first:

 

The ideal diagram is on the left and the real diagram is on the right. The diagram on the right is similar to the one I was looking at in the Bosch Book.

 

In the ideal diagram, the intake and exhaust strokes occur at atmospheric pressure (i.e. no pumping loss at all - the cylinder vents to atmospheric pressure instantly when the exhaust valve opens).

 

I've dicked up a bit in my interpretation of the "real" curve, however . I was assuming that a system with significant backpressure would exhibit a pressure rise in the cylinder as the piston forced the gas out (i.e. the piston was trying to force the gas out faster than the system would allow). What I didn't appreciate was that although the "real" curve isn't showing a pressure rise, is it showing a constant positive pressure which the piston must still work against, which is still a pumping loss. DOH.

 

So: One point to Ian because dropping the back pressure will reduce this residual cylinder pressure and therefore the pumping losses leading to increased fuel efficiency (all other things being equal).

 

I'm going to claim a point back for correctly assuming that the net effect on MPG would be minimal, however

 

I need to go back and re-read what I put in tbourner's thread now...

[Digsy]

Okay...I don't think I've contradicted myself here (yet )

 

IIRC, the main thrust of tbourner's post was that off-boost and up to a certain point, a turbo engine will be more fuel efficient than an NA of the same displacement because even when off-bost the windmilling turbos will be blowing air into the cylinder in the induction stroke, hence reducing pumping losses.

 

My reply (in summary) was that there might be some benefits in the intake and exhaust strokes, but I'd guessed that they wouldn't be much. In fact. referring back to my pervious posts, then if there is a pressure rise across the turbo even when it is only windmilling, then it would raise the cylinder pressure during the induction stroke (making it closer to the "ideal" curve for that portion). However, because the turbine impeller has to offer an obstruction to the exhaust gas it would increase the backpressure on the exhaust stroke, moving that further away from the ideal curve.

 

I made a similar mis-assumption in that post as I did earlier today: That you can only get rid of pumping losses by raising the pressure on the induction stroke to above atmospheric. Actually, all you have to do is raise it to above what they would be in the equivalent NA engine:

 

The losses on compression in an NA will be higher because of the higher compression ratio (typically about 10:1 as opposed to 8.5:1 ish on the turbo version). I'm not sure about the pumping losses on the induction stroke as I don't know what impact the manifold pressure would have on actually forcing the piston down in the bore - my guess is not much. On the exhaust stroke you could run lots of overlap and use the turbo to "blow down" the cylinder but I'm not sure you would want to as the point of blowing down is to maximise available volume for the next charge but you get a better charge with forced induction anyway.

The pressure from a windmilling turbo (if there is any) wouldn't push the piston down in the bore, but it would help reduce the vacuum resisting the downward motion. The net result would be a reduction in pumping losses on the induction stroke.

 

However, the main thrust of my post was that because we are talking about an essentially off-boost situation, the turbo engine would be producing less torque than its NA brother at a given throttle position by virtue of its lower compression ratio, hence if you drove it at the same road load in a car the net result would be a decrease in MPG.

[Ian C]

 

So: One point to Ian

 

[tbourner]

That's cool Digs, as I said I hadn't factored in the compression ratio idea. 1 point to you (although not a point against Ian as it's sort of a different argument!!).

 

I'm still confused, but I'm going to claim that I'm on a steep learning curve and didn't know anything at all about how an engine works about 6 months ago!!

[Digsy]

That's cool Digs, as I said I hadn't factored in the compression ratio idea. 1 point to you (although not a point against Ian as it's sort of a different argument!!).

 

I'm still confused, but I'm going to claim that I'm on a steep learning curve and didn't know anything at all about how an engine works about 6 months ago!!

He's never going to let me live that down, you know. He used the banana and eveything.

 

[tbourner]

to the banana though.

 

 

Slightly off topic, but isn't banana a difficult word to type quickly!! I nearly wrote bananan!!

[Digsy]

to the banana though.

 

 

Slightly off topic, but isn't banana a difficult word to type quickly!! I nearly wrote bananan!!

In geek-speak, I believe that is actually called a "banana error".

[Max Headroom]

Thats very helpful not knowing what car you have, what mods you have, what petrol you using and how many miles you do a week

 

The point I was trying to make, perhaps unsuccessfully is that if your are worried about MPG you should buy a mondeo diesel, the supra is a high performance car, when I bought it I knew that the mpg was not going to be at the top of my wish list. Its a beautiful car enjoy it. I know that my answer was not the best but thats the way I am.

[tbourner]

True, I always get people talking to me about my car, and they always say "What's the 0-60?" first off, and then "What's the fuel consumption like?".

I usually say "I don't know" or "does it matter"!

[Digsy]

...but it can be helpul to at least know what your mpg is and benchmark it against others if you think it is off. I do it routinely beavause I think its one good way to monitor youe engine's health.

[Max Headroom]

...but it can be helpul to at least know what your mpg is and benchmark it against others if you think it is off. I do it routinely beavause I think its one good way to monitor youe engine's health.

I dont go too deeply into all the technical data, I am not an engineer, I'm an Optical Dispensing Assistant, I know the mpg isnt going to be great, but if I was too benchmark it considering the different mpg on this thread where would I start, 15 or 33 mpg? I dont really care about mpg, I care about my car. thank you

[Steve W2]

The point I was trying to make, perhaps unsuccessfully is that if your are worried about MPG you should buy a mondeo diesel, the supra is a high performance car, when I bought it I knew that the mpg was not going to be at the top of my wish list. Its a beautiful car enjoy it. I know that my answer was not the best but thats the way I am.

 

I'm not the one worried about my MPG. I was providing information in answer to to the original post of the thread, backing it up with actual data taken from both of my cars.

 

The whole 'if you want good MPG go buy a diesel' thing is bollocks. You say you don't care about your fuel consumption because it's a lovely car, but I bet you would if you were getting 2 MPG.

 

It wouldn't be lovely then, it would be broken, and that it part of the reason that people are alos interested in the cars fuel consumption.

 

Likewise, if you bought a diesel, you wouldn't just say 'it's a shit car but will have ace MPG' and forget about it. You would want to know what MPG you were getting out of your diesel.

[toyotasuprauk]

Roughly I'm getting 21mpg from my NA manual without any motorway driving. Improves a bit on the motorway. That's with Super Unleaded which I always find less economical than normal 95.

 

Quoting my own post here but just out of interest I used normal 95 Ron on my last 2 fills ups. The first one didn't make much difference perhaps because there was still some Super unleaded in the tank but on the second fill my MPG increased to 24mpg.

[250horses]

Quoting my own post here but just out of interest I used normal 95 Ron on my last 2 fills ups. The first one didn't make much difference perhaps because there was still some Super unleaded in the tank but on the second fill my MPG increased to 24mpg.

 

Did you notice any reduction in performance ?? Always wondered whether Opimax was worth it - I always use it, but maybe should try normal unleaded instead

[tbourner]

May only make a little difference to an NA, they do have a knock sensor I believe but I don't know how much of a difference that'd make.

 

On a TT it's essential to use Optimax or similar, especially if it's a jap. Otherwise you'll find yourself getting tons of det.

[250horses]

May only make a little difference to an NA, they do have a knock sensor I believe but I don't know how much of a difference that'd make.

 

On a TT it's essential to use Optimax or similar, especially if it's a jap. Otherwise you'll find yourself getting tons of det.

 

Cheers Id heard that before but was never sure - off to Shell from now onwards

[toyotasuprauk]

There was no noticeable difference in performance, but when I try super again I may notice a slight increase?

[Steve W2]

There was no noticeable difference in performance, but when I try super again I may notice a slight increase?

 

Plus you car is NA, charlie's is a TT and shouldn't use anything less than Super unleaded.

 

Good information though dude. These are sort of performance facts that we need.