When ULSP was first introduced (at least when it was first labelled as such) in late December I also noticed a drop in mpg from around 37-39 to 35-36. This would tend to fit in with what your other readers have found. Since then my mpg has improved and at the last couple of fill-ups was around the 40-41 mpg mark.
However, when I noticed the drop in mpg the ambient temperature dropped to around -5 to -10 C in the morning. At the moment the temperature is between 15 to 20 C and higher in the evening. My theory is that ULSP makes little if any difference to fuel consumption and it's the low temperatures that caused the increase in consumption rather than the fuel.
It probably goes to show how difficult it is to measure fuel consumption accurately.
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Rod
You have a point here. When I monitored the fuel consumption of my car more closely than I do now it was clear that 'winter' consumption was definitely several mpg worse that 'summer' consumption. It was still very noticable on long journeys, so it wasn't solely due to the extra period the car spent on 'choke'. I'm sure this effect still exists, plus the best 'single trip' figures I've had have always been in the summer.
The reasons - certainly the longer periods of cold running, but also probably the different summer/winter petrol formulations - it's more volatile in winter. The data I have show that its density and CV can vary by about 4%, which I guess is the annual variation. As we buy fuel by volume, not mass, the density and CV variations are very significant of course!
Regards
john
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I don't suppose the high winter fuel consumption has anything to do with having the headlights/heating on, automatic chokes staying in longer and mothers taking their precious brood to school saves them getting wet and shrinking in the rain. In summer the M32 is empty, come winter its solid from one end to the other.
From a technical view it wouldn't have anything to do with the air mass being more dense due to the lower temperatures. More air more fuel?
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denser air will expand better than thin air, but the atmospheric pressure is usually lower in winter and wetter. Old escort Xr3 engines used to inject water vapour to 'cool' the air which meant better compression (I think?!?).
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OK guys, a few misconceptions here. Apologies to those who know this but an internal combustion engine works by drawing in air, heating it by combusting the fuel, and the subsequent expansion increases the pressure in the cylinder so forcing the piston down and producing work.
Increasing the density of the air drawn in results in a greater mass of air in the cylinder, and a greater potential power output, provided the correct amount of fuel is burned. That's why engines lose power at high altitude - they (a) rely on atmospheric pressure to fill the cylinder on the inlet stroke, and (b) are affected by the lower density of the air, so the cylinders are filled less effectively.
Using a water spray as a power enhancing device has been used for many years, typically on aero engines, and recently on gas turbine power stations in hot climates. (the power output of gas turbines is significantly affected by increasing ambient temperatures, precisely because the mass flow of air drops with increasing air temperature). The idea is that a fine water spray evaporates, drawing the latent heat of evaporation from the surrounding air so reducing the temperature of the air and increasing its density. Dense air does not 'expand better' - it's simply that an engine can produce more power when the cylinders contain a greater mass of air - which is exactly how super- and turbo-charging increase power.
Now, from that point of view an engine should be more efficient in winter when air temperatures are lower. However, this seems rarely to be the case. I think the posts have identified many of the offsetting effects - different fuel specs, more time on rich mixture, more traffic, more drag from colder lubricating oil, especially transmission oil which takes much longer to heat up than the engine oil, colder and so stiffer tyres, increased electrical load (20 amps extra = 1/3 bhp which could be 1 -2% of the engine output if you're running at 30 - 40 mph) etc. It's my view that these factors greatly outweigh any gain from the lower air inlet temperature.
regards
john
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John Slaughter wrote:
>
> Using a water spray as a power enhancing device has been used
> for many years, ........The idea is that a fine water spray evaporates....
Hi John,
I presume the amount of water spray has to be very carefully adjusted so that it gets drawn into the combustion system only as vapour and effectively increases the humidity and density of the incoming air. As water vapour has no calorific value (CN=0) does this also decrease the NOx emissions? Maybe by reducing peak combustion temperatures? Or am I talking out my rear end?
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Stuart
Yes, absolutely right. A very fine spray is used because the intention is that the water is fully evaporated, so maximising the cooling effect on the air. In terms of mass, the mass of water is a very small proportion of the mass of air, and the performance benefit to the engine far outweighs the downside of adding a non-combustible gas to the air, which is 80% non-combustible nitrogen anyway. The big gain is from the temperature drop of the air rather than the increased density brought about by adding the water vapour.
As for a de-NOx effect - I'm not sure it would be significant in this way, but certainly water injection is used as a method of reducing peak combustion temperature and so reducing NOx production in gas turbines. In this case the water (or sometimes steam) is sprayed directly into the combustion chamber of the GT, where it directly reduces the combustion temperature. The ratio mass of water/mass of fuel can be quite high. I've not heard of air inlet water spray being noted as a de-NOx technique, but I imagine the increased humidity could have only a minor effect.
Regards
John
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If I remember correctly I read an article in the MG Owners club magazine for a steam injection kit which claimed to solve all manner of ills including the use of 4 star in the MGC engine which preferred 5 star (Leaded of course, I am talking nearly 20 years ago). One of the arguments was that this was the reason why your engine seemed to run better on cold damp days. I didn't fully understand the Physics then and I'll have to re-read some of these messages now but I wonder if anyone heard of one of these kits being fitted? I've certainly not seen any sign of one for sale recently in the classic car press so it obviously didn't live up to its claims.
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Water mist is allegedly used in current rally cars and was used in some Formula 1 cars of the 1980s. A kit is on sale in the current issue of Demon Tweeks (sic) performance catalogue for £361.67. It claims to reduce pinking in engines running higher than standard turbo boost.
Wonder if you could adapt it to blast insect corpses of the windscreen ....
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Guys,
In the mid 80's (here he goes again) a mate and I both had Saab Turbos. Mine was a 1981 99 2dr Turbo that was really a road version of the rally car. It had some of the best mid-range top gear acceleration times of any car on the road. Steve's was a 900 Turbo hatch.
Anyway all we had to interest us in those days was what we could do to the cars and I remember one tempting fittment was a water injection kit. I seem to remember it was about £300 even then.
Unusually it was sold by the local dealers as a genuine fit Saab approved item, and for DIY as well. I think it was just a bottle, valves and some tubing.....perhaps a pump as well?
In the end neither of us purchased, perhaps too busy replacing the expensive tyres chewed through at an alarming rate.
Thinking back Saab really deserve a pat on the back for introducing turbos to the mass market. The cars were very quick for the time and nothing went wrong with our engines over a couple of years of very hard business use.
David
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Colder engines do take longer to heat up, but only by a minute or so. in summer the car is warmer and more fuel will evapourate through the filler cap and other small gaps in the system.
I am experimenting with the super unleaded, as I do 400 miles a week and my performance is averaging 25-28 mpg, I would like to see which offers the best economy.
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It's an interesting suggestion that your consumption variations were due to changes in temperature. Unfortunately logic would suggest that as one buys by volume, in winter the fuel should be colder and thus denser, and one would get more carbon and thus more miles from one's litre. This is not what you got?
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I can see what you are saying about the density of the fuel in winter compared to summer but as most of the storage tanks are underground there is very little difference in the temperature it is stored at around the year (the Earth is a surprisingly good insulator).
This is an issue, however, for the petrol station because the fuel will leave the refinery at a typically higher temperature than the one it is stored at. This means that they could be selling more mass per litre than they actually bought it at. It wouldn't be much but it could dent a retailer's profit margin. Maybe a petrol station manager could confirm this?
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Rod
Yes this is very true. It all comes from buying vehicle fuel by volume, when what you are really buying is energy. So the diesel buyers, with a product 13% more dense but similar CV do very nicely, thank you! Also gives the sellers the chance to juggle the spec. too.
In industry, you work out energy costs on a £/gigajoule basis, not on the tonnage purchased.
Regards
John
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honest john wrote:
>
> Answer to Stuart: threads don't drop off the bottom of the
> list, you just have to keep clicking on older topics. There
> must be about 50 - 60 pages of them.
Hej John,
For the last few weeks if I click on the older messages link I can go down about 5 pages and then the link disappears. I had assumed that this was a demon way of saving space on the system, yet you can still find the old messages under search mode so they are still there.
I have my view set on collapsed threads. Any ideas? Is it a problem this end?
Cheers,
S
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You can get by this and back to the very beginning by choosing a thread at the bottom of the last page you can get to, and then once read (if you desire) hit the go to topic link and you will find the previous topics.
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Yes, this is one of the quirks of Phorum. But overall we thought it was by far the best of the bulletin board programs available.
Collapsed view is fine, by the way.
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Thanks John I had missed that little tweak which works fine. Could not understand what I was doing different.
Cheers,
Stuart
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