Sorry mech1, the only truth is that an engine runs best in COLD weather, this because the air is more dense, hence the need for an intercooler on efficient turbocharged engines, the intercooler cools the incoming air making it more dense thus allowing more to enter the combustion chamber at any given manifold pressure.
The issue of the temp that the engine runs at is not directly related however the reason that cooling systems are pressurised is to stop the coolant boiling, simply the boiling point of any liquid increases in realtion to its pressure.
Regards.
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The issue of the temp that the engine runs at is not directly related.
cannot see that.after all any engine unless running at correct temperature controlled by thermostat.will send a message to ecu saying the temp is wrong causing either under or over fueling.apart from the fact if thermostat gives up there is then no control of the temp.whichever way you look at it engine temp is a major factor in the economy of engine turbo or not.dont dissagree with the pressurisation as it has to be.
besides what would be the point of having a temperature control on a engine if it is allowed to run cool all the time.ie no thermostat installed. besides your fuel consumption would be something to complain about
I could be wrong here but thought the idea of intercooler was to increase oxygen by cooling so giving a more efficient burn.Like I say could be wrong
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Hi, In saying that the temp the engine runs at is not directly related I mean that two different engines might be designed to run at temps 15deg c apart however both will run more efficiently in cool damp air than in hot dry air. Of course if an engine is over heating or over cooling, i.e. outside it's design tolerances, it will not run efficiently.
Yes an intercooler increases oxygen, it does this by cooling the air, making it denser, so more air (therefore oxygen) is contained within a given volume at a given pressure.
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Summer and winter blends of diesel and petrol at oil refineries
are different and have different calorific values. This (in my opinion) is the main reason for the MPG difference.
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Interesting point, GIM.
'have different calorific values'. Is this true? And why? And do you have any evidence.
I know that winter Diesel is different to summer diesel as it mustn't freeze. but I didn't know that petrol was any different.
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\'have different calorific values\'. Is this true? And why? And do you have any evidence.
I know that winter blend diesel has up to 25% jet fuel in it to help with waxing and that winter grade petrol has much more butane in it....I work at a refinery so I could find out further details if you are interested.
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Petrol has not as much hydrocarbons in.which means as its calorific value is low it burns easier and needs less oxygen to burn.kerosene/paraffin ie jet fuel is higher in calorific value and needs more oxygen to burn.but paraffin I think would be added but in a small amount to increase its volatility during cold weather.
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What are you talking about, mech1? What is petrol if not hydrocarbons? Ignoring things like detergent additives, if I thought my fuel had non-hydrocarbons in, I would complain to someone! Alcohol, perhaps?
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Think I wrote wrong so fair do`s But I think you got the jist.Having a conversation and writing a post dont mix.if you see my point?
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I think you would find it hard to vary the calorific value of a petroleum fuel by anything like 5%. Coal varies a good deal (think of the exhaustive testing done on steam engines using Welsh or Yorkshire coal) but it can contain all sorts of non-fuel, unlike hydrocarbon oil.
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You may also find in hot humid conditions the engine would be even more efficient.the same also applies to none turbo`d cars
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I also forgot to mention unless I am wrong is that the intercooler cools down the air that is compressed during turbo boost.as if it did not the engine would suffer detonation as fuel is injected rather than when intended as in spark plug firing off.ie temperature of the air is higher than it should be
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Re runs better in hot humid conditions, this is wrong, any engine runs better in cool damp air because the air is denser and contains more oxygen, the moisture inhibits detonation.
We are rather going round in circles here though take it from me cool air is best hence the need for intercoolers to make turbos efficient.
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One of my pilot friends relishes the prospect of dry and cold days to take his Moth for a trip: cold air is dense than warm air, producing more lift, and the engine runs better. Note: "dry and cold". Denser air is harder for any vehicle to push through; and, even if the engine is running more efficiently, it is also constantly battling against the low ambient temperature. Hence, I guess, fuel consumption increases in winter, as the greater efficiency of the engine is outweighed by the air-density and ambient-temperature factors. But I'm no expert.
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Hi Roger, once again an engine runs better in cool damp air, the dense air provides more oxygen and the moisture inhibits preignition in both diesel and petrol engines, in modern engines this allows their management systems to optimise injector / ignition timing.
This increased theromdynamic efficiency will far out way the increase in drag that denser air brings. The reasons for increased consumption in winter are numerous, increased use of lights, slowing more for corners, more stop start traffic etc etc.
With regard to flight, yes cold air provides more lift so less of the aircraft's power is used keeping it in the air, however an aero engine will also run better in cool damp conditions for reasons outlined above.
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Well, if the calorific value per unit volume is 5% lower in winter than in summer, there you have it. Perhaps, GIM, you would be able to check this out for us? (If on the other hand the winter calorific value is HIGHER, then it's back to the drawing board!)
And yes, 5% appears reasonable from this link
www.uvi.edu/Physics/SCI3xxWeb/Energy/GasolineFAQ.h...l
which suggests 42-44MJ/L calorific value for 'gasoline' which is a 5% range.
I know unleaded petrol contains considerably more aromatic hydrocarbons than leaded used to, in order to boost the octane rating once the 'lead' (actually a complex hydrocarbon containing lead) was removed. I'm guessing now, but I assume that there is a correlation between 'octane' rating and calorific value?
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I'm guessing now, but I assume that there is a correlation between 'octane' rating and calorific value?
I sense another "Optimax is better..." thread in the air!
Mapmaker,
If you do a forum search on "Optimax", you'll find some useful info on Calorific/RON values etc. You may have to trawl through some lengthy threads .....don't say I didn't warn you!
:-)
Chad.
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Thanks, Cheddar. I like the Back Room: you learn something every day. I still can't help thinking that ambient temperature must be a significant factor in itself.
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Question
Petrol or diesel for cars is purchased by volume rather than weight. The weight of a litre of petrol on a hot day in summer must be less than on a freezing cold day in winter. Does this mean that motoring in winter is more economical? And is this why airline companies purchase their fuel by weight rather than volume?
Answers
The density of any given fuel decreases with increase of temperature. So the mass will generally be less for a given volume in summer. For an engine operating at a fixed efficiency the energy produced is proportional to the mass of fuel burned. So, all things being equal, winter motoring should be more economical.
Because the fuel companies adjust the composition of their petrol or diesel by season to ensure easy starting in winter and to avoid vaporisation in summer, it is almost impossible to tell how the energy content of a volume of fuel will vary over the year. The fuel consumption also depends on the efficiency with which the carburettor or injection system copes with temperature changes and, in winter, the car will take longer to warm up and the oil will be thicker, all of which will reduce economy. So the actual seasonal difference will be negligible in monetary terms.
Airlines buy their fuels by volume, the same as motorists. It does not seem possible to get a meter which will give an accurate direct reading of the mass of a liquid delivered. Aircraft fuel gauges read in kilograms and consumption is calculated in kilograms per hour.
All flight planning is done using fuel in kilograms. It is also necessary to know the aircraft weight, including the fuel, to calculate takeoff performance and speeds.
The gauges are checked by converting the volume uplift into kilograms using the density of fuel. The density of JET A1 fuel varies from 0·80 at around freezing to about 0·775 at 35 °C, a difference of about 3 per cent. When it is necessary to carry maximum fuel this can make quite a difference. On a Boeing 767, a full tank of fuel weighs about 80 500 at 30 °C and 83 000 kilograms at 0 °C.
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That depends how cool?
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RE GIM. Many years ago I was waiting for a flight from Thesaloniki to Gatwick, the temp was high 30's and we were delayed while an Olympic 737 took 10 or 12 attempts to land due to undercarriage problems. Once we were in the air and some where over the Alps the capitain explained that we would have to refuel at Ostend before flying over British airspace due to the high temps in Greece causing the av fuel to expand and not achieve a full fill. Once at altitude, approx 30000ft the fuel contracted by about 5% which left us with below the minimum for CAA regs. Wasn't helped by the BAC111 (shows how long ago it was) being at the limit of it's range.
However this temp difference would have been approx +40 to -30, the difference between summer and winter motoring is nowhere near this approx 70 deg C range.
Regards.
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