Bearing in mind that a diesel engine is less economical in cold weather, does it mean that the hotter the weather gets the more economical the engine is ?
Or after 40C of really hot weather the economy dies off ?
Just a general question as its so so hot today.
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Yes diesel is a lot more economical in hot weather compared with cold or cooler weather, less friction to overcome so less fuel is required, also the warmer fuel/air compressed mixture ignites easier so less fuel is injected into the cylinders to compensate.
My Focus 1.6 TDCi easily does in excess of 80+mpg on A/B roads during the type of weather we are having at the moment, 73f - 82f. I don't know where the exact optimal economy point is though, we don't get temperatures of 40c in the UK.
Economy has a lot to do with how a car is driven coupled with the drivers understanding & application of a particular vehicles engine technology/characteristics, gearing etc.
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certainly takes a LOT less time for my Mondy diesel to warm up in the morning once ambient temps are above 14 - 15 celsius.
Mike, I'm not sure about less fuel being injected ... VAG PDs and (I believe) most modern common-rail systems have fuel coolers and fuel temp sensors, which allow the ECU to compensate for the lower density of hotter fuel, by injecting more per cycle.
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So why does my V70 have an intercooler to cool the air down after it has been heated by being compressed in the turbo?
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IIRC from my first year thermodynamics, the efficiency of the engine is actually related to the temperature of the engine itself. Hence the reason diesel engines deliver poorer fuel economy in the winter is because they take so long to come up to operating temperature. I would have thought the impact from friction is negligible.
I would argue the other way and put that a diesel engine properly warming up but running in a cold environment will be more efficient. The intercooler is there to reduce the temperature of the air entering the engine (and hence increase the density and thus the mass of fuel in the engine assuming it's running stoichiometrically).
When I was flying in Phoenix the high density altitude (temps hit 40+C most days) made getting decent performance from non-turbocharged engines a nightmare.
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so it's denser when it goes into the cylinder.
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Mike
Seriously, 80 mpg? Is that the 90 or 110 hp version. My fiesta TDCI 90hp does 65mpg. What's the secret?
Peanut
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Seriously, 80 mpg?
Quite. If Ford doesn't claim it, I doubt that you will achieve it. My car calculates anything between 28 and 999 mpg depending where my 'throttle' foot is, but overall the answer is usually 62-65, somewhat less than Peugeot's extra-urban textbook figure. I get about the same answer from the miles and the litres.
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I think that the cause of better economy in many diesels during hot spells, is because most diesels use hot-wire ammeters to measure air volume or MAF sensors by their more usual name.
These crudely measure the air volume based upon how much the sensor is 'cooled'. So if, like today it is over 30 degrees, the sensed Mass of the air is less than if it was 0 degrees and as such less diesel is subsequently injected.
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I thought the mass of warm air was a lot lower than cold air? Hence the air quality getting into the fuel mixture is better quality in colder weather?
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I thought the mass of warm air was a lot lower than cold air? Hence the air quality getting into the fuel mixture is better quality in colder weather?
This is true from an aerodynamics point of view, but my point is that the WAY air mass is measured relyys on the cooling of the MAF sensor, which will be less 'cooled' in hot weather than in cold weather, thus telling the ECU that there is less air mass and thus causing less fuel to be injected.
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>>These crudely measure the air volume
No, they measure mass - hence the name.
As they work on the cooling effect as described, they automatically compensate for the changes in air density at different temperatures, and, if working properly, correctly report the air mass, which is the value needed by the engine management unit. At sensible cost, it's by far the best way of measuring the air mass injested.
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Fine, but at bottom a fixed mass of car will need a certain amount of energy input to complete a given journey, which will come from burning a certain mass of hydrocarbon and a proportional amount of air. When the fuel is measured 'hot' its mass will be less tho the litres may be the same; to balance that, the car's lubrication may be thinner, air-drag may be less, and perhaps rolling resistance of softer tyres on hotter roads may be more ...
But presumably in summer the refineries use higher-carbon (i.e.less volatile) diesel than in winter, which means denser and therefore higher energy-content, diesel? So mpg apparently improves?
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I always thought it was the different blends of diesel in winter and summer that accounted for the difference.
I thought all internal combustion engines "preferred" colder, denser air.
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It's beneficial to fill-up early in the day as the temperatures are lower and the fuel is more dense, so you are getting more "bang for your buck" - myth or fact?
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There may be a marginal advantage I suppose, but as the fuel is kept in underground tanks I would have thought its temperature is pretty constant. Perhaps a more significant consideration is how recently it has been delivered from the tanker?
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Brim to brim actually, the trip computer is fairly accurate but brim to brim more so.
I use BP Ultimate diesel, synthetic oil, keep tyres at correct pressure, use high gears as much as possible, light right foot, minimum braking, only use air con when necessary. It's also been tuned for economy which provides upto 10% additional fuel economy although I still use to beat the manufacturers fuel figures prior to tuning.
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MT
I would have thought that the less powerful car would get better mpg. Curious. How many miles has it done - would that affect it much?
Peanut
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Seriously, 80 mpg?
Yes seriously, it's the 109ps, and I can easily better Fords claimed urban & extra urban data. Only during cold winter periods does the Focus economy drop below the claimed figures.
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I can easily better Fords claimed urban & extra urban data.
Hmmm. Did you calculate it, or your car's computer?
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Colinh, I would have thought that idea would apply more to petrol as this evaporates but diesel doesn't evaporate (just thinking of spilt diesel on the forecourt floor - always seems to stay there)
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...It's beneficial to fill-up early in the day as the temperatures are lower and the fuel is more dense...
Cleverer people than me have worked out this comes to a drop or two a tankful, which is well within the the accuracy limits of the pump or what's left in the pipe, and that tiny quantity matters not, anyway.
I think I read somewhere those who buy and sell by the supertanker load have a calculation to take temperature into account.
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Peanut, the tuning boosted torque and power and economy benefits as a result in like for like driving. The standard 1.6 TDCi 90ps and 109ps both have spare untapped power/torque & economy, there is little difference in their overall economy in standard form.
Last year it was noticable how easy it was for manufacturers to suddenly come out with an economy version as soon as the market was ready (when oil went high last year). In many cases they simply released some of the spare uptapped power/torque & economy as far as the engine was concerned.
Manufacturers decide to sell their cars (engines) for a variety of commercial reasons. The Focus currently has a 1.6 diesel engine with 2 different outputs, 1.8 & 2.0 diesel each with 1 specific output.
In the future (and including now) a lot of manufacturers will be offering fewer engine capacity options (cost reductions, increased fuel economy, price positioning etc.) but will offer a specific engine capacity with various output options. As such manufacturers will be forced to free-up some of the potential spare power/torque capacity they currently have available in some of their diesel turbo engines, the same applies to petrol turbo engines.
The Focus III (due next year) will have a 1.6 TDCi option offering upto 128bhp, plus 2.0-litre with 168bhp or 197bhp, with improved fuel economy and reduced emissions. It'll be interesting to see how improved the fuel economy is compared with the current Focus II.
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