Question about machine that runs on the road.
 

sorry - could not resist, prob get nil replies and serves me right too . . .!

Being generally a nerd on fuel economy - I have an engine (which is attached to a car) and it is a diesel (I hate Diesels in aeroplanes as they tend to bancrupt operators . . .)

Back to the diesel engine then. I noticed that in the winter I get 10 miles to the quid or 125 miles on a tenner - and yet in the summer, the fuel consumption is considerably higher . . . ?

I checked for leaks and changed back to my old fuel supplier in case other fuels were less volatile. volotile, whatever, and yea, I am using more fuel in the summer.

How does that interest pprune . . ?

Just wondered if it could be air density helping the combustion along in the winter by being more dense but messing it up by being . . less dense in the winter?

(Moderators should be onto me within about five seconds, but at least it is technical - ( I bet it ends up in Jet Blast)

Edited edition = less dense in the (summer)

Temperature.
Engine draws in cold air in winter, cold air more dense, more dense more oxygen, more oxygen more efficient use of suck-squeeze-bang-blow. Principal of turbo-supercharging. Compressing (charging air) makes it warm, loses thermal efficiency so add intercooler to cool charge down.

The speed record breaking inter-war cars often had a compartment next to the air intake which before a run was attempted was filled with crushed ice in an attempt to cool the air down.



The answers all in air temperature.

I always find a diesel to be better in summer as there are less freezing agents in the fuel. Right up till the A/C goes on and the roads become congested!

Honest John has a this quote in a discussion on this theme...

Do you use the air conditioning most of the time in summer?

G

Depends also on wether or not there are winter or summer diesel blends where you are. When I drove a Mercedes diesel for several years the fuel consumption would be consistently higher in one season over the other. But that had a lot to do with additives in the diesel on account of emission controls.

- Peter.

RedhillPhil,

We have several anecdotal stories that absolutely contradict the "colder is better" story.

Cough described a 17% loss of fuel economy in winter. My gasoline vehicle gets 60 mpg in summer and 50 mpg in winter (that's miles and gallons, gentlemen) which is also 17%.

Is there some way to resolve these contradictions?

Air is denser when cold. :sad:

Note that the denser air has the ability to hold more fuel that lighter air. This may appear counterintuative, but airflow to an engine is virtually constant (except when the engine is warming up on choke) Warming up the engine, simply aids combustion.
Given the average, there is only so much that can be managed, given the numerous single points that are common. ie, the computer will try to balance the OAT, intake velocity, intake volume with the fuel source. All fuel is not equal, so IF your vehicle has a fuel sensor, it can optimize that curve, if it does not, it will relegate the same average metering.
Average fuel metering, winter vs summer, creates a significant difference in economy.
As you know from aviation, there may be different additives, depending on weather, but also in general.
Diesel is especially notorious for a plethora of additives, meant to 'cut' the volume, the temp range, or just to get rid of PCB oil.

Do you get D1 or D2?
D1 is a thinner blend, and you wont get as good a mileage
Diesel can be cut with 5 to 10% biodiesel as well, this will actually increase capacity in diesel...
D2 with 10% bio is a monster of a blend! :ok:

Volume variation with temperature.
Hot fuel expands so lower mass, cold fuel contracts so more mass/per gallon.
It is the mass of the fuel that determines it's energy value.
Most (warm) countries sell fuel by volume uncorrected. Canada IIRC (for example) sells it's fuel corrected to Standard day conditions (cause it makes more money for the fuel companies).

One should get slightly better fuel consumption in the summer...

Because in the summer you do not have a very cold engine,a cold engine has higher friction and it will take longer for the oil etc to get to normal temp during cold weather !

Another factor in the winter is that you may have your engine idling for 5 mins or so while you deice your windscreen :) this does affect your average mpg :)

In the winter my diesel was doing 52mpg (avg),it is now doing 54mpg(avg)...what does affect diesel mpg is short journeys!
On most modern diesels even the cruising speed does not affect the mpg dramatically,a high speed long distance motorway dash will not cost me any more than 2 mpg.

BTW owing to the unusual hot weather recently...I have been using air con quite a bit ...dashboard still reads 54 mpg :)

Oil viscosity graph :)

https://www.google.co.uk/url?sa=t&rc...sJgbL_RLOvwZYQ

In general under the same conditions in regards to driving style i do get a better fuel economy in summer. However, during the warm season i'm much more often inclined to drive with the roof down, considerably faster than 210km/h and using the aircondition. That really drives up fuel consumption. Winter tires (mandatory here by law) are usually limited to only 210km/h which limits the speed, not to mention unfavourable conditions like snow, ice and so on limit speed even more.

He he, but Denti, April first was some time ago...

We all know that aerodynamic profile drag increases as the square of the speed.
Does anyone know what proportion of the power required to drive a car along is needed to overcome that drag, and what proportion is needed to overcome internal friction, tyre hysteresis, etc?

@AB have to say i do not really understand that comment. Was it about the speeds? We do not have a speed limit over here, driving faster than 200km/h is quite common if conditions (traffic, roadwork) allow it.

Dont forget most diesel engines are DI - direct injection so the only place where they 'mix' together with air is in the cylnder itself.

As RedPhil says, the colder the air, the more dense, so the mass drawn into the cylinders, is higher on cold days.

Hotter days less air mass = less power.
More throttle input to compensate= more fuel burnt.

Yes you may get a little more power on a cold damp day,but for the reasons I posted previously and also as others have alluded to about winter fuel composition...any increase in engine efficiency is wiped out by the other factors previously mentioned.
I have driven diesels for 30 years and always get better mpg in the summer :)

You get as much power as you need to push the car along. The power vs air density curve may affect racing performance, where max power makes a difference. But not for day to day driving.

There are two things going on: Friction (engine and tire) vs temperature and engine efficiency vs temperature.

Air drag (like lift) goes down as temperature goes up. Engine friction is probably relatively constant once warmed up (being dependent on the thermostat set point). But engine pumping losses are lower for less dense air (warmer). And this probably affects diesels more so than petrol engines given their 'wide open throttle' operation.

The other factor is the additional energy needed to heat colder air up than warmer air. Thermodynamics isn't my specialty, but given displacement and RPM, the mass flow and resulting cylinder PV curves would reveal the work difference.
:8

EEngr may have the resolution. My 60/50 summer/winter experience comes from thirteen years of driving a hybrid that displays the instantaneous full consumption, as well as cumulative.

On a familiar piece of road, I can tell the headwind component just from looking at the instantaneous fuel consumption.

The resolution may be that the improved engine performance in winter is overwhelmed by the increasing drag. So far, I haven't had to worry about lift.

In a diesel car the instantaneous mpg readout fluctuates wildly even with minute 'throttle' pedal movement - it is a useless measure for any practical purpose !!
I only ever use the avg readout.
Wind resistance/drag coefficient will vary between car models - mine is a fairly draggy shape which is why I lose 2 mpg on a long fast motorway run...in previous and more sleek cars - long distance motorway driving has actually given a better mpg.