I sort of knew that petrol cars performed better in cool weather. I remember reading an article in a recent edition of EVO Magazine comparing the M5 to the maserati quatroporte and a merc cls AMG, which enthused about the M5 V10 'coming alive' in the cold european mountain air.

I have recently been wondering if it is the other way round with diesel engines. In cold weather, my engine feels reigned in and less smooth. Now the weather is warming up (20 odd degrees C at the moment in UK - which is good for us ) my car is running silky smooth and really feels more edgy and responsive.

Znod - respect to your post - I didn't understand it entirely, but I think you have basically proven that petrol engines respond better in cool weather.

Can anyone explain why it seems to be the reverse with diesels?

 

Diesel engine uses compression ignition compare with Petrol engine which uses a spark plug to ignite the fuel.

The theory of compression ignition is that by compressing air its temperature raises, so the typical engine compression ratio for diesel engine is 25:1 much higher than a petrol engine.

Diesel engine works by compressing air and injecting fuel into the combustion chamber when the piston are at the very top. By injecting diesel fuel into the hot air it will burn and hence creates power.

But in very cold weather, diesel fuel thickens and increases in viscosity and forms wax crystals or a gel. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner which make it hard to start and not running smoothly.

Petrol engine burn air and gas mixture which is already mixed prior to entering into the combustion chamber so cold weather doesn't affect the petrol engine as much.

 

interesting topic,

As I' am posting from Saudi Arabia, Heat is a common factor which we have to live with it, yesterday the temperature was reading 46.5C (still in the begining of June)

I will not go technical since this is something i am not good at, however, what i am experiencing is something very weired!!!

once i drive for a certain period inside the city with such high temperature and full A/C i am having ticking noise coming from the front side.. after consulting our local BMW dealer ship and after so manyvisits they could finally tell me that the exhaust shield (cover to prevent the heat to be transmitted to the car) is expending/deforming . by doing so it is touching some area and the ticking noise is being transmitted in the car body... whenever the car cools down these noises disapear...

does anyone experienced this and what was the solution

Regards

 

Yeah, Znod's analysis is very good and correct - good refresher from my aviation science studies...
for something a touch more on the practical side I'll just refer you to this little calculator.

calculator



You will be able to see for yourself the relative difference of engine power at various conditions. Just get temp and pressure reading from your local airport in your area from any aviation weather site. METAR's ( 4 letter code for airports ) you will be able to find here :


METAR codes

then input here airport weather

to get real time info on pressure, temp, dew point etc...


good luck..

 

I agree but I am far more interested in the ambient temperature of my gin in the summer. I generally like it cooler than 65 degrees when the temperature is above one hundred. This allows me to kill more brain cells. By removing these unneeded cells I can make more money so I can pay you smart guys to worry about these important issues for me.

 

Good explaining!!!

 

Thank you for reading along, my friend.

 

I think the goverment is still trying to find me from the 60's. I'd be afraid to apply.

 

The three big weather-related factors can be thought of as temperature, altimiter setting, and dew point. For my 48 passes, here are some Pearson correlation coefficients for unadjusted ET.

Temperature = .474
Altimeter setting = -.526
Dew point = .06
Density altitude = .616

Thus, it is hard to say whether temperature or altimeter setting has the most effect on ET. But, the main point is that it really is necessary to think in terms of density altitude (DA) in thinking about the effects of ET on weather. As indicated earlier DA can be expressed as a function of temperature, altimeter setting, dew point, and altitude.

The same story is told when one looks at the correlations between DA and each other factor:

Temperature = .882
Altimeter setting = -0.647
Dew point = .208

From an overall perspective, what you have been noticing is the SOP correlation between probably the most noticable component of density altitude and performance. But, for explanatory purposes, one must consider all of the components--and generally other things. For example, I can explain over 99% of the variation in my adjusted ET data with the following multiple regression coefficients.

AS = Altimeter Setting in Hg
DPF = Dew point in F
Pro RR RWHP = All Loss Pro RR RWHP
DA = Density Altitude
UET = Unadjusted ET
Calculated AET = ET calculated with regression coefficients
AET = weather and altitude adjusted to 500' ET caculated with online calculators

Note that temperature does not enter into the regression equation because of its very high correlation with density altitude.


_________________________Variables and Coefficients__________________________
Interecept........AS..........DPF.......Pro RR RWHP......DA...........UET..Calculated AET......AET
0.83133.....-.016848..-0.000027.0.000038394.-0.000176. .98328.....13.549..........13.554
.................................................. .................................................. ......13.662..........13.668
R2..................0.993430160................... ...............................................13. 621..........13.627
Adjusted R2....0.992648036................................. ..................................13.657.......... 13.663
SE..................0.006352941................... ...............................................13. 688...........13.686
.................................................. .................................................. .......13.731..........13.729
Term.................Coefficient.................. .................................................1 3.655...........13.652
Intercept..........0.831335390.................... .............................................13.70 4...........13.703
AS..................-0.016848111....................................... ..........................13.721...........13.719
DPF................-0.000027431....................................... ..........................13.736...........13.734
Pro RR FWHP....0.000038394............................... ......................................etc......... .......etc. (through pass 48)
DA.................-0.000175801
UET.................0.983281715

 

The first site you link is the best I have found. And, for weather matters, check this one out. It's excellent. If I wanted to know more about many of the things in my posts, then these are the first places to go online IMO.